Rosetta Rendezvous Mission with Comet 67P/Churyumov-Gerasimenko
Rosetta is a deep space mission of ESA , it is the first mission designed to both orbit and land on a comet. ESA selected the mission in Nov. 1993 as the third cornerstone mission in its long-term science program, called 'Horizon 2000'. The goal of the Probe is to rendezvous with the Comet 67P/Churyumov-Gerasimenko and map its surface in fine detail. It will also land a package of instruments (the Philae Lander) to study some of the most primitive, unprocessed material in the solar system. 1)
Comets are among the most beautiful and least understood nomads of the night sky. To date, half a dozen of these most heavenly of heavenly bodies have been visited by spacecraft in an attempt to unlock their secrets. All these missions have had one thing in common: the high-speed flyby. Like two ships passing in the night (or one ship and one icy dirtball), they screamed past each other at hyper velocity — providing valuable insight, but fleeting glimpses, into the life of a comet. That is, until Rosetta.
Launched in March 2004 and expected to reach the comet by 2014, Rosetta will be the first mission to revolve around the comet's nucleus and deliver a lander to its surface. The spacecraft will be located at a distance of 600 million km or 4AU (astronomical units) from the sun upon the comet.
Table 1: Some background on the Rosetta mission 2)
Legend to Figure 1: The Rosetta Stone is a granodiorite stele inscribed with a decree issued at Memphis in 196 BC on behalf of King Ptolemy V. The decree is given in three languages: Egyptian hieroglyphs (top), Demotic (middle), and ancient Greek (bottom). Champollion used the Greek to decipher the hieroglyphs (a full decipherment was published in 1824). The Rosetta Stone provided the key to the modern understanding of Egyptian hieroglyphs. The Rosetta Stone has a size of 114.4 cm x 72.3 cm x 27.93 cm. 3)
The measurements goals of the Rosetta mission are: 4)
- a global characterization of the nucleus
- the determination of its dynamic properties
- the surface morphology and composition
- the determination of chemical, mineralogical and isotopic compositions of volatiles and refractories in the cometary nucleus
- the determination of the physical properties and interrelation of volatiles and refractories in the cometary nucleus
- studies of the development of cometary activity and the processes in the surface layer of the nucleus and inner coma, that is dust/gas interaction
- studies of the evolution of the interaction region of the solar wind and the outgassing comet during perihelion approach.
Rosetta is truly an international enterprise, involving more than 50 industrial contractors from 14 European countries and the United States. The prime spacecraft contractor is Airbus Defence and Space (formerly EADS Astrium, GmbH, Friedrichshafen, Germany), responsible for building the spacecraft. Other contributions were provided by Airbus Defence and Space UK (spacecraft platform), and Airbus Defence and Space France (spacecraft avionics) and Alenia Spazio (assembly, integration and verification) are major subcontractors.
Rosetta comprises a large orbiter, which was designed to operate for a decade, and a lander. Each of these components carries a large array of scientific instruments that will perform the most extensive study of a comet to date. The orbiter will revolve around the comet from 1km distance examining the nucleus and environment of the comet.
The Rosetta orbiter is a platform of size 2.8 m x 2.1 m x 2 m (an aluminum box) with a payload support module, which houses the 11 scientific instruments, mounted on the top and a bus support module, housing the subsystems, on the base. Two sets of solar panels with 14 m length having an area of 64m2 extend from the side of the spacecraft and a 2.2 m diameter steerable, high-gain antenna dish sticks out from the front. The lander is attached to the back of the orbiter. The two solar panel wings rotate at ±180° to capture the maximum sunlight. 5) 6)
The spacecraft is built around a vertical thrust tube, whose diameter corresponds to the 1.194 m Ariane-5 interface. This tube contains two large, equally sized propellant tanks (each of 1106 liter), the upper one containing fuel, and the lower one containing the (heavier) oxidizer. The Orbiter also carries 24 thrusters for trajectory and attitude control. Each of these thrusters pushes the spacecraft with a force of 10 Newton, equivalent to that experienced by someone holding a bag of 10 apples. Over half the launch mass of the entire spacecraft, about 1670 kg, is made up of propellant.
Figure 2: Artist's rendition of the deployed Rosetta spacecraft (image credit: ESA)
The spacecraft is three-axis stabilized. Attitude is maintained by four reaction wheels as well as using two star trackers, sun sensors, navigation cameras, and three laser gyro packages. Power is supplied by the solar arrays. The solar cells employed are 200 µm Si solar cells of LLIT (Low Intensity, Low Temperature) type sized 37.75 mm in width and 61.95 mm in length (Figure 3). The cover glass is 100 µm thick ceria doped micro-sheet designated curb mount glass (CMG). The cover-glass covers the solar cell completely. The solar arrays will provide 395 W at 5.25 AU and 850 W at 3.4 AU, when comet operations begin. Power is stored in four 10 Ah NiCd batteries which supply the 28 V bus power.
The Rosetta deep space mission of ESA represents a rather special case of solar cell technology use. At its destination in 2014, the spacecraft is at a distance of about 675 million km from Earth, corresponding to 4.5 AU, a distance almost as far out as Jupiter , where sunlight levels are only 4% of those on Earth. In LEO (Low Earth Orbit), Rosetta is the most powerful spacecraft that ESA ever built, at 12 kW of installed solar power generation provided by two solar arrays, covered with hundreds of thousands of specially developed non-reflective silicon cells. But at the deep space distance of Comet 67P/Churyumov-Gerasimenko, the total solar power available is only ~400 W. - Rosetta is the first deep space mission ever to rely entirely on solar power generation beyond the main asteroid belt (with sunlight levels of only 3-4% as those in LEO) - without the use of RTG (Radioisotope Thermoelectric Generator) technology (as is being done by all other deep space satellites).
Illumination reduces with distance from the Sun by what is called the inverse square law – if one goes twice as far away only a quarter the solar intensity is available, at three times the distance, only one ninth of the intensity is available. This means that temperatures experienced by the spacecraft also fall with distance, though in principle this is good news for solar cell designers, since cell efficiency increases as the temperature goes down. 7) 8) 9) 10)
In practice however, in low solar intensities with temperatures dropping below –100°C, standard solar arrays show worse-than-expected performance due to unpredictable degradation of individual cells. To overcome this problem, the LLIT (Low Intensity Low Temperature) specific solar cell technology was developed. The resulting single-junction silicon cells are flying on ESA’s Rosetta comet chaser, which is venturing three times further from the Sun than Earth.
Legend to Figure 4: The image was taken in 2002 showing Rosetta being checked in ESA/ESTEC in Noordwijk, The Netherlands. One hinged wing is supported on a rig to allow it to unfurl safely in Earth gravity instead of weightlessness. Each wing is made up of five hinged panels, the steerable pair of wings together stretches 32 m tip-to-tip from the box-shaped spacecraft. 11)
At Rosetta's encounter with the Comet, the spacecraft is experiencing only 11% of Earth-level solar illumination — but still better than the 4% when it was furthest from the Sun. But instead of going nuclear, Rosetta runs solely on LILT (Low-Intensity, Low-Temperature) silicon solar cells, a new European technology devised for this mission, optimized for deep-space conditions. - The same is true of Rosetta’s Philae lander, whose batteries are designed to be recharged by the LILT cells covering its body.
RF communications: Communications is maintained via the high-gain antenna, a fixed 0.8 m medium-gain antenna, and two omnidirectional low gain antennas. Rosetta utilizes an S-band telecommand uplink and S- and X-band telemetry and science-data downlinks, with data transmission rates from 5 to 20 kbit/s. The communication equipment includes a 28 W RF X-band TWTA (Traveling Wave Tube Amplifier) and a dual 5 WRF S/X band transponder. Onboard heaters keep the instrumentation from freezing during the period the spacecraft is far from the sun.
The Rosetta spacecraft had to be designed for a high level of reliability as the main scientific mission is starting more than 10 years after launch, and also for a high level of availability during the early years of the mission cruise, which also contains many key technical and scientifically valuable events including three Earth swingbys, one Mars swingby, two asteroid flybys, several deep space trajectory correction maneuvers, and regularly scheduled onboard system and payload checkouts.
These outstanding efforts will assure that Rosetta will contribute significantly to answer open questions in solar system research such as: How pristine are comets? How does cometary activity work? Are the craters on comets from impacts or from other processes? What is the internal structure of cometary nuclei? How does cometary material look like and what is it made of? Are there internal heat sources that trigger normal activity and outbursts? What are the main physical and chemical processes in the coma? How does solar wind – comet interaction change at the different activity levels from 3 AU to perihelion? Are comets candidates that delivered prebiotic molecules and water to Earth?
Comets remain the poorest understood solar system objects. The future measurements of Rosetta orbiting around a comet for several months and delivering a lander to the surface will open a whole new field of research. And Rosetta will provide a much better understanding of comets and solar system formation, much as the Rosetta Stone did in our understanding of the Egyptian culture (Ref. 4).
NAVCAM (Navigation Camera): The Rosetta spacecraft uses a single camera with a 5º field of view and 12 bit 1024 x 1024 pixel resolution, allowing for visual tracking on each of the spacecraft approaches to the asteroids and finally to the comet.
Figure 5: Exploded view of major spacecraft components (image credit: ESA, AOES Medialab)
Table 2: Overview of some spacecraft parameters
Thermal louvers: Throughout its mission, the Rosetta spacecraft is exposed to extreme cold and hot temperatures. In the early and late stages of its prolonged expedition, the spacecraft will sweep across the inner Solar System, where sunlight is plentiful. However, in order to rendezvous with Comet 67P/Churyumov-Gerasimenko, Rosetta will have to probe beyond the asteroid belt, more than 5 times the Earth's distance from the Sun. In those frigid regions, the solar energy levels are only 4% of the those that we enjoy on our balmy planet. 12)
Since it is not feasible to wrap a spacecraft in multiple layers of warm clothing for periods of deep freeze, then strip these away when sunbathing is the order of the day, the ESA team has been obliged to come up with alternative ways of regulating temperature.
Designers have provided Rosetta with louvers — high-tech venetian blinds which control the spacecraft's heat loss. Lovingly polished by hand, these assemblies of thin metal blades must be handled like precious antiques, since any scratching, contamination or fingerprints will degrade their heat reflecting qualities.
The principle behind the louvers is quite simple. When Rosetta is cruising around the inner Solar System and basking in the warmth of the Sun, surface temperatures may soar to 130°C, and even internal equipment can reach 50°C. At such times, it is vital to stop the spacecraft from overheating, so the louvers are left fully open, allowing as much heat as possible to escape into space from Rosetta's radiators.
However, during its prolonged deep space exploration and comet rendezvous phases, when temperatures plummet to -150°C, heat conservation is the order of the day. Since the spacecraft's limited internal power supply - equivalent to the output from three ordinary light bulbs - then becomes the main source of warmth, it is essential to trap as much heat as possible. This means completely closing the louvers in order to prevent any heat from escaping.
Some 14 of these louver panels cover an area of 2.25 m2 on the Rosetta spacecraft, placed over its radiators across the side and back of the spacecraft. The louvers open and close on a fully passive basis, requiring no power to operate. Instead they work on a ‘bimetallic’ thermostat principle. The blades are moved by coiled springs made up in this case of a trio of different metals that expand and contract at differing rates, precisely tailored to rotate as required. Designed by Spain’s Sener company, the louvers were extensively tested by ESA’s Mechanical Systems Laboratory in advance of Rosetta’s 2004 launch. 13)
Figure 6: Photo of a louver panel (image credit: ESA–A. Le Floc'h)
Figure 7: Photo of the Rosetta spacecraft with thermal blankets, released on January 19, 2004, ready for testing in the Large Space Simulator, at ESA/ESTEC (image credit: ESA) 14)
Legend to Figure 7: Temperature control was a major headache for the designers of the Rosetta spacecraft. Near the Sun, overheating has to be prevented by using radiators to dissipate surplus heat into space. In the outer Solar System, the hardware and scientific instruments must be kept warm (especially when in hibernation) to ensure their survival.
Figure 8: Rosetta and Philae — the lander is attached to the orbiter during integration of the spacecraft (image credit: ESA)
Legend to Figure 9: Rosetta is the first spacecraft to journey beyond the main asteroid belt and rely solely on solar cells for power generation. The new solar-cell technology used on its two giant solar panels allow it to operate over 800 million km from the Sun, where light levels are only 4% of those on Earth.
Launch: The Rosetta spacecraft was launched on March 2, 2004 on Ariane 5G+ vehicle from Kourou, French Guinea.
Orbit and mission event overview:
The original target of the Rosetta mission was comet 46P/Wirtanen. A failure of an Ariane-5 rocket in December 2002 forced ESA to postpone the initially scheduled January 2003 launch and to re-target Rosetta, now heading for Comet 67P/Churyumov-Gerasimenko (Ref. 4).
Rosetta could not head straight for the comet. Instead it began a series of looping orbits around the Sun that brought it back for three Earth fly-bys and one Mars fly-by. Each time, the spacecraft changed its velocity and trajectory as it extracted energy from the gravitational field of Earth or Mars. During these planetary fly-bys, the science teams checked out their instruments and, in some cases, took the opportunity to carry out science observations coordinated with other ESA spacecraft such as Mars Express, Envisat and Cluster. 15)
During the 10 year trek across our solar system, Rosetta will travel five times the distance Sun-Earth, and will pass through the asteroid belt into deep space beyond 5 AU solar distance before it reaches its destination, the periodic comet 67P/Churyumov-Gerasimenko. On its way to 67P/Churyumov-Gerasimenko the spacecraft will employ four planetary gravity assist maneuvers (Earth-Mars-Earth-Earth) to acquire sufficient energy to reach the comet (Figure 10). Each of the fly-bys required months of intense preparation. In particular the fly-by of Mars in February 2007 was a critical operation: the new mission trajectory to 67P/Churyumov-Gerasimenko required that Rosetta fly past Mars at just 250 km from the surface, and spend 24 minutes in its shadow.
In between the last two Earth swingbys Rosetta will fly by the main belt asteroid 2867 Steins at a distance of 1700 km and at a relative velocity of 9 km/s on September 5, 2008. After the third Earth swingby Rosetta will enter the main asteroid belt again and fly by the main belt asteroid 21 Lutetia at a distance of 3000 km and a speed of 15 km/s on July 10, 2010. The spacecraft will enter a hibernation phase in July of 2011. In January 2014 Rosetta will come out of hibernation and begin a series of rendezvous maneuvers for comet 67P/Churyumov-Gerasimenko in May 2014.
The rendezvous maneuver 2 at~4.5 AU from the Sun will lower the spacecraft velocity relative to that of the comet to about 25 m/s and put it into the near comet drift phase, starting May 22, 2014 until the distance is about 10,000 km from the comet (Figure 3). It will be performed on the basis of a ground-based determination of the orbit from dedicated astrometric observations, before the comet is detected by the on-board cameras. The final point of the near-comet drift phase, the CAP (Comet Acquisition Point), is reached at a Sun distance of less than 4 AU. As soon as the spacecraft with a maximum relative velocity of about 1 m/s. The time and direction of the Rosetta-Philae separation will be chosen such that the landing package arrives with minimum vertical and horizontal velocities relative to the local (rotating) surface. After delivery of the lander on November 10, 2014 at a solar distance of 3 AU, the spacecraft will be injected into an orbit which is optimized for receiving the data transmitted from the lander and to relay them to the Earth. To adjust the payload operations sequences, the lander can be commanded via the orbiter.
Table 3: Milestones of the Rosetta mission
Figure 11: Schematic showing the spacecraft maneuvers close to comet 67P/Churyumov-Gerasimenko (image credit: ESA)
Sensor complement: (OSIRIS, ALICE, VIRTIS, MIRO, ROSINA, COSIMA, MIDAS, CONSERT, GIADA,RPC, RSI)
The Orbiter’s scientific sensor complement includes 11 experiments and a small Lander, which will conduct its own scientific investigations. Scientific consortia from institutes across Europe and the United States have provided these state-of-the-art instruments.
The instruments on the Rosetta Orbiter will examine every aspect of the small cosmic iceberg. Wide and Narrow Angle Cameras will image the comet’s nucleus to determine their volume, shape, bulk density and surface properties. Three spectrometers operating at different wavelengths will analyze the gases in the near-nucleus region, measure the comet’s production rates of water and carbon monoxide/dioxide, and map the temperature and composition of the nucleus (Ref. 4).
Our knowledge of the nucleus should be revolutionized by the CONSERT experiment, which will probe the comet’s interior by transmitting and receiving radio waves that are reflected and scattered as they pass through the nucleus. 16)
Four more instruments will examine the comet’s dust and gas environment, measuring the composition and physical characteristics of the particles, e.g. population, size, mass, shape and velocity. The comet’s plasma environment and interaction with the electrically charged particles of the solar wind will be studied by the Rosetta Plasma Consortium and the Radio Science Investigation.
Figure 12: The Rosetta spacecraft and its scientific payload (image credit: ESA) 17)
OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System)
Multi-color imaging with a WAC (Wide Angle Camera) and a NAC (Narrow Angle Camera) to obtain high-resolution images of the comet’s nucleus. PI: Holger Sierks, MPS (Max Planck Institute for Solar System Research), Katlenburg-Lindau, Germany.
The objective of the OSIRIS assembly is to observe the cometary rotation, and to study the physical and chemical processes that occur in, on, and near the cometary nucleus. It also maps the cometary morphology, which will help Rosetta’s lander (Philae) to find a suitable spot for setting down in the comet’s surface. -The strength of OSIRIS is the coverage of the whole nucleus and its immediate environment with excellent spatial and temporal resolution and the spectral sensitivity across the whole reflected solar continuum up to the onset of thermal emission. This provides a context for the interpretation of the results from Philae. 18) 19) 20)
The OSIRIS cameras were provided by a consortium of 9 institutes from 5 European countries and from ESA, under the leadership of the MPS . The participating institutes of the consortium are: MPS (Katlenburg-Lindau, Germany), LAM (Laboratoire d’Astrophysique de Marseille), Marseille, France; UPD (University of Padova), Padova, Italy; IAA (Instituto de Astrofísica de Andalucía ), Granada, Spain; University of Uppsala (Sweden); ESA/ESTEC, Noordwijk, The Netherlands; UPM (Universidad Politécnica de Madrid), Madrid, Spain; INTA (National Institute for Aerospace Technology),Madrid, Spain; IDA (Institute of Computer and Network Engineering at the TU Braunschweig),Braunschweig, Germany.
Figure 13: Photo of the OSIRIS cameras (image credit: MPS)
After the launch of Rosetta (March 2, 2004), OSIRIS was activated on several occasions before the arrival to the main target, comet 67P/Churyumov-Gerasimenko. It was commissioned in seven slots between March 2004 and June 2005, and it performed several important scientific observations:
• A monitoring campaign of comet 9P/Tempel 1 around the Deep Impact event on 4 July 2005
• The fly-by of asteroid 2867 Steins on 5 September 2008
• Two Earth swing-bys in Nov. 2007 and Nov. 2009
• The observation of the remnant of a collision between two main-belt asteroids in February 2010 - The flyby of asteroid 21 Lutetia on 10 July 2010
• Early observation of the comet from more than 1AU distance in March 2011.
Figure 14: Specification of the OSIRIS assembly
Science objectives: The OSIRIS science objectives for the comet nucleus, the gases and dust produced by the comet and for the asteroid flybys are:
Figure 15: Nucleus objectives of OSIRIS
Table 4: Dust objectives of OSIRIS
Table 5: Gas objectives of OSIRIS
Table 6: Asteroid flyby objectives of OSIRIS
Table 7: Mars and Martian satellites flyby objectives of OSIRIS
Table 8: Earth/Moon system flyby objectives of OSIRIS
ALICE (Ultraviolet Imaging Spectrometer)
Analyses gases in the coma and tail and measures the comet’s production rates of water and carbon monoxide/dioxide. ALICE is a NASA instrument providing UV spectroscopy in the band 70-205 nm. PI: Alan Stern, SwRI (Southwest Research Institute), Boulder, CO, USA. The ALICE UV spectrometer will analyze gases in the coma and the tail, and it measures the comet’s production rates of water and carbon monoxide or dioxide. It will also provide information on the surface composition of the nucleus. 21) 22)
Instrument: Light enters the Alice telescope through a 40 x 40 mm entrance aperture and is collected and focused by an off-axis paraboloidal primary mirror onto the approximately 0.1° x 6° spectrograph entrance slit. After passing through the entrance slit, the light falls onto the toroidal holographic grating of a Rowland Circle style imaging spectrograph, where it is dispersed onto a microchannel plate detector. The 2D (1024 x 32 pixel) format MCP detector uses dual, side-by-side, solar-blind photocathodes of potassium bromide (KBr) and cesium iodide (CsI). The predicted spectral resolving power (λ/Δλ) of Alice is in the range of 105 - 330 for an extended source that fills the instantaneous FOV (Field of View) defined by the size of the entrance slit. 23)
Table 9: Summary of ALICE characteristics 24)
VIRTIS (Visible and Infrared Thermal Imaging Spectrometer)
An imaging spectrometer that combines three data channels in one instrument. Two of the data channels are designed to perform spectral mapping. The third channel is devoted to spectroscopy. Maps and studies the nature of the solids and the temperature on the surface of the nucleus. Also identifies comet gases, characterizes the physical conditions of the coma and helps to identify the best landing sites. Provides VIS and IR mapping (spectroscopy) in the region 0.25-5 µm. PI: Angioletta Coradini, IAS-CNR, Rome, Italy. 25)
Instrument: The optical subsystems are housed inside a common structure - the cold box - cooled to 130 K by a radiative surface supported on a truss having low thermal conductivity. On the pallet supporting the truss, two sets of electronics and two cryogenic coolers for the detectors are mounted. The cold box is rigidly mounted on the pallet but thermally isolated from it. The pallet and cold box together form the optics module, which is mounted inside the spacecraft arranged so that the observing axes of the optical subsystems are normal to the nadir (comet) pointing wall of the spacecraft. The electronics module, containing the digital electronics and power supply, is mounted separately. 26)
The mapping channel optical system is a Shafer telescope matched through a slit to an Offner grating spectrometer. The Shafer telescope consists of five aluminum mirrors mounted on an aluminum optical bench. The primary mirror is a scanning mirror driven by a torque motor. The Offner spectrometer consists of a relay mirror and a spherical convex diffraction grating, both made of glass.
The mapping channel utilizes a silicon charge coupled device (CCD) to detect wavelengths from 0.25 µm to 1 µm and a mercury cadmium telluride (HgCdTe) infrared focal plane array (IRFPA) to detect from 0.95 µm to 5 µm. The IRFPA is cooled to 70 K by a Stirling cycle cooler. The cold tip of the cooler is connected to the IRFPA by copper thermal straps. The CCD is operated at 155 K and is mounted directly on the spectrometer.
The high resolution channel is an echelle spectrometer. The incident light is collected by an off-axis parabolic mirror and then collimated by another off-axis parabola before entering a cross-dispersion prism. After exiting the prism, the light is diffracted by a flat reflection grating, which disperses the light in a direction perpendicular to the prism dispersion. The low groove density grating is the echelle element of the spectrometer and achieves very high spectral resolution by separating orders seven through sixteen across a two-dimensional detector array.
The high-resolution channel employs a HgCdTe IRFPA to perform detection from 2 to 5 μm. The detector is cooled to 70 K by a Stirling cycle cooler.
Table 10: Summary of VIRTIS characteristics
• Aug. 1, 2014: ESA’s Rosetta spacecraft has made its first temperature measurements of its target comet, finding that it is too hot to be covered in ice and must instead have a dark, dusty crust. The observations were made by the VIRTIS instrument between 13 and 21 July, when Rosetta closed in from 14 000 km to the comet to just over 5000 km. 27)
• On 14 July, 2014, the entire surface of the comet occupied one of VIRTIS's pixels, allowing the scientists to estimate the mean temperature of the nucleus – around 205 K. While this may seem rather cold, it is somehow warmer than the scientists expected, providing the scientists with some first clues on the composition and the physical properties of the surface of the nucleus.
• July 8, 2014: First measurements of VIRTIS on board Rosetta have been probing the surface temperature on the nucleus of comet 67P/C-G. 28)
MIRO (Microwave Instrument for the Rosetta Orbiter)
The MIRO investigation addresses the nature of the cometary nucleus, outgassing from the nucleus and development of the coma as strongly interrelated aspects of cometary physics. During the flybys of the asteroids and, the MIRO instrument will measure the near surface temperature of these asteroids and search for outgassing activity in an effort to understand better the relationship between comets and asteroids.
PI: Sam Gulkis of NASA/JPL, Pasadena, CA. The MIRO investigation was conceived and designed functionally by the investigation team consisting of 19 scientists from 6 different institutions, and the MIRO project office, located at NASA/JPL.
Figure 16: Illustration of the MIRO instrument (image credit: MIRO Team)
The five objectives of the MIRO instrument are to: 29)
1) Characterize the abundances of major volatile species and key isotope ratios in the nucleus ices.
The MIRO instrument will measure absolute abundances of key volatile species - H2O, CO, CH3OH, and NH3 - and quantify fundamental isotope ratios - 17O,16O and 18O, 16O - in a region within several kilometers from the surface of the nucleus, nearly independent of orbiter to nucleus distance.
Water and carbon monoxide are chosen for observation because they are believed to be the primary ices driving cometary activity. Methanol is a common organic molecule, chosen because it is a convenient probe of gas excitation temperature by virtue of its many transitions. Knowledge of ammonia abundance has important implications for the excitation state of nitrogen in the solar nebula. By providing measurements of isotopic species abundances with extremely high mass discrimination, the MIRO experiment can use isotope ratios as a discriminator of cometary origins. The MIRO investigation will combine measurements of the variation of outgassing rates with heliocentric distance with models of gas votalization and transport in the nucleus to quantify the intrinsic abundances of volatiles within the nucleus.
2) Study the processes controlling outgassing in the surface layer of the nucleus.
The MIRO experiment will measure surface outgassing rates for H2O, CO, and other volatile species, as well as nucleus subsurface temperatures to study key processes controlling the outgassing of the comet nucleus.
The MIRO experiment will measure surface outgassing rates for H2O, CO, and other volatile species, as well as nucleus subsurface temperatures to study key processes controlling the outgassing of the comet nucleus.
3) Study the processes controlling the development of the inner coma. -MIRO will measure density, temperature, and kinematic velocity in the transition region close to the surface of the nucleus.
Measurements of gas density, temperature, and flow field in the coma near the surface of the nucleus will be used to test models of the important radiative and dynamical processes in the inner coma, and thus improve our understanding of the causes of observed gas and dust structures. The high spectral resolution and sensitivity will provide a unique capability to observe Doppler-broadened spectral lines at very low temperatures.
4) Globally characterize the nucleus subsurface to depths of a few centimeters or more.
The MIRO instrument will map the nucleus and determine the subsurface temperature distribution to depths of several centimeters or more. Morphological features on scales as small as 5 m will be identified and correlated with regions of outgassing.
The combination of global outgassing and temperature observations from MIRO and in situ measurements from the Rosetta lander will provide important insights into the origins of outgassing regions and of the thermal inertia of subsurface materials in the nucleus.
5) Search for low levels of gas in the asteroid environment.
The MIRO instrument will search for low levels of gas in the vicinity of asteroids and measure subsurface temperature to provide information on the presence of water ice, and on near surface thermal characteristics and the presence or absence of a regolith.
MIRO is configured both as a continuum radiometer and a very high spectral resolution line receiver. Center-band operating frequencies are near 190 GHz (1.6 mm) and 562 GHz (0.5 mm). The spatial resolution of the instrument, operating in the submillimeter band, is approximately 5 m at a distance of 2 km from the nucleus. The MIRO spectrometer is tuned to measure four volatile species - H2O, CO, CH3OH, and NH3 and the isotopes of water —H217O and H218O. These four species have all been measured to be present in comets. The spectral resolution is sufficient to observe individual, thermally broadened, line shapes at all temperatures down to 10 K or less. The MIRO experiment will use these species as probes of the physical conditions within the nucleus and coma. The basic quantities measured by MIRO are surface temperature, and gas abundance, velocity, and temperature of each species, along with their spatial and temporal variability. This information will be used to infer coma structure and outgassing processes, including the nature of the nucleus/coma interface. 30) 31)
Instrument: MIRO consists of an assembly of two heterodyne radiometers:
- Millimeterwave receiver (at 190 GHz , ~ 1.6 mm)
- Submillimeterwave receiver (at 562 GHz, 0.5 mm).
The mm and sub-mm radiometers are configured with a broadband continuum detector for the determination of the brightened temperature of the comet nucleus and the target asteroids. The sub-mm receiver is configured as a very high resolution spectrometer for the observation of the eight molecular transitions. The instrument is constituted of four separate physical modules, interconnected by a harness. The sensor unit is mounted on the spacecraft payload plane using the baseplate as the interface. The telescope boresight direction is aligned with the Rosetta payload line of sight. The optical bench is mounted on the undersite of the baseplate, under the telescope and inside the spacecraft. The mm and sub-mm wave receiver front ends, the calibration mechanism and the quasi-optics for coupling the telescope to the receivers are installed on the optical bench. The Sensor Backend Electronic Unit contains the intermediate frequency processor, the PLL (Phase Locked Loop) and the frequency sources. Due to his power consumption, it mounted next to a louvred radiator internal to the spacecraft. The Electronic Unit contains the CTS (Chirp Transform Spectrometer), including instrument computer and power conditioning circuits. The USO (Ultra Stable Oscillator) is self contained and thermally controlled. Those items are presented in Figure 17.
Table 11: Key parameters of MIRO
MIRO post launch results: The MIRO instrument appears to be fully functional at the end of commissioning. The measured key parameters appear to be better than expected.
Table 12: Key parameters were measured using the Earth as target, they are summarized in this table
ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis)
Comets are believed to be the most pristine bodies in the solar system. They were created 4600 million years ago far away from the sun and have remained for most of the time of their existence far outside of Pluto’s orbit. They are small enough to have experienced almost no internal heating. They therefore present a reservoir of well-preserved material from the time of the Solar System’s creation. They offer clues to the origin of the Solar System’s material and to the processes that led from the solar nebula to the formation of planets. In contrast to meteorites (the other primitive material available for investigations), comets have retained the volatile part of the solar nebula. Several interesting questions on the history of the Solar System materials can therefore be answered only by studying comets. In particular, the composition of the volatile material — the main goal of the ROSINA instrument.
ROSINA is the main mass spectrometer on the orbiter of ESA’s Rosetta mission to comet 67P/Churyumov-Gerasimenko. It consists of two mass spectrometers for neutrals and ions. ROSINA’s primary objective is to determine the basic properties of the gas in the comet’s atmosphere and ionosphere such as composition, temperature and velocity. 32) 33) 34) 35)
The ROSINA research team comes from many institutions: Physical Institute, University of Bern, Bern Switzerland; BIRA (Belgian Institute for Space Aeronomy), Brussels, Belgium; CESR (Centre d'Etude Spatiale des Rayonnements), Toulouse, France; IPSL (L'Institut Pierre Simon Laplace), Saint Maur, France; Lockheed Martin Advanced Technology Center, Palo Alto, CA, USA; MPS (Max Plack Institute for Solar System Research), Katlenburg-Lindau, Germany; Technische Universität Braunschweig, Germany; University of Michigan, Ann Arbor, MI, USA; SwRI (Southwest Research Institute), San Antonio, TX, USA; University of Giessen, Giessen, Germany. PI: Kathrin Altwegg,Hans Balsiger (honorary PI), University of Bern, Switzerland.
Science objectives: ROSINA’s main goal is to determine the elemental, isotopic, and molecular composition of the comet’s atmosphere and ionosphere. In addition, the scientists are interested in the temperature and the bulk velocity of the gas as well as the reactions of the gas and ions with the dust emitted by the comet. These results may render important implications for questions regarding the origin of comets, the relation between cometary and interstellar material, and the origin and evolution of the Solar System.
To accomplish these demanding objectives, ROSINA has unprecedented capabilities:
• A wide mass range from 1 amu (atomic mass unit) to more than 300 amu. This makes it possible to detect light atoms such as hydrogen as well as heavy organic molecules.
• A high mass resolution of more than 3000 m/Δm. This means that ROSINA is, for example, able to resolve CO from N2 and 13C from 12CH.
• A wide dynamic range of 1010
• High sensitivity (more than 10-5 A/mbar) to accommodate large differences in ion and neutral gas concentrations and severe changes in the ion and gas flux as the comet’s activity develops between aphelion and perihelion.
Instrument: ROSINA consists of two mass spectrometers for neutrals and primary ions with complementary capabilities and a pressure sensor. The total mass of the ROSINA assembly is 36 kg with a power consumption of 53 W max.
• The DFMS (Double Focusing Magnetic mass Spectrometer) has a mass range from 1 amu to 150 amu and a mass resolution of 3000 at 1 percent peak height. It is optimized for very high mass resolution and a large dynamic range.
• The RTOF (Reflectron Time Of Flight) mass spectrometer with a mass range from 1 amu to more than 300 amu and a high sensitivity.
• The two pressure gages, COPS (COmet Pressure Sensor), provide density and velocity measurements of the cometary gas.
Figure 18: Photo of the ROSINA DFMS (Double Focusing Mass Spectrometer), Image credit: University of Bern
Figure 19: Photos of the ROSINA RTOF instruments (image credit: MPS)
Figure 20: Photo of the COPS devices (image credit: University of Bern)
COSIMA (Cometary Secondary Ion Mass Analyzer)
The instrument will analyze the characteristics of dust grains emitted by the comet, including their composition and whether they are organic or inorganic. COSIMA is a dust mass spectrometer (SIMS, m/µm ~2000). COSIMA was built by a consortium led by the MPS (Max Planck Institute for Solar System Research), Katlenburg-Lindau, Germany - in collaboration with Laboratoire de Physique et Chimie de l'Environnement et de l'Espace, CNRS, Université d’Orléans, France; Institut d'Astrophysique Spatiale, CNRS, Université Paris Sud, Orsay, France; FMI (Finnish Meteorological Institute), Helsinki, Finland; Universität Wuppertal, Wuppertal, Germany; von Hoerner und Sulger GmbH, Schwetzingen, Germany; Universität der Bundeswehr, Neubiberg, Germany; Institut für Physik, Forschungszentrum Seibersdorf , Seibersdorf, Austria; Institut für Weltraumforschung, Österreichische Akademie der Wissenschaften, (Graz, Austria; PI: Martin Hilchenbach, (formerly Jochen Kissel), MPS. 36) 37)
COSIMA is a secondary ion mass spectrometer equipped with a dust collector, a primary ion gun, and an optical microscope for target characterization. Dust from the near comet environment is collected on a target. The target is then moved under a microscope where the positions of any dust particles are determined. The cometary dust particles are then bombarded with pulses of indium ions from the primary ion gun. The resulting secondary ions are extracted into the time-of-flight mass spectrometer.
Science objectives: COSIMA will perform in-situ measurements on individual dust particles emitted by the target comet and collected by COSIMA dust collector subsystem. From the resulting data it will be possible to determine:
• The elemental composition of solid cometary particles to characterize comets in the framework of the solar system chemistry
• The isotopic composition of key elements in solid cometary particles such as H, C, Mg, Ca, Ti in order to establish boundary conditions for models of the origin and evolution of comets and thereby of the solar system
• The chemical states of the elements
• Variations of the chemical and isotopic composition between individual particulate components
• Changes in composition that occur as functions of time ("short-term variations") and orbital position
• The variability of the composition of different comets by comparing the results to those obtained previously from comet Halley
• The presence of an organic component that is not associated with a rocky phase
• The molecular composition of the organic phase of the solid cometary particles
• The molecular composition of the inorganic phase of the solid cometary particles
• The chemical state of the organic matter characterized by its saturation degree oxidation state and bond types.
Instrument: The core of the COSIMA instrument is a TOF (Time of Flight) secondary SIMS (Ion Mass Spectrometer) equipped with a dust collector, a primary ion gun, and an optical microscope (COSISCOPE) for target characterization. Once one of the targets on the target wheel has been exposed to cometary dust it is moved in front of the microscope and imaged under shallow angle illumination provided by light emitting diodes. On-board image evaluation detects the presence and location of dust particles with diameters exceeding a few µm and calculates their position relative to the target reference point. Once the presence of features of interest is established, the target is moved in front of the mass spectrometer. Three nanosecond duration pulses of indium-115 with an energy of 10 keV and about 10 µm in diameter from the primary ion gun hit the selected feature. Secondary ions from the cometary matter are extracted by the SIL (Secondary Ion extraction Lens) into the TOF section. After passing deflection plates for beam steering the ions travel through a field free section. Next they pass a two stage reflector, return through the drift section to the ion detector. Its main element is a single stage microsphere plate, where the ions are detected. The arrival time of each ion is measured with an accuracy of about 2 ns.
Precision in the timing of the primary ion pulses, the correct selection of the dimensions and the voltages of the mass spectrometer and the accurate measurement of the secondary ion flight time are needed to obtain high mass resolution in the COSIMA instrument. A mass resolution of 2000 is achieved for ions having a flight time of 16 µs, which occurs for ion masses of above 28 Daltons (atomic mass units).
Figure 21: Photo of the COSIMA flight model (image credit: MPS)
Operations: Since launch and commissioning, the instrument health has been monitored in checkouts every 6 months. The instrument proved to be in good health during the checkouts performed so far. Operations have included mechanism tests, instrument calibration, ion source maintenance, and interference check between COSIMA and other instruments. The instrument will be re-commissioned in April and July 2014, just before the science phase of Rosetta at comet 67P/Churyumov-Gerasimenko.
• Sept. 8, 2014: The COSIMA team presented an image of the first dust grains collected by the COSIMA instrument when Rosetta was at a distance of less than 100 km from the nucleus of comet 67P/Churyumov-Gerasimenko. On Aug. 11, the first of COSIMA’s 24 target plates were exposed to space. On Aug. 24, when the COSIMA team took a look at the image of the plate, they saw a number of large dust grains from the comet on a target that had been pristine when examined one week before. A first examination of the plate indicates that the largest two grains are about 50 µm and 70 µm in width, comparable to the width of a human hair. 38) 39)
• August 8, 2014: Now that comet 67P/Churyumov-Gerasimenko is within reach, Rosetta’s mass spectrometer COSIMA, is beginning to reach for cometary dust. 40)
• In early April 2014, the project uploaded new software, and after switching the COSIMA instrument off and then back on, the previous tests proved successful and COSIMA was up and running with a fresh memory. On their way to or from the spacecraft, these data travel about 40 minutes through the inner Solar System. 41)
MIDAS (Micro-Imaging Dust Analysis System)
MIDAS is designed to collect and image dust particles collected in the vicinity around the comet. The measurement principle is based on atomic force microscopy. This technique allows for true three-dimensional imaging at nanometer-scale resolution. The instrument is built to investigate the smallest grain size fraction released from the comet's surface; it will also investigate the structural complexity of grain cluster up to a few µm. More than 60 collector facets, which can be individually exposed into the dust stream, and a total of 16 imaging sensors, guarantee continuous observations throughout the mission lifetime. 42)
Scientific objectives: Dust particles emitted from comet nuclei form a major source of information for the understanding of primitive matter in our Solar System. It represents remnant material from the early times of the formation comets, asteroids and planets some 4.5 billion years ago. The prime scientific objective of the MIDAS experiment is to image the micro-topography and micro-textural units of cometary dust particles; this provides important information about the characteristics and nature of these particles, for example, about the composition of their primary building blocks. In addition, sub-features on clean crystal surfaces provide insight into either the growth conditions (twinning, screw dislocations) and/or storage environment conditions (dissolution marks).
Following the mapping of single particles with a resolution in the few nanometer range, many statistical parameters describe the cometary environment. This comprises the statistical evaluation of the collected particles according to size, volume and shape, but also temporal and spatial variations of the particle flux can be deduced.
In summary, MIDAS will meet the scientific objectives that have been established for this instrument when the following information can be obtained during the rendezvous with the comet:
• 3D images of single particles with a resolution better than 10 nm
• Search for "very small particles" (10 nm)
• Search for evidence of euhedral (well-formed, sharp-faced) crystals
• Possible detection of ferro-magnetic minerals
• Size distribution of particles
• Variation of particle fluxes on time scales between hours and days.
The instrument was developed by an international collaborative team led by IWF (Institut für Weltraumforschung - Space Research Institute), Graz, Austria; ESA/ESTEC, Noordwijk, The Netherlands; Physics Department of the University of Kassel, Germany; Institute of Applied Systems Technology of Joanneum Research, Graz, Austria; Austrian Research Centers Seibersdorf (now Ruag Space, Vienna); Vienna University of Technology, Vienna, Austria. PI: Mark Bentley, IWF, Graz, Austria. 43) 44)
Instrument: Dust grains in the size range from 4 µm down to 4 nm will be imaged in three dimensions by means of atomic force microscopy (AFM). AFM makes use of tiny physical forces (van der Waals, interatomic, magnetic, etc.) that act on a sensor in closest distance to a surface.
The sensor is a 600 µm long cantilever arm with an extremely sharp 7 µm long tip mounted underneath. The sensor is controlled by a piezoelectric mechanical system that scans above the surface and senses its topography. The dust particles enter the instrument via a funnel penetrating the spacecraft's hull and hit the collector surface. Sixty-four of these targets (coated silicon facets) are mounted on the perimeter of the dust collector wheel. The facet exposed to the dust stream is rotated and presented to the microscope, which approaches the surface automatically and starts the scanning (imaging) process.
Figure 22: Photo of the MIDAS instrument, inside view (left) and flight model (right), image credit: IWF
Table 13: Performance parameters of the MIDAS instrument
In the test sequence of the MIDAS instrument, which ran over five contact passes from the ground station to the spacecraft, the following results were obtained:
- Complete electronics checkout
- Cover opening by a pyrolytic device
- Unlocking of all clamp mechanisms
- Movement of linear stage and approach mechanism out of launch position
- Verification of all motors and mechanisms
- Verification of all 16 sensors by resonance search
- Verification of the scanner by imaging calibration surfaces
- Initial characterization of mechanical noise environment.
The overall result of the MIDAS commissioning phase and subsequent instrument checkout demonstrated full functionality and performance of the instrument up to the moment the spacecraft (and payload) was put into hibernation in June 2011.
CONSERT (Comet Nucleus Sounding Experiment by Radiowave Transmission)
CONSERT is a time domain transponder that operates between one module that will land on the comet surface and another that will orbit the comet. A radio signal is transmitted from the orbiting component of the instrument and passes through the comet nucleus to the component on the comet surface. The signal is received on the lander, where some data is extracted, and then immediately re-transmitted back to the orbiter, where the main experiment data collection occurs. The variations in phase and amplitude that occur as the radio waves pass through different parts of the cometary nucleus will be used to perform tomography of the nucleus and determine the dielectric properties of the nuclear material. 45) 46) 47)
The overall science objective of the CONSERT investigation is to gather information about the geometrical structure and electrical properties of the deep interior of the comet nucleus. Inferences about the composition of the interior of the comet will then be made from the measured electrical properties. The main scientific objectives are:
• To measure the mean dielectric properties and, through modelling, to set constraints on the cometary composition (like material and porosity)
• To detect large-scale embedded structures (several tens of meters), and stratifications
• To detect small scale irregularities within the comet.
CONSERT probes the comet’s interior by studying radio waves that are reflected and scattered by the nucleus. The CONCERT instrument provides radio sounding and nucleus tomography. The instrument's mass is limited to 3 kg. PI: Wlodek Kofman, LPG (Laboratoire de Planetologie de Grenoble), CNRS/UJF, Grenoble, France.
Instrument: CONSERT works as a time domain transponder. The indirect and apparently complicated transponding procedure reduces the required accuracy of the clocks on the Orbiter and Lander, and makes it possible to stay within the constraints on mass and power consumption imposed on the space experiment. The CONSERT experiment on the orbiter and on the lander both consist of a transmit/receive antenna and a transmitter and receiver contained in a common box.
A 90 MHz radio signal, phase modulated with pseudo-randomly encoded data is transmitted from the orbiter towards the comet. The transmission lasts about 25 µs. The signal propagates through the comet nucleus and is received on the lander. The transmission cycle is repeated every 200 ms. The received signal is digitised and accumulated in the lander in order to increase the signal to noise ratio. Once the accumulation is finished, the signal is compressed to obtain a time/space resolution corresponding to 100 ns which corresponds to about 20 m in the comet. After the signal processing on the lander, which determines the position of the strongest path, the lander transmits the same pseudo-random code with a delay corresponding to that of the strongest path. The transmission cycle again lasts about 25 microseconds. The signal propagates back to the orbiter along virtually the same path, since the orbiter does not travel far during the measurement cycle. The signal is received on the orbiter, accumulated and stored in the memory in order to be sent to Earth. A complete measurement cycle lasts about 1 s.
GIADA (Grain Impact Analyzer and Dust Accumulator)
GIADA will measure the number, mass, momentum and velocity distribution of dust grains in the near-comet environment. GIADA will analyze both grains that travel directly from the nucleus to the spacecraft and those that arrive from other directions having had their ejection momentum altered by solar radiation pressure. 48) 49) 50)
GIADA is a contamination monitor providing dust velocity and impact momentum measurement. PI: Luigi Colangelo, INAF, Naples, Italy.
The primary scientific objectives of GIADA are:
• Dust flux measurement for "direct" and "reflected" grains: Two populations of cometary grains exist: "direct" (coming directly from the nucleus) and "reflected" grains (coming from the Sun direction, under the action of the solar radiation pressure). The two populations undergo very dissimilar dynamic evolution in the coma and have different times of ejection from the nucleus. In the case of Rosetta, "direct" and "reflected" grains can be collected simultaneously. The relative amount will depend on the probe position along its orbit. GIADA will be able to monitor grain fluxes coming from different directions and will allow, for the first time, discrimination between the two dust populations. This task is fundamental to the determination of the original dust size distribution. In turn, this information is required to define the dust mass loss rate.
• Analysis of the dust velocity distribution : The dust ejection velocity depends both on the grain size and on time. Moreover, grains with a given size have a wide dust velocity distribution. GIADA will allow the measurement of scalar velocity and momentum for grains coming from the nucleus direction so as to give mass and impact velocity of each analyzed "direct" grain. From this information it will be possible to derive grain mass and ejection velocity from the nucleus surface. For the first time we will obtain:
- the size dependence of the dust ejection velocity
- the relation between most probable dust velocity and dust mass
- the velocity distribution for each dust mass
- the link between velocity dispersion and dust mass.
• Study of dust evolution in the coma : Once ejected from the nucleus, grains may change their physical properties due to several processes, including, for example, fragmentation. These modifications may alter the grain size distribution. The size distribution of grains collected by GIADA in the nucleus direction should not be affected by the dust velocity dispersion. Thus, changes in the dust distribution at different nucleus distances can be linked directly to actual variations in the dust size distribution and correlation can be found with dust fragmentation and/or with emission from active areas on the nucleus.
• Correlation of dust changes with nucleus evolution and emission anisotropy : The dust environment characteristics depend on the comet-Sun distance and on the time evolution of the nucleus. The continuous monitoring by GIADA of dust flux and dynamic properties will offer the best opportunity to characterize the time evolution of the dust environment as a function of heliocentric distance. Nucleus imaging will allow us to link observed changes to the nucleus evolution and to its spin state.
• Determination of dust to gas ratio : One of the crucial parameters characterizing the comet nucleus is the dust to gas ratio. Dust flux monitoring by GIADA is needed to estimate the dust to gas ratio. This will be possible in combination with results of other experiments.
• Other objectives : The data provided by GIADA about dust fluxes and grain dynamic properties are very important for the correct interpretation of images of the coma and nucleus and mass spectrometer data. GIADA will help in the selection of the surface science package landing site. The characterization of dust emitting areas, and possibly of the dust population of different active areas, will be necessary for the site selection process to achieve a proper balance between safety and scientific interest.
GIADA will play an important role for the health and the safety of various experiments and the spacecraft itself, as it will be able to provide information about dust flux in several directions. Optical surfaces of experiments and other devices pointing to the nucleus will be polluted by the dust flux. GIADA data will allow the prediction of deposition rates and informed decision making for mission planning and operations. Data from GIADA will be the only resource to predict and allow control of the performance degradation of critical devices such as passive radiators and solar panels.
Instrument: The instrument comprises three modules: GIADA 1 measures momentum, scalar velocity and mass of single grains entering the instrument by the GDS (Grain Detection System) and the IS (Impact Sensor), placed in cascade. The GIADA 2 module contains the MBS (Micro Balances System); it controls the acquisition of data from the sensors and the operation of the other subsystems. It also provides the power supply for the whole experiment. The GIADA 3 module measures the cumulative dust flux and fluence from different directions by means of five microbalances. One microbalance points towards the nucleus, while the other four cover the widest possible solid angle. 51)
In the GDS, four laser diodes with their foreoptics are used to form a thin (3 mm) light curtain (100 cm2). For each grain passing through it, the scattered/reflected light is detected by two series of four detectors (photodiodes) placed at 90º with respect to the sources. In front of each photodiode a Winston cone is placed to achieve a uniform sensitivity in the detection area.
The IS is a thin (0.5 mm) aluminum square diaphragm (sensitive area 100 cm2) equipped with five piezoelectric sensors, placed below the corners and its center. When a grain impacts the sensing plate, flexural waves are generated on the plate, and are detected by the piezoelectric crystals. The maximum displacement of these systems is directly proportional to the impulse imparted, and the displacement of the crystal produces a proportional potential. Through calibration, a known impulse may be equated with a specific charge produced on the electrodes of the PZT crystals. The detected signal is proportional to the momentum of the incident grain through the factor (1+e), where e is the coefficient of restitution.
When a grain enters GIADA 1, the GDS gives a first estimate of the grain speed and starts a time counter that is stopped when the IS detects the grain impact and the momentum is measured. In this way, for each entering grain, speed, TOF (Time of Flight), momentum and, therefore, mass are measured.
The microbalances in GIADA 3 each consist of two quartz crystals oscillating at a frequency of about 15 MHz, one acting as a sensor, the other as a reference. The measured physical quantity is the beat frequency between the two crystals. The resonance frequency of the sensor quartz oscillator, exposed to the dust environment, changes due to the variation of its mass as a result of material accretion, while the reference crystal is not exposed to the dust flux. Thus, the output signal is proportional to the mass deposited on the sensor and dust flux and fluence are measured in time. The use of a reference crystal ensures extremely small dependence on temperature and power supply fluctuations and, thus, high sensitivity.
Figure 23: Two views of the GIADA instrument (image credit: INAF)
RPC (Rosetta Plasma Consortium)
Six sensors measure the physical properties of the nucleus; examine the structure of the inner coma;monitor cometary activity; and study the comet’s interaction with the solar wind. The RPC assembly consists of:
- Langmuir Probe (LAP). PI: Anders Eriksson (formerly Rolf Boström), IRF Uppsala, Sweden
- Ion and Electron Sensor (IES). PI: Jim Burch, SRI, San Antonio, TX, USA
- Flux Gate Magnetometer (MAG). PI: Karl-Heinz Glassmeier, IGEP, Braunschweig, Germany
- Ion Composition Analyzer (ICA). PI: Rickard Lundin, IRF, Kiruna, Sweden
- Mutual Impedance Probe (MIP). PI: Jean-Gabriele Trotignon, LPCE/CNRS, Orleans, France
- Plasma Interface Unit (PIU). PI: Chris Carr, Imperial College, London, UK.
• The physical properties of the cometary nucleus and its surface. Emphasis will be given to determination of the electrical properties of the crust, its remnant magnetization, surface charging and surface modification due to solar wind interaction, and early detection of cometary activity.
• The inner coma structure, dynamics, and aeronomy. Charged particle observation will allow a detailed examination of the aeronomic processes in the coupled dust-neutral gas-plasma environment of the inner coma, its thermodynamics, and structure such as the inner shocks.
• The development of cometary activity, and the micro- and macroscopic structure of the solar-wind interaction region as well as the formation and development of the cometary tail.
In order to realize these investigations extensive in-situ monitoring of the plasma electrons and ions, their composition, distribution, temperature, density, flow velocity, and the magnetic field will be necessary. These measurements will improve the understanding of the coupling processes of cometary dust, gas, and plasma as well as its interaction with the solar wind. The plasma and fields measurements thus provide complementary information to that of other Rosetta instruments for a deeper understanding of the overall physics and chemistry of an active comet.
The flybys of asteroid Steins and asteroid Lutetia have provided an opportunity to study in detail the physics of the solar wind - asteroid interaction. RPC has excellent capabilities for the investigation of this interaction. It has also been possible to study the magnetic and electric conductivity properties of the asteroids.
ICA (Ion Composition Analyzer): ICA measures the three-dimensional velocity distribution and mass distribution of positive ions. The mass resolution is sufficient to differentiate between the major particle species such as protons, helium, oxygen, molecular ions, and heavy ion clusters (dusty plasma). The ICA comprises an electrostatic arrival angle filter, a hemispherical electrostatic analyzer employed as an energy filter, and a magnetic deflection momentum filter. Particles are detected using a large micro channel plate and a two-dimensional anode array.
IES (Ion and Electron Sensor): The IES will simultaneously measure the flux of electrons and ions in the plasma surrounding the comet over an energy range from around one electron volt, which approaches the limits of detectability, up to 22 keV. IES consists of two electrostatic analyzers, one for electrons and one for ions, which share a common entrance aperture. The charged particle optics for IES employs a toroidal top-hat geometry along with electrostatic angle deflectors to achieve an electrostatically scanned field of view of 90º x 360º. 54)
LAP (Langmuir Probe): The LAP instrument will measure the density, temperature and flow velocity of the cometary plasma. It comprises two spherical sensors mounted at the tip of deployable booms, with the sensors capable of being swept in potential to measure the current-voltage characteristic of the intervening plasma, which provides information on the electron number density and temperature. The probes can be held at a fixed bias potential to measure plasma density fluctuations and by a time-of-flight analysis of the signals from the two probes the plasma flow velocity can be determined. 55)
MAG (Flux Gate Magnetometer): The MAG will measure the magnetic field in the region where the solar wind plasma interacts with the comet. It consists of two triaxial fluxgate magnetometer sensors mounted on a 1.5 m deployable boom that points away from the comet nucleus. One sensor is mounted near the outboard tip of the boom and one is mounted part way along the boom. The use of two sensors allows the effects of the spacecraft's own magnetic field to be minimized. MAG will also study any magnetic field possessed by the comet nucleus, in cooperation with the ROMAP magnetometer experiment on the Rosetta lander.
MIP (Mutual Impedance Probe): MIP will derive the electron gas density, temperature, and drift velocity in the inner coma of the comet by measuring the frequency response of the coupling impedance between two dipoles. MIP will also investigate the spectral distribution of natural waves in the 7 kHz to 3.5 MHz frequency range and monitor the dust and gas activity of the nucleus.
PIU (Plasma Interface Unit): PIU acts as an interface between the five instruments that make up RPC and the Rosetta spacecraft by providing a single path for the transmission of scientific and housekeeping data to the ground and for the receipt and processing of commands sent from the ground. The PIU also takes power from the spacecraft and converts, conditions and manages it for the RPC instruments. PIU also performs on-board data processing for the MAG sensor unit, which has no data processing capability of its own. 56)
RSI (Radio Science Investigation)
RSI makes use of the communication system that the Rosetta spacecraft uses to communicate with the ground stations on Earth. Either one-way or two-way radio links can be used for the investigations. In the one-way case, a signal generated by an ultra-stable oscillator on the spacecraft is received on Earth for analysis. In the two way case, a signal transmitted from the ground station is transmitted back to Earth by the spacecraft. In either case, the downlink may be performed in either X-band or both X-band and S-band. — PI: Martin Pätzold, University of Cologne, Germany. 57)
The goal of RSI is to investigate the nondispersive frequency shifts (classical Doppler) and dispersive frequency shifts (due to the ionized propagation medium), the signal power and the polarization of the radio carrier waves. Variations in these parameters will yield information on the motion of the spacecraft, the perturbing forces acting on the spacecraft and the propagation medium.
Science objectives: Doppler data provide time-resolved measurements of the spacecraft motion and the plasma state and thus may be used for physical investigation of the nucleus and the inner coma of the comet. In particular, the following scientific objectives may be addressed by an analysis of dual-frequency one-way or two-way radiometric tracking data, together with information provided by other Rosetta experiments, for example the remote imaging system (OSIRIS):
• Gravity Field and Dynamics
- Cometary mass and bulk density
- Cometary gravity field coefficients
- Cometary moments of inertia and spin state
- Cometary orbit, light shift, thermal properties of the nucleus
- Asteroid mass and bulk density
• Cometary Nucleus
- Size and shape (from spacecraft occultation observations)
- Internal structure (from nucleus sounding)
- Internal structure (from nucleus sounding)
- Rotation, precession and nutation rates (from bistatic radar)
• Cometary Coma
- Distribution of mm - dm size particles (from coma sounding)
- Plasma content of the inner coma (from coma sounding)
- Gas and dust mass flux (from non-gravitational perturbations of the spacecraft)
• Solar Corona Science
- Electron content of the inner corona, solar wind acceleration, search for coronal mass ejections, turbulence.
Instrument: The two-way radio link is established by transmitting an uplink radio signal either at S-band or X-band to the spacecraft. The received uplink carrier frequency is transponded to downlinks at X-band and S-band upon multiplying by the constant transponder ratios 240/221 and 880/221, respectively, in order that the ratio of the two downlinks is 880/240 = 11/3. This radio mode takes advantage of the superior frequency stability inherent to the hydrogen maser in the ground station on Earth. This mode is used for all RSI gravity science applications, routine tracking observations when in orbit during the escort phase, and for the sounding of the solar corona.
The one-way radio link is used only during an occultation of the spacecraft by the nucleus as seen from Earth. This enables radio sounding of the immediate vicinity of the nucleus and perhaps even the nucleus itself, should the solid cometary body prove to be penetrable by microwaves. These one-way occultation experiments require an USO (Ultra-Stable Oscillator) added to the radio subsystem. The prime purpose of the USO is to serve as a phase-coherent frequency reference for the simultaneous one-way downlink transmissions at S-band and X-band. The required stability (Allan variance) of the USO is about Δf/f ~10-13 at 10-1000 seconds integration time. The one-way radio link can be transmitted either while receiving a non-coherent uplink or without any uplink contact at all.
Ground segment: Ground stations include antennas, associated equipment and operating systems in the tracking complexes of Perth (ESA, 35 m), Australia, and the DSN (Deep Space Network) of NASA, (34 m) in California, Spain and Australia. A tracking pass consists of typically eight to ten hours of visibility. Measurements of the spacecraft range and carrier Doppler shift can be obtained whenever the spacecraft is visible. In the two-way mode the ground station transmits an uplink radio signal at S-band (if the spacecraft receiver operates at S-band) or at X-band and receives the dual-frequency simultaneous downlink at X-band and S-band. The information about signal amplitude, received frequency and polarization is extracted and stored as a function of ground receive time.
SREM (Standard Radiation Environment Monitor)
In addition to these scientific experiments the orbiter is also equipped with a SREM device to monitor the high energetic, ionizing particle environment aboard the spacecraft. The objective of SREM is to provide a continuous, almost uninterrupted measurement of the high energetic particles encountered by Rosetta and provide this information for mission analysis purposes.
Philae (Rosetta Lander)
The Rosetta lander Philae can be considered a scientific spacecraft of its own that is carried and delivered by the Rosetta orbiter to the comet. Upon proposal by various scientists, lead by Helmut Rosenbauer from the MPS (Max Planck Institute for Solar System Research) in Katlenburg-Lindau, the 10 scientific instruments and the various spacecraft subsystems are provided by a consortium of spaceflight agencies and research institutes from 6 European countries and by ESA. The Philae lander is provided by a consortium led by DLR, MPS, CNES and ASI. DLR played a major role in building the lander and runs the LCC (Lander Control Center) at DLR Cologne, which is preparing for and overseeing the difficult task of landing on the comet, a feat never before accomplished. 58) 59) 60) 61) 62) 63)
The goal of Philae's mission is to land successfully on the surface of a comet, and transmit data from the surface about the comet's composition. The scientific goals of the mission focus on "elemental, isotopic, molecular and mineralogical composition of the cometary material, the characterization of physical properties of the surface and subsurface material, the large-scale structure and the magnetic and plasma environment of the nucleus.”
The SONC (Science Operations and Navigation Center) is located at CNES in Toulouse, France. Both centers are directly connected to the RMOC (Rosetta Mission Operations Center) at ESOC, Darmstadt. The Rosetta science operations planning is performed at the RSGS (Rosetta Science Ground Segment) at ESAC, near Madrid. - The responsibility for the Lander delivery lies at ESA. However, close cooperation between the partners is envisaged, to reach the challenging task of the first successful landing on a comet. 64)
Figure 24: Artist’s impression of the 100 kg Philae lander (image credit: ESA, DLR)
The Philae lander is designed to touch down on the comet's surface after being deployed from the main spacecraft body and "falling" from a height of 25 km at about 1 m/s towards the comet along a ballistic trajectory. Upon contact, it will deploy two harpoons to anchor itself to the surface, and the legs are designed to dampen the initial impact to avoid bouncing, because the comet's escape velocity is only around 0.5 m/s.
Communications with Earth will use the orbiter spacecraft as a relay station to reduce the electrical power needed. The mission duration on the surface is planned to be at least one week, but an extended mission lasting months is possible.
Rosetta-Philae RF link: The transceiver is a full duplex S-band transmission set for digital data developed specifically for space applications. The conception made by Syrlinks was done with drastic objectives for mass and power consumption. For this, the use of commercial parts was decided leading to a low cost product widely used afterwards on the Myriade platform family. The transceiver is composed of a transmitter, a receiver and a reception filter for dual antenna use (Figure 25). The filter protects the receiver from out-of-band signals, particularly from the transmitter. The two functions (receiver and transmitter) are fully independent and can be activated separately. Technical details are given in the Table 14 and an illustration in Figure 26. 65) 66)
There are two transceivers on both sides of the RF link. The redundancy is activated with RF switches on orbiter side (1 Tx/1 Rx active) and with diplexer on lander side (1 Tx/2 Rx active). The choice of implementing identical RF chains for transmission and reception on the orbiter and the lander has given great advantages, such as cutting procurement costs and simplifying qualification, integration and testing.
With 1W RF output power and 1 dBi gain (@ 60º) patch antennas, link establishment is possible for distances up to 150 km.
The lander telecommunication system answers to a request-to-send protocol from the orbiter at any time. This handshake protocol, which implies full duplex equipment and which was specifically designed for Rosetta mission ensures a desired quality of transmission even when the relative geometry and visibility between the orbiter and the lander is not favorable.
In the housekeeping telemetry available at orbiter side, one parameter is particularly interesting to get information beyond its intrinsic value: the RSSI (Received Signal Strength Indicator). From this raw telemetry value, it is possible to extract the received power level on orbiter side, which can be then processed.
Lander bus: The main structure of the lander is made from carbon fiber, shaped into a plate maintaining mechanical stability, a platform for the science instruments, and a hexagonal "sandwich" to connect all the parts. The main body rests on a tripod landing gear, with ice screws and sensors integrated in the feet. All instruments and the drill are depicted in their deployed configuration. The open face of Philae with instruments exposed to the cometary environment is colloquially termed “balcony”.
The total mass is about 100 kg. Its "hood" is covered with solar cells for power generation.
Battery assembly: The lander energy storage is based on two types of sources: primary batteries for short term activities (1000 Wh), secondary batteries for long term activities(140 Wh). The assembly includes the associated electronics.
Legend to Figure 27: The figure shows the overall structure, the subsystems and the experiment compartment of the Lander. Some instruments are not visible in the drawing: specifically, the instruments in charge of analyzing the samples distributed by the SD2 (CIVA, COSAC, PTOLEMY), and the down-looking camera (ROLIS).
Figure 28: Side view schematics of the inner structure of the lander compartment showing the location of COSAC and PTOLEMY systems, the CONSERT antennas, the SESAME dust sensor and various CIVA cameras (image credit: Philae Team)
The general tasks of Philae are to get a first in situ analysis of primitive material from the early solar system and to study the structure of a cometary nucleus which reflects growth processes in the early solar system and to provide ground truth for Rosetta Orbiter instruments. The scientific objectives of the Lander are:
• determining the composition (elemental, isotopic, mineralogical and molecular) of the cometary surface material
• measuring the physical properties (thermal, electrical, mechanical) of the cometary surface material
• describing the large-scale structure (panoramic imaging, particles and magnetic field, and internal heterogeneity)
• monitoring the cometary activity (day/night cycle, changing distance to the Sun, outbursts).
Legend of Figure 29: The ejection maneuver takes place at an altitude on the order of 1 km only; the Lander eject velocity partly cancels Rosetta’s orbital velocity, such that Philae moves on an comet-surface crossing ellipse, stabilized by a flywheel and the optional use of a cold-gas thrusters (in z direction). After touchdown on the moving comet surface, the cold-gas system is activated to provide a hold-down thrust until the harpoons have safely anchored the Lander.
Philae sensor complement:
The Rosetta Lander carries a further nine experiments, as well as a drilling system to take samples of subsurface material. The Lander instruments are designed to study in situ for the first time the composition and structure of the surface and subsurface material on the nucleus. The science payload of the lander consists of ten instruments with a mass of ~27 kg, making up nearly one-third of the mass of the lander. 67)
Table 15: The payload of the Rosetta lander Philae lead investigators: Jean-Pierre Bibring and Hermann Boehnhard
For the collection of samples and the deployment of instruments it is important to note that the Lander can be rotated around its z (vertical) axis by 360º defining a “working circle” around the Lander body axis. Thus, arbitrary locations can be accessed by the sampling drill (SD2), the down-looking camera (ROLIS), and the APXS sensor; the MUPUS-PEN and the SESAME-PP electrodes are attached to the latter two instruments. Also the stereo camera pair of CIVA will be able to image a full panoramic of 360º using the Lander rotation capability.
Note that the landing gear also provides a tilting capability. This capability had to be drastically reduced in range (to ±5º) after the change of target comet in 2003 to ensure a safe landing.
The science operations of Philae are divided into various phases. During the 10 year cruise, check-ups, calibrations, software and command up-loads are scheduled as well as occasional observation campaigns for CIVA-P (flybys and ROMAP (flybys, solar wind, comet tail crossings).
After arrival at the comet, global mapping by the Orbiter instruments and the selection of a landing site, the Separation–Descent–Landing phase begins. Immediately before release from the Orbiter, thermal preparation and battery charging are foreseen. Immediately after the eject, the Landing Gear is unfolded, thereby releasing the CONSERT antennas. Then, the ROMAP boom is deployed. A telemetry contact to the Orbiter will be established a few minutes after release until well after landing. During descent (30–60 min) to the comet’s surface, scientific measurements (images by ROLIS, magnetic field measurements by ROMAP-MAG, dust impact by SESAME-DIM and -CASSE, calibrations of SESAME-CASSE and MUPUS-TM) will be performed to monitor the cometary environment between the Orbiter and the surface of the nucleus, to observe the nucleus while approaching, to characterize remotely the landing site and to document the touchdown event of the Lander at the surface. ROLIS descent images will be taken until touchdown and MUPUS-ANC measurements during the actual anchoring.
During the “first science sequence” of approximately five days, Philae will be operated mainly on primary batteries, thus minimizing sensitivity to landing geometry (solar irradiance of the cells). In the first 60 hours following the touch-down, all instruments will work in their baseline mode at least once at full completion of their relevant science goals. In particular a full panorama of the landing site will be taken by CIVA-P immediately after landing and cometary samples will be acquired by SD2, both from the surface and from the maximum depth reachable with drill (i.e., about 0.2 m); these samples will then be processed by the relevant instruments (COSAC, Ptolemy, CIVA-M). MUPUS-PEN and APXS will be deployed and thermal conductivity, thermal diffusivity, strength measurements be made by MUPUS and the first X-ray and alpha spectra will be recorded by APXS. CONSERT will sound the nucleus over at least one full Orbiter orbit relative to the Lander. ROMAP will observe the daily variation of magnetic field and the plasma properties. All three parts of SESAME will perform measurements (PP only after MUPUS-PEN and/or APXS have been deployed). The Lander resources should enable at least a partial redo of this sequence over the following 60 hours, if partial failure (e.g., in data transmission) had happened. If performed successfully, the first sequence will secure a “minimum science success” of the Lander mission. 68)
During the “long-term science mission” (up to three months until r = 2 AU is reached) all instruments will be operated mostly sequentially, powered by the solar cells and buffered by the secondary (rechargeable) batteries. The Lander has enough flexibility to allow—by rotation around its body axis—the optimized orientation of the solar cells with respect to the local time, to drill several boreholes, and to measure physical properties all around the landing site.
The data volume to be uplinked to Earth is 235 Mbit during descent and the first five days, and 65 Mbit during each subsequent 60 hour period. However, depending on actual telemetry coverage and Orbiter requirements, a significantly larger data volume is expected.
With the current best estimate of the comet environment, about 52–65 hours of primary mission operations are feasible (incl. a 30% system margin). Primary power during the first science sequence is 15–20 W; the solar cells generate 10 W during the day at 3 AU.
The long-term operations then rely entirely on the solar generator; the end of life will be determined either by overheating (the thermal system is designed for a range of 2–3 AU) or by insufficient power if the solar cell degradation (mainly by dust deposition) becomes too severe.
Figure 30: Artist’s impression (not to scale) of the Rosetta orbiter deploying the Philae lander to comet 67P/Churyumov–Gerasimenko (image credit: ESA, C. Carreau, ATG medialab) 69)
Rosetta Mission Status
Mission status: (Note: Rosetta was launched on 2 March 2004 and completed science observations on 30 Sept. 2016)
• October 28, 2020: After years of detective work, the second touchdown site of Rosetta's Philae lander has been located on Comet 67P/Churyumov-Gerasimenko in a site that resembles the shape of a skull. Philae left its imprint in billions-of-years-old ice, revealing that the comet's icy interior is softer than cappuccino froth. 70)
- Philae descended to the surface of the comet on 12 November 2014. It rebounded from its initial touchdown site at Agilkia and embarked on a two-hour flight, during which it collided with a cliff edge and tumbled towards a second touchdown location. Philae eventually came to a halt at Abydos, in a sheltered spot that was only identified in Rosetta imagery 22 months later, a few weeks before the conclusion of the Rosetta mission.
- ESA's Laurence O'Rourke, who played the leading role in finding Philae in the first instance, was also determined to locate the previously undiscovered second touchdown site.
- "Philae had left us with one final mystery waiting to be solved," says Laurence. "It was important to find the touchdown site because sensors on Philae indicated that it had dug into the surface, most likely exposing the primitive ice hidden underneath, which would give us invaluable access to billions-of-years-old ice."
- Together with a team of mission scientists and engineers, he set about pulling together data from both Rosetta and Philae instruments to find and confirm the ‘missing' touchdown site.
Figure 31: Philae creates eye of the skull. The comet topography at Philae's second touchdown site resembles the shape of a skull with a pointed ‘hat' when viewed from above. This gif shows the feature that resembles a skull face, with Philae superimposed for scale (Philae's ‘body' measures about 1 m across, and each leg is 1.5 m long). Philae's body compressed into the ice-dust scenery to create the skull's right eye. The dark region just above the skull's right eye is the entrance to a gap between the two boulders nicknamed ‘skull-top crevice', where Philae acted like a windmill to pass between them.
Figure 32: How Philae left its mark during touchdown two. This graphic presents the data collected by Philae's ROMAP instrument – a magnetometer boom – during the time of the second touchdown on Comet 67P/Churyumov-Gerasimenko on 12 November 2014, matched with imagery showing evidence of the key moments of Philae's interaction with the surface. Signatures were created in the magnetic data of the ROMAP boom relative to the lander body when the boom physically moved as it struck a surface (the boom sticks out 48 cm from the lander body). This created a characteristic set of spikes in the magnetic data, which provided an estimate of the duration of Philae stamping into the ice. The data could also be used to constrain the acceleration of Philae during these contacts. The data shows that Philae spent nearly two full minutes at touchdown site two, making contact with the surface multiple times. - The data plot is labelled with five different events, corresponding to the imagery. Initially travelling in a downward direction, Philae slides down the edge of a boulder (1) and flips vertically, rotating like a windmill to pass between two boulders (2) exposing layers of ice in the crevice walls with its feet. A dust wall was created by the windmill action, pushing through the dust that had heaped up between the boulders up to that point in time. The crevice is about 2.5 m long and is curved with a width of 1-1.5 m, allowing Philae to pass through. Philae then stamps a 25 cm imprint of the top of the lander into the comet's surface (3) – a hole made by the top of the SD2 (Sampling, Drilling and Distribution device) tower that sticks up above the top of Philae can be recognized. Philae then climbed out of the crevice, being pushed down once more by an overhang (4a), knocking off material in the process, with its top surface then creating an impression in the dust corresponding to the ‘eye' of the feature that resembles a skull face (4b). The ROMAP data also reveal information about the speed and direction of travel. When Philae first entered the region it was travelling downwards at 20 cm/s and forwards at 10 cm/s. When it left the area it was travelling upwards at less than 1 cm/s and forwards at 9 cm/s. It never flew higher than one meter above the surface in its final flight to touchdown point three, 30 m away (image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; Data: ESA/Rosetta/Philae/ROMAP; Analysis: O'Rourke et al (2020)
The star of the show
- Although a bright patch of ‘sliced ice' observed in high-resolution images from Rosetta's OSIRIS camera proved crucial in confirming the location, it was Philae's magnetometer boom, ROMAP, that turned out to be the star of the show. The instrument was designed to make magnetic field measurements in the comet's local environment, but for the new analysis the team looked at changes recorded in the data that arose when the boom – which sticks out 48 cm from the lander – physically moved as it struck a surface. This created a characteristic set of spikes in the magnetic data as the boom moved relative to the lander body, which provided an estimate of the duration of Philae stamping into the ice. The data could also be used to constrain the acceleration of Philae during these contacts.
Figure 33: Animation showing how Rosetta's Philae lander moved through touchdown site two on Comet 67P/Churyumov-Gerasimenko on 12 November 2014. Initially travelling in a downward direction, Philae slides down the edge of a boulder (1) and flips vertically, rotating like a windmill to pass between two boulders (2) exposing layers of ice in the crevice walls with its feet. A dust wall was created by the windmill action, pushing through the dust that had heaped up between the boulders up to that point in time. The crevice is about 2.5 m long and is curved with a width of 1–1.5 m, allowing Philae to pass through. Philae then stamps a 25 cm imprint of the top of the lander into the comet's surface (3) – a hole made by the top of the SD2 (Sampling, Drilling and Distribution device) tower that sticks up above the top of Philae can be recognized. Philae then climbed out of the crevice, knocking off material from an overhang (4a) and was pushed down again with its top surface, creating an impression in the dust corresponding to the ‘eye' of the feature that resembles a skull face (4b). The colors correspond to the data presented in the accompanying annotated infographic [video credit: Image: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; Data: ESA/Rosetta/Philae/ROMAP; Analysis: O'Rourke et al (2020)]
- ROMAP's data were cross-correlated with those collected by Rosetta's RPC magnetometer at the same time to determine Philae's attitude and exclude any influence from the background magnetic field of the plasma environment around the comet.
- "We weren't able to make all the measurements we planned in 2014 with Philae, so it is really amazing to use the magnetometer like this, and to combine data from both Rosetta and Philae in a way that was never intended, to give us these wonderful results," says Philip Heinisch, who led the analysis of the ROMAP data.
- A reanalysis of the touchdown data found that Philae had spent nearly two full minutes at the second touchdown site, making at least four distinct surface contacts as it ploughed across it. One particularly notable imprint revealed in the images was created as Philae's top surface sank 25 cm into the ice on the side of a crevice, leaving identifiable marks of its drill tower and sides. The spikes in the magnetic field data arising from the boom movement showed that it took Philae three seconds to make this particular depression.
Figure 34: Where is 'skull face'? Rosetta's Philae lander touched down on Comet 67P/Churyumov-Gerasimenko on 12 November 2014 and made multiple contacts with the surface before arriving at its final resting place. Its second touchdown site was recently identified just 30 meters away from its final position, in a location that resembles the shape of a skull with a pointed ‘hat' when viewed from above. This animation shows the location of ‘skull face' hidden in the shadowed cliffs of the comet: when the exposure of the image is enhanced, the touchdown site is revealed. The animation points out the various features of the ‘skull'. For example, Philae's body compressed into the ice-dust scenery to create the skull's right eye [video credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; Analysis: O'Rourke et al (2020)]
- "The shape of the boulders impacted by Philae reminded me of a skull when viewed from above, so I decided to nickname the region ‘skull-top ridge' and to continue that theme for other features observed," says Laurence.
- "The right ‘eye' of the ‘skull face' was made by Philae's top surface compressing the dust while the gap between the boulders is ‘skull-top crevice', where Philae acted like a windmill to pass between them."
- Analysis of images and data from OSIRIS and Rosetta's spectrometer VIRTIS confirmed that the bright exposure was water-ice covering an area of about 3.5 m2. Although the ice was mostly in shadow at the time of the landing, the Sun was directly illuminating the area when the images were taken months later, lighting it up like a beacon to stand out against everything around it. The ice was brighter than the surrounds because it had not been previously exposed to the space environment and undergone space weathering.
Figure 35: A light shining in the darkness. Rosetta's Philae lander touched down on Comet 67P/Churyumov-Gerasimenko on 12 November 2014 and made multiple contacts with the surface before arriving at its final resting place. This animated gif focuses on the second touchdown site, which is characterized by a bright patch of exposed water-ice covering an area of about 3.5 m2. Although the ice was mostly in shadow at the time of the landing, the Sun was directly illuminating the area when these images were taken 22 months later, lighting it up like a beacon to stand out against everything around it. In this animated gif, every second image has its brightness/contrast fully reduced to show how the ice in the crevice is brighter than all of the surrounding regions [image credit: ESA/Rosetta/Philae/ROLIS/DLR; all other images: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; Analysis: O'Rourke et al (2020)]
- "It was a light shining in the darkness," says Laurence, noting that it was located just 30 m away from where Philae was finally imaged on the comet surface.
- While an exciting conclusion in the search for the second touchdown site, the study also provides the first in situ measurement of the softness of the icy-dust interior of a boulder on a comet.
- "The simple action of Philae stamping into the side of the crevice allowed us to work out that this ancient, billions-of-years-old, icy-dust mixture is extraordinarily soft – fluffier than froth on a cappuccino, or the foam found in a bubble bath or on top of waves at the seashore," adds Laurence.
- The study also allowed an estimate of the boulder's porosity – how much empty space exists between the ice-dust grains inside the boulder – of about 75%, which is in line with the value measured previously for the whole comet in a separate study. The same study showed that the comet is homogeneous anywhere in its interior on all size scales down to about one meter. This implies that the boulders represent the overall state of the comet's interior when it formed some 4.5 billion years ago.
- "This is a fantastic multi-instrument result that not only fills in the gaps in the story of Philae's bouncy journey, but also informs us about the nature of the comet," says Matt Taylor, ESA's Rosetta project scientist. "In particular, understanding the strength of a comet is critical for future lander missions. That the comet has such a fluffy interior is really valuable information in terms of how to design the landing mechanisms, and also for the mechanical processes that might be needed to retrieve samples."
Figure 36: Flight over Abydos valley. Rosetta's Philae lander touched down on Comet 67P/Churyumov-Gerasimenko on 12 November 2014 and made multiple contacts with the surface before arriving at its final resting place in a location named Abydos. This video presents a flyover of the Abydos valley including Philae's second touchdown location in a region nicknamed ‘Skull-top ridge', as viewed on a high-resolution Digital Terrain Model reconstructed from OSIRIS imagery. The Abydos valley is a boulder-strewn location with the touchdown area located on the edge of a hill-like feature that towers 30 m above the valley at its maximum point. - The flyover begins with the skull-top boulders circled, before the viewer flies towards them, and make a 360º flight around. Although the fine details of Philae's interaction with touchdown site two are not shown in this shape model, the ice contained inside the crevice itself is clearly visible. - For orientation, at 45 s, skull-top ‘hat' is to the left of the crevice, and ‘skull face' is to the right. Skull-top hat has a length of 5 m and a maximum height of 4 m. The skull face boulder is over 6 m in length. Both boulders are about 2.5 m wide. Although the boulders have a distinct shape from the initial fly in perspective, the challenge in finding them from other viewing points becomes clear when seen from other perspectives [video credit: Video prepared by and music composed by Gerhard Paar (Joanneum Research Forschungsgesellschaft mbH); Analysis: O'Rourke et al (2020)]
• September 21, 2020: ESA's Rosetta mission has revealed a unique kind of aurora, an exciting phenomenon seen throughout the Solar System, at its target comet, Comet 67P/Churyumov-Gerasimenko. 71)
Figure 37: ESA's Rosetta mission has revealed a unique kind of aurora, an exciting phenomenon seen throughout the Solar System, at its target comet, Comet 67P/Churyumov-Gerasimenko. This image shows the key stages of the mechanism by which this aurora is produced: as electrons stream out into space from the Sun and approach the comet, they are accelerated and go on to break down molecules in the comet's environment. This destructive process can throw out excited atoms, which then ‘de-excite' to produce the observed aurora. To reveal the auroral nature of the emissions, the study relies on a set of in situ and remote-sensing instruments aboard Rosetta (RPC, Rosina, Virtis, Miro and Alice), as shown to the right of the infographic in the spacecraft schematic [image credit: ESA (spacecraft: ESA/ATG medialab)]
- At Earth, auroras form as charged particles from the Sun interact with our planet's magnetic field to create shimmering displays of color and light in the high-latitude sky. While these light displays have been seen at various planets and moons in the Solar System, and around a more distant star, observations from ESA's comet-chasing Rosetta mission now reveal unique auroral emissions at the spacecraft's target comet: 67P/Churyumov-Gerasimenko (67P/C-G).
- These emissions are created as charged particles stream towards the comet from the Sun – a flow known as the solar wind – and interact with the gas surrounding the comet's icy, dusty nucleus.
- "The glow surrounding 67P/C-G is one of a kind," says Marina Galand of Imperial College London, UK, lead author of the new study. 72) "By digging into data from numerous instruments on Rosetta and linking them together, we've discovered that this glow is auroral in nature: it's caused by a mix of processes, some seen at Jupiter's moons Ganymede and Europa and others at Earth and Mars."
- These processes define how the envelope of gas (or coma) around 67P/C-G becomes excited – that is, made to glow – and how the particles causing this excitation are given a boost of energy and sped up. 67P/C-G's aurora shows up in ultraviolet light, as seen in martian auroras detected by ESA's Mars Express (although the ‘colors' of the auroras at 67P/C-G and Mars differ).
- To reveal more than what one instrument can offer, Marina and colleagues connected various measurements made by Rosetta, both in situ and remote-sensing, using a physics-based model.
- "By doing this, we didn't have to rely upon just a single dataset from one instrument," says Marina. "Instead, we could draw together a large, multi-instrument dataset to get a better picture of what was going on. This enabled us to unambiguously identify how 67P/C-G's ultraviolet atomic emissions form, and to reveal their auroral nature."
Figure 38: Animation of ultraviolet aurora being produced at Rosetta's comet (video credit: ESA)
- The ultraviolet emissions Rosetta observed at comet 67P/C-G have shown up before, and were thought to be ‘dayglow', a process caused by solar light particles (photons) interacting with cometary gas.
- However, this study shows that these emissions are aurora instead: they are driven not by photons, but by electrons in the solar wind that have been accelerated in the comet's nearby environment. "These electrons then interact with molecules in the coma to produce the auroral glow. The process by which the electrons are accelerated is similar to some of the processes that drive auroras at Earth and Mars, despite 67P/C-G lacking an intrinsic magnetic field," adds Marina. "In fact, the magnetic environments of moons, planets, and comets are all very different, so it's exciting and intriguing that we see auroras at all of them."
- Exploring the emission at 67P/C-G will enable scientists to assess how the particles comprising the solar wind change over time, something that is crucial for understanding space weather throughout the Solar System. It also confirms that ultraviolet auroras can occur at comets, and brings insight into how these exciting, and often spectacular, light shows form at different Solar System objects.
- Following its rendezvous with 67P/C-G in 2014, Rosetta has provided a wealth of data that has underpinned our exploration of how the Sun and solar wind interact with comets. The spacecraft was the first to orbit a comet's nucleus, the first to fly alongside a comet as it travelled into the inner Solar System, the first to dispatch a robotic lander to a cometary nucleus, and the first to image a cometary surface at such a high spatial resolution.
- "And now, it's the first to spot an ultraviolet aurora at a comet! Auroras are inherently exciting – and this excitement is even greater when we see one somewhere new, or with new characteristics," adds ESA's Rosetta project scientists Matt Taylor.
- "This multi-instrument analysis brings together more pieces of the puzzle in our understanding of both auroras throughout the Solar System, and of the various phenomena we see around comets.
- "It uses data from numerous instruments aboard Rosetta, and is a great example of why we fly several different instruments, with different goals and techniques and areas of focus, on any given mission: to get a full picture of the objects and processes going on in our cosmic environment."
• September 07, 2020: A permeable heart with a hardened facade –the resting place of Rosetta's lander on comet 67P/Churyumov-Gerasimenko is revealing more about the interior of the 'rubber duck' shaped-body looping around the Sun. 73)
Figure 39: The graphic shows the signal connecting the CONSERT instrument on Philae, on the surface of the comet, to the one on the Rosetta orbiter. The fan like appearance is a result of the motion of Rosetta along its orbit, with the colors marking the separate signal paths as the orbit evolves. The image at the bottom shows the signals in more detail, propagating inside the comet from Philae to the points from where they leave the comet to the orbiter. The curving is a result of the projection of its paths on the bumpy surface of the comet. The bluer color indicates more shallow paths (just a few centimeters), while the redder tones show where the signals penetrated below 100 m in depth. (image credit: ESA/Rosetta/Philae/CONSERT)
- A recent study suggests that the comet's interior is more porous than the material near the surface. The results confirm that solar radiation has significantly modified the comet's surface as it travels through space between the orbits of Jupiter and Earth. Heat from the Sun triggers an ejection and subsequent falling back of material. 74)
- Location, location, location. That was key for the radar instrument on the Rosetta spacecraft and its Philae lander, which was designed to probe the comet's nucleus. The CONSERT experiment involved two antennas sending precise signals to each other. But when Philae went missing upon landing on November 2014, scientists had to work with estimated values.
- Philae operated for over two days on the surface – 63 hours, to be precise.
- "We managed to define the region where the lander was with a margin of about 150 m. The real landing site was in this region," explains Wlodek Kofman, emeritus principal investigator of CONSERT.
- It took nearly two years to find out where Philae was. In September 2016 the exact position of Philae was retrieved within the area identified by CONSERT.
- Precise 3D models of the comet with Philae in the picture "allowed us to revisit the measurements and improve our analysis of the interior," says Wlodek.
- The time for the signal to travel between the two radars offers insights into the comet's nucleus, such as porosity and composition. The team discovered that rays propagated at different velocities, indicating varying densities within the comet.
- The discussion is still open, but Wlodek believes that "this strongly suggests that the less dense interior has kept its pristine nature." Known as the most primitive objects in our cosmic neighborhood, comets might hold, deep inside, valuable clues about the formation of our Solar System.
• March 13, 2020: Scientists have detected ammonium salts on the surface of Comet 67P/Churyumov-Gerasimenko (Figure 40) by analyzing data collected by the Visible, Infrared and Thermal Imaging Spectrometer (VIRTIS) on ESA's Rosetta mission between August 2014 and May 2015. 75)
Figure 40: As VIRTIS mapped the comet during the first half of Rosetta's mission, the data revealed how its surface is as dark as coal, and slightly reddened by a mixture of carbon-based compounds and opaque minerals. The instrument also spotted local patches of water and carbon dioxide ice, along with a puzzling but almost ubiquitous absorption feature around the infrared wavelength of 3.2 µm. The nature of the compound that could cause such a feature, however, remained unclear until now [image credit: O. Poch, IPAG, UGA/CNES/CNRS (left); ESA/Rosetta/NavCam – CC BY-SA IGO 3.0 (right)]
- The new study, led by Olivier Poch of Institut de Planétologie et d'Astrophysique de Grenoble, France, and published in the journal Science, adds to the complementary measurements obtained in the comet's atmosphere, or coma, using another instrument, the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA), which were published in Nature Astronomy earlier this year.
- As VIRTIS mapped the comet during the first half of Rosetta's mission, the data revealed how its surface is as dark as coal, and slightly reddened by a mixture of carbon-based compounds and opaque minerals. The instrument also spotted local patches of water and carbon dioxide ice, along with a puzzling but almost ubiquitous absorption feature around the infrared wavelength of 3.2 µm. The nature of the compound that could cause such a feature, however, remained unclear until now.
- To solve the enigma, a team of scientists have been creating ‘artificial' cometary surfaces in the laboratory (an example is shown in this image on the left), testing their properties and comparing them to the VIRTIS observations. With this aim, the scientists have produced fine particles of water ice containing dark dust grains and a variety of compounds. Then, they exposed these particles to cometary-like conditions – vacuum and low temperature. After several hours, all the ice had sublimated, leaving a surface made of porous dust, analogous to a cometary surface.
- The result of such experiments indicate that the mysterious absorption feature observed by VIRTIS at Comet 67P is predominantly due to salts of ammonium (NH4+), which are mixed with dark dust and detected on all kinds of terrains across the comet.
- The presence of these salts might considerably increase the amount of nitrogen that scientists had previously expected to find on this comet, and possibly on other comets as well. The findings agree with the recent detection, by the ROSINA instrument on Rosetta, of gases produced by the sublimation of ammonium salts on dust grains ejected from the comet.
- A similar absorption feature has also been observed on several asteroids, both in the main belt and in Jupiter's Trojan family, as well as on Jupiter's moon Himalia, suggesting that these bodies contain ammonium salts, too. The presence of these salts might hint at a link in the chemical composition between asteroids, comets and possibly the proto-solar nebula, providing a tantalizing scenario for the delivery of nitrogen – a key element for the chemistry of life as we know it on Earth – to the inner planets of the Solar System.
• January 20, 2020: Observations from ESA's Rosetta spacecraft are shedding light on the mysterious make-up of Comet 67P/Churyumov-Gerasimenko, revealing a mix of compounds thought to be essential precursors to life – including salts of ammonium and a particular type of hydrocarbons. 76)
- These new studies suggest the comet gleaned this material from the presolar cloud where the Solar System formed 4.6 billion years ago.
- Comets have a nucleus consisting of ice and dust, from which material sublimates when warmed by sunlight to form a gaseous enveloping ‘coma'. These comas contain a mix of molecules that largely match theoretical predictions, with one outstanding exception: nitrogen gas, which is usually present in far smaller amounts than expected.
- "The reason behind this nitrogen depletion has remained a major open question in cometary science," says Kathrin Altwegg of the University of Bern, Switzerland, principal investigator for the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) instrument and lead author of a new study. 77)
- "Using ROSINA observations of Comet 67P, we discovered that this ‘missing' nitrogen may in fact be tied up in ammonium salts that are difficult to detect in space."
- Ammonia, a molecule comprising one nitrogen and three hydrogen atoms, is one of the main carriers of volatile nitrogen, and readily combines with various acids found in both the space between stars and in cometary ice to form salts. These ammonium salts are thought to be the starting point for far more complex compounds – such as urea and glycine, the latter of which was also found on Rosetta's comet – that are known to be precursors to life as we know it on Earth.
- "Finding ammonium salts on the comet is hugely exciting from an astrobiology perspective," adds Kathrin. "This discovery highlights just how much we can learn from these intriguing celestial objects."
- Kathrin and colleagues used ROSINA data gathered during the final phase of the mission, from when Rosetta was performing close flyovers of the comet in September 2016. These data were somewhat serendipitous: Rosetta ventured closer to 67P than ever before, reaching just 1.9 km above the comet's surface, and became completely enshrouded by dust.
- "Because of the dusty environment at the comet, and the rotation of Earth, we were not able to readily communicate with Rosetta via our antennas at the time and had to wait until the next morning to reestablish our communication link," says Kathrin.
- "None of us slept well that night! But both Rosetta and ROSINA ended up behaving perfectly, flawlessly measuring the most abundant and most diverse mass spectra yet, and revealing many compounds we had never spotted on 67P before."
- Adding to this investigation of 67P, another recent study made use of a different instrument on Rosetta, the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS), which operated until May 2015, to explore the properties of the comet's nucleus.
- The researchers explored several million infrared spectra gathered with VIRTIS and discovered clear signs of hydrogen and carbon chains known as organic aliphatic compounds – the first time such compounds have been spotted on the surface of a cometary nucleus.
- "Where – and when – these aliphatic compounds came from is hugely important, as they are thought to be essential building blocks of life as we know it," explains lead author Andrea Raponi of INAF, the National Institute for Astrophysics in Italy.
- "The origin of material such as this found in comets is crucial to our understanding of not only our Solar System, but planetary systems throughout the Universe."
- Comets are flung inwards towards the Sun from the outer reaches of the Solar System, and may deliver material to the inner planets as they travel. This material is thought to have either come from the young, still-forming Sun, or from the interstellar medium.
- "We found that the nucleus of Comet 67P has a composition similar to the interstellar medium, indicating that the comet contains unaltered presolar material," says co-author Fabrizio Capaccioni, also of INAF and principal investigator for VIRTIS.
- "This composition is also shared by asteroids and some meteorites that we have found on Earth, suggesting that these ancient, rocky bodies locked up various compounds from the primordial cloud that went on to form the Solar System."
Figure 41: This image shows salts of ammonium chloride (NH4Cl), image credit: University of Bern
- "This may mean that at least a fraction of the organic compounds in the early Solar System came directly from the wider interstellar medium – and thus that other planetary systems may also have access to these compounds," adds Raponi.
- Rosetta explored Comet 67P for over two years, ending its mission by crash-landing on the comet on 30 September 2016.
- "Although Rosetta operations ended over three years ago, it is still offering us an incredible amount of new science and remains a truly ground-breaking mission," adds Matt Taylor, ESA's Rosetta Project Scientist.
- "These studies tackled a couple of open questions in cometary science: why comets are depleted in nitrogen, and where comets got their material from. Inspiring discoveries such as these help us to understand a great deal more about not only comets themselves, but the history, characteristics and evolution of our entire cosmic neighborhood."
• January 15,2020: Astronomers using the combined powers of ESA's Rosetta mission and the ground-based Atacama Large Millimeter/submillimeter Array (ALMA) have traced the journey of phosphorus – one of life's building blocks – from star-forming regions to comets. 78)
- An essential element for life as we know it, phosphorus is present in our DNA and cell membranes. But how it arrived on the early Earth is something of a mystery.
- A new study combining data collected by the Rosetta mission at Comet 67P/Churyumov–Gerasimenko with ALMA observations of the star-forming region AFGL 5142 has revealed, for the first time, where molecules containing phosphorus form in the Universe. The research also shows how this element is carried in comets and how a particular molecule – phosphorus monoxide – may have played a crucial role in starting life on our planet. 79)
- "Life appeared on Earth about four billion years ago, but we still do not know the processes that made it possible," says Víctor Rivilla of INAF – Arcetri Astrophysical Observatory in Florence, Italy, who lead the new study.
- New stars and planetary systems arise in cloud-like regions of gas and dust in between stars, making these interstellar clouds the ideal places to start the search for life's building blocks.
- By observing such a stellar nursery with ALMA, Víctor and collaborators could pinpoint the formation sites of phosphorus-bearing molecules, which appear to be created while massive stars are formed. As flows of gas from young massive stars open up cavities in the surrounding cloud material, molecules containing phosphorus form on the cavity walls through the combined action of shocks and radiation from the infant star.
- The astronomers have also shown that phosphorus monoxide, which combines phosphorus with one oxygen atom, is the most abundant phosphorus-bearing molecule in the cavity walls of this particular star-forming region.
- Following the cosmic trail of this life-enabling compound, the team took their investigations closer to home, looking at comets in the Solar System.
- If the cavity walls of an interstellar cloud collapses to form a star, especially low-mass ones like our Sun, phosphorus monoxide can freeze out and get trapped in the icy dust grains that remain around the new star. Even before the star is fully formed, those dust grains come together to form pebbles, rocks and ultimately comets, which become transporters of phosphorus monoxide.
- The ROSINA instrument on Rosetta, which collected data at Comet 67P/C-G for over two years, had already found hints of phosphorus at the comet in 2016, but it was not clear what molecule had carried it there. After the ALMA observations suggested that phosphorus monoxide would be a very likely candidate, ROSINA scientists went back to their data and eventually found evidence of this molecule at Rosetta's comet.
- "Phosphorus is essential for life as we know it," says co-author and ROSINA Principal Investigator Kathrin Altwegg from University of Bern, Switzerland.
- "As comets most probably delivered large amounts of organic compounds to the Earth, the phosphorus monoxide found in Comet 67P/C-G may strengthen the link between comets and life on Earth."
- This first sighting of phosphorus monoxide at a comet helps astronomers draw a connection between star-forming regions, where the molecule is created, all the way to our planet.
Figure 42: This ALMA image shows a detailed view of the star-forming region AFGL 5142. A bright, massive star in its infancy is visible at the center of the image. The flows of gas from this star have opened up a cavity in the region, and it is in the walls of this cavity (shown in color), that phosphorus-bearing molecules like phosphorus monoxide are formed. The different colors represent material moving at different speeds. A joint study linking the presence of phosphorus monoxide in this star-forming region with analogous observations performed by the Rosetta mission at Comet 67P/Churyumov–Gerasimenko showed how this compound may have played a crucial role in starting life on our planet (image credit: ALMA (ESO/NAOJ/NRAO), Rivilla et al.)
Figure 43: This wide-field view shows the region of the sky, in the constellation of Auriga, where the star-forming region AFGL 5142 is located. This view was created from images forming part of the Digitized Sky Survey 2 (image credit: ESO/Digitized Sky Survey 2. Acknowledgement: Davide De Martin)
- This first sighting of phosphorus monoxide at a comet helps astronomers draw a connection between star-forming regions, where the molecule is created, all the way to our planet.
- "The combination of the ALMA and ROSINA data has revealed a sort of chemical thread during the whole process of star formation, in which phosphorus monoxide plays the dominant role," explains Víctor.
- "The detection of phosphorus monoxide was clearly thanks to an interdisciplinary exchange between telescopes on Earth and instruments in space," adds Altwegg.
Figure 44: Our Solar System condensed from a cloud of gas and dust over 4.6 billion years ago. As the newborn planets settled in their orbits, gravitational perturbations are thought to have disrupted swarms of comets into the inner Solar System, impacting the rocky planets. As well as inheriting ingredients during the planet-forming process itself, comets are also believed to have delivered some of the basic ingredients for life to Earth, leading to life as we know it today (image credit: ESA) 80)
- This and similar results are a testament to the synergy between pioneering space science missions, such as the fleet operated by ESA, and cutting-edge ground-based astronomical facilities like the ones run in Chile by the European Southern Observatory (ESO), ALMA's European executive partner.
- "This study takes one of Rosetta's major results regarding prebiotic compounds – the detection of phosphorus, in this case – and digs further into the data," says Matt Taylor, ESA Rosetta project scientist.
- "It is an inspiring demonstration of the broader impact and importance of the cometary observations made by Rosetta to the broader astronomy community, deepening our understanding of how stars and planetary systems form and evolve."
• November 12, 2019: On 12 November 2014 Philae became the first spacecraft to land on a comet as part of the successful Rosetta mission to study Comet 67P/Churyumov-Gerasimenko. Five years later, and after the mission's official end in 2016, Rosetta is continuing to provide insights into the origins of our Solar System. 81)
- Rosetta's instruments have already discovered that the comet contained oxygen, organic molecules, noble gases and 'heavy' or deuterated water different to that found on Earth. - As scientists continue to analyze data from Rosetta's instruments, including the ionized gas or plasma, the results are improving our understanding of comets. Mission data is also being delivered to an archive as a future resource.
- Rosetta orbits the Sun every 6.5 years and will pass the Earth again, visible from ground-based telescopes, in 2021. ESA's future Comet Interceptor mission will build on Rosetta's success when it performs a flyby of a comet. But, unlike Rosetta, the comet will be new to our Solar System.
Figure 45: This video contains interviews with Charlotte Goetz, Research Fellow, ESA; Kathrin Altwegg, ROSINA instrument principal investigator, Rosetta/University of Bern; Colin Snodgrass, Comet Interceptor principal investigator (video credit: ESA)
• September 30, 2019: Friday 30 September 2016 was a bitter-sweet day for space exploration: the incredible Rosetta spacecraft reached the end of its hugely successful mission, fittingly, by touching down on the surface of the comet it had been studying from orbit for the previous two years. 82)
Figure 46: This image was captured by the spacecraft's wide-angle OSIRIS camera during the final hour of the mission from an altitude of about 400 m above the surface of Comet 67P/Churyumov-Gerasimenko. Its final resting place is not far from the top center of the image; see also this breathtaking sequence of images covering the final hours of the mission (image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA – CC BY-SA 4.0)
- Rosetta arrived at the comet on 6 August 2014 after a ten year journey through space, and deployed lander Philae to its surface on 12 November 2014. Rosetta continued to study the icy, dusty object from near and far as the comet reached its closest approach to the Sun in August 2015 and moved towards the outer Solar System again.
- Conducting science until the very end, the descent gave Rosetta the opportunity to collect unique data on the comet's gas, dust and plasma environment very close to its surface, as well as take very high-resolution images and temperature measurements.
- While the mission operations have concluded, the science certainly continues. Intense activities also surround the preservation of Rosetta's highest resolution and best calibrated data in ESA's Planetary Science Archive, securing the mission's legacy for future generations.
- Last week marked another milestone as the final Science Working Team meeting was held at ESA's technical facility in the Netherlands. It was the 52nd of such meetings, the first having been held in the late 1990s. The meeting closed out the formal aspect of the mission and archiving activities and enabled teams to reflect on their efforts over the last decades. In addition, several days were dedicated to the latest and ongoing science activities, which are delving deep into the cross-instrument analysis of the comet. A number of the topics discussed are also presented in a recently published special edition of Astronomy and Astrophysics.
• September 18, 2019: Scientists analyzing the treasure trove of images taken by ESA's Rosetta mission have turned up more evidence for curious bouncing boulders and dramatic cliff collapses. 83)
- Rosetta operated at Comet 67P/Churyumov-Gerasimenko between August 2014 and September 2016, collecting data on the comet's dust, gas and plasma environment, its surface characteristics and its interior structure.
- As part of the analysis of some 76,000 high-resolution images captured with its OSIRIS camera, scientists have been looking for surface changes. In particular, they are interested in comparing the period of the comet's closest approach to the Sun – known as perihelion – with that after this most active phase, to better understand the processes that drive surface evolution.
Figure 47: Bouncing boulder on Comet 67P/C-G. An example of a boulder having moved across the surface of Comet 67P/Churyumov-Gerasimenko's surface, captured in Rosetta's OSIRIS imagery. The image was taken with the narrow-angle camera and shows the boulder in the lower third of the image [image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA (CC BY-SA 4.0)]
Figure 48: An example of a boulder having moved across the surface of Comet 67P/Churyumov-Gerasimenko's surface, captured in Rosetta's OSIRIS imagery [image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA (CC BY-SA 4.0); Analysis: J-B. Vincent et al (2019)]
Evolution of a bouncing boulder: The first image (left) provides a reference view of the comet, along with a close-up of the region under study. The smaller insets on the right show before and after images of the region containing the bouncing boulder, captured on 17 March 2015 and 19 June 2016, respectively. Impressions of the boulder have been left in the soft regolith covering the comet's surface as it bounced to a halt. It is thought to have fallen from the nearby cliff, which is about 50 m high. The graphic at the bottom illustrates the path of the boulder as it bounced across the surface, with preliminary measurements of the ‘craters' calculated.
- Loose debris is seen all over the comet, but sometimes boulders have been caught in the act of being ejected into space, or rolling across the surface. A new example of a bouncing boulder was recently identified in the smooth neck region that connects the comet's two lobes, an area that underwent a lot of noticeable large-scale surface changes over the course of the mission. There, a boulder about 10 m-wide has apparently fallen from the nearby cliff, and bounced several times across the surface without breaking, leaving ‘footprints' in the loosely consolidated surface material.
- "We think it fell from the nearby 50 m-high cliff, and is the largest fragment in this landslide, with a mass of about 230 tons," said Jean-Baptiste Vincent of the DLR Institute for Planetary Research, who presented the results at the EPSC-DPS conference in Geneva today.
- "So much happened on this comet between May and December 2015 when it was most active, but unfortunately because of this activity we had to keep Rosetta at a safe distance. As such we don't have a close enough view to see illuminated surfaces with enough resolution to exactly pinpoint the ‘before' location of the boulder."
- Studying boulder movements like these in different parts of the comet helps determine the mechanical properties of both the falling material, and the surface terrain on which it lands. The comet's material is in general very weak compared with the ice and rocks we are familiar with on Earth: boulders on Comet 67P/C-G are around one hundred times weaker than freshly packed snow.
- Another type of change has also been witnessed in several locations around the comet: the collapse of cliff faces along lines of weakness, such as the dramatic capture of the fall of a 70 m-wide segment of the Aswan cliff observed in July 2015. But Ramy El-Maarry and Graham Driver of Birkbeck, University of London, may have found an even larger collapse event, linked to a bright outburst seen on 12 September 2015 along the northern-southern hemisphere divide.
Figure 49: An outburst event on Comet 67P/Churyumov-Gerasimenko took place on 12 September 2015 and is thought to be associated with one of the most dramatic cliff collapses captured during the lifetime of the Rosetta mission [image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA (CC BY-SA 4.0)]
- "This seems to be one of the largest cliff collapses we've seen on the comet during Rosetta's lifetime, with an area of about 2000 m2 collapsing," said Ramy, also speaking at EPSC-DPS today.
- During perihelion passage, the southern hemisphere of the comet was subjected to high solar input, resulting in increased levels of activity and more intensive erosion than elsewhere on the comet.
- "Inspection of before and after images allow us to ascertain that the scarp was intact up until at least May 2015, for when we still have high enough resolution images in that region to see it," says Graham, an undergraduate student working with Ramy to investigate Rosetta's vast image archive.
- "The location in this particularly active region increases the likelihood that the collapsing event is linked to the outburst that occurred in September 2015."
- Looking in detail at the debris around the collapsed region suggests that other large erosion events have happened here in the past. Ramy and Graham found that the debris includes blocks of variable size ranging up to tens of meters, substantially larger than the boulder population following the Aswan cliff collapse, which is mainly comprised of boulders a few meters diameter.
- "This variability in the size distribution of the fallen debris suggests either differences in the strength of the comet's layered materials, and/or varying mechanisms of cliff collapse," adds Ramy.
- Studying comet changes like these not only gives insight into the dynamic nature of these small bodies on short timescales, but the larger scale cliff collapses provide unique views into the internal structure of the comet, helping to piece together the comet's evolution over longer timescales.
- "Rosetta's datasets continue to surprise us, and it's wonderful the next generation of students are already making exciting discoveries," adds Matt Taylor, ESA's Rosetta project scientist. 84)
Figure 50: Cliff collapse before and after. Before and after a cliff collapse on Comet 67P/Churyumov-Gerasimenko. In the upper panels the yellow arrows show the location of a scarp at the boundary between the illuminated northern hemisphere and the dark southern hemisphere of the small lobe at times before and after the outburst event (September 2014 and June 2016, respectively). The lower panels show close-ups of the upper panels; the blue arrow points to the scarp that appears to have collapsed in the image after the outburst. Two boulders (1and 2) are marked for orientation [image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA (CC BY-SA 4.0)]
• August 12, 2019: Last week marked five years since ESA's Rosetta probe arrived at its target, a comet named 67P/Churyumov-Gerasimenko (or 67P/C-G). Tomorrow, 13 August, it will be four years since the comet, escorted by Rosetta, reached its perihelion – the closest point to the Sun along its orbit. This image, gathered by Rosetta a couple of months after perihelion, when the comet activity was still very intense, depicts the nucleus of the comet with an unusual companion: a chunk of orbiting debris (circled). At that time, the spacecraft was at over 400 km away from the comet's center. 85)
Figure 51: Animated sequence of images obtained by ESA's Rosetta probe at Comet 67P/Churyumov-Gerasimenko on 21 October 2015. The sizeable chunk in this view was spotted by astrophotographer Jacint Roger from Spain, who mined the Rosetta archive, processed some of the data, and posted the finished images on Twitter as an animated GIF. Scientists at ESA and in the OSIRIS instrument team are now looking into this large piece of cometary debris in greater detail. Dubbed a ‘Churyumoon' by researcher Julia Marín-Yaseli de la Parra, the chunk appears to span just under 4 m in diameter. - Modelling of the Rosetta images indicates that this object spent the first 12 hours after its ejection in an orbital path around 67P/C-G at a distance of between 2.4 and 3.9 km from the comet's center. Afterwards, the chunk crossed a portion of the coma, which appears very bright in the images, making it difficult to follow its path precisely; however, later observations on the opposite side of the coma confirm a detection consistent with the orbit of the chunk, providing an indication of its motion around the comet until 23 October 2015. Scientists have been studying and tracking debris around 67P/C-G since Rosetta's arrival in 2014. The object pictured in this sequence is likely the largest chunk detected around the comet, and will be subject to further investigations [image credit: ESA/Rosetta/MPS/OSIRIS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA/J. Roger (CC BY-SA 4.0)]
- Comet 67P/C-G is a dusty object. As it neared its closest approach to the Sun in late July and August 2015, instruments on Rosetta recorded a huge amount of dust enshrouding the comet. This is tied to the comet's proximity to our parent star, its heat causing the comet's nucleus to release gases into space, lifting the dust along. Spectacular jets were also observed, blasting more dust away from the comet. This disturbed, ejected material forms the ‘coma', the gaseous envelope encasing the comet's nucleus, and can create a beautiful and distinctive tail.
- A single image from Rosetta's OSIRIS instrument can contain hundreds of dust particles and grains surrounding the 4 km-wide comet nucleus. Sometimes, even larger chunks of material left the surface of 67P/C-G – as shown here.
- The sizeable chunk in this view was spotted a few months ago by astrophotographer Jacint Roger from Spain, who mined the Rosetta archive, processed some of the data, and posted the finished images on Twitter as an animated GIF. He spotted the orbiting object in a sequence of images taken by Rosetta's OSIRIS narrow-angle camera on 21 October 2015. At that time, the spacecraft was at over 400 km away from 67P/C-G's center. The animated sequence is available for download here.
• April 26, 2019: Two-and-a-half years have passed since the operational phase of the Rosetta mission came to an end in September 2016. However, scientific evaluation of the enormous amounts of data from the instruments on the spacecraft and the Philae lander is still ongoing. The team of scientists working on the VIRTIS instrument have now published new findings relating to the surface temperature and thermal effects on the 'duck-shaped' Comet 67P / Churyumov-Gerasimenko, Germany's scientific contributions to VIRTIS are led by the German Aerospace Center (DLR). 86) 87)
- The Visible InfraRed and Thermal Imaging Spectrometer (VIRTIS) acquired infrared images of the comet from on board the Rosetta orbiter during August and September 2014, approximately one year before the comet reached perihelion – the point in its orbit closest to the Sun. During the period under consideration, the comet was still distant from the Sun, and its level of activity was low. The researchers converted the images into thermal maps.
- Temperature is the most important parameter for deriving the gas and dust activity typical of comets. First, the VIRTIS team measured the average temperature of the comet's nucleus on its daytime side. While the average surface temperature over the two months was approximately minus 60ºC, the scientists also identified places that were significantly warmer, at around minus 43ºC. These included a cavity in the surface, where the inner walls reflected the thermal radiation and thus led to stronger warming, referred to as self-heating.
- Self-heating also occurs at the 'duck's neck' connecting the two lobes of the comet. Temperatures were higher here than the laws of black-body radiation would imply. Assuming a dust-dominated surface a few millimeters thick and minimal sublimation of volatile substances, self-heating is attributable to surface roughness. The self-heating effect is enhanced by the striking concave shape of the 'neck'.
- Another significant finding concerns the thermal gradients caused by sudden shadows cast alternately onto the 'neck' by the two lobes of the comet during solar illumination. These localized shadows on the 'neck' created extreme temperature differences within the space of just a few minutes, which might be 10 times greater than normal daily variations in temperature on other areas of the surface. "To better investigate seasonal temperature effects on the nucleus, we concentrated on a region named Imhotep, which is relatively flat and far from the 'neck', and where the self-heating effect is significantly lower," says Gabriele Arnold of the DLR Institute of Planetary Research. "For this area, we compared the observations performed by VIRTIS with those of the Microwave Instrument for the Rosetta Orbiter (MIRO), another instrument on board Rosetta. MIRO made it possible to measure the temperature in larger depressions on the comet. The findings of the two instruments can be explained by the theory that the Imhotep region has a thin surface layer consisting mainly of loose dust."
- Imhotep was also observed a few months later, when the comet was much closer to the Sun. The temperature values obtained by VIRTIS were much higher than before, but lower than expected, given that the scientists were working on the assumption that the surface layer consisted only of loose dust. This led the researchers to conclude that the composition of the uppermost layers must have changed over time. The quantity of volatiles within it must have increased, resulting in a higher degree of sublimation and more intense comet activity. This in turn can cause surface temperatures to be lower than would be reached by a layer consisting solely of dust.
- All the observational evidence suggests a comet nucleus with thermal behavior that is dominated by phenomena associated with the morphology and chemical and physical state of the thin uppermost surface layer, which is only a few centimeters thick. In the subsurface, the nucleus is thought to remain essentially unchanged and has only been weakly influenced by previous approaches to the Sun.
- Gabriele Arnold sums up: "The work that has just been published shows that the ongoing evaluation of the large quantity of data acquired will continue to provide unique findings for comet research and the study of the early Solar System, even years after the end of the Rosetta mission."
Figure 52: Measured and modelled surface temperature (image credit: VIRTIS study team, DLR)
• April 23, 2019: From a distance of five million kilometers to within 20 meters, ESA's Rosetta spacecraft captured images of Comet 67P/Churyumov-Gerasimenko from all angles. 88)
- Between the first and the last images lies one of humankind's greatest space adventures to rendezvous with and follow a comet as it orbited the Sun, and deploy a lander to its surface.
- Figure 53 is just one of almost 70,000 images taken with Rosetta's high-resolution imaging system OSIRIS that are now available via a new online and mobile-friendly ‘comet viewer' created in a joint project with the Department of Information and Communication at Flensburg University of Applied Sciences, and the Max Planck Institute for Solar System Research, who lead the OSIRIS team.
- The image viewer hosts the full archive, but also has subsections organizing image sets into themes: for example, images showing towering cliffs and bizarre cracks on the comet surface, or those focusing on spectacular dust fountains as the comet launched gas and dust jets into space as its surface ices were warmed as it came closer to the Sun on its orbit.
- The collection of OSIRIS images captured the farewell of lander Philae as it dropped towards the surface of the comet, and later, towards the end of the mission, the feverish search for the hidden robot.
- Within the new comet viewer, each of the nearly 70,000 images is supplemented with the date on which it was taken, the distance to the comet, and a short accompanying text briefly describing what is seen in the image. The images can be downloaded in full resolution and can also be directly shared to Twitter and Facebook.
- For users who wish to delve deeper or use the archive for research purposes, the images are also available in scientific data format; in addition, there is information available on the filters used, focal lengths, and exposure times as well as references to the scientific documentation and evaluation software.
- The tool supplements the official ESA Archive Image Browser which also hosts the images taken by Rosetta's navigation camera made available throughout the mission, and ESA's Planetary Science Archive, for which the OSIRIS image archive was completed in June 2018.
- Read more: https://www.mps.mpg.de/One-comet-70000-images
Figure 53: Seen from afar, the comet is usually likened to a duck in shape, but in this enchanting close-up view its profile resembles that of a cat's face seen side-on. The two ‘ears' of the cat make up the twin peaks either side of the ‘C. Alexander Gate' – named for US Rosetta Project Scientist Claudia Alexander who passed away in July 2015. These impressive cliffs lie at the border between the Serqet and Anuket regions on the comet's head. The image was taken on 6 October 2014 from a distance of 18.6 km to the comet [image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA (CC BY-SA 4.0)]
• April 12, 2019: ESA's Rosetta mission was an incredible feat of science and engineering. Studying a comet up close, and even landing a probe on its surface, was an apt demonstration of ESA's abilities. Even though the mission ended in 2016, its benefits live on in the ESA Academy's Rosetta Science Operations Scheduling Legacy Workshops. The 2019 workshop has just been completed by 30 university students from 12 different ESA Member States and Canada. 89)
- "The friendly and international environment, together with the knowledge acquired during these four days were the best parts of the workshop," reviewed a Spanish student from the Polytechnic University of Catalonia. "This workshop was one of the few opportunities that I have had, as a student, to meet international science operation experts with the same interests and motivations and a common goal, while learning from their shared experiences in a mission that every space technology student is very fond of. I also found important the fact that this expert-student flow of information did not happen only during the lectures, but also in less formal scenarios, such as over coffee breaks or dinner. The workshop exceeded all my expectations and I had no idea how much I could learn in just four days. What an amazing week!"
- The students were excited to learn about science operations scheduling, and how it was planned for the Rosetta mission. A combination of lectures and exercises using the ESA's Mission Analysis and Payload Planning Software (MAPPS) kept each day varied.
- The students were divided into groups of three, each with a different background. They were supported by the experts that had scheduled the Rosetta mission. Various scenarios were proposed, such as Rosetta's arrival at the comet, and the search for the Philae lander after it bounced on the comet's surface. These situations provided context, and gave ample opportunity to both utilize the planning software, and draw upon the experience of the experts. In one scenario, students had to schedule the pictures that needed to be taken for Philae's descent, and then scour the Rosetta archive to find and analyze them.
- "During this week I embarked on a journey with Rosetta, alongside the people that dedicated their careers to see this mission succeed," said a Portuguese student from the Delft University of Technology. "From arriving at the comet to its final descent, Rosetta unveiled the secrets of 67P, and produced invaluable science during its lifetime. Hopefully the students involved in the workshop will be the future scientists and engineers exploring our Solar System and beyond, following the footsteps of Rosetta!"
- The accompanying lectures were designed to support and complement the exercises. They also allowed the students to gain insight into the many and varied aspects of the Rosetta mission. Some were broad, such as an overview of Rosetta; while others were more specific, such as planning the end of mission operations.
- Throughout the week, each student group had the opportunity to present the results of an exercise to the rest of the participants. This allowed them in-depth discussions, which were evaluated so that the students could claim ECTS credit(s) on their return to their respective universities. In addition, the students enjoyed a guided tour of ESEC-Redu and the PROBA control and operations rooms, which proved to be particularly illuminating.
Figure 54: This second edition of the workshop was held from 2 to 5 April 2019 at ESA Academy's Training and Learning Facility in ESEC-Galaxia, Belgium.. Present to share expertise with the students were seven experts, scientists and engineers, from ESA and the Max Planck Institute for Solar System Research. Among them were the Rosetta Project Scientist and Spacecraft Operations Manager. Personally involved with the Rosetta mission, the trainers had unrivalled knowledge and invaluable viewpoints, and they peppered the week with stories and anecdotes (image credit: ESA)
- A Spanish student from the University Carlos III of Madrid found this to be a formative experience: "There was T. S. Eliot quote I read a long time ago, which goes ‘Only those who risk going too far can possibly find out how far one can go'. I have always tried to follow that ideal. The second edition of the Rosetta Science Operations Scheduling Legacy Workshop proved me that ESA not only follow that ideal, but also achieve it. I truly wish one day I will work at ESA."
Figure 55: Photo of the students and trainers at the ESA Academy's Training Center (image credit: ESA)
• February 18, 2019: Feeling stressed? You're not alone. ESA's Rosetta mission has revealed that geological stress arising from the shape of Comet 67P/Churyumov–Gerasimenko has been a key process in sculpting the comet's surface and interior following its formation. 90)
Figure 56: Single frame enhanced NavCam image taken on 27 March 2016, when Rosetta was 329 km from the nucleus of Comet 67P/Churyumov-Gerasimenko. The scale is 28 m/pixel and the image measures 28.7 km across (image credit: ESA/Rosetta/NavCam – CC BY-SA IGO 3.0)
- Small, icy comets with two distinct lobes seem to be commonplace in the Solar System, with one possible mode of formation a slow collision of two primordial objects in the early stages of formation some 4.5 billion years ago. A new study using data collected by Rosetta during its two years at Comet 67P/C-G has illuminated the mechanisms that contributed to shaping the comet over the following billions of years.
- The researchers used stress modelling and three-dimensional analyses of images taken by Rosetta's high resolution OSIRIS camera to probe the comet's surface and interior.
- "We found networks of faults and fractures penetrating 500 meters underground, and stretching out for hundreds of meters," says lead author Christophe Matonti of Aix-Marseille University, France.
- "These geological features were created by shear stress, a mechanical force often seen at play in earthquakes or glaciers on Earth and other terrestrial planets, when two bodies or blocks push and move along one another in different directions. This is hugely exciting: it reveals much about the comet's shape, internal structure, and how it has changed and evolved over time."
- The model developed by the researchers found shear stress to peak at the center of the comet's ‘neck', the thinnest part of the comet connecting the two lobes.
- "It's as if the material in each hemisphere is pulling and moving apart, contorting the middle part – the neck – and thinning it via the resulting mechanical erosion," explains co-author Olivier Groussin, also of Aix-Marseille University, France.
- "We think this effect originally came about because of the comet's rotation combined with its initial asymmetric shape. A torque formed where the neck and ‘head' meet as these protruding elements twist around the comet's center of gravity."
- The observations suggest that the shear stress acted globally over the comet and, crucially, around its neck. The fact that fractures could propagate so deeply into 67P/C-G also confirms that the material making up the interior of the comet is brittle, something that was previously unclear.
- "None of our observations can be explained by thermal processes," adds co-author Nick Attree of the University of Stirling, UK. "They only make sense when we consider a shear stress acting over the entire comet and especially around its neck, deforming and damaging and fracturing it over billions of years."
- Sublimation, the process of ices turning to vapor and resulting in comet dust being dragged out into space, is another well-known process that can influence a comet's appearance over time. In particular, when a comet passes closer to the Sun, it warms up and loses its ices more rapidly – perhaps best visualized in some of the dramatic outbursts captured by Rosetta during its time at Comet 67P/C–G.
- The new results shed light on how dual-lobe comets have evolved over time.
Figure 57: These images show how Rosetta's dual-lobed comet, 67P/Churyumov-Gerasimenko, has been affected by a geological process known as mechanical shear stress. The comet's shape is shown in the left two diagrams from top and side perspectives, while the four frames on the right zoom in on the part marked by the overlaid black box (the comet's ‘neck'). The red arrow points to the same spot in both images, seen from a different perspective. - The two central frames show this part of the neck as imaged by Rosetta's OSIRIS camera, and used in a new study exploring how the comet's shape has evolved over time. The two frames on the right highlight different features on the comet using these images as a background canvas. Red lines trace fracture and fault patterns formed by shear stress, a mechanical force often seen at play in earthquakes or glaciers on Earth and other terrestrial planets. This occurs when two bodies or blocks push and move along one another in different directions, and is thought to have been induced here by the comet's rotation and irregular shape. Green marks indicate terraced layers [image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; C. Matonti et al. (2019)] 91) 92)
Legend to Figure 58: The comet is thought to have formed this long ago in the primordial disc of the Solar System, perhaps as two small objects slowly collided and stuck together. Comets form in the icy outer Solar System and are stored there in vast clouds before beginning their journey inwards; comet 67P/C-G is thought to have entered the giant planet region hundreds of thousands to millions of years of ago. By this point a form of geological erosion named mechanical shear stress had taken hold, and was the dominant process sculpting and shaping the comet's surface and interior. A new study using data from Rosetta found this stress to peak in the region connecting the two lobes of the comet: the ‘neck'. This neck bore the brunt of mechanical erosion, fracturing and thinning over time – as shown in the diagram by the cross-hatched lines.
The final steps cover the time period from tens of thousands of years ago to present day, a period during which sublimation erosion was dominant in shaping the comet's surface and interior. This kind of erosion takes place as the Sun warms ices within the comet, causing the ice to turn to gas and escape to space, carrying cometary material along with it. This weakened the comet's neck further, and the force grew stronger as it travelled inwards from Jupiter's orbit towards Mars.
It is important to note that the red arrows do not imply cometary rotation; instead, they represent shear deformation, and illustrate the torque generated at the neck.
- Comets are thought to have formed in the earliest days of the Solar System, and are stored in vast clouds at its outer edges before beginning their journey inwards. It would have been during this initial ‘building' phase of the Solar System that 67P/C-G got its initial shape.
Figure 59: First impressions of the Kuiper Belt object Ultima Thule (left) revealed a surprisingly familiar appearance to the comet that ESA's Rosetta spacecraft explored for more than two years (right), [image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute; right: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0]
Legend to Figure 59: First impressions of the Kuiper Belt object Ultima Thule (left) revealed a surprisingly familiar appearance to the comet that ESA's Rosetta spacecraft explored for more than two years (right). - NASA's New Horizons flew by Ultima Thule on 1 January 2019, with subsequent images and data suggesting that its two lobes are rather more 'squashed' like a pancake, with respect to Comet 67P/Churyumov-Gerasimenko. Ultima Thule, which sits beyond the orbit of Neptune in the outskirts of the Solar System, is about 34 km long, with the two lobes measuring about 19.5 and 14.2 km across. — By comparison, Comet 67P/C-G's two lobes measure 4.1 x 3.3 x 1.8 km and 2.6 x 2.3 x 1.8 km. The comet likely originated from the Kuiper Belt and now orbits around the Sun on a 6.5 year journey that takes it from just beyond the orbit of Jupiter at its most distant, to between the orbits of Earth and Mars at its closest.
- Excitingly, NASA's New Horizons probe recently returned images from its flyby of Ultima Thule, a trans-Neptunian object located in the Kuiper belt, a reservoir of comets and other minor bodies at the outskirts of the Solar System.
- The data revealed that this object also has a dual-lobed shape, even though somewhat flattened with respect to Rosetta's comet.
- "The similarities in shape are promising, but the same stress structures don't seem to be apparent in Ultima Thule," comments Christophe.
- As more detailed images are returned and analyzed, time will tell if it has experienced a similar history to 67P/C-G or not.
- "Comets are crucial tools for learning more about the formation and evolution of the Solar System," says Matt Taylor, ESA's Rosetta Project Scientist.
- "We've only explored a handful of comets with spacecraft, and 67P is by far the one we've seen in most detail. Rosetta is revealing so much about these mysterious icy visitors and with the latest result we can study the outer edges and earliest days of the Solar System in a way we've never been able to do before."
• February 11, 2019: It is always reassuring to catch that first familiar glimpse of home after a great adventure, but for our space-faring satellites the return visit is brief and of a practical nature: to use the planet's immense gravity to sling it onto a new trajectory. 93)
- These ‘gravity assists' are fleeting encounters, but enough to change the spacecraft's speed and direction such that it can eventually enter orbit around another world.
- This delicate view of Earth was captured in 2007 on the second of three Earth flybys made by ESA's comet-chasing Rosetta spacecraft on its ten year journey to Comet 67P/Churyumov-Gerasimenko. The spacecraft also got a boost from Mars to set it on course with its destination.
- The first ever interplanetary gravity slingshot took place on 5 February 1974, when NASA's Mariner 10 flew past Venus en route to flybys of Mercury.
Figure 60: The First Spacecraft to Use Gravity Assist - Mariner 10 (video credit: NASA) 94)
- The ESA-JAXA BepiColombo mission – whose name is inherited from Giuseppe Colombo who originally proposed to NASA the interplanetary trajectories that would allow Mariner-10 multiple Mercury flybys by using gravity assists at Venus – will make nine flybys of Earth, Venus and Mercury to reach the innermost planet and eventually enter orbit about it.
- Similarly, ESA's upcoming Solar Orbiter mission will use Venus gravity assists to change its inclination to get a better look at the Sun's poles. And ESA's Jupiter Icy Moons Explorer will first dive into the inner Solar System to use Earth, Venus and Mars to set course for the gas giant Jupiter.
- But Earth remains home to a fleet of satellites busy performing a number of different activities from orbit: while some are peering far away into the cosmos, our Earth Observation missions are watching diligently over our precious planet, taking its ‘pulse' and helping us to better understand how to care for it. The Sun-illuminated crescent seen around Antarctica in this beautiful image (Figure 61) certainly evokes a feeling of fragility and reminds us of our special place in space.
Figure 61: The image was taken by the OSIRIS camera on Rosetta about two hours before closest approach during the 13 November 2007 flyby, when the spacecraft was 75,000 km from Earth. The mission went on to become the first to rendezvous with and land on a comet, and the first to follow and study a comet on its orbit around the Sun (image credit: ESA ©2005 MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/ UPM/DASP/IDA)
• January 31, 2019: Engineering models are an important part of spacecraft operations – acting as faithful and realistic testbeds for all sorts of trials and tricks too risky to attempt, first-go, on the original. Models like this also serve as mementos of our human endeavors in space, which are so often hard to visualize, and even harder to get close to. 95)
- The original Rosetta probe carried out its final maneuver at 20:50 GMT (22:50 CEST) on 29 September 2016, setting itself down on comet 67P/Churyumov–Gerasimenko and sending its final image from just 24 or so meters above the surface.
- While we no longer receive updates from the plucky comet-chaser, our 'super model' – seen here at night in its new glassed-in pavilion – reminds us every day of what a remarkable achievement this was.
Figure 62: This life-size copy of the world-famous Rosetta spacecraft is living out its retirement at ESA's European Space Operations Center in Darmstadt, Germany (image credit: ESA/J. Mai)
• December 17, 2018: An old friend of ESA, Comet 46P/Wirtanen, is crossing our skies this month. — The comet nucleus is at the core of the brightest spot at the center of the image, and the green diffuse cloud is its coma. The green color is caused by molecules – mainly CN (cyanogen) and C2 (diatomic carbon) – that are ionized by sunlight as the comet approaches the Sun. A hint of the comet's tail is visible to the upper left; the diagonal stripes are star trails. 96) 97)
- A bright comet with a period of 5.5 years, 46P had been chosen in the 1990s as the target of ESA's Rosetta mission. However, a launch delay from 2003 to 2004 meant the spacecraft would not be able to rendezvous with that comet at its closest approach to the Sun in 2013, prompting the Rosetta team to select a new target, the now famed 67P/Churyumov–Gerasimenko.
- Comet 46P was at perihelion, the closest point to the Sun along its orbit, on 12 December, and kept moving towards our planet, reaching the closest distance to Earth on 16 December.
- Astronomers across the world – professional, student and amateur alike – have been observing the comet recently, and will keep doing so in coming weeks as it moves away from the Sun along its orbit.
Figure 63: This image was taken by Wouter Van Reeven at ESA/ESAC (European Space Astronomy Center) near Madrid, Spain, on 14 December 2018. It is a composite of 132 individual images, each with a 10 second exposure, using a William Optics ZS 71 ED (71 mm refractor) telescope and a Canon EOS 700D DSLR camera (ISO: 3200). The field of view spans 2.8º x 1.8º (image credit: ESA/ESAC Astronomy Club / W. Van Reeven)
• December 12, 2018: A new study reveals that, contrary to first impressions, Rosetta did detect signs of an infant bow shock at the comet it explored for two years – the first ever seen forming anywhere in the Solar System. 98) 99)
- From 2014 to 2016, ESA's Rosetta spacecraft studied Comet 67P/Churyumov-Gerasimenko and its surroundings from near and far. It flew directly through the ‘bow shock' several times both before and after the comet reached its closest point to the Sun along its orbit, providing a unique opportunity to gather in situ measurements of this intriguing patch of space.
- Comets offer scientists an extraordinary way to study the plasma in the Solar System. Plasma is a hot, gaseous state of matter comprising charged particles, and is found in the Solar System in the form of the solar wind: a constant stream of particles flooding out from our star into space.
- As the supersonic solar wind flows past objects in its path, such as planets or smaller bodies, it first hits a boundary known as a bow shock. As the name suggests, this phenomenon is somewhat like the wave that forms around the bow of a ship as it cuts through choppy water.
Figure 64: Artist's impression of the infant bow shock detected by ESA's Rosetta spacecraft at Comet 67P/Churyumov-Gerasimenko. The spacecraft detected signs of a forming bow shock around 50 times closer to the comet's nucleus than anticipated in the case of 67P. This boundary was observed to be asymmetric, wider than the fully developed bow shocks observed at other comets, and moving in unexpected ways. - Rosetta detected the bow shock as the boundary changed position responding to the upstream magnetic field flipping from one side to the other. As a result, the spacecraft found itself alternatively outside of the shock (left frame) and behind it (right frame). It is the first time a bow shock in such an early formation stage has been detected anywhere in the Solar System (image credit: ESA)
- Bow shocks have been found around comets, too – Halley's comet being a good example. Plasma phenomena vary as the medium interacts with the surrounding environment, changing the size, shape, and nature of structures such as bow shocks over time.
- Rosetta looked for signs of such a feature over its two-year mission, and ventured over 1500 km away from 67P's center on the hunt for large-scale boundaries around the comet – but apparently found nothing.
- "We looked for a classical bow shock in the kind of area we'd expect to find one, far away from the comet's nucleus, but didn't find any, so we originally reached the conclusion that Rosetta had failed to spot any kind of shock," says Herbert Gunell of the Royal Belgian Institute for Space Aeronomy, Belgium, and Umeå University, Sweden, one of the two scientists who led the study.
- "However, it seems that the spacecraft actually did find a bow shock, but that it was in its infancy. In a new analysis of the data, we eventually spotted it around 50 times closer to the comet's nucleus than anticipated in the case of 67P. It also moved in ways we didn't expect, which is why we initially missed it."
- On 7 March 2015, when the comet was over twice as far from the Sun as the Earth and heading inwards towards our star, Rosetta data showed signs of a bow shock beginning to form. The same indicators were present on its way back out from the Sun, on 24 February 2016.
- This boundary was observed to be asymmetric, and wider than the fully developed bow shocks observed at other comets.
- "Such an early phase of the development of a bow shock around a comet had never been captured before Rosetta," says co-lead Charlotte Goetz of the Institute for Geophysics and Extraterrestrial Physics in Braunschweig, Germany.
- "The infant shock we spotted in the 2015 data will have later evolved to become a fully developed bow shock as the comet approached the Sun and became more active – we didn't see this in the Rosetta data, though, as the spacecraft was too close to 67P at that time to detect the ‘adult' shock. When Rosetta spotted it again, in 2016, the comet was on its way back out from the Sun, so the shock we saw was in the same state but ‘unforming' rather than forming."
- Herbert, Charlotte, and colleagues explored data from the Rosetta Plasma Consortium, a suite of instruments comprising five different sensors to study the plasma surrounding Comet 67P. They combined the data with a plasma model to simulate the comet's interactions with the solar wind and determine the properties of the bow shock.
- The scientists found that, when the forming bow shock washed over Rosetta, the comet's magnetic field became stronger and more turbulent, with bursts of highly energetic charged particles being produced and heated in the region of the shock itself. Beforehand, particles had been slower-moving, and the solar wind had been generally weaker – indicating that Rosetta had been ‘upstream' of a bow shock.
- "These observations are the first of a bow shock before it fully forms, and are unique in being gathered on-location at the comet and shock itself," says Matt Taylor, ESA Rosetta Project Scientist. "This finding also highlights the strength of combining multi-instrument measurements and simulations. It may not be possible to solve a puzzle using one dataset, but when you bring together multiple clues, as in this study, the picture can become clearer and offer real insight into the complex dynamics of our Solar System – and the objects in it, like 67P."
Legend to Figure 65: The comet is represented in grey in the left-hand frame, while the small cyan satellite represents the simulated Rosetta spacecraft. The simulation reconstructs the plasma conditions when Rosetta spotted an infant bow shock in the process of ‘unforming': this infant bow shock can be seen as the sweeping red-yellow curve. The colors show the proton density for the region – the number of protons found within a cubic cm – as indicated in the bar at the top, with red-yellow being a high and black-blue a low density. In this frame, the Sun is on the right-hand side, meaning that the solar wind flows from right to left.
As Rosetta circles around the comet in this simulation, crossing the bow shock twice, the associated patterns observed in proton and water ion abundances and energies can be seen in the upper and central panels on the right, with the moving cyan line representing Rosetta's location. This demonstrates how the major types of charged particles in the vicinity of the forming bow shock change along with Rosetta's simulated location, and how the distribution of protons becomes wider in the region of the shock than in the surrounding solar wind (shown in the two vertical stripes of red in the top right panel). The bars to the right indicate the differential particle flux to the instrument – the number of particles passing through a given area per unit time and solid angle (a measure of the instrument's field of view) – with red being high and black being low. The scales to the left indicate particle energies.
The magnetic field strength around the comet can be seen in the bottom-right panel. As the simulated Rosetta crosses the shock, two spikes appear in the field strength, which also correlate with the shock-related effects seen in the upper two right-hand panels.
• October 01, 2018: On 30 September 2016, ESA's Rosetta spacecraft came closer than ever to the target it had studied from afar for more than two years, concluding its mission with a controlled impact onto the surface of Comet 67P/Churyumov-Gerasimenko (67P/C-G). 100)
- This second comet landing followed the pioneering endeavour of Rosetta's lander, Philae, which became the first probe to successfully touch down on a comet on 12 November 2014.
- With a suite of 11 scientific instruments on board, Rosetta collected an impressive amount of images and other data at this now iconic comet, scanning its surface, probing its interior, scrutinizing the gas and dust in its surroundings, and exploring its plasma environment. Scientists have been using these measurements to advance our understanding of comets as well as of the history of our Solar System.
- This image shows a portion of 67P/C-G as viewed by Rosetta on 22 September 2014, only one and a half months after the spacecraft had made its rendezvous with the comet. At the time, the spacecraft was 28.2 km from the comet center (around 26.2 km from the surface). Amateur astronomer Jacint Roger Perez, from Spain, selected and processed this view by combining three images taken in different wavelengths by the OSIRIS narrow-angle camera on Rosetta.
- Seen in the center and left of the frame is Seth, one of the geological regions on the larger of the two comet lobes, which declines towards the smoother Hapi region on the comet's ‘neck' that connects the two lobes. The landscape in the background reveals hints of the Babi and Aker regions, both located on the large lobe of 67P/C-G. For a wider image of this region in the overall context of the comet see here.
- The sharp profile in the lower part of the image shows the Aswan cliff, a 134 m-high scarp separating the Seth and Hapi regions. Observations performed by Rosetta not long before the comet's perihelion, which took place on 13 August 2015, revealed that a chunk of this cliff had collapsed – a consequence of increased activity as the comet drew closer to the Sun along its orbit.
Figure 66: An evocative image of Rosetta's comet to recall the end of its trailblazing mission two years ago (image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; J. Roger – CC BY SA 4.0)
• August 6, 2018: Over its lifetime Rosetta extensively mapped the comet's surface, which has since been divided into 26 geological regions named after Ancient Egyptian deities. The entire comet has been likened to a duck in shape, with a small ‘head' attached to a larger ‘body'. 101)
Figure 67: This image shows a section of 67P/C-G as viewed by Rosetta's high-resolution camera OSIRIS on 10 February 2016. Amateur astronomer Stuart Atkinson, from the UK, selected and processed this view from the OSIRIS image archive. It is a crop of a larger image that shows a slightly wider view of the comet's ‘Bes' region on body of the comet, which takes its name from the protective deity of households, children and mothers (image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA – CC BY SA 4.0; Acknowledgement: S Atkinson)
- The image shows the uneven, shadowed surface of the comet in detail; particularly prominent just to the right of center is an upright feature surrounded by scattered depressions, rocky outcrops and debris.
• July 18, 2018: As Japan's Hayabusa-2 drew closer to its target Ryugu asteroid, a strange new planetoid came into view – but one with a somewhat familiar shape. This distinct ‘spinning top' asteroid class has been seen repeatedly in recent years, and might give a foretaste of things to come for ESA's proposed Hera mission. 102)
- Hayabusa2 is currently just 20 km away from the 900-m wide asteroid. The view from its navigation camera reveals a spinning body with an enlarged ridge of material around its equator – a bulge suggesting Ryugu may once have been spinning much faster.
Figure 68: Image of Ryugu, captured with ONC-T (Optical Navigation Camera – Telescopic) at 12:50 p.m. (JST), June 26, 2018 (image credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, AIST)
- As ESA's space scientist Michael Küppers followed Hayabusa-2's approach, he recalled Europe's own asteroid first encounter, just under a decade ago on 5 September 2008, when Rosetta performed a flyby of the Steins asteroid en route to its final destination, comet 67P/Churyumov-Gerasimenko.
Figure 69: Asteroid Steins seen from a distance of 800 km, taken by the OSIRIS imaging system from two different perspectives. The effective diameter of the asteroid is 5 km, approximately as predicted. At the top of the asteroid (as shown in this image), a large crater, approximately 1.5-km in size, can be seen. Scientists were amazed that the asteroid survived the impact that was responsible for the crater (image credit: ESA ©2008 MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA) 103)
Figure 70: Sequential views from Japan's Hayabusa-2 mission as it approached the Ryugu asteroid during June 2018 (image credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu and AIST)
- "At 6 km across, Šteins (Steins) was much larger, but had a similar diamond shape," says Michael. "Personally I wasn't surprised to see this again with Ryugu, because it has turned up with many smaller asteroids in recent years. The current thinking is this shape is due to asteroids being set spinning rapidly, and the resulting centrifugal force moving material away from the poles and towards the equator. As for what causes such a spin, this probably comes down to the so-called YORP (Yarkovsky–O'Keefe–Radzievskii–Paddack) effect."
- The YORP effect, named after four different researchers who worked on asteroids, is triggered by the warming of asteroids by sunlight. The asteroids re-radiate this energy as heat, which gives rise to a tiny amount of thrust. Eventually Newton's Third Law – ‘every action has an equal and opposite reaction' – exerts itself. And due to their irregular shapes, some parts of asteroids generate more thrust than others, leading to a turning force like wind past a windmill.
- "The resulting centrifugal force could continue to the point that material is actually thrown out into space," adds Michael, "leading to the creation of the binary or multiple asteroid systems that make up 15% of all asteroids so far discovered. Some might also crumble apart altogether. For larger asteroids YORP is less likely to influence shape, as their ratio between mass and surface area is much higher."
- Today Michael is serving as project scientist on ESA's Hera mission study, planned as humankind's first mission to a binary asteroid system if approved at next year's ESA Council meeting at ministerial level. His role is to work with external scientists to come up with mission requirements, and make early plans for operations and data analysis.
- Hera's target is the Didymos system, with a 780 m main body orbited by a smaller 160 m ‘Didymoon'. NASA's DART spacecraft will impact this smaller body in 2022 to measurably shift its orbit, ahead of Hera's arrival in 2026 – the two missions combining in an audacious, full-scale planetary defence test.
• June 21, 2018: All high-resolution images and the underpinning data from Rosetta's pioneering mission at Comet 67P/Churyumov-Gerasimenko are now available in ESA's archives, with the last release including the iconic images of finding lander Philae, and Rosetta's final descent to the comet's surface. 104)
- The Archive Image Browser also hosts images captured by the spacecraft's Navigation Camera, while the Planetary Science Archive contains publicly available data from all eleven science instruments onboard Rosetta – as well as from ESA's other Solar System exploration missions.
- The final batch of high-resolution images from Rosetta's OSIRIS camera covers the period from late July 2016 to the mission end on 30 September 2016. It brings the total count of images from the narrow- and wide-angle cameras to nearly 100 000 across the spacecraft's 12 year journey through space, including early flybys of Earth, Mars and two asteroids before arriving at the comet.
- The spacecraft's trajectory around the comet changed progressively during the final two months of the mission, bringing it closer and closer at its nearest point along elliptical orbits. This allowed some spectacular images to be obtained from within just two kilometers of the surface, highlighting the contrasts in exquisite detail between the smooth and dusty terrain, and more consolidated, fractured comet material.
- One particularly memorable sets of images captured in this period were those of Rosetta's lander Philae following the painstaking effort over the previous years to determine its location. With Rosetta flying so close, challenging conditions associated with the dust and gas escaping from the comet, along with the topography of the local terrain, caused problems with getting the best line-of-sight view of Philae's expected location, but the winning shot was finally captured just weeks before the mission end.
- In the mission's last hours as Rosetta moved even closer towards the surface of the comet, it scanned across an ancient pit and finally sent back images showing what would become its resting place. Even after the spacecraft was silent, the team were able to reconstruct a last image from the final telemetry packets sent back when Rosetta was within about 20 m of the surface.
• January 22, 2018: Perhaps you live in a part of the world where you regularly experience snow storms or even dust storms. But for many of us, the weather forms a natural part of everyday conversation – more so when it is somewhat extreme, like a sudden blizzard that renders transport useless or makes you feel highly disoriented as you struggle to fix your sights on recognizable landmarks.
- ESA's Rosetta mission had a similar experience, for more than two years, as it flew alongside Comet 67P/Churyumov–Gerasimenko between 2014 and 2016. It endured the endless impacts of dust grains launched by gaseous outpourings as the comet's surface ices were warmed by the heat of the Sun, evaporating into space and dragging the dust along. 105)
- The image of Figure 71 was taken two years ago, on 21 January 2016, when Rosetta was flying 79 km from the comet. At this time Rosetta was moving closer following perihelion in the previous August, when the comet was nearer to the Sun and as such at its most active, meaning that Rosetta had to operate from a greater distance for safety.
- As can be seen from the image, the comet environment was still extremely chaotic with dust even five months later. The streaks reveal the dust grains as they passed in front of Rosetta's camera, captured in the 146 second exposure.
- Excessive dust in Rosetta's field of view presented a continual risk for navigation: the craft's startrackers used a star pattern recognition function to know its orientation with respect to the Sun and Earth. On some occasions flying much closer to the comet, and therefore through denser regions of outflowing gas and dust, the startrackers locked on to dust grains instead of stars, creating pointing errors and in some cases putting the spacecraft in a temporary safe mode.
- Despite its dangers, the dust was of high scientific interest: three of Rosetta's instruments studied tens of thousands of grains between them, collectively analyzing their composition, their mass, momentum and velocity, and profiling their 3D structure. Studying the smallest and the most pristine grains ejected is helping scientists to understand the building blocks of comets.
- Two years before the image was taken, 20 January 2014, Rosetta was only just waking up from 31 months of deep-space hibernation. It arrived at its destination after 10 years in space in August 2014, and released the lander Philae three months later. Rosetta made unique scientific observations of the comet until reaching its grand finale on 30 September 2016 by descending to the comet's surface. By the end of the mission, more than a hundred thousand images had been taken by the high-resolution OSIRIS camera (including the one shown here) and the navigation camera, the majority of which are available to browse in the Archive Image Browser.
Figure 71: This stormy day image was taken with the OSIRIS narrow angle camera two years ago, on 21 January 2016, when Rosetta was flying 79 km from the comet (image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)
• October 26, 2017: Last year, a fountain of dust was spotted streaming from Rosetta's comet, prompting the question: how was it powered? Scientists now suggest the outburst was driven from inside the comet, perhaps released from ancient gas vents or pockets of hidden ice. 106)
- The plume was seen by ESA's Rosetta spacecraft on 3 July 2016, just a few months before the end of the mission and as Comet 67P/Churyumov–Gerasimenko was heading away from the Sun at a distance of almost 500 million km.
- "We saw a bright plume of dust blowing away from the surface like a fountain," explains Jessica Agarwal of the Max Planck Institute for Solar System Research in Göttingen, Germany, and lead author of the new paper. "It lasted for roughly an hour, producing around 18 kg of dust every second."
- Alongside a steep increase in the number of dust particles flowing from the comet, Rosetta also detected tiny grains of water-ice. The images showed the location of the outburst: a 10 m-high wall around a circular dip in the surface.
- Previous plumes, collapsing cliffs and similar features have been seen on the comet, but spotting this one was especially fortunate: as well as imaging the location in detail, Rosetta also sampled the ejected material itself.
- "This plume was really special. We have great data from five different instruments on how the surface changed and on the ejected material because Rosetta was, by chance, flying through the plume and looking at the right part of the surface when it happened," adds Jessica. "Rosetta hasn't provided such detailed and comprehensive coverage of an event like this before."
- Initially, scientists thought that the plume might have been surface ice evaporating in the sunlight. However, Rosetta's measurements showed there had to be something more energetic going on to fling that amount of dust into space. "Energy must have been released from beneath the surface to power it," says Jessica. "There are evidently processes in comets that we do not yet fully understand."
- How such energy was released remains unclear. Perhaps it was pressurized gas bubbles rising through underground cavities and bursting free via ancient vents, or stores of ice reacting violently when exposed to sunlight.
- "One of Rosetta's major goals was to understand how a comet works. For example, how does its gaseous envelope form and change over time?" says Matt Taylor, ESA's Rosetta Project Scientist. -"Outbursts are interesting because of this, but we weren't able to predict when or where they would occur – we had to be lucky to capture them. Having full, multi-instrument coverage of an outburst like this and its effect on the surface is really valuable for revealing how these events are driven. Rosetta scientists are now combining measurements from the comet with computer simulations and laboratory work to find out what drives such plumes on comets." 107)
Figure 72: Comet plume (image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)
Figure 73: Comet plume in context (image credit: Comet image (left): ESA/Rosetta/NavCam, CC BY-SA 3.0 IGO; comet model: ESA; all others: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)
• September 28, 2017: Scientists analyzing the final telemetry sent by Rosetta immediately before it shut down on the surface of the comet last year have reconstructed one last image of its touchdown site. After more than 12 years in space, and two years following Comet 67P/Churyumov–Gerasimenko as they orbited the Sun, Rosetta's historic mission concluded on 30 September with the spacecraft descending onto the comet in a region hosting several ancient pits. It returned a wealth of detailed images and scientific data on the comet's gas, dust and plasma as it drew closer to the surface. 108)
Figure 74: A final image from Rosetta, shortly before it made a controlled impact onto Comet 67P/Churyumov–Gerasimenko on 30 September 2016, was reconstructed from residual telemetry. The image has a scale of 2 mm/pixel and measures about 1 m across (image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)
- But there was one last surprise in store for the camera team, who managed to reconstruct the final telemetry packets into a sharp image. "The last complete image transmitted from Rosetta was the one that we saw arriving back on Earth in one piece moments before the touchdown at Sais," says Holger Sierks, principal investigator for the OSIRIS camera at the Max Planck Institute for Solar System Research in Göttingen, Germany. - "Later, we found a few telemetry packets on our server and thought, wow, that could be another image."
- During operations, images were split into telemetry packets aboard Rosetta before they were transmitted to Earth. In the case of the last images taken before touchdown, the image data, corresponding to 23,048 bytes per image, were split into six packets.
- For the very last image the transmission was interrupted after three full packets were received, with 12,228 bytes received in total, or just over half of a complete image. This was not recognized as an image by the automatic processing software, but the engineers in Göttingen could make sense of these data fragments to reconstruct the image.
Legend to Figure 75: In order to give a feeling of scale, this artist impression of the Rosetta spacecraft is superimposed on an OSIRIS wide-angle camera image of the region in which it landed on 30 September 2016. Also marked on the image are the approximate locations of the final two images taken by the spacecraft from around 20 m altitude. The cross indicates the estimated center of touchdown of Rosetta. -The background image measures about 55 m across, while the final images are about 1 m across. For comparison, Rosetta measures 32 m from tip to tip, and its solar panels are a little more than 2 m high each. — Note that the positioning of the spacecraft on the image is not an accurate representation of the actual landing.
Legend to Figure 76: Annotated image indicating the approximate locations of some of Rosetta's final images. Note that due to differences in timing and viewing geometry between consecutive images in this graphic, the illumination and shadows vary.
- Top left: a global view of Comet 67P/Churyumov–Gerasimenko shows the area in which Rosetta touched down in the Ma'at region on the smaller of the two comet lobes. This image was taken by the OSIRIS narrow-angle camera on 5 August 2014 from a distance of 123 km.
- Top right: an image taken by the OSIRIS narrow-angle camera from an altitude of 5.7 km, during Rosetta's descent on 30 September 2016. The image scale is about 11 cm/pixel and the image measures about 225 m across. The final touchdown point, named Sais, is seen in the bottom right of the image and is located within a shallow, ancient pit. Exposed, dust-free terrain is seen in the pit walls and cliff edges. Note the image is rotated 180º with respect to the global context image at top right.
- Middle: an OSIRIS wide-angle camera image taken from an altitude of about 331 m during Rosetta's descent. The image scale is about 33 mm/pixel and the image measures about 55 m across. The image shows a mix of coarse and fine-grained material.
- Bottom right: the penultimate image, which was the last complete image taken and returned by Rosetta during its descent, from an altitude of 24.7±1.5 m.
- Bottom left: the final image, reconstructed after Rosetta's landing, was taken at an altitude of 19.5±1.5 m. The image has a scale of 2 mm/pixel and measures about 1 m across.
• June 8, 2017: The challenging detection, by ESA's Rosetta mission, of several isotopes of the noble gas xenon at Comet 67P/Churyumov-Gerasimenko has established the first quantitative link between comets and the atmosphere of Earth. The blend of xenon found at the comet closely resembles U-xenon, the primordial mixture that scientists believe was brought to Earth during the early stages of Solar System formation. These measurements suggest that comets contributed about one fifth the amount of xenon in Earth's ancient atmosphere. 109)
- Xenon – a colorless, odorless gas which makes up less than one billionth of the volume of Earth's atmosphere – might hold the key to answer a long-standing question about comets: did they contribute to the delivery of material to our planet when the Solar System was taking shape, some 4.6 billion years ago? And if so, by how much?
- The noble gas xenon is formed in a variety of stellar processes, from the late phases of low- and intermediate-mass stars to supernova explosions and even neutron star mergers. Each of these phenomena gives rise to different isotopes of the element. As a noble gas, xenon does not interact with other chemical species, and is therefore an important tracer of the material from which the Sun and planets originated, which in turns derives from earlier generations of stars.
- "Xenon is the heaviest stable noble gas and perhaps the most important because of its many isotopes that originate in different stellar processes: each one provides an additional piece of information about our cosmic origins," says Bernard Marty from CRPG-CNRS and Université de Lorraine, France. Bernard is the lead author of a paper reporting Rosetta's discovery of xenon at Comet 67P/C-G, which is published today in Science. 110)
- It is because of this special 'fingerprint' that scientists have been using xenon to investigate the composition of the early Solar System, which provides important clues to constrain its formation. Over the past decades, they sampled the relative abundances of its various isotopes at different locations: in the atmosphere of Earth and Mars, in meteorites deriving from asteroids, at Jupiter, and in the solar wind – the flow of charged particles streaming from the Sun.
Figure 77: The blend of isotopes of the noble gas xenon detected by ESA's Rosetta mission at Comet 67P/Churyumov-Gerasimenko, compared with the mixture of xenon measured in other regions of the Solar System. All abundances are normalized with respect to the abundance observed in the solar wind, the flow of charged particles streaming from the Sun (shown as a yellow line), image credit: Data from B. Marty et al., 2017 and references therein
- The blend of xenon present in the atmosphere of our planet contains a higher abundance of heavier isotopes with respect to the lighter ones; however, this is a result of lighter elements escaping more easily from Earth's gravitational pull and being lost to space in greater amounts. By correcting the atmospheric composition of xenon for this runaway effect, scientists in the 1970s calculated the composition of the primordial mixture of this noble gas, known as U-xenon, that was once present on Earth.
- This U-xenon contained a similar mix of light isotopes to that of asteroids and the solar wind, but included significantly smaller amounts of the heavier isotopes.
- "For these reasons, we have long suspected that xenon in the early atmosphere of Earth could have a different origin from the average blend of this noble gas found in the Solar System," says Bernard.
- One of the explanations is that Solar System xenon derives directly from the protosolar cloud, a mass of gas and dust that gave rise to the Sun and planets, while the xenon found in the Earth's atmosphere was delivered at a later stage by comets, which in turn might have formed from a different mix of material.
- With ESA's Rosetta mission visiting Comet 67P/Churyumov-Gerasimenko, an icy fossil of the early Solar System, scientists could finally gather the long-sought data to test this hypothesis.
- "Searching for xenon at the comet was one of the most crucial and challenging measurements we performed with Rosetta," says Kathrin Altwegg from the University of Bern, Switzerland, principal investigator of ROSINA, the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis, which was used for this study.
- Xenon is very diffuse in the comet's thin atmosphere, so the navigation team had to fly Rosetta very close – 5 km to 8 km from the surface of the nucleus – for a period of three weeks so that ROSINA could obtain a significant detection of all the relevant isotopes.
- Flying so close to the comet was extremely challenging because of the large amount of dust that was lifting off the surface at the time, which could confuse the star trackers that were used to orient the spacecraft.
- Eventually, the Rosetta team decided to perform this operation in the second half of May 2016. This period was chosen as a compromise so that enough time would have passed after the comet's perihelion, in August 2015, for the dust activity to be less intense, but not too much for the atmosphere to be excessively thin and the presence of xenon hard to detect.
- As a result of the observations, ROSINA identified seven isotopes of xenon, as well as several isotopes of another noble gas, krypton; these brought to three the inventory of noble gases found at Rosetta's comet, following the discovery of argon from measurements performed in late 2014.
- "These measurements required a long stretch of dedicated time solely for ROSINA, and it would have been very disappointing if we hadn't detected xenon at Comet 67P/C-G, so I'm really glad that we succeeded in detecting so many isotopes," adds Kathrin.
- Further analysis of the data revealed that the blend of xenon at Comet 67P/C-G, which contains larger amounts of light isotopes than heavy ones, is quite different from the average mixture found in the Solar System. A comparison with the on-board calibration sample confirmed that the xenon detected at the comet is also different from the current mix in the Earth's atmosphere.
- By contrast, the composition of xenon detected at the comet seems to be closer to the composition that scientists think was present in the early atmosphere of Earth.
- "This is a very exciting result because it is the first discovery of a candidate for the hypothesized U-xenon," explains Bernard.
- "There are some discrepancies between the two compositions, which indicate that the primordial xenon delivered to our planet could derive from a combination of impacting comets and asteroids."
- In particular, Bernard and his colleagues were able to establish the first quantitative link between comets and our planet's gaseous shroud: based on the Rosetta measurements at Comet 67P/C-G, 22 percent of the xenon once present in Earth's atmosphere could originate from comets – the rest being delivered by asteroids.
- This result is not in contradiction with the isotopic measurements of water at Rosetta's comet, which were significantly different to that found on Earth. In fact, given the trace amounts of xenon in Earth's atmosphere and the much larger amount of water in the oceans, comets could have contributed to atmospheric xenon without having a significant impact on the composition of water in the oceans.
- The contribution inferred from the xenon measurements, instead, agrees with the possibility that comets have been significant carriers of pre-biotic material – such as phosphorus and the amino acid glycine, which were also detected by Rosetta at the comet – that was crucial to the emergence of life on Earth.
- Finally, the difference between the blend of xenon found at the comet – which was incorporated in the nucleus at the time of its formation – and the xenon observed across the Solar System indicates that the protosolar cloud from which the Sun, planets, and small bodies were born was a rather inhomogeneous place in terms of its chemical composition.
- "This conclusion is in accord with previous measurements performed by Rosetta, including the unexpected detections of molecular oxygen (O2) and di-sulphur (S2), and the high deuterium-to-hydrogen ratio observed in the comet water," adds Kathrin.
- Additional evidence for the inhomogeneous nature of the protosolar cloud came also from anther study based on ROSINA observations, published in May in Astronomy & Astrophysics, which revealed that the mixture of silicon isotopes seen at the comet is different from what is measured elsewhere in the Solar System.
• March 21, 2017: Rosetta scientists have made the first compelling link between an outburst of dust and gas and the collapse of a prominent cliff, which also exposed the pristine, icy interior of the comet. 112) Growing fractures, collapsing cliffs, rolling boulders and moving material burying some features on the comet's surface while exhuming others are among the remarkable changes documented during Rosetta's mission. 113) 114) 115)
Figure 78: A 3D view of the Aswan cliff before and after part of it collapsed. The cliff was originally observed to have a 70 m-long, 1 m-wide fracture separating an overhanging block 12 m across from the main plateau. After the collapse, bright, pristine material is observed in the cliff wall, with new debris at the foot of the cliff (image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; F. Scholten & F. Preusker)
- Sudden and short-lived outbursts were observed frequently during Rosetta's two-year mission at Comet 67P/Churyumov–Gerasimenko. Although their exact trigger has been much debated, the outbursts seem to point back to the collapse of weak, eroded surfaces, with the sudden exposure and heating of volatile material likely playing a role.
- In a study published today in Nature Astronomy, scientists make the first definitive link between an outburst and a crumbling cliff face, which is helping us to understand the driving forces behind such events. 116)
Legend to Figure 79: Left: images of the 70 m-long, 1 m-wide fracture at the top of the 134 m-high Aswan cliff in the Seth region of Comet 67P/Churyumov–Gerasimenko (marked with arrow). The last image of the fracture still present was taken on 4 July 2015 (not shown here). Center: a broad plume of dust is imaged by Rosetta's navigation camera on 10 July 2015, which can be traced back to an area on the comet that encompasses the Seth region (the Aswan cliff is included within the marked rectangle). Right: two example images taken after the cliff collapse, showing the exposed material in the cliff face (top) and the new outline of the cliff top (bottom).
- The first close images of the comet taken in September 2014 revealed a 70 m-long, 1 m-wide fracture on the prominent cliff-edge subsequently named Aswan, in the Seth region of the comet, on its large lobe.
- Over the course of the following year as the comet drew ever closer to the Sun along its orbit, the rate at which its buried ices turned to vapor and dragged dust out into space increased along the way. Sporadic and brief, high-speed releases of dust and gas punctuated this background activity with outbursts.
- One such outburst was captured by Rosetta's navigation camera on 10 July 2015, which could be traced back to a portion of the comet's surface that encompassed the Seth region.
Legend to Figure 80: Sequence of images showing different views of the Aswan cliff collapse on Comet 67P/Churyumov–Gerasimenko. The first image shows the fracture long before it gave away on 10 July 2015. Images taken on 15 July and 26 December show the bright, pristine material exposed in the cliff collapse, which is thought to have occurred on 10 July. Although not obvious from these images, the brightness had faded by about 50% by the 26 December image, showing that much of the exposed water-ice had already sublimated by that time. The images from 2016 show different views of the new cliff top. By August 2016, much of the cliff face had returned to the average brightness of the comet. — Arrows are used to mark the fracture and the exposed water-ice, and to delineate the new cliff top outline.
- The next time the Aswan cliff was observed, five days later, a bright and sharp edge was spotted where the previously identified fracture had been, along with many new meter-sized boulders at the foot of the 134 m-high cliff. "The last time we saw the fracture intact was on 4 July, and in the absence of any other outburst events recorded in the following ten-day period, this is the most compelling evidence that we have that the observed outburst was directly linked to the collapse of the cliff," says Maurizio Pajola, the study leader.
- The event also provided a unique opportunity to study how the pristine water-ice otherwise buried tens of meters inside the comet evolved as the exposed material turned to vapor over the following months (Figure 81).
Figure 81: Comet cliff collapse in 3D. Anaglyph images of the Aswan cliff showing the overhang before (left) and after (right) it collapsed. The anaglyph images were prepared for evaluating the volume of overhang that detached in July 2015. Note the orientation between the two images is different (image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; M. Pajola)
- Indeed, after the event, the exposed cliff face was calculated to be at least six times brighter than the overall average surface of the comet nucleus. By 26 December 2015 the brightness had faded by half, suggesting much of the water-ice had already vaporised by that time. — And by 6 August 2016, most of the new cliff face had faded back to the average, with only one large, brighter block remaining.
Figure 82: Fallen cliff debris. Color-coded plot showing the number and size distribution of boulders at the bottom of the Aswan cliff in the Seth region of Comet 67P/Churyumov–Gerasimenko before and after a large section collapsed on 10 July 2015. Significantly more smaller boulders are identified than larger pieces of debris (image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; Pajola et al. (2017)
- In addition, the team had a clear ‘before and after' look at how the crumbling material settled at the foot of the cliff. By counting the number of new boulders seen after its collapse, the team estimated that 99% of the fallen debris was distributed at the bottom of the cliff, while 1% was lost to space.
- This corresponds to around 10,000 tons of removed cliff material, with at least 100 tons that did not make it to the ground, consistent with estimates made for the volume of dust in the observed plume. Furthermore, the size range of the new debris, between 3 m and 10 m, is consistent with the distributions observed at the foot of several other cliffs identified on the comet.
- "We see a similar trend at the foot of other cliffs that we have not been so fortunate to have before and after images, so this is an important validation of cliff collapse as a producer of these debris fields," says Maurizio.
- But what actually led to the cliff suddenly collapsing at this particular moment?
Legend to Figure 83: Much of the comet's regular activity can be linked back to the steady erosion of cliff walls that are initially fractured by thermal or mechanical erosion. These fractures propagate into the underlying mixture of ice and dust. As the ices sublimate, the gases escape through the fractures, acting a bit like nozzles to focus the gas flows and picking up dust a long the way to create the distinct collimated jets observed in Rosetta's images. Continued cracking, heating and sublimation eventually leads to sudden collapse of the cliff wall – the likely source of more-transient outburst events. At the same time, the debris that falls to the foot of the cliff also exposes previously hidden material, contributing to the observed outflow.
- An earlier study suggested that both rapid daily changes in heating or longer-term seasonal changes can create thermal stresses that lead to fracturing and subsequent exposure of volatile materials, triggering a rapid outburst that can cause the weakened cliff to collapse.
- Even though the Aswan cliff region had been experiencing large temperature changes in the months before the collapse, interestingly, the collapse occurred at local night, ruling out a sudden extreme temperature change as the immediate trigger.
- Instead, both daily and seasonal temperature variations may have propagated fractures deeper into the subsurface than previously considered, predisposing it to the subsequent collapse.
- "If the fractures permeated volatile-rich layers, heat could have been transferred to these deeper layers, causing a loss of deeper ice," explains Maurizio. "The gas released by the vaporising material could further widen the fractures, leading to a cumulative effect that eventually led to the cliff collapse. Thanks to this particular event at Aswan, we think that the cumulative effect led by strong thermal gradients could be one of the most important weakening factors of the cliff structure."
- "Rosetta's images already suggested that cliff collapses are important in shaping cometary surfaces, but this particular event has provided the missing ‘before–after' link between such a collapse, the debris seen at the foot of the cliff, and the associated dust plume, supporting a general mechanism where comet outbursts can indeed be generated by collapsing material," says Matt Taylor, ESA's Rosetta project scientist.
• December 15, 2016: ESA's Rosetta completed its incredible mission on 30 September, collecting unprecedented images and data right until the moment of contact with the comet's surface. Rosetta's signal disappeared from screens at ESA's mission control at 11:19:37 GMT, confirming that the spacecraft had arrived on the surface of Comet 67P/Churyumov–Gerasimenko and switched off some 40 minutes earlier and 720 million kilometers from Earth. 117)
- One of the final pieces of information received from Rosetta was sent by its navigation star trackers: a report of a ‘large object' in the field of view – the comet horizon.
Figure 84: Imaging ‘footprints' of Rosetta's OSIRIS camera during the descent to the comet's surface. A primary focus was the pit named Deir el-Medina, as indicated by the number of footprints indicated in blue. The trail of orange and red squares reflect the change in pointing of the camera towards the impact site, subsequently named Sais. The final image was acquired at about 20 m above the surface, and the touchdown point was only 33 m from the center of the predicted landing ellipse (image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)
- Reconstruction of the final descent showed that the spacecraft gently struck the surface only 33 m from the target point. The accuracy once again highlighted the excellent work of the flight dynamics specialists who supported the entire mission. The spot, just inside an ancient pit in the Ma'at region on the comet's ‘head', was named Sais, after a town where the Rosetta Stone was originally located.
- Numerous images were taken of the neighboring pit, capturing incredible details of its layered walls that will be used to help decipher the comet's geological history.
- The final image was acquired about 20 m above the impact point. In addition, a number of Rosetta's dust, gas and plasma analysis instruments collected data.
- The pressure of the gas outflow from the comet was seen to rise as the surface neared. Scans revealed temperatures between about –190ºC and –110ºC down to a few centimeters below the surface. The variation was most likely due to shadows and local topography as Rosetta flew across the surface.
- A last measurement of water vapor emission was made on 27 September, estimating the comet was emitting the equivalent of two tablespoons of water per second. During its most active period in August 2015, estimates were in the region of two bathtubs' worth of water every second.
Figure 85: Rosetta's last image of Comet 67P/Churyumov-Gerasimenko, taken with the OSIRIS wide-angle camera shortly before impact, at an estimated altitude of about 20 m above the surface (image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)
Legend to Figure 85: The initially reported 51 m was based on the predicted impact time. Now that this has been confirmed, and following additional information and timeline reconstruction, the estimated distance is now thought to be around 20 m, and analysis is ongoing. The image scale is about 2 mm/pixel and the image measures about 96 cm across.
Figure 86: Comet landing sites in context: Rosetta's planned impact point in Ma'at shown in context with Philae's first and final touchdown sites. All three sites are on the smaller of Comet 67P/Churyumov–Gerasimenko's two lobes (image credit: CIVA: ESA/Rosetta/Philae/CIVA; NAVCAM: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0; OSIRIS: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; ROLIS: ESA/Rosetta/Philae/ROLIS/DLR) 118)
Legend to Figure 86: The insets show close-up details of the three sites. Philae's first touchdown in Agilkia was captured by the lander's descent camera ROLIS; the image shown here was taken from a height of just 9 m above the surface on 12 November 2014, and has a resolution of 0.95 cm/pixel. The view at Philae's final touchdown site, known as Abydos, was taken by the lander's CIVA camera on 13 November 2014; the image shown here is a two-image mosaic, and includes one of the lander's feet.
- The first indications from spectral readings show there to be no significant differences in surface composition at the high resolutions obtained all the way down, and there was no obvious indication of small icy patches near the landing site. The measurements also suggest an increase in very small dust grains – possibly around a millionth of a millimeter – close to the surface.
- The last observation of the gas coma surrounding the comet was made the day before the final descent. Carbon dioxide was still being outgassed, at a greater distance from the Sun than when the comet was approaching it. Stable solar wind conditions reigned during the final measurements of the solar wind and interplanetary magnetic field, providing ‘quiet' background values that will be important for calibration. Decreasing comet plasma densities were observed from about 2 km above the surface, with no obvious detection of local outgassing from the Ma'at pits.
- Magnetic field measurements down to an estimated 11 m above the surface confirmed the previous observations of the comet as a non-magnetic body.
- No large dust particles were collected during the descent, in itself an interesting result. First impressions are that the observed water vapor production was too low to lift dust grains above a detectable size from the surface.
- "It's great to have these first insights from Rosetta's last set of data," says Matt Taylor, ESA's Rosetta Project Scientist. "Operations have been completed for over two months now, and the instrument teams are very much focused on analyzing their huge datasets collected during Rosetta's two-plus years at the comet.
• November 17, 2016: As Rosetta's comet approached its most active period last year, the spacecraft spotted carbon dioxide ice – never before seen on a comet – followed by the emergence of two unusually large patches of water ice. 119) 120) 121)
- The carbon dioxide ice layer covered an area comparable to the size of a football pitch, while the two water ice patches were each larger than an Olympic swimming pool and much larger than any signs of water ice previously spotted at the comet.- The three icy layers were all found in the same region, on the comet's southern hemisphere.
- A combination of the complex shape of the comet, its elongated path around the Sun and the substantial tilt of its spin, seasons are spread unequally between the two hemispheres of the double-lobed Comet 67P/Churyumov-Gerasimenko.
- When Rosetta arrived in August 2014, the northern hemisphere was still undergoing its 5.5 year summer, while the southern hemisphere was in winter and much of it was shrouded in darkness. — However, shortly before the comet's closest approach to the Sun in August 2015, the seasons changed and the southern hemisphere experienced a brief but intense summer, exposing this region to sunlight again.
- In the first half of 2015, as the comet steadily became more active, Rosetta observed water vapour and other gases pouring out of the nucleus, lifting its dusty cover and revealing some of the comet's icy secrets. In particular, on two occasions in late March 2015, Rosetta's visible, infrared and thermal imaging spectrometer, VIRTIS, found a very large patch of carbon dioxide ice in the Anhur region, in the comet's southern hemisphere.
- This is the first detection of solid carbon dioxide on any comet, although it is not uncommon in the Solar System – it is abundant in the polar caps of Mars, for example. "We know comets contain carbon dioxide, which is one of the most abundant species in cometary atmospheres after water, but it's extremely difficult to observe it in solid form on the surface," explains Gianrico Filacchione from Italy's INAF-IAPS Istituto di Astrofisica e Planetologia Spaziali, who led the study.
- In the comet environment, carbon dioxide freezes at -193°C, much below the temperature where water turns into ice. Above this temperature, it changes directly from a solid to a gas, hampering its detection in ice form on the surface. By contrast, water ice has been found at various comets, and Rosetta detected plenty of small patches on several regions. "We hoped to find signs of carbon dioxide ice and had been looking for it for quite a while, but it was definitely a surprise when we finally detected its unmistakable signature," adds Gianrico.
- The patch, consisting of a few percent of carbon dioxide ice combined with a darker blend of dust and organic material, was observed on two consecutive days in March. This was a lucky catch: when the team looked at that region again around three weeks later, it was gone.
- Assuming that all of the ice had turned into gas, the scientists estimated that the 80 m x 60 m patch contained about 57 kg of carbon dioxide, corresponding to a 9 cm-thick layer. Its presence on the surface is likely an isolated rare case, with the majority of carbon dioxide ice being confined to deeper layers of the nucleus.
- Gianrico and his collaborators believe the icy patch dates back a few years, when the comet was still in the cold reaches of the outer Solar System and the southern hemisphere was experiencing its long winter. At that time, some of the carbon dioxide still outgassing from the interior of the nucleus condensed on the surface, where it remained frozen for a very long while, and vaporised only as the local temperature finally rose again in April 2015.
- This reveals a seasonal cycle of carbon dioxide ice, which unfolds over the comet's 6.5 year orbit, as opposed to the daily cycle of water ice, also spotted by VIRTIS shortly after Rosetta's arrival.
Figure 87: First detection of carbon dioxide at a comet (image credit: data: ESA/Rosetta/VIRTIS/INAF-IAPS/OBS DE PARIS-LESIA/DLR; Reprinted with permission from G. Filacchione et al., Science 10.1126/science.aag3161 (2016); context image: ESA/Rosetta/NavCam – CC BY-SA IGO 3.0)
- Interestingly, shortly after the carbon dioxide ice had disappeared, Rosetta's OSIRIS narrow-angle camera detected two unusually large patches of water ice in the same area, between the southern regions of Anhur and Bes (Figure 88). "We had already seen many meter-sized patches of exposed water ice in various regions of the comet, but the new detections are much larger, spanning some 30 m x 40 m each, and they persisted for about 10 days before they completely disappeared," says Sonia Fornasier from LESIA–Observatoire de Paris and Université Paris Diderot, France, lead scientist of the study focusing on seasonal and daily surface color variations.
- These ice-rich areas appear as very bright portions of the comet surface reflecting light that is bluer in color compared with the redder surroundings. Scientists have experimented with mixtures of dust and water ice to show that, as the concentration of ice in them increases, the reflected light becomes gradually bluer in color, until reaching a point where equal amounts of light are reflected in all colors.
- The two newly detected patches contain 20–30% of water ice mixed with darker material, forming a layer up to 30 cm thick of solid ice. One of them was likely lurking underneath the carbon dioxide ice sheet revealed by VIRTIS about a month before.
Figure 88: Large patches of water ice found on comet surface (image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; Reprinted with permission from S. Fornasier et al., Science 10.1126/science.aag2671 (2016), Ref. 121)
• September 30, 2016: ESA's historic Rosetta mission has concluded as planned, with the controlled impact onto the comet it had been investigating for more than two years. Confirmation of the end of the mission arrived at ESA's control center in Darmstadt, Germany at 11:19 UTC (13:19 CEST) with the loss of Rosetta's signal upon impact into 67P/Churyumov-Gerasimenko, 718 million km from Earth. Controlled hard-landing has become a common way to end the missions of planetary probes. But while most have been very high-velocity impacts, Rosetta's touchdown was made at a sedate walking pace of 2 km/h. 122) 123) 124)
- Rosetta carried out its final maneuver last night at 20:50 UTC (22:50 CEST), setting it on a collision course with the comet from an altitude of about 19 km. Rosetta had targeted a region on the small lobe of Comet 67P/Churyumov–Gerasimenko, close to a region of active pits in the Ma'at region.
- The descent gave Rosetta the opportunity to study the comet's gas, dust and plasma environment very close to its surface, as well as take very high-resolution images. - Pits are of particular interest because they play an important role in the comet's activity. They also provide a unique window into its internal building blocks.
- The information collected on the descent to this fascinating region was returned to Earth before the impact. It is now no longer possible to communicate with the spacecraft.
- "Rosetta has entered the history books once again," says Johann-Dietrich Wörner, ESA's Director General. "Today we celebrate the success of a game-changing mission, one that has surpassed all our dreams and expectations, and one that continues ESA's legacy of 'firsts' at comets."
- "Thanks to a huge international, decades-long endeavor, we have achieved our mission to take a world-class science laboratory to a comet to study its evolution over time, something that no other comet-chasing mission has attempted," notes Alvaro Giménez, ESA's Director of Science. "Rosetta was on the drawing board even before ESA's first deep-space mission, Giotto, had taken the first image of a comet nucleus as it flew past Halley in 1986. "The mission has spanned entire careers, and the data returned will keep generations of scientist busy for decades to come."
- "As well as being a scientific and technical triumph, the amazing journey of Rosetta and its lander Philae also captured the world's imagination, engaging new audiences far beyond the science community. It has been exciting to have everyone along for the ride," adds Mark McCaughrean, ESA's senior science advisor.
- Many surprising discoveries have already been made during the mission, not least the curious shape of the comet that became apparent during Rosetta's approach in July and August 2014. Scientists now believe that the comet's two lobes formed independently, joining in a low-speed collision in the early days of the Solar System. Long-term monitoring has also shown just how important the comet's shape is in influencing its seasons, in moving dust across its surface, and in explaining the variations measured in the density and composition of the coma, the comet's ‘atmosphere'.
- Some of the most unexpected and important results are linked to the gases streaming from the comet's nucleus, including the discovery of molecular oxygen and nitrogen, and water with a different ‘flavor' to that in Earth's oceans.- Together, these results point to the comet being born in a very cold region of the protoplanetary nebula when the Solar System was still forming more than 4.5 billion years ago.
- While it seems that the impact of comets like Rosetta's may not have delivered as much of Earth's water as previously thought, another much anticipated question was whether they could have brought ingredients regarded as crucial for the origin of life. - Rosetta did not disappoint, detecting the amino acid glycine, which is commonly found in proteins, and phosphorus, a key component of DNA and cell membranes. Numerous organic compounds were also detected by Rosetta from orbit, and also by Philae in situ on the surface.
- Overall, the results delivered by Rosetta so far paint comets as ancient leftovers of early Solar System formation, rather than fragments of collisions between larger bodies later on, giving an unparalleled insight into what the building blocks of the planets may have looked like 4.6 billion years ago.
- "Just as the Rosetta Stone after which this mission was named was pivotal in understanding ancient language and history, the vast treasure trove of Rosetta spacecraft data is changing our view on how comets and the Solar System formed," says project scientist Matt Taylor. "Inevitably, we now have new mysteries to solve. The comet hasn't given up all of its secrets yet, and there are sure to be many surprises hidden in this incredible archive. So don't go anywhere yet – we're only just beginning."
Figure 89: Rosetta's OSIRIS narrow-angle camera captured this image of Comet 67P/Churyumov-Gerasimenko at 06:53 UTC from an altitude of about 8.9 km during the spacecraft's final descent on 30 September. The image scale is about 17 cm/pixel and the image measures about 350 m across (image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)
Figure 90: Headed for the abyss? This photo was made from 1.2 km high just a few minutes before impact. The scene measures just 33 m wide (image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)
• September 27, 2016: Over the past two years, Rosetta has kept a close eye on many properties of Comet 67P/Churyumov-Gerasimenko, tracking how these changed along the comet's orbit. A very crucial aspect concerns how much water vapor a comet releases into space, and how the water production rate varies at different distances from the Sun. For the first time, Rosetta enabled scientists to monitor this quantity and its evolution in situ over two years. 125)
- In a new study led by Kenneth C. Hansen of the University of Michigan, Ann Arbor, MI, USA, measurements of water production rate based on data from ROSINA, the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis, are compared with water measurements from other Rosetta instruments. 126)
- The combination of all instruments shows an overall increase of the production of water, from a few tens of thousands of kg per day when Rosetta first reached the comet, in August 2014, to almost 100 000 000 kg per day around perihelion, the closest point to the Sun along the comet's orbit, in August 2015. In addition, ROSINA data show that the peak in water production is followed by a rather steep decrease in the months following perihelion.
- "We were pleasantly surprised to find such a good agreement between the data collected by all the various instruments in this unprecedented study of the water production rate's evolution for a Jupiter-family comet," says Hansen. The scientists analyzed almost two years' worth of data from ROSINA, which detects neutral water molecules with its DFMS (Double-Focussing Mass Spectrometer). "This is by no means trivial: ROSINA performs measurements locally, at specific points around the comet, and we need a model to extend them to the entire atmosphere," adds Hansen.
Figure 91: The water production rate measured at Comet 67P/C-G as a function of the comet's distance from the Sun (in astronomical units, AU). The measurements are from ROSINA-DFMS (blue diamonds); MIRO (yellow circles); VIRTIS-H (solid green triangles); VIRTIS-M (unfilled green triangles); RPC-ICA (red triangles). The dust production rate, estimated from ground-based observations, is indicated in tan crosses. The data span the period between June 2014 and May 2016 (image adapted from Ref. 126)
- The simplest model would be a spherical distribution of the outgassing centered around the nucleus but, given the complex shape and season cycle of Comet 67P/C-G, this would be a very crude approximation. For this reason, the ROSINA team developed a series of numerical simulations to accurately describe the comet's production of water, which are presented in a separate study led by Nicolas Fougere also of the University of Michigan. 127)
- From these simulations, which showed that the water production rate at a comet like 67P/C-G is highly inhomogeneous, Hansen and his colleagues derived an empirical model, which they then used to transform the local ROSINA measurements into estimates of the overall water production rate.
- The results revealed that, during the first several months of observations, when the comet was at distances between 3.5 and 1.7 astronomical units (au) from the Sun, water was predominantly produced in the comet's northern hemisphere.
Figure 92: The water production rate predicted by simulations as Comet 67P/C-G approached the Sun, from August 2014 (left) to May 2015 (right), before the equinox that marked the end of the northern summer. Images adapted from Hansen et al. (2016); animations courtesy of K.C. Hansen.
- Then, in May 2015, the equinox marked the end of the 5.5-year long northern summer and the beginning of the short and intense southern summer. At that time, the comet was about 1.7 au from the Sun, and scientists expected that the peak of water production would drift slowly from the northern to the southern hemisphere; instead, this transition happened more abruptly than predicted. This was likely due to the complex shape of the nucleus, which causes highly variable illumination conditions including self-shadowing effects.
- As expected, the production of water peaked between the end of August and early September 2015, about three weeks after the comet's perihelion, which took place on 13 August, 1.24 au from the Sun. The data hint at possible variations in the water production rate at this epoch: these might be due to the spacecraft's motion relative to the comet, but could also be an indication of actual changes to the outgassing dynamics, and will be subject of future in-depth investigation.
Figure 93: The water production rate predicted by simulations after the equinox of Comet 67P/C-G, which marked the beginning of the southern summer, and covering several weeks around the comet's perihelion. Images adapted from Hansen et al. (2016); animations courtesy of K.C. Hansen
- In addition to the ROSINA measurements, Hansen and his colleagues collated a series of previously published measurements of the water production rate at 67P/C-G. These include observations performed with the Microwave Instrument for the Rosetta Orbiter (MIRO) shortly before and after Rosetta had reached the comet, data from the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) obtained between November 2014 and January 2015, and measurements from the Ion Composition Analyzer, part of the Rosetta Plasma Consortium (RPC) suite of instruments, obtained between October 2014 and April 2015.
- RPC-ICA does not detect water directly, but rather measures the ratio of differently ionized Helium ions; since He+ ions arise mainly from collisions between alpha particles (He2+) from the solar wind and neutral molecules, such as water, found in the comet's atmosphere, this ratio can be used to estimate the amount of water produced at the comet.
- Hansen and his collaborators have found some small discrepancies between the various data sets: for example, the measurements from ROSINA yield systematically higher values than those from VIRTIS. One possible reason for this is the different nature of the two experiments: ROSINA samples the gas in the coma at the spacecraft's position, while VIRTIS tends to observe closer to the nucleus, where the water production activity is potentially more confined than it is further out in the coma. The difference in measurements techniques and the discrepancy could potentially indicate an extended source of water in the coma itself, for example icy grains that are lifted into the coma and turn into gas a few kilometers above the surface.
- Another difference was found between the MIRO measurements, which indicate a rising trend in the water production rate from June to September 2014, and the first months of ROSINA data, starting in August, pointing to an almost constant rate in the same period. "This could be explained if a sudden surge in the water production happened around the time of the first MIRO measurement, a few weeks before Rosetta's rendezvous with 67P/C-G, and the beginning of ROSINA observations," says Hansen.
• September 23, 2016: Brief but powerful outbursts seen from Comet 67P/Churyumov–Gerasimenko during its most active period last year have been traced back to their origins on the surface. In the three months centered around the comet's closest approach to the Sun, on 13 August 2015, Rosetta's cameras captured 34 outbursts. These violent events were over and above regular jets and flows of material seen streaming from the comet's nucleus. The latter switch on and off with clockwork repeatability from one comet rotation to the next, synchronized with the rise and fall of the Sun's illumination. 128) 129)
- By contrast, outbursts are much brighter than the usual jets – sudden, brief, high-speed releases of dust. They are typically seen only in a single image, indicating that they have a lifetime shorter than interval between images – typically 5–30 minutes. A typical outburst is thought to release 60–260 tons of material in those few minutes.
- On average, the outbursts around the closest approach to the Sun occurred once every 30 hours – about 2.4 comet rotations. Based on the appearance of the dust flow, they can be divided into three categories. One type is associated with a long, narrow jet extending far from the nucleus, while the second involves a broad, wide base that expands more laterally. The third category is a complex hybrid of the other two.
- "As any given outburst is short-lived and only captured in one image, we can't tell whether it was imaged shortly after the outburst started, or later in the process," notes Jean-Baptiste Vincent, lead author of the paper published in MNRAS (Monthly Notices of the Royal Astronomical Society). "As a result, we can't tell if these three types of plume ‘shapes' correspond to different mechanisms, or just different stages of a single process. - But if just one process is involved, then the logical evolutionary sequence is that an initially long narrow jet with dust is ejected at high speed, most likely from a confined space. -Then, as the local surface around the exit point is modified, a larger fraction of fresh material is exposed, broadening the plume ‘base'. - Finally, when the source region has been altered so much as not to be able to support the narrow jet anymore, only a broad plume survives."
- The OSIRIS cameras on board ESA's Rosetta spacecraft have monitored the activity of comet 67P-Churyumov-Gerasimenko (67P) across varying heliocentric distances (4 AU to 1.24 AU) and different seasons on the nucleus (sub solar latitude between +45º and -55 º). One of the striking discoveries of Rosetta has been the clockwork repeatability of jets from one rotation to the next. Jets are very dynamic by nature, depending on the complex hydrodynamics of the gas and dust streams interacting with the local topography, and controlled by local thermal conditions. They grow and fade with the solar illumination as the nucleus rotates, but the same exact features can be observed from one rotation to the next. Figure 94 shows an example of this phenomenon. This, of course, put constraints on the thermophysics and volatile content of active areas, which need to ensure the sustainability and repeatability of the jets we observed.
Figure 94: Example of two images acquired on 2015-08-09T12.09.49 (left) and 2015-08-10T00.23.00 (right), almost one rotation apart (Rotation period - images separation = 5min33s). Both images contrast is stretched to the same level (5% of the same maximum brightness value). Field of view 1x1 degree, distance = 305 km, resolution = 5.7 m/px (image credit: ESA/Rosetta/NAVCAM - CC BYSA IGO 3.0)
- In the study, monitoring data acquired by the OSIRIS Narrow Angle and Wide Angle Cameras (NAC & WAC), as well as Rosetta's navigation camera (NAVCAM) were used to increase the temporal coverage. Around perihelion, OSIRIS monitoring campaigns were run on a weekly basis, with a set of images acquired every 1/2 h for slightly longer than the current nucleus rotation period (12h18m10s at perihelion). After noticing the first outbursts in July 2015, the cadence of images was increased in each observation, and the time was reduced between monitoring campaigns to a few days. In addition to the OSIRIS data, transient events in the navigation images were also looked into, acquired about every 4 hours during the whole mission.
To distinguish between outbursts and other short lived features, the following definition was established: An outburst is identified by a sudden brightness increase in the coma, associated to a release of gas and dust over a duration very short with respect to the rotation period of the nucleus. Typically detected in one image only, depending on the observing cadence. The dust plume is typically one order of magnitude brighter than the usual jets. Plume morphology as a criterion was not imposed.
Following this definition, 34 events were identified in the data set, listed in a Table. Among them, 26 were detected with OSIRIS NAC, 3 by OSIRIS WAC, and 5 by the NAVCAM. A visual catalog of the brightest evens is provided in Figure 95.
- Figure 96 shows all outburst sources projected on a topographic map of 67P, and on a morphological map displaying the regions boundaries. All sources but one are located in the southern hemisphere, between 0 and -50º of latitude, i.e. around the sub solar latitude for this period (it varied from -30 to -55º). This is consistent with previous observations showing that active sources in general migrate with the Sun. Outburst sources are not evenly distributed along this latitude. Some clustering in three main areas were observed: (1) The Anhur-Aker boundary (big lobe), (2) the Anuket-Sobek boundary (big lobe), and (3) the Wosret-Maftet boundary (small lobe). These areas are characterized by steep scarps, cliffs, and pits, which contrast with the overall flatter morphology of the Southern hemisphere. It is interesting to note that beyond those three areas, it seems like all outbursts sources are located close to morphological boundaries, i.e. areas where we observe discontinuities in the local terrain, either textural or topographic. This seems to indicate a link between morphology and outbursts, although it is not clear which one influences the other.
Figure 96: Maps of all summer outbursts detected by the OSIRIS cameras (blue dots) and Rosetta's NAVCAM (red dots). The top panel plots the sources over a topographic maps in which the gray shading represents the local gravitational slope (white=flat, black=vertical wall). Dotted ellipses represent the estimated uncertainty for the few outbursts whose source was not observed directly (image credit: ESA/Rosetta/NAVCAM - CC BYSA IGO 3.0)
Legend to Figure 96: Note that this map is a 2D representation of a bi-lobate, strongly concave object, and therefore presents significant distortions. To guide the reader, white dashed lines of the boundary of the two lobes are indicated: the map is centered on the small lobe, the big lobe covers the left-right-bottom edges of the map, and the contact area between the two lobes covers mainly the top of the map (regions Hapi, Neith, Sobek). The 3 main clusters of outbursts sources are located around longitudes 60º (big lobe), 300º (southern neck), and 315º (small lobe).
• September 9, 2016: Squeezing out unique scientific observations until the very end, Rosetta's thrilling mission will culminate with a descent on 30 September towards a region of active pits on the comet's ‘head'. The region, known as Ma'at, lies on the smaller of the two lobes of Comet 67P/Churyumov–Gerasimenko. It is home to several active pits more than 100 m in diameter and 50–60 m in depth – where a number of the comet's dust jets originate. 130) 131)
- The walls of the pits also exhibit intriguing meter-sized lumpy structures called ‘goosebumps', which scientists believe could be the signatures of early ‘cometesimals' that assembled to create the comet in the early phases of Solar System formation.
- Rosetta will get its closest look yet at these fascinating structures on 30 September: the spacecraft will target a point adjacent to a 130 m-wide, well-defined pit that the mission team has informally named Deir el-Medina, after a structure with a similar appearance in an ancient Egyptian town of the same name.
- Like the archaeological artefacts found inside the Egyptian pit that tell historians about life in that town, the comet's pit contains clues to the geological history of the region. Rosetta will target a point very close to Deir el-Medina, within an ellipse about 700 x 500 m.
- Since 9 August, Rosetta has been flying elliptical orbits that bring it progressively closer to the comet – on its closest flyby, it may come within 1 km of the surface, closer than ever before.
- "Although we've been flying Rosetta around the comet for two years now, keeping it operating safely for the final weeks of the mission in the unpredictable environment of this comet and so far from the Sun and Earth, will be our biggest challenge yet," says Sylvain Lodiot, ESA's spacecraft operations manager. "We are already feeling the difference in gravitational pull of the comet as we fly closer and closer: it is increasing the spacecraft's orbital period, which has to be corrected by small maneuvers. But this is why we have these flyovers, stepping down in small increments to be robust against these issues when we make the final approach."
- The final flyover will be complete on 24 September. Then a short series of maneuvers needed to line Rosetta up with the target impact site will be executed over the following days as it transfers from flying elliptical orbits around the comet onto a trajectory that will eventually take it to the comet's surface on 30 September.
- The collision maneuver will take place in the evening of 29 September, initiating the descent from an altitude of about 20 km. Rosetta will essentially free-fall slowly towards the comet in order to maximize the number of scientific measurements that can be collected and returned to Earth before its impact.
- A number of Rosetta's scientific instruments will collect data during the descent, providing unique images and other data on the gas, dust and plasma very close to the comet. The exact complement of instruments and their operational timeline remains to be fixed, because it depends on constraints of the final planned trajectory and the data rate available on the day.
Figure 97: Rosetta's planned impact site (image credit: ESA)
Figure 98: Planned maneuvers of Rosetta during September for final impact on Comet 67P/Churyumov-Gerasimenko (image credit: ESA)
• September 7, 2016: Rosetta's dust-analyzing COSIMA (COmetary Secondary Ion Mass Analyzer) instrument has made the first unambiguous detection of solid organic matter in the dust particles ejected by Comet 67P/Churyumov-Gerasimenko, in the form of complex carbon-bearing molecules. 132) 133)
- The optical images (Figure 99) of two of the dust grains collected and analyzed by COSIMA, named Kenneth and Juliette, which show the signature of carbon-based organics. They were collected in May and October 2015, respectively.
Figure 99: COSIMA images of Comet 67P/C-G dust particles Kenneth and Juliette (image credit: ESA/Rosetta/MPS for COSIMA Team MPS/CSNSM/UNIBW/TUORLA/IWF/IAS/ESA/BUW/MPE/LPC2E/LCM/FMI/UTU/LISA/UOFC/vH&S/ Fray et al. (2016))
• September 5, 2016: Less than a month before the end of the mission, Rosetta's high-resolution camera has revealed the Philae lander wedged into a dark crack on Comet 67P/Churyumov–Gerasimenko. 134) 135)
Figure 100: The images were taken on 2 September by the OSIRIS narrow-angle camera as the orbiter came within 2.7 km of the surface and clearly show the main body of the lander, along with two of its three legs .The images also provide proof of Philae's orientation, making it clear why establishing communications was so difficult following its landing on 12 November 2014. (image credit: Main image and lander inset: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; context: ESA/Rosetta/ NavCam – CC BY-SA IGO 3.0)
- "With only a month left of the Rosetta mission, we are so happy to have finally imaged Philae, and to see it in such amazing detail," says Cecilia Tubiana of the OSIRIS camera team, the first person to see the images when they were downlinked from Rosetta yesterday.
- "After months of work, with the focus and the evidence pointing more and more to this lander candidate, I'm very excited and thrilled that we finally have this all-important picture of Philae sitting in Abydos," says ESA's Laurence O'Rourke, who has been coordinating the search efforts over the last months at ESA, with the OSIRIS and Lander Science Operations and Navigation Center (SONC, CNES) teams.
- Philae was last seen when it first touched down at Agilkia, bounced and then flew for another two hours before ending up at a location later named Abydos, on the comet's smaller lobe. After three days, Philae's primary battery was exhausted and the lander went into hibernation, only to wake up again and communicate briefly with Rosetta in June and July 2015 as the comet came closer to the Sun and more power was available.
- However, until today, the precise location was not known. Radio ranging data tied its location down to an area spanning a few tens of meters, but a number of potential candidate objects identified in relatively low-resolution images taken from larger distances could not be analyzed in detail until recently.
- While most candidates could be discarded from analysis of the imagery and other techniques, evidence continued to build towards one particular target, which is now confirmed in images taken unprecedentedly close to the surface of the comet.
- At 2.7 km, the resolution of the OSIRIS narrow-angle camera is about 5 cm/pixel, sufficient to reveal characteristic features of Philae's 1 m-sized body and its legs, as seen in these definitive pictures.
- "This remarkable discovery comes at the end of a long, painstaking search," says Patrick Martin, ESA's Rosetta Mission Manager. "We were beginning to think that Philae would remain lost forever. It is incredible we have captured this at the final hour."
- "Now that the lander search is finished we feel ready for Rosetta's landing, and look forward to capturing even closer images of Rosetta's touchdown site," adds Holger Sierks, principal investigator of the OSIRIS camera.
Figure 101: Close-up image of Philae (image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)
• August 25, 2016: In unprecedented observations made earlier this year, Rosetta unexpectedly captured a dramatic comet outburst that may have been triggered by a landslide. Nine of Rosetta's instruments, including its cameras, dust collectors, and gas and plasma analyzers, were monitoring the comet from about 35 km in a coordinated planned sequence when the outburst happened on 19 February, 2016. 136) 137)
Figure 102: Outburst of Comet 67P/Churyumov-Gerasimenko on Feb. 19, 2016 observed by 9 instruments of the Rosetta Spacecraft (image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)
- "Over the last year, Rosetta has shown that although activity can be prolonged, when it comes to outbursts, the timing is highly unpredictable, so catching an event like this was pure luck," says Matt Taylor, ESA's Rosetta Project Scientist. "By happy coincidence, we were pointing the majority of instruments at the comet at this time, and having these simultaneous measurements provides us with the most complete set of data on an outburst ever collected."
- The data were sent to Earth only a few days after the outburst, but subsequent analysis has allowed a clear chain of events to be reconstructed, as described in a paper led by Eberhard Grün of the Max-Planck-Institute for Nuclear Physics, Heidelberg, accepted for publication in Monthly Notices of the Royal Astronomical Society.
- A strong brightening of the comet's dusty coma was seen by the OSIRIS wide-angle camera at 09:40 UTC, developing in a region of the comet that was initially in shadow.
- Over the next two hours, Rosetta recorded outburst signatures that exceeded background levels in some instruments by factors of up to a hundred. For example, between about 10:00–11:00 UTC, ALICE saw the ultraviolet brightness of the sunlight reflected by the nucleus and the emitted dust increase by a factor of six, while ROSINA and RPC detected a significant increase in gas and plasma, respectively, around the spacecraft, by a factor of 1.5–2.5. In addition, MIRO recorded a 30°C rise in temperature of the surrounding gas.
- Shortly after, Rosetta was blasted by dust: GIADA recorded a maximum hit count at around 11:15 UTC. Almost 200 particles were detected in the following three hours, compared with a typical rate of 3–10 collected on other days in the same month.
- At the same time, OSIRIS narrow-angle camera images began registering dust grains emitted during the blast. Between 11:10 UTC and 11:40 UTC, a transition occurred from grains that were distant or slow enough to appear as points in the images, to those either close or fast enough to be captured as trails during the exposures.
- In addition, the star trackers, which are used to navigate and help control Rosetta's attitude, measured an increase in light scattered from dust particles as a result of the outburst. - The star trackers are mounted at 90° to the side of the spacecraft that hosts the majority of science instruments, so they offered a unique insight into the 3D structure and evolution of the outburst.
Figure 103: Evolution of a comet outburst (credit of image: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; all data from Grün et al. (2016))
- By examining all of the available data, scientists believe they have identified the source of the outburst. "From Rosetta's observations, we believe the outburst originated from a steep slope on the comet's large lobe, in the Atum region," says Eberhard.
• July 28, 2016: Detailed analysis of data collected by Rosetta show that comets are the ancient leftovers of early Solar System formation, and not younger fragments resulting from subsequent collisions between other, larger bodies. — Understanding how and when objects like Comet 67P/Churyumov-Gerasimenko took shape is of utmost importance in determining how exactly they can be used to interpret the formation and early evolution of our Solar System (Figure 104). 138)
- A new study addressing this question led by Björn Davidsson of the Jet Propulsion Laboratory, California Institute of Technology in Pasadena (USA), has been published in Astronomy & Astrophysics. 139)
- If comets are primordial, then they could help reveal the properties of the solar nebula from which the Sun, planets and small bodies condensed 4.6 billion years ago, and the processes that transformed our planetary system into the architecture we see today.
- The alternative hypothesis is that they are younger fragments resulting from collisions between older 'parent' bodies such as icy trans-Neptunian objects (TNOs). They would then provide insight into the interior of such larger bodies, the collisions that disrupted them, and the process of building new bodies from the remains of older ones.
- "Either way, comets have been witness to important Solar System evolution events, and this is why we have made these detailed measurements with Rosetta – along with observations of other comets – to find out which scenario is more likely," says Matt Taylor, ESA's Rosetta project scientist.
- During its two-year sojourn at Comet 67P/Churyumov-Gerasimenko, Rosetta has revealed a picture of the comet as a low-density, high-porosity, double-lobed body with extensive layering, suggesting that the lobes accumulated material over time before they merged.
- The unusually high porosity of the interior of the nucleus provides the first indication that this growth cannot have been via violent collisions, as these would have compacted the fragile material. Structures and features on different size scales observed by Rosetta's cameras provide further information on how this growth may have taken place.
- Earlier work showed that the head and body were originally separate objects, but the collision that merged them must have been at low speed in order not to destroy both of them. The fact that both parts have similar layering also tells us that they must have undergone similar evolutionary histories and that survival rates against catastrophic collision must have been high for a significant period of time.
- Merging events may also have happened on smaller scales. For example, three spherical 'caps' have been identified in the Bastet region on the small comet lobe, and suggestions are that they are remnants of smaller cometesimals that are still partially preserved today.
- At even smaller scales of just a few meters across, there are the so-called 'goosebumps' and 'clod' features, rough textures observed in numerous pits and exposed cliff walls in various locations on the comet.
- While it is possible that this morphology might arise from fracturing alone, it is actually thought to represent an intrinsic 'lumpiness' of the comet's constituents. That is, these 'goosebumps' could be showing the typical size of the smallest cometesimals that accumulated and merged to build up the comet, made visible again today through erosion due to sunlight.
- According to theory, the speeds at which cometesimals collide and merge change during the growth process, with a peak when the lumps have sizes of a few meters. For this reason, meter-sized structures are expected to be the most compact and resilient, and it is particularly interesting that the comet material appears lumpy on that particular size scale.
- Further lines of evidence include spectral analysis of the comet's composition showing that the surface has experienced little or no in situ alteration by liquid water, and analysis of the gases ejected from sublimating ices buried deeper within the surface, which finds the comet to be rich in supervolatiles such as carbon monoxide, oxygen, nitrogen and argon.
- These observations imply that comets formed in extremely cold conditions and did not experience significant thermal processing during most of their lifetimes. Instead, to explain the low temperatures, survival of certain ices and retention of supervolatiles, they must have accumulated slowly over a significant time period.
- "While larger TNOs in the outer reaches of the Solar System appear to have been heated by short-lived radioactive substances, comets don't seem to show similar signs of thermal processing. We had to resolve this paradox by taking a detailed look at the time line of our current Solar System models, and consider new ideas," says Björn. Björn and colleagues propose that the larger members of the TNO population formed rapidly within the first one million years of the solar nebula, aided by turbulent gas streams that rapidly accelerated their growth to sizes of up to 400 km.
- Around three million years into the Solar System's history, gas had disappeared from the solar nebula, only leaving solid material behind. Then, over a much longer period of around 400 million years, the already massive TNOs slowly accreted further material and underwent compaction into layers, their ices melting and refreezing, for example. Some TNOs even grew into Pluto or Triton-sized objects.
- Comets took a different path. After the rapid initial growth phase of the TNOs, leftover grains and 'pebbles' of icy material in the cold, outer parts of the solar nebula started to come together at low velocity, yielding comets roughly 5 km in size by the time gas has disappeared from the solar nebula. The low speeds at which the material accumulated led to objects with fragile nuclei with high porosity and low density. - This slow growth also allowed comets to preserve some of the oldest, volatile-rich material from the solar nebula, since they were able to release the energy generated by radioactive decay inside them without heating up too much.
- The larger TNOs played a further role in the evolution of comets. By 'stirring' the cometary orbits, additional material was accreted at somewhat higher speed over the next 25 million years, forming the outer layers of comets. The stirring also made it possible for the few kilometer-sized objects in size to bump gently into each other, leading to the bi-lobed nature of some observed comets.
- "Comets do not appear to display the characteristics expected for collisional rubble piles, which result from the smash-up of large objects like TNOs. Rather, we think they grew gently in the shadow of the TNOs, surviving essentially undamaged for 4.6 billion years," concludes Björn. "Our new model explains what we see in Rosetta's detailed observations of its comet, and what had been hinted at by previous comet flyby missions."
- "Comets really are the treasure-troves of the Solar System," adds Matt. "They give us unparalleled insight into the processes that were important in the planetary construction yard at these early times and how they relate to the Solar System architecture that we see today."
Figure 104: Profile of a primordial comet (image credit: Center: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0; Insets: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; Fornasier et al. (2015); ESA/Rosetta/MPS for COSIMA Team MPS/CSNSM/UNIBW/TUORLA/IWF/IAS/ESA/BUW/MPE/LPC2E/LCM/FMI/UTU/LISA/UOFC/vH&S; Langevin et al. (2016))
• On July 27, 2016, the ESS (Electrical Support System) on Rosetta, which is used to communicate with Philae, will be switched off to save energy before September 30, the day the Rosetta mission will come to an end. 140)
• July 15, 2016: This CometWatch image (Figure 105) was taken with Rosetta's NAVCAM on 9 July 2016, when the spacecraft was 11.7 km from the nucleus of Comet 67P/Churyumov-Gerasimenko. The close-up view shows a portion of the Khonsu region on the larger of the two comet lobes. Khonsu is part of the southern hemisphere of 67P/C-G. 141)
- The image reveals a variety of fractured and smooth terrains, with a great number of boulders of all sizes, including several large ones. It also includes a three-layered structure with a balancing boulder on top, which was also portrayed in previous images, for example the NAVCAM view featured as CometWatch 13 June, which shows the same region but from a broader perspective.
Figure 105: Enhanced NAVCAM image of Comet 67P/C-G taken on 9 July 2016, 11.7 km from the nucleus. The scale is 1 m/pixel and the image measures about 1 km (image credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0)
• June 30, 2016: Rosetta is set to complete its mission in a controlled descent to the surface of its comet on 30 September. The mission is coming to an end as a result of the spacecraft's ever-increasing distance from the Sun and Earth. It is heading out towards the orbit of Jupiter, resulting in significantly reduced solar power to operate the craft and its instruments, and a reduction in bandwidth available to downlink scientific data. 142)
- Combined with an ageing spacecraft and payload that have endured the harsh environment of space for over 12 years – not least two years close to a dusty comet – this means that Rosetta is reaching the end of its natural life.
- Unlike in 2011, when Rosetta was put into a 31-month hibernation for the most distant part of its journey, this time it is riding alongside the comet. Comet 67P/Churyumov-Gerasimenko's maximum distance from the Sun (over 850 million km) is more than Rosetta has ever journeyed before. The result is that there is not enough power at its most distant point to guarantee that Rosetta's heaters would be able to keep it warm enough to survive.
- Instead of risking a much longer hibernation that is unlikely to be survivable, and after consultation with Rosetta's science team in 2014, it was decided that Rosetta would follow its lander Philae down onto the comet.
- The final hours of descent will enable Rosetta to make many once-in-a-lifetime measurements, including very-high-resolution imaging, boosting Rosetta's science return with precious close-up data achievable only through such a unique conclusion. — Communications will cease, however, once the orbiter reaches the surface, and its operations will then end.
- "We're trying to squeeze as many observations in as possible before we run out of solar power," says Matt Taylor, ESA Rosetta project scientist. "30 September will mark the end of spacecraft operations, but the beginning of the phase where the full focus of the teams will be on science. That is what the Rosetta mission was launched for and we have years of work ahead of us, thoroughly analyzing its data."
- Rosetta's operators will begin changing the trajectory in August ahead of the grand finale such that a series of elliptical orbits will take it progressively nearer to the comet at its closest point.
- "Planning this phase is in fact far more complex than it was for Philae's landing," says Sylvain Lodiot, ESA Rosetta spacecraft operations manager. "The last six weeks will be particularly challenging as we fly eccentric orbits around the comet – in many ways this will be even riskier than the final descent itself. - "The closer we get to the comet, the more influence its non-uniform gravity will have, requiring us to have more control on the trajectory, and therefore more maneuvers – our planning cycles will have to be executed on much shorter timescales."
- A number of dedicated maneuvers in the closing days of the mission will conclude with one final trajectory change at a distance of around 20 km about 12 hours before impact, to put the spacecraft on its final descent.
- In the meantime, science will continue as normal, although there are still many risks ahead. Last month, the spacecraft experienced a 'safe mode' while only 5 km from the comet as a result of dust confusing the navigation system. Rosetta recovered, but the mission team cannot rule out this happening again before the planned end of the mission.
• May 27, 2016: Ingredients regarded as crucial for the origin of life on Earth have been discovered at the comet that ESA's Rosetta spacecraft has been probing for almost two years. They include the amino acid glycine, which is commonly found in proteins, and phosphorus, a key component of DNA and cell membranes. 143) 144)
- Scientists have long debated the important possibility that water and organic molecules were brought by asteroids and comets to the young Earth after it cooled following its formation, providing some of the key building blocks for the emergence of life. While some comets and asteroids are already known to have water with a composition like that of Earth's oceans, Rosetta found a significant difference at its comet – fuelling the debate on their role in the origin of Earth's water.
- But new results reveal that comets nevertheless had the potential to deliver ingredients critical to establish life as we know it. Amino acids are biologically important organic compounds containing carbon, oxygen, hydrogen and nitrogen, and form the basis of proteins.
- Hints of the simplest amino acid, glycine, were found in samples returned to Earth in 2006 from Comet Wild-2 by NASA's Stardust mission. However, possible terrestrial contamination of the dust samples made the analysis extremely difficult.
- Now, Rosetta has made direct, repeated detections of glycine in the fuzzy atmosphere or 'coma' of its comet. "This is the first unambiguous detection of glycine at a comet," says Kathrin Altwegg, principal investigator of the ROSINA instrument that made the measurements, and lead author of the paper. "At the same time, we also detected certain other organic molecules that can be precursors to glycine, hinting at the possible ways in which it may have formed."
- The measurements were made before the comet reached its closest point to the Sun – perihelion – in August 2015 in its 6.5 year orbit. The first detection was made in October 2014 while Rosetta was just 10 km from the comet. The next occasion was during a flyby in March 2015, when it was 30–15 km from the nucleus.
- A sample mass spectrum at 75, 45, 31, and 30 dalton is shown in Figure106. The number of ionized particles registered on the detector is given as a function of the position on the detector, which corresponds to mass/charge ratio (m/z). Glycine (C2H5NO2), methylamine (CH5N), and ethylamine (C2H7N) can be found on mass 75, 31, and 45 dalton, respectively.
- Glycine was also seen on other occasions associated with outbursts from the comet in the month leading up to perihelion, when Rosetta was more than 200 km from the nucleus but surrounded by a lot of dust. "We see a strong link between glycine and dust, suggesting that it is probably released perhaps with other volatiles from the icy mantles of the dust grains once they have warmed up in the coma," says Kathrin.
- Glycine turns into gas only when it reaches temperatures just below 150°C, meaning that usually little is released from the comet's surface or subsurface because of the low temperatures. This accounts for the fact that Rosetta does not always detect it. "Glycine is the only amino acid that is known to be able to form without liquid water, and the fact we see it with the precursor molecules and dust suggests it is formed within interstellar icy dust grains or by the ultraviolet irradiation of ice, before becoming bound up and conserved in the comet for billions of years," adds Kathrin.
- Another exciting detection made by Rosetta and described in the paper is of phosphorus, a key element in all known living organisms. For example, it is found in the structural framework of DNA, in cell membranes and in transporting chemical energy within cells for metabolism. "There is still a lot of uncertainty regarding the chemistry on early Earth and there is of course a huge evolutionary gap to fill between the delivery of these ingredients via cometary impacts and life taking hold," says co-author Hervé Cottin. "But the important point is that comets have not really changed in 4.5 billion years: they grant us direct access to some of the ingredients that likely ended up in the prebiotic soup that eventually resulted in the origin of life on Earth."
- "The multitude of organic molecules already identified by Rosetta, now joined by the exciting confirmation of fundamental ingredients like glycine and phosphorous, confirms our idea that comets have the potential to deliver key molecules for prebiotic chemistry," says Matt Taylor, ESA's Rosetta project scientist.
Figure 106: ROSINA DFMS (Double Focusing Mass Spectrometer) mass spectra (9 July 2015) for masses 30, 31, 45, and 75 dalton. Integration time is 20 s per spectrum. Error bars represent 1σ counting statistics (image credit: Rosetta/ROSINA Team)
• April 18, 2016: Captured in this curious view (Figure 107) are the Anuket region and its surroundings on Comet 67P/Churyumov–Gerasimenko. The image was taken by Rosetta’s navigation camera (NavCam) on 13 March 2016, from a distance of 17 km, and measures about 1.5 km across. 145)
- However, if we briefly suspend disbelief and set our imagination free, we might be tricked into recognizing the profile of a face, with the forehead and eyebrows on the left, a nose pointing upwards and even the hint of a smile. This is an effect of pareidolia, a psychological phenomenon whereby humans tend to identify familiar shapes in the vague patterns of random images.
- In reality, the fictional face shows the rough landscape of Anuket, a region of rugged terrains on the small comet lobe and declining towards the large lobe, which is located beyond the lower-right corner of the image. What might appear as a forehead, towards the left, is in fact the surface of Serqet, a small region comprising flat and smooth terrains and a few boulders. The sharp cliff separating Serqet and Anuket contributes to the optical illusion, suggesting the profile of an eye socket and eyebrow.
- Along the sharp boundary between Serqet and Anuket – the eyebrow – is a round, crescent feature, known as C. Alexander Gate. It is dedicated to Dr Claudia J. Alexander, who was the US Rosetta Project Scientist and passed away in July 2015.
- The image also depicts Serqet casting a dramatic shadow onto parts of Ma’at, the smooth, dusty region visible in the lower-right part of the image. Another striking element is the border between Ma’at and Anuket, highlighting the difference between the almost featureless appearance of Ma’at and the rugged terrains of Anuket.
- A hint of the Hathor region is also visible, albeit cast in shadow, on the lower part of the image. You can use the comet viewer tool to aid navigation around the comet’s regions.
- Rosetta is currently around 30 km from the nucleus, and will continue studying the comet from up close until the end of September, when it will be maneuvered into a controlled impact on the comet.
Figure 107: Image of the Anuket region and its surroundings on Comet 67P/Churyumov–Gerasimenko, acquired on March 13, 2016 with the NAVCAM from a distance of 17 km (image credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0)
• April 7, 2016: Comet 67P/Churyumov-Gerasimenko was seen changing color and brightness by Rosetta’s VIRTIS (Visible and InfraRed Thermal Imaging Spectrometer), as more water-ice was exposed near its surface as it moved close to the Sun between August and November 2014. 146) 147)
- In the three-month study period the comet moved from about 542 million km to 438 million km from the Sun, and the spacecraft-to-comet distance varied from about 100 km to 10 km, resulting in a range of illumination conditions and viewing geometries. In general, the darkest portions of the comet, containing dry dust made out of a mixture of minerals and organics, reflect light at redder wavelengths, while active regions and the occasional ice-rich exposure is bluer.
- The VIRTIS study shows that even in the first three months of study at the comet, global average changes are noticeable, with an overall trend of the comet becoming brighter and more water-ice-rich. This is particularly notable in the Imhotep region, which becomes overall bluer over time.
Figure 108: Illustration of the comet's orbit in the period August-November 2014 (image credit: ESA/ATG medialab)
Figure 109: Comet 67p/Churyumov-Gerasimenko was seen changing color and brightness as more water-ice was exposed near its surface as it moved closer to the Sun between August and November 2014 (image credit: ESA/Rosetta/VIRTIS/INAF-IAPS/OBS De PARIS-LESIA/DLR; G. Filacchione et al., 2016)
Legend to Figure 109: Variation of visible spectral slope (color) over time. Red corresponds to more organic-rich material; blue indicates more active regions (such as Hapi) and water-ice rich exposures. The transition from redder to bluer spectra is most clearly seen in the Imhotep region.
• March 11, 2016: ESA’s Rosetta spacecraft has revealed a surprisingly large region around its host comet devoid of any magnetic field. The Rosetta magnetometer RPC-MAG has been exploring the plasma environment of comet 67P/Churyumov-Gerasimenko since August 2014. The first months were dominated by low-frequency waves which evolved into more complex features. However, at the end of July 2015, close to perihelion, the magnetometer detected a region that did not contain any magnetic field at all. 148) 149)
- These signatures match the appearance of a diamagnetic cavity as was observed at comet 1P/Halley in 1986 when ESA's Giotto found a vast magnetic-free region extending more than 4000 km from the nucleus. This was the first observation of something that scientists had until then only thought about but had never seen.
- The cavity at Rosetta is more extended than previously predicted by models and features unusual magnetic field configurations, which need to be explained. According to the team's analysis of the data acquired by the Rosetta Plasma Consortium instrumentation confirms the existence of a diamagnetic cavity. However, the size is larger than predicted by simulations. One possible explanation are instabilities that are propagating along the cavity boundary and possibly a low magnetic pressure in the solar wind. This conclusion is supported by a change in sign of the Sun-pointing component of the magnetic field. Evidence also indicates that the cavity boundary is moving with variable velocities ranging from 230-500 m/s.
Legend to Figure 110: The magnetic field-free region, shown here in blue, is caused by the interaction of the comet and the solar wind, shown in purple. The solar wind is a flow of electrically charged particles streaming from the Sun (located beyond the left edge of the image) and carrying its magnetic field across the Solar System. When it approaches the comet, which is pouring lots of gas into space, its flow is obstructed and slowed. Eventually, the solar wind stops entirely, diverting its flow around the comet, and also its magnetic field is unable to penetrate the environment around the comet, creating a region devoid of magnetic field called a diamagnetic cavity. — Scientists expected that such a diamagnetic cavity could form at Rosetta’s comet around perihelion, but only extend to 50–100 km from the nucleus, and since the spacecraft was at greater distances from the comet at the time, they did not expect to detect it. Instead, they measured almost 700 magnetic field-free regions with the RPC-MAG magnetometer on Rosetta since June 2015, revealing that the cavity is much bigger than expected. The reason for that is likely an oscillating perturbation, or instability (thin wiggly line), that propagates and gets amplified along the boundary (thick line) between the solar wind and the magnetic field-free cavity, causing the latter to grow in size and allowing Rosetta to detect it.
- Prior to Rosetta arriving at Comet 67P/Churyumov-Gerasimenko, scientists had hoped to observe such a magnetic field-free region in the environment of this comet. The spacecraft carries a magnetometer as part of the Rosetta Plasma Consortium suite of sensors (RPC-MAG), whose measurements were already used to demonstrate that the comet nucleus is not magnetized.
- However, since Rosetta’s comet is much less active than Comet Halley, the scientists predicted that a diamagnetic cavity could form only in the months around perihelion – the closest point to the Sun on the comet’s orbit – but that it would extend only 50–100 km from the nucleus. During 2015, the increased amounts of dust dragged into space by the outflowing gas became a significant problem for navigation close to the comet. To keep Rosetta safe, trajectories were chosen such that by the end of July 2015, a few weeks before perihelion, it was some 170 km away from the nucleus. As a result, scientists considered that detecting signs of the magnetic field-free bubble would be impossible.
- “We had almost given up on Rosetta finding the diamagnetic cavity, so we were astonished when we eventually found it,” says Charlotte Götz of the Institute for Geophysics and extraterrestrial Physics in Braunschweig, Germany. Charlotte is the lead author of a new study, published in the journal Astronomy and Astrophysics. “We were able to detect the cavity, on many occasions, because it is much bigger and dynamic than we had expected,” adds Charlotte.
Figure 111: The decrease in magnetic field strength measured by Rosetta’s RPC-MAG instrument at Comet 67P/Churyumov–Gerasimenko on 26 July 2015 at a distance of about 170 km from the comet (image credit: ESA/Rosetta/RPC/IGEP/IC)
• Feb. 12, 2016: Silent since its last call to mothership Rosetta seven months ago, the Philae lander is facing conditions on Comet 67P/Churyumov–Gerasimenko from which it is unlikely to recover. 150) 151)
- Rosetta, which continues its scientific investigations at the comet until September before its own comet-landing finale, has in recent months been balancing science observations with flying dedicated trajectories optimized to listen out for Philae. But the lander has remained silent since 9 July 2015.
- “The chances for Philae to contact our team at our lander control center are unfortunately getting close to zero,” says Stephan Ulamec, Philae project manager at the German Aerospace Center, DLR. “We are not sending commands any more and it would be very surprising if we were to receive a signal again.”
- Philae’s team of expert engineers and scientists at the German, French and Italian space centers and across Europe have carried out extensive investigations to try to understand the status of the lander, piecing together clues since it completed its first set of scientific activities after its historic landing on 12 November 2014.
- A story with incredible twists and turns unfolded on that day. In addition to a faulty thruster, Philae also failed to fire its harpoons and lock itself onto the surface of the comet after its seven-hour descent, bouncing from its initial touchdown point at Agilkia, to a new landing site, Abydos, over 1 km away. The precise location of the lander has yet to be confirmed in high-resolution images.
- A reconstruction of the flight of the lander suggested that it made contact with the comet four times during its two-hour additional flight across the small comet lobe. After bouncing from Agilkia it grazed the rim of the Hatmehit depression, bounced again, and then finally settled on the surface at Abydos (Figure 135).
- Even after this unplanned excursion, the lander was still able to make an impressive array of science measurements, with some even as it was flying above the surface after the first bounce.
- Once the lander had made its final touchdown, science and operations teams worked around the clock to adapt the experiments to make the most of the unanticipated situation. About 80% of its initial planned scientific activities were completed.
- In the 64 hours following its separation from Rosetta, Philae took detailed images of the comet from above and on the surface, sniffed out organic compounds, and profiled the local environment and surface properties of the comet, providing revolutionary insights into this fascinating world.
- But with insufficient sunlight falling on Philae’s new home to charge its secondary batteries, the race was on to collect and transmit the data to Rosetta and across 510 million km of space back to Earth before the lander’s primary battery was exhausted as expected. Thus, on the evening of 14–15 November 2014, Philae fell into hibernation.
- The last images of Philae will probably be acquired in the summer of 2016, when the Rosetta spacecraft images the lander during close fly-bys. "When we see how Philae is positioned, we will be able to better interpret certain data, such as the measurements of the CONSERT radar experiment." In approximately six years, Philae and Rosetta, which will be landed on the comet in September 2016 at the end of its mission, will be closer to Earth – and Comet 67/P Churyumov-Gerasimenko will have circled the Sun once again.
• Feb. 4, 2016: There are no large caverns inside Comet 67P/Churyumov-Gerasimenko. ESA’s Rosetta mission has made measurements that clearly demonstrate this, solving a long-standing mystery. Comets are the icy remnants left over from the formation of the planets 4.6 billion years ago. A total of eight comets have now been visited by spacecraft and, thanks to these missions, we have built up a picture of the basic properties of these cosmic time capsules. While some questions have been answered, others have been raised. 152)
- Comets are known to be a mixture of dust and ice, and if fully compact, they would be heavier than water. However, previous measurements have shown that some of them have extremely low densities, much lower than that of water ice. The low density implies that comets must be highly porous. But is the porosity because of huge empty caves in the comet’s interior or it is a more homogeneous low-density structure?
- In a new study led by Martin Pätzold, the authors have shown that Comet 67P/Churyumov-Gerasimenko is also a low-density object, but they have also been able to rule out a cavernous interior. This result is consistent with earlier results from Rosetta’s CONSERT radar experiment showing that the double-lobed comet’s ‘head’ is fairly homogenous on spatial scales of a few tens of meters. 153)
- The most reasonable explanation then is that the comet’s porosity must be an intrinsic property of dust particles mixed with the ice that make up the interior. In fact, earlier spacecraft measurements had shown that comet dust is typically not a compacted solid, but rather a ‘fluffy’ aggregate, giving the dust particles high porosity and low density, and Rosetta’s COSIMA and GIADA instruments have shown that the same kinds of dust grains are also found at 67P/Churyumov-Gerasimenko.
- Pätzold’s study team made their discovery by using the RSI (Radio Science Experiment) to study the way the Rosetta orbiter is pulled by the gravity of the comet, which is generated by its mass. The effect of the gravity on the movement of Rosetta is measured by changes in the frequency of the spacecraft’s signals when they are received at Earth. It is a manifestation of the Doppler effect, produced whenever there is movement between a source and an observer, and is the same effect that causes emergency vehicle sirens to change pitch as they pass by.
- In this case, Rosetta was being pulled by the gravity of the comet, which changed the frequency of the radio link to Earth. ESA’s 35 m antenna at the New Norcia ground station in Australia is used to communicate with Rosetta during routine operations. The variations in the signals it received were analyzed to give a picture of the gravity field across the comet. Large internal caverns would have been noticeable by a tell-tale drop in acceleration.
- ESA’s Rosetta mission is the first to perform this difficult measurement for a comet. “Newton’s law of gravity tells us that the Rosetta spacecraft is basically pulled by everything,” says Martin Pätzold, the principal investigator of the RSI experiment. “In practical terms, this means that we had to remove the influence of the Sun, all the planets – from giant Jupiter to the dwarf planets – as well as large asteroids in the inner asteroid belt, on Rosetta’s motion, to leave just the influence of the comet. Thankfully, these effects are well understood and this is a standard procedure nowadays for spacecraft operations.”
- Next, the pressure of the solar radiation and the comet’s escaping gas tail has to be subtracted. Both of these ‘blow’ the spacecraft off course. In this case, Rosetta’s ROSINA instrument is extremely helpful as it measures the gas that is streaming past the spacecraft. This allowed Pätzold and his colleagues to calculate and remove those effects too.
- Whatever motion is left is due to the comet’s mass. For Comet 67P/Churyumov-Gerasimenko, this gives a mass slightly less than 10 billion tons. Images from the OSIRIS camera have been used to develop models of the comet’s shape and these give the volume as around 18.7 km3, meaning that the density is 533 kg/m3.
- Extracting the details of the interior was only possible through a piece of cosmic good luck. The comet’s strange shape was revealed as Rosetta drew nearer for its rendezvous in August 2014. Luckily for RSI, the double-lobed structure meant that the differences in the gravity field would be much more pronounced, and therefore easier to measure from far away.
- In September 2016, Rosetta will be guided to a controlled impact on the surface of the comet. The maneuver will provide a unique challenge for the flight dynamics specialists at ESA/ESOC (European Space Operations Center) in Darmstadt, Germany. As Rosetta gets nearer and nearer, the complex gravity field of the comet will make navigating harder and harder. But for RSI, its measurements will increase in precision. This could allow the team to check for caverns just a few hundred meters across.
• January 13, 2016: Observations made shortly after Rosetta’s arrival at its target comet in 2014 have provided definitive confirmation of the presence of water ice. Although water vapor is the main gas seen flowing from comet 67P/Churyumov–Gerasimenko, the great majority of ice is believed to come from under the comet’s crust, and very few examples of exposed water ice have been found on the surface. However, a detailed analysis by Rosetta’s VIRTIS infrared instrument reveals the composition of the comet’s topmost layer: it is primarily coated in a dark, dry and organic-rich material but with a small amount of water ice mixed in. 154) 155)
- In the latest study, which focuses on scans between September and November 2014, the team confirms that two areas several tens of meters across in the Imhotep region that appear as bright patches in visible light, do indeed include a significant amount of water ice. The ice is associated with cliff walls and debris falls, and was at an average temperature of about –120ºC at the time.
- In those regions, pure water ice was found to occupy around 5% of each pixel sampling area, with the rest made up of the dark, dry material. The abundance of ice was calculated by comparing Rosetta’s VIRTIS infrared measurements to models that consider how ice grains of different sizes might be mixed together in one pixel.
- The data reveal two different populations of grains: one is several tens of micrometers in diameter, while the other is larger, around 2 mm. These sizes contrast with the very small grains, just a few micrometers in diameter, found in the Hapi region on the ‘neck’ of the comet, as observed by VIRTIS in a different study.
- The Rosetta scientists are now analyzing data captured later in the mission, as the comet moved closer to the Sun in mid-2015, to see how the amount of ice exposed on the surface evolved as the heating increased.
Figure 112: Two exposures of water ice identified by Rosetta’s VIRTIS instrument in the Imhotep region of Comet 67P/Churyumov–Gerasimenko in September–November 2014 (image credit: ESA/Rosetta/NavCam – CC BY-SA IGO 3.0)
Legend to Figure 112: The main image was taken on 17 September 2014 from a distance of about 28.8 km from the comet center. The two insets show oblique views of the two icy exposures. The left hand image was taken on 20 September 2014 from a distance of 27.9 km. The right hand image was taken on 15 September 2014 from a distance of 29.9 km.
• Dec. 31, 2015: Images from Rosetta taken over the holiday period. 156)
Figure 113: The single-frame OSIRIS narrow-angle camera image was taken on 31 December 2015, when Rosetta was 79.6 km from the nucleus of Comet 67P/Churyumov–Gerasimenko. The scale is 1.44 m/pixel (image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)
• Nov. 30, 2015: A new 3D shape model of Comet 67P/Churyumov-Gerasimenko has been released by ESA’s Rosetta archive team (Figure 114). The model includes images taken by Rosetta’s NAVCAM up until mid-late July 2015, and reveals parts of the comet’s southern hemisphere that were not included in earlier shape models. 157)
• November 12, 2015: As announced in June 2015 along with confirmation of the mission’s extension, Rosetta teams are planning to end the operational phase of the mission in a controlled impact of the orbiter on the surface of Comet 67P/Churyumov-Gerasimenko at the end of September 2016. While the specific details of the trajectories and impact site are still under discussion, ESA’s Rosetta Spacecraft Operations Manager Sylvain Lodiot, Project Scientist Matt Taylor, and mission manager Patrick Martin, share some background information on the planning of this dramatic mission finale. 158)
• Nov. 12, 2015: One year since Philae made its historic landing on a comet, mission teams remain hopeful for renewed contact with the lander, while also looking ahead to next year’s grand finale: making a controlled impact of the Rosetta orbiter on the comet. - Rosetta arrived at Comet 67P/Churyumov–Gerasimenko on 6 August 2014, and after an initial survey and selection of a landing site, Philae was delivered to the surface on 12 November. 159) 160) 161)
- After touching down in the Agilkia region as planned (Figure 115), Philae did not secure itself to the comet, and it bounced to a new location in Abydos. Its flight across the surface is depicted in a new animation, using data collected by Rosetta and Philae to reconstruct the lander’s rotation and attitude.
- In the year since landing, a thorough analysis has also now been performed on why Philae bounced. There were three methods to secure it after landing: ice screws, harpoons and a small thruster. The ice screws were designed with relatively soft material in mind, but Agilkia turned out to be very hard and they did not penetrate the surface.
- The harpoons were capable of working in both softer and harder material. They were supposed to fire on contact and lock Philae to the surface, while a thruster on top of the lander was meant to push it down to counteract the recoil from the harpoon. - Attempts to arm the thruster the night before failed: it is thought that a seal did not open, although a sensor failure cannot be excluded.
- Then, on landing, the harpoons themselves did not fire. “It seems that the problem was either with the four ‘bridge wires’ taking current to ignite the explosive that triggers the harpoons, or the explosive itself, which may have degraded over time,” explains Stephan Ulamec, Philae lander manager at the DLR German Aerospace Center. “In any case, if we can regain contact with Philae, we might consider an attempt to retry the firing.”
- The reason is scientific: the harpoons contain sensors that could measure the temperature below the surface. Despite the unplanned bouncing, Philae completed 80% of its planned first science sequence before falling into hibernation in the early hours of 15 November when the primary battery was exhausted. There was not enough sunlight in Philae’s final location at Abydos to charge the secondary batteries and continue science measurements.
- The hope was that as the comet moved nearer to the Sun, heading towards closest approach in August, there would be enough energy to reactivate Philae. Indeed, contact was made with the lander on 13 June 2015 but only eight intermittent contacts were made up to 9 July. The problem was that the increasing sunlight also led to increased activity on the comet, forcing Rosetta to retreat to several hundred kilometers for safety, well out of range with Philae.
- However, over the past few weeks, with the comet’s activity now subsiding, Rosetta has started to approach again. This week it reached 200 km, the limit for making good contact with Philae, and today it dips to within 170 km.
- In the meantime, the lander teams have continued their analysis of the data returned during the contacts in June and July, hoping to understand the status of Philae when it first woke up from hibernation. “We had already determined that one of Philae’s two receivers and one of the two transmitters were likely no longer working,” says Koen Geurts, Philae’s technical manager at DLR’s Lander Control Center in Cologne, Germany, “and it now seems that the other transmitter is suffering problems. Sometimes it did not switch on as expected, or it switched off too early, meaning that we likely missed possible contacts.” The team is taking this new information into account to determine the most promising strategy to regain regular contact. But it’s a race against time: with the comet now heading out beyond the orbit of Mars, temperatures are falling.
Figure 115: The area surrounding Philae’s first touchdown point, Agilkia (circled) on comet 67P/Churyumov–Gerasimenko. The large depression is the Hatmehit region. The dashed line marks the comet’s equator. This image is a composite of five frames from the OSIRIS narrow-angle camera (image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)
• October 2015 (Flight dynamics analysis for the approach of Rosetta with the comet): When the Rosetta spacecraft approached comet Churyumov-Gerasimenko in August 2014 after ten years of flight, almost nothing was known about its target’s gravity field and rotation, except for the rotation period. Since knowledge of these parameters is essential for trajectory design, a flexible strategy had to be devised to cope with a large range of possible values. In addition, the strategy had to be robust against contingencies like missed orbit maneuvers. The first simulations showed that the initial plan was not feasible. After several iterations, a strategy meeting these requirements was found. It consisted of a Comet Approach Phase, where the relative velocity was gradually reduced by orbit maneuvers of decreasing size, and an Initial Characterization Phase, where the physical parameters of the comet were determined from a sequence of hyperbolic arcs forming a pyramid-like orbit. These parameters would subsequently be used to adapt the trajectories for the next phase of the mission, where the spacecraft would be put into a bound orbit around the comet. The strategy chosen was successfully validated by a navigation analysis in which the operations that would be performed during the real approach were simulated, and finally confirmed when it was put into action during the real operations. 162)
- In preparation for the approach of spacecraft Rosetta to comet 67P/Churyumov-Gerasimenko, an extensive number of simulations were performed. These included both closed-loop and open-loop simulations, where the more realistic closed-loop simulations were used to validate the idea of open-loop simulations, which are less realistic but require less effort.
- The first simulations clearly indicated that the original plan, which foresaw a direct insertion into a bound orbit at the end of the approach phase, was not feasible since the comet mass and attitude could not be determined with sufficient accuracy before the insertion maneuver. Instead, after several iterations, a new strategy was devised: The initial characterization of the comet was done from hyperbolic arcs at cometocentric distances between 50 and 120 km. The velocity in these arcs was chosen such that the influence of the initially poorly known comet gravity force on the spacecraft orbit was large enough to provide an accurate determination of the comet mass, but still small enough to allow a reasonably accurate orbit prediction. Orbit maneuvers were performed regularly to keep the spacecraft close to the comet, in such a way that the comet-spacecraft vector formed two pyramids with the nucleus at the apex.
- From these pyramid orbits, the mass, rotational state and shape of the comet could be estimated. Moreover, landmarks on the surface of the nucleus could be identified with sufficient accuracy for a safe insertion into a bound orbit. Following the insertion, the spacecraft entered the Global Mapping and the Close Observation phases, in which the knowledge about the comet properties was further improved by observations from successively closer orbits around the comet.
- The validity of the revised strategy was finally fully confirmed when it was put into action during the actual operations, without the need for any modifications. All orbit maneuvers during the approach performed well within the expected uncertainties (163) 164)), the comet was detected at the earliest possible stage using the OSIRIS NAC and the derivation of optical measurements from the camera images worked flawlessly. 165) 166)
• October 28, 2015: ESA’s Rosetta spacecraft has made the first in situ detection of oxygen molecules outgassing from a comet, a surprising observation that suggests they were incorporated into the comet during its formation. Rosetta has been studying Comet 67P/Churyumov–Gerasimenko for over a year and has detected an abundance of different gases pouring from its nucleus. Water vapor, carbon monoxide and carbon dioxide are the most prolific, with a rich array of other nitrogen-, sulphur- and carbon-bearing species, and even ‘noble gases’ also recorded. 167) 168)
- Oxygen is the third most abundant element in the Universe, but the simplest molecular version of the gas, O2, has proven surprisingly hard to track down, even in star-forming clouds, because it is highly reactive and readily breaks apart to bind with other atoms and molecules. For example, oxygen atoms can combine with hydrogen atoms on cold dust grains to form water, or a free oxygen split from O2 by ultraviolet radiation can recombine with an O2 molecule to form ozone (O3).
- Despite its detection on the icy moons of Jupiter and Saturn, O2 had been missing in the inventory of volatile species associated with comets until now. “We weren’t really expecting to detect O2 at the comet – and in such high abundance – because it is so chemically reactive, so it was quite a surprise,” says Kathrin Altwegg of the University of Bern, and principal investigator of the ROSINA-DFMS (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis-Double Focusing Mass Spectrometer) instrument.
- The research team analyzed more than 3000 samples collected around the comet between September 2014 and March 2015 to identify the O2. They determined an abundance of 1–10% relative to H2O, with an average value of 3.80 ± 0.85%, an order of magnitude higher than predicted by models describing the chemistry in molecular clouds. The amount of molecular oxygen detected showed a strong relationship to the amount of water measured at any given time, suggesting that their origin on the nucleus and release mechanism are linked. By contrast, the amount of O2 seen was poorly correlated with carbon monoxide and molecular nitrogen, even though they have a similar volatility to O2. In addition, no ozone was detected.
Figure 116: Rosetta has made the first detection of molecular oxygen at a comet. The results presented in this graphic are based on ROSINA-DFMS observations between September 2014 and March 2015 when Rosetta was still on the approach to the Sun along its orbit (image credit: Spacecraft: ESA/ATG medialab; comet: ESA/Rosetta/NavCam – CC BY-SA IGO 3.0; Data: A. Bieler et al. (2015)
October 2015 (Rosetta navigation during lander delivery phase and
reconstruction of Philae descent trajectory and rebound): 169) 170)
- Rosetta’s SPD (Lander Delivery Phase) started on October 28, 2014 with the execution of the SDP-1 maneuver that drove the spacecraft away from the “Close Observation” 10 km orbit, in a transition arc to the 30 km parking orbit, in preparation for the lander delivery sequence to be executed on November 12. The main objectives of this phase were: to safely deliver Philae into its descent trajectory towards the target landing site; to keep the communication link with the lander during descent and immediately after landing; and additionally to take images of the lander’s descent and landing.
- Trajectory: Figure 117 shows, in white, the terminator orbits flown by Rosetta the month before lander delivery (from 20 km to 10 km distance); and in red the trajectory flown by Rosetta for lander delivery:
Legend to Figure 117: (1) Rosetta is inserted in a circular 30 km parking orbit with an orbital plane tilted 15º from the terminator plane (plane separating day/night). Rosetta flies almost a full orbit revolution around the comet, half of it spent in the night side of the comet (2). Once Rosetta arrives at the target point (3) for the initiation of the lander delivery sequence, the pre-delivery maneuver (83.6 cm/s) is executed, driving the S/C in a hyperbolic trajectory, almost in collision course, with 5 km miss-distance. A mission constraint did not allow to drive the orbiter in a collision trajectory to the comet, even if the plan included a subsequent maneuver that would avoid the collision.
At a distance of 22.6 km (4), Philae separates from Rosetta. The nominal separation’s relative velocity was 18.76 cm/s. Taking into account the mass of Rosetta and Philae and the fraction of Rosetta’s propellant intervening in the separation, it results that, nominally, the lander would receive a ΔV of 17.4 cm/s and Rosetta of 1.1 cm/s. The ΔV received by the lander redirected its trajectory towards the target point on the comet’s surface. The direction in which the separation is performed fixes the orientation of the lander’s Z-axis, which is stabilized by the angular momentum stored in its flywheel. The descent trajectory was designed such that at touchdown (7 hours after separation) the lander’s Z-axis was parallel to the local vertical (defined as the normal to the local surface), and that the incoming relative velocity with respect to the surface was also parallel to the local vertical.
In the meantime, Rosetta executes the post-delivery maneuver to avoid passing at 5 km distance from the comet, staying in the day side of the comet and ensuring communications with Philae during descent. Philae’s antenna is mounted pointing along the lander’s +Z axis and, by design, it is not pointing to Rosetta after separation. Therefore, a communication link during descent would not be possible if Rosetta did not execute the post-delivery maneuver. Afterwards, the mission enters in the Relay Phase (5), in which Rosetta performs a series of maneuvers to keep its trajectory in the region where the lander visibilities durations are maximized, in order to support the scientific operations of the lander on the comet’s surface.
- Navigation analysis: In the scope of the design of the lander delivery trajectory, navigation analysis was performed to assess the achievable landing accuracy. Monte Carlo simulations were run reproducing the navigation process: randomly generating a “real-world” trajectory; based on it, simulated observations are generated and fed to the OD (Orbit Determination); from the OD solution, the landing trajectory is optimized; and finally, the resulting maneuvers and separation conditions are perturbed and applied to the real-world to propagate the trajectory and compute the achieved landing point.
- The results of this analysis showed that the landing uncertainty was covering an area of roughly 500 m radius around the target landing point and that the uncertainty in the landing time was of about 40 min. Figure 118 shows the landing points of several simulated trajectories plotted on top of a NAVCAM image. 171)
- On Nov. 12, 2014, Rosetta released Philae. The small lander, after 7 h descent, finally touched softly the ground of comet Churyumov-Gerasimenko. Unfortunately its anchoring system failed and Philae experienced a 2 h bouncing trajectory. It landed 1 km away from its target site. Nevertheless it was operated during 57 h performing its FSS (First Science Sequence. The FSS, made possible with the two batteries, should have been followed by the LTS (Long Term Science Sequence) but Philae was not well illuminated and fell “asleep”. One of the last Philae actions was to rotate its head slightly (+22° around Z axis) to improve solar arrays illumination. 172)
The wake-up of Philae occurred on June 13, 2015. A very short and unstable RF link was established between Philae and Rosetta. The lander was still not properly illuminated. The final position and attitude of Philae were key data to forecast an eventual wake-up. But as Philae was not equipped with system dedicated to position and attitude monitoring, the only way to determine missing data was to examine all collected measurements, housekeeping as well as scientific data. During the next weeks, Rosetta and Philae teams tried to assess the Philae’s status and to improve the RF links. Unfortunately, due to comet outgassing, it was not possible to reduce the distance between the two spacecrafts significantly. The last telecommunication occurred on July 9, 2015.
The Science Operation Navigation Center was responsible to coordinate and to realize activities necessary to determine the Philae attitude and position. Thanks to a collective effort, a possible landing area and an attitude were successfully estimated. Equivalent work was also realized to achieve the reconstruction, both of trajectory and attitude, of the descent and bouncing on the surface.
• Sept.28, 2015: Two comets collided at low speed in the early Solar System to give rise to the distinctive ‘rubber duck’ shape of Comet 67P/Churyumov–Gerasimenko, say Rosetta scientists. -The origin of the comet’s double-lobed form has been a key question since Rosetta first revealed its surprising shape in July 2014. Two leading ideas emerged: did two comets merge or did localized erosion of a single object form the ‘neck’? 173)
- Now, scientists have an unambiguous answer to the conundrum. By using high-resolution images taken between 6 August 2014 and 17 March 2015 to study the layers of material seen all over the nucleus, they have shown that the shape arose from a low-speed collision between two fully fledged, separately formed comets.
- “It is clear from the images that both lobes have an outer envelope of material organized in distinct layers, and we think these extend for several hundred meters below the surface,” says Matteo Massironi, lead author from the University of Padova, Italy, and an associate scientist of the OSIRIS team. “You can imagine the layering a bit like an onion, except in this case we are considering two separate onions of differing size that have grown independently before fusing together.”
- The results of the study are reported in the journal Nature and were presented on Sept. 28, 2015 at the European Planetary Science Congress in Nantes, France. 174)
Legend to Figure 119: A selection of high-resolution OSIRIS images used to identify patterns in Comet 67P/Churyumov–Gerasimenko’s extensive layering.
- Top left: main terraces (green) and exposed layers (red dashed lines) seen in the Seth region on the comet’s large lobe. The terraces become more inclined towards the comet neck region. The close-up shows terraces in two locations (thin white and yellow arrows) together with examples of parallel lineaments (large white arrows) that define a continuous stratification.
- Bottom left: outline of exposed layers (red dashed lines) primarily in the Imhotep and Ash region on the comet’s large lobe. The terraces in Ash change their dip direction from that in Seth to very slightly dip towards Imhotep. Some layers are also indicated on the comet’s small lobe in the background. The close-up shows the details of the parallel layers in a section along the Imhotep-Ash boundary.
- Top right: main layers (red dashed lines) and cross-cutting fractures (blue dashed lines) in the Hathor cliff face on the comet’s small lobe. No abrupt change in the orientation of the layers is seen between Hathor and Ma’at. The close-up shows stratification in an alcove at the Hathor-Anuket boundary, providing a view of the Anuket inner structure, which appears to extend under Ma’at. Terraces on Anuket (white arrows) are seen in different orientations to neighboring regions. Taken together, this reinforces the idea that Hathor represents the inner comet structure that has been exposed, with Anuket as the remnant.
- Bottom right: layers (white dashed lines) at the boundary of Anubis and Seth on the comet’s large lobe. This continuous scarp suggests the thickness of the Seth region is about 150 m. The three arrow heads point to a terrace margin in Anubis and the single white arrow points to a terrace in the adjacent Atum region.
To reach their conclusion, Matteo and his colleagues first used images to identify over 100 terraces seen on the surface of the comet, and parallel layers of material clearly seen in exposed cliff walls and pits. A 3D shape model was then used to determine the directions in which they were sloping and to visualize how they extend into the subsurface.
It soon became clear that the features were coherently oriented all around the comet’s lobes and in some places extended to a depth of about 650 m. “This was the first clue that the two lobes are independent, reinforced by the observation that the layers are inclined in opposite directions close to the comet’s neck,” says Matteo. “To be sure, we also looked at the relationship between the local gravity and the orientations of the individual features all around the reconstructed comet surface.”
Broadly speaking, layers of material should form at right angles to the gravity of an object. The team used models to compute the strength and direction of the gravity at the location of each layer. In one case, they modelled the comet as a single body with a center of mass close to the neck. In the other, they worked with two separate comets, each with its own center of mass. The team found that orientation of a given layer and the direction of the local gravity are closer to perpendicular in the model with two separate objects, rather than in the one with a single combined nucleus (Ref. 173).
Legend to Figure 120: The methods used by Rosetta scientists to determine that Comet 67P/Churyumov–Gerasimenko’s shape arises from two separately forming comets.
- Left: high-resolution OSIRIS images were used to visually identify over 100 terraces (green) or strata – parallel layers of material (red dashed lines) – in exposed cliff walls and pits all over the comet surface (top: Hathor and surrounding regions on comet’s small lobe; bottom: Seth region on comet’s large lobe).
- Middle: a 3D shape model was used to determine the directions in which the terraces/strata are sloping and to visualize how they extend into the subsurface. The strata ‘planes’ are shown superimposed on the shape model (left panel) and alone (right panel) and show the planes coherently oriented all around the comet, in two separate bounding envelopes (scale bar indicates angular deviation between plane and local gravity vector).
- Right: local gravity vectors visualized on the comet shape model perpendicular to the terrace/strata planes further realize the independent nature of the two lobes (Ref. 173).
• Sept. 23, 2015: Comets are celestial bodies comprising a mixture of dust and ices, which they periodically shed as they swing towards their closest point to the Sun along their highly eccentric orbits. As sunlight heats the frozen nucleus of a comet, the ice in it – mainly water but also other 'volatiles' such as carbon monoxide and carbon dioxide – turns directly into a gas. This gas flows away from the comet, carrying dust particles along. Together, gas and dust build up the bright halo and tails that are characteristic of comets. 175)
- Rosetta arrived at Comet 67P/Churyumov–Gerasimenko in August 2014 and has been studying it up close for over a year. On 13 August 2015, the comet reached the closest point to the Sun along its 6.5-year orbit, and is now moving back towards the outer Solar System.
- A key feature that Rosetta's scientists are investigating is the way in which activity on the comet and the associated outgassing are driven, by monitoring the increasing activity on and around the comet since Rosetta's arrival.
- Scientists using Rosetta's Visible, InfraRed and Thermal Imaging Spectrometer, VIRTIS, have identified a region on the comet's surface where water ice appears and disappears in sync with its rotation period (Figure 121). Their findings are published in the journal Nature. The results are based on images and spectra taken at visible and infrared wavelengths of light on 12–14 September 2014 with VIRTIS. 176)
- The research team found a mechanism that replenishes the surface of the comet with fresh ice at every rotation: this keeps the comet 'alive'," says Maria Cristina De Sanctis from INAF-IAPS in Rome, Italy. The team studied a set of data taken in September 2014, concentrating on a 1 km2 region on the comet's neck. At the time, the comet was about 500 million km from the Sun and the neck was one of the most active areas. As the comet rotates, taking just over 12 hours to complete a full revolution, the various regions undergo different illumination. "We saw the tell-tale signature of water ice in the spectra of the study region but only when certain portions were cast in shadow," says Maria Cristina. "Conversely, when the Sun was shining on these regions, the ice was gone. This indicates a cyclical behavior of water ice during each comet rotation."
- The data suggest that water ice on and a few cm below the surface 'sublimates' when illuminated by sunlight, turning it into gas that then flows away from the comet. Then, as the comet rotates and the same region falls into darkness, the surface rapidly cools again. However, the underlying layers remain warm owing to the sunlight they received in the previous hours, and, as a result, subsurface water ice keeps sublimating and finding its way to the surface through the comet's porous interior. But as soon as this 'underground' water vapor reaches the cold surface, it freezes again, blanketing that patch of comet surface with a thin layer of fresh ice. - Eventually, as the Sun rises again over this part of the surface on the next comet day, the molecules in the newly formed ice layer are the first to sublimate and flow away from the comet, restarting the cycle.
- "We suspected such a water ice cycle might be at play at comets, on the basis of theoretical models and previous observations of other comets but now, thanks to Rosetta's extensive monitoring at 67P/Churyumov–Gerasimenko, we finally have observational proof," says Fabrizio Capaccioni, VIRTIS principal investigator at INAF-IAPS in Rome, Italy. From these data, it is possible to estimate the relative abundance of water ice with respect to other material. Down to a few cm deep over the region of the portion of the comet nucleus that was surveyed, water ice accounts for 10–15% of the material and appears to be well-mixed with the other constituents.
- The scientists also calculated how much water vapor was being emitted by the patch that they analyzed with VIRTIS, and showed that this accounted for about 3% of the total amount of water vapor coming out from the whole comet at the same time, as measured by Rosetta's MIRO microwave sensor. "It is possible that many patches across the surface were undergoing the same diurnal cycle, thus providing additional contributions to the overall outgassing of the comet," adds Fabrizio Capaccioni. The scientists are now busy analyzing VIRTIS data collected in the following months, as the comet's activity increased around the closest approach to the Sun.
- Left, top: Comet 67P/Churyumov-Gerasimenko based on four images taken by Rosetta's navigation camera on 2 September 2014.
- Left, bottom: images of Comet 67P/Churyumov–Gerasimenko taken with Rosetta's Visible, InfraRed and Thermal Imaging Spectrometer, VIRTIS (left), and maps of water ice abundance (middle) and surface temperature (right).
- The images were taken on 12 (top), 13 (middle) and 14 September (bottom) and focus on Hapi, a region on the comet's 'neck', one of the most active spots on the nucleus at the time.
- By comparing these images and maps, the scientists have found that water ice is present on colder patches, while it is less abundant or absent on warmer patches. In addition, water ice was only detected on a patch of the surface when it was cast in shadow. This indicates a cyclical behavior of water ice during each comet rotation.
- Right: the daily water ice cycle. During the local day, water ice on and a few centimeters below the surface sublimates and escapes; during the local night, the surface rapidly cools while the underlying layers are still warm, so subsurface water ice continues sublimating and finding its way to the surface, where it freezes again. On the next comet day, sublimation starts again, beginning from water ice in the newly formed surface layer.
• Sept. 15, 2015: New results from PTOLEMY – the OU (Open University) led instrument on the Rosetta mission’s Philae lander, suggest that Comet 67P/Churyumov-Gerasimenko may be giving of different gases from different parts of its surface, making it heterogeneous in nature. PTOLEMY – the gas analysis instrument on board Philae, has taken measurements of the concentration of volatile molecules at the lander’s final resting site, Abydos. Its findings have shown the presence of both water (H2O) and carbon dioxide (CO2), but of very little carbon monoxide (CO). These findings follow the first set of results published by the Ptolemy team last month which reported the presence of organic compounds in the surface dust on Comet 67P. 178) 179)
- The Ptolemy team have been surprised by the results as, based on the findings of the ROSINA instrument on board the Rosetta orbiter, they were expecting to see larger concentrations of CO on the surface. ROSINA, like PTOLEMY, is a mass spectrometer and at the time of landing was analyzing the gases rising from the surface some 30 km above Comet 67P. Results by ROSINA, acquired shortly before landing (published in January 2015), found that the concentration of CO, although variable, was up to four times that of CO2, whereas the PTOLEMY measurements found that CO was about ten times less than CO2.
- According to Andrew Morse, lead author on the paper, these findings could suggest that either the coma gas composition changes through various chemical reactions as it moves away from the comet, or that the gas vaporised from the comet varies by location, making it a heterogeneous comet. He says: “Though it is a possibility that carbon monoxide is produced in the coma as it moves away from the comet, a more probable account of such a large change would be that the gases released from the comet’s surface differs according to location.”
- One hypothesis is that a heterogeneous comet is the result of it being accumulated from diverse building blocks during its formation in the solar system. Alternatively it is the result of uneven heating in its journey into the inner solar system. The ROSINA instrument could help to answer this by making further measurements of the water coming off the comet’s surface. Andrew Morse adds: “Our results, from Comet 67P’s surface, has both surprised us as well as opened up a variety of new questions about how comets form and how they work. We’re eagerly awaiting new results which should help us to clarify whether Comet 67P is indeed heterogeneous in nature or if there is another explanation. Either way, these results offer up an important piece of the complex, yet fascinating puzzle of how comets are formed.”
- Co-author Geraint Morgan says: “The questions raised by PTOLEMY show the value of landers, even high risk ones, in establishing a ‘ground truth’ measurement on the surface to compare with on-going measurements from orbiting spacecraft. Despite Philae’s eventful journey, the data produced could be the key to help the Rosetta mission unlock the secrets of the Solar System.”
• August 13: 2015: ESA’s Rosetta today witnessed Comet 67P/Churyumov–Gerasimenko making its closest approach to the Sun. The exact moment of perihelion occurred at 02:03 GMT this morning when the comet came within 186 million km of the Sun. 180)
- In the year that has passed since Rosetta arrived, the comet has travelled some 750 million km along its orbit towards the Sun, the increasing solar radiation heating up the nucleus and causing its frozen ices to escape as gas and stream out into space at an ever greater rate. These gases, and the dust particles that they drag along, build up the comet’s atmosphere – coma – and tail.
- The activity reaches its peak intensity around perihelion and in the weeks that follow – and is clearly visible in the spectacular images returned by the spacecraft in the last months. One image taken by Rosetta’s navigation camera was acquired at 01:04 GMT, just an hour before the moment of perihelion, from a distance of around 327 km.
- The scientific camera is also taking images today – the most recent available image was taken at 23:31 GMT on 12 August, just a few hours before perihelion. The comet’s activity is clearly seen in the images, with a multitude of jets stemming from the nucleus, including one outburst captured in an image taken at 17:35 GMT yesterday.
Figure 122: This series of images of Comet 67P/Churyumov–Gerasimenko was captured by Rosetta’s OSIRIS narrow-angle camera on 12 August 2015, just a few hours before the comet reached the closest point to the Sun along its 6.5-year orbit, or perihelion (image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)
Legend to Figure 122: The image at left was taken at 14:07 GMT, the middle image at 17:35 GMT, and the final image at 23:31 GMT. The images were taken from a distance of about 330 km from the comet. The comet’s activity, at its peak intensity around perihelion and in the weeks that follow, is clearly visible in these spectacular images. In particular, a significant outburst can be seen in the image captured at 17:35 GMT.
- “Activity will remain high like this for many weeks, and we’re certainly looking forward to seeing how many more jets and outburst events we catch in the act, as we have already witnessed in the last few weeks,” says Nicolas Altobelli, acting Rosetta project scientist.
- Rosetta’s measurements suggest the comet is spewing up to 300 kg of water vapor – roughly the equivalent of two bathtubs – every second. This is a thousand times more than was observed this time last year when Rosetta first approached the comet. Then, it recorded an outflow rate of just 300 g per second, equivalent to two small glasses of water.
- Along with gas, the nucleus is also estimated to be shedding up to 1000 kg of dust per second, creating dangerous working conditions for Rosetta.
• August 11, 2015: In the approach to perihelion over the past few weeks, Rosetta has been witnessing growing activity from Comet 67P/Churyumov–Gerasimenko, with one dramatic outburst event proving so powerful that it even pushed away the incoming solar wind . 181)
- The period around perihelion is scientifically very important, as the intensity of the sunlight increases and parts of the comet previously cast in years of darkness are flooded with sunlight. Although the comet’s general activity is expected to peak in the weeks following perihelion, much as the hottest days of summer usually come after the longest days, sudden and unpredictable outbursts can occur at any time – as already seen earlier in the mission.
- On 29 July, Rosetta observed the most dramatic outburst yet, registered by several of its instruments from their vantage point 186 km from the comet. They imaged the outburst erupting from the nucleus, witnessed a change in the structure and composition of the gaseous coma environment surrounding Rosetta, and detected increased levels of dust impacts (Figure 124). Perhaps most surprisingly, Rosetta found that the outburst had pushed away the solar wind magnetic field from around the nucleus.
- A sequence of images taken by Rosetta’s scientific camera OSIRIS show the sudden onset of a well-defined jet-like feature emerging from the side of the comet’s neck, in the Anuket region. It was first seen in an image taken at 13:24 GMT, but not in an image taken 18 minutes earlier, and has faded significantly in an image captured 18 minutes later. The camera team estimates the material in the jet to be travelling at 10 m/s at least, and perhaps much faster. “This is the brightest jet we’ve seen so far,” comments Carsten Güttler, OSIRIS team member at the MPS (Max Planck Institute for Solar System Research) in Göttingen, Germany.
- The decrease in magnetic field strength was measured by Rosetta’s RPC-MAG instrument during the outburst event on 29 July 2015. This is the first time a ‘diamagnetic cavity’ has been detected at Comet 67P/Churyumov–Gerasimenko and is thought to be caused by an outburst of gas temporarily increasing the gas flux in the comet’s coma, and pushing the pressure-balance boundary between it and incoming solar wind farther from the nucleus than expected under ‘normal’ levels of activity (Figure 123).
Figure 124: Rosetta’s scientific camera OSIRIS shows the sudden onset of a well-defined jet-like feature emerging from the side of the comet’s neck, in the Anuket region on July 29, 2015 (image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)
- Soon afterwards, the comet pressure sensor of ROSINA detected clear indications of changes in the structure of the coma, while its mass spectrometer recorded changes in the composition of outpouring gases. For example, compared to measurements made two days earlier, the amount of carbon dioxide increased by a factor of two, methane by four, and hydrogen sulphide by seven, while the amount of water stayed almost constant.
- During an outburst of gas and dust from Comet 67P/Churyumov–Gerasimenko on 29 July 2015, Rosetta’s ROSINA instrument detected a change in the composition of gases compared with previous days. Figure 125 shows the relative abundances of various gases after the outburst, compared with the measurements two days earlier. For example, the amount of carbon dioxide (CO2) increased by a factor of two, methane (CH4) by four, and hydrogen sulphide (H2S) by seven, while the amount of water (indicated by the horizontal black line) stayed almost constant.
• August 10, 2015: What a difference a year can make. Rosetta arrived at Comet 67P/Churyumov–Gerasimenko on 6 August 2014, achieving rendezvous at a distance of 100 km before moving even closer to the nucleus in the following weeks. The image shown on the left was taken with the navigation camera, NAVCAM, on rendezvous day, when Rosetta was about 121 km out. 182)
Figure 126: Images of Comet 67P/Churyumov–Gerasimenko on 6 August 2014 (left) and on 6 August 2015 (right) with increased exposure to the Sun’s energy and its resulting activity (image credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0)
• On August 6, 2015, ESA’s Rosetta mission celebrates one year at Comet 67P/Churyumov–Gerasimenko, with its closest approach to the Sun now just one week away. 183)
- It’s been a long but exciting journey for Rosetta since its launch in 2004, featuring Earth, Mars and two asteroid flybys before arriving at its ultimate destination on 6 August 2014. Over the following months, the mission became the first ever to orbit a comet and the first to soft land a probe – Philae – on its surface.
• July 30, 2015: Complex molecules that could be key building blocks of life, the daily rise and fall of temperature, and an assessment of the surface properties and internal structure of the comet are just some of the highlights of the first scientific analysis of the data returned by Rosetta's lander Philae last November. 184)
- Early results from Philae's first suite of scientific observations of Comet 67P/Churyumov-Gerasimenko were published today in a special edition of the journal Science (Science Special Issue, July 31, 2015). 185)
- Data were obtained during the lander's seven-hour descent to its first touchdown at the Agilkia landing site, which then triggered the start of a sequence of predefined experiments. But shortly after touchdown, it became apparent that Philae had rebounded and so a number of measurements were carried out as the lander took flight for an additional two hours some 100 m above the comet, before finally landing at Abydos.
- Meanwhile, Ptolemy sampled ambient gas entering tubes at the top of the lander and detected the main components of coma gases – water vapor, carbon monoxide and carbon dioxide, along with smaller amounts of carbon-bearing organic compounds, including formaldehyde.
- A timeline of the science operations that Rosetta's lander Philae performed between 12 and 15 November 2014, following touchdown on the surface of Comet 67P/Churyumov–Gerasimenko. This is an update on the original first science sequence. Following Philae's unexpected flight across the surface of the comet, the planned first science sequence had to be adapted according to the new situation. The graphic shows the approximate times (to the nearest 15 minutes) that each of Philae's 10 instruments was activated; however, it does not indicate the success of data acquired. 186)
Figure 127: Philae's adapted first science sequence (image credit: ESA)
• July 20, 2015: On 9 July 2015 at 19:45 CEST, Philae reported back to the team at DLR/LCC (Lander Control Center) only to then go back to 'silent mode'. Since then, the team has been working hard to get back in contact with the lander and operate it to conduct scientific measurements. "We sent a command to turn on the ROMAP (Rosetta Lander Magnetometer and Plasma Monitor), but have not seen a response," explains DLR's Philae project leader Stephan Ulamec. Using an identical model in the MUSC (Microgravity User Support Center) at DLR, the engineers are currently testing various commands, with which they want to enable and optimize Philae. "In the telemetry received, we have observed signs that Philae could have moved and that its antennas are thus perhaps more concealed or their orientation might have changed." 187)
Figure 128: Testing with the Philae lander ground model (image credit: DLR)
- Philae's move: In the data previously sent by Philae from the surface of Comet 67P/Churyumov-Gerasimenko about its condition, the lander has also transmitted information about the sunlight reaching its solar panels. “This profile – where panels are receiving a great deal of sunlight – has clearly changed between June and July,” says Ulamec. “This cannot be explained only by the course of the seasons on the comet.” The lander could have moved, for example, due to outgassing during the comet's awakening. After a not entirely smooth landing on 12 November 2014, Philae finally halted at a crater rim on uneven terrain – for this reason, even a slight change in its position could mean that its antennas are now obstructed by more objects above it. This would affect communication with Philae.
- Blind commands as backup: It is also possible that one of the lander's two radio receiver units is damaged and that one of the transmitter units is not fully functional. However, Philae is programmed to switch back and forth between the two transmitters periodically. This could also explain why contact with Philae is irregular. "We have therefore tested a command on our ground model that will cause Philae to only interact with the functional transmitter." This command has been transmitted to the lander. This 'blind commanding' – without the lander sending a confirmation – should make it possible for it to receive the command and execute it as soon as it is supplied with solar energy during the comet day and switches on.
- The engineers at the LCC are also testing another command on the ground model of Philae; they want to try to activate a 'work package' on the lander that was successfully executed in November 2014 during the landing and is still stored by Philae. At that time, the team at the LCC had supplied the lander with a kind of 'emergency program', so that it could still operate five instruments without communication. This occurred as the engineers at the consoles had to adjust their plans to adapt to the evolving situation with a new landing site. "With this work package, the thermal probe MUPUS measured temperatures, ROMAP and SESAME conducted measurements, and PTOLEMY and COSAC researched in 'sniff' mode," says Ulamec. "All of these instruments require no detailed commands, but the stored work package must first of all be retrieved." If this idea works, once Philae switches on, it would start to conduct scientific measurements and then send the data to Earth.
- Interaction between lander and orbiter: Until 24 July 2015, the Rosetta orbiter will fly an orbit that satisfies the requirements of the lander and follow a path that is favorable for communication between the two spacecraft. Then, Rosetta will fly with its 11 instruments over the southern hemisphere of the comet, which is now increasingly illuminated by the Sun. Here, the attempts to communicate with Philae will alternate with the priorities for observation with the orbiter instruments. The comet's increasing activity – with its gas and dust ejections –does not allow the orbiter to fly very close to the comet's surface. On 12 July 2015, Rosetta’s star trackers were once again affected by the dusty environment. For this reason, the orbiter is now flying at a safer distance of 170–190 km.
- Of course, the Philae lander team at DLR has not given up. "The lander is obviously still functional, because it sends us data, albeit at irregular intervals and at surprising times," says Ulamec. "There have been several times when we feared that the lander would not switch back on, but it has repeatedly taught us otherwise."
• July 13, 2015: Rosetta’s investigations of its comet are continuing as the mission teams count down the last month to perihelion – the closest point to the Sun along the comet’s orbit – when the comet’s activity is expected to be at its highest.“Perihelion is an important milestone in any comet’s calendar, and even more so for the Rosetta mission because this will be the first time a spacecraft has been following a comet from close quarters as it moves through this phase of its journey around the Solar System,” notes Matt Taylor, ESA’s Rosetta project scientist. “We’re looking forward to reaching perihelion, after which we’ll be continuing to monitor how the comet’s nucleus, activity and plasma environment changes in the year after, as part of our long-term studies.” 188)
Figure 129: The orbit of Comet 67P/Churyumov-Gerasimenko and its approximate location around perihelion, the closest the comet gets to the Sun. The positions of the planets are correct for 13 August 2015 (image credit: ESA)
• July 10, 2015: The Philae lander communicated with the Rosetta orbiter again between 19:45 and 20:07 CEST (Central European Standard Time) on 9 July 2015 and transmitted measurement data from the CONSERT (COmet Nucleus Sounding Experiment by Radiowave Transmission) instrument. Although the connection failed repeatedly after that, it remained completely stable for those 12 minutes. "This sign of life from Philae proves to us that at least one the lander's communication units remains operational and receives out commands," said German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) engineer Koen Geurts, a member of the lander control team at DLR Cologne. The mood had been mixed over the last few days; Philae had not communicated with the team in the DLR/LCC (Lander Control Center) since 24 June 2015. After an initial test command to turn on the power to CONSERT on 5 July 2015, the lander did not respond. Philae’s team began to wonder if the lander had survived on Comet 67P/Churyumov-Gerasimenko. 189)
- Commanded from the ground successfully: "We never gave up on Philae and remained optimistic," said Geurts. There was great excitement when Philae 'reported in' on 13 June 2015 after seven months of hibernation and sent data about its health. The lander was ready to perform its tasks, 300 million km away from Earth. However, Philae has to communicate with the ground stations through Rosetta, which acts as a radio relay. Restrictions on the orbiter's approach, orbiting around the comet, have not permitted regular communication with the lander. The data sent on 24 June did not suggest that the lander had experienced technical difficulties. Now, Philae's internal temperature of 0ºC gives the team hope that the lander can charge its batteries; this would make scientific work possible regardless of the 'time of day' on the comet.
- Currently, DLR's lander team is evaluating the data that were received. "We can already see that the CONSERT instrument was successfully activated by the command we sent on 9 July," explained Geurts. Even now, Philae is causing the team some puzzlement: "We do not yet have an explanation for why the lander has communicated now, but not over the past few days.” The trajectory of the orbiter, for example, has not changed over the last three weeks. However, one thing is certain; Philae has survived the harsh conditions on the comet and is responding to commands from the LCC team. "This is extremely good news for us," said Geurts.
• July 1, 2015: A number of the dust jets emerging from Rosetta’s comet can be traced back to active pits that were likely formed by a sudden collapse of the surface. These ‘sinkholes’ are providing a glimpse at the chaotic and diverse interior of the comet. 190)
- In a study reported in the science journal Nature, 18 quasi-circular pits have been identified in the northern hemisphere of the comet, some of which are the source of continuing activity. The pits are a few tens to a few hundreds of meters in diameter and extend up to 210 m below the surface to a smooth dust-covered floor. Material is seen to be streaming from the most active pits. The study team observes jets arising from the fractured areas of the walls inside the pits. These fractures mean that volatiles trapped under the surface can be warmed more easily and subsequently escape into space, according to Jean-Baptiste Vincent from the Max Planck Institute for Solar System Research, lead author of the study. 191)
Left: 18 pits have been identified in high-resolution OSIRIS images of Comet 67P/Churyumov–Gerasimenko’s northern hemisphere. The pits are named after the region they are found in, and some of them are active. The context image was taken on 3 August 2014 by the narrow-angle camera from a distance of 285 km; the image resolution is 5.3 m/pixel.
Middle, top: close-up of the active pit named Seth_01 reveals small jets emanating from the interior walls of the pit. The close-up also shows the complex internal structure of the comet. The image is a section of an OSIRIS wide-angle camera image capture on 20 October 2014 from a distance of 7 km from the comet surface. Seth_01 measures about 220 m across.
Right, top: context image showing fine structure in the comet’s jets as seen from a distance of 28 km from the comet’s surface on 22 November 2014. The image was taken with the OSIRIS wide-angle camera and has a resolution is 2.8 m/pixel. In both images the contrast is deliberately stretched in order to see the details of the activity. The active pits in this study contribute a small fraction of the observed activity.
Left, bottom: how the pits may form through sinkhole collapse. 1. Heat causes subsurface ices to sublimate (blue arrows), forming a cavity (2). When the ceiling becomes too weak to support its own weight, it collapses, creating a deep, circular pit (3, red arrow). Newly exposed material in the pit walls sublimates, accounting for the observed activity (3, blue arrows).
• June 26. 2015: Despite a new trajectory for Rosetta and a reduction of the distance between the orbiter and Comet 67P/Churyumov-Gerasimenko from 200 to 180 km, contact with the Philae lander remains irregular and short. After the initial contact on 13 June 2015, Philae has reported to the DLR/LCC (Lander Control Center) in Cologne a total of six times. However, for the last three possibilities calculated for establishing a connection with Philae, no data could be received. "Right now, we are playing with the geometry between the Rosetta orbiter and the Philae lander," says DLR's Philae Project Manager, Stephan Ulamec. "The most recent contact – on 24 June 2015 – lasted 20 minutes; then, the line went dead again." Now, the DLR and ESA mission teams are analyzing which measures will make better contact with Philae possible. 193)
- On June 27, ESA will begin new maneuvers, which will move the Rosetta orbiter 20 km closer to the comet's surface and Philae by 30 June 2015. The team at the DLR control center hopes that contact with Philae at a distance of 160 km will then be regular and stable. The next few days will show whether changes in the geometry between the lander and orbiter improves communication with Philae.
- One possible reason for the lander's current silence could be a failure of Philae's communications equipment caused by poor conditions during hibernation. Analysis of the data received so far by the team at DLR has shown that while one of the communications units is compromised, the other unit has worked thus far without problems. "To continue conducting scientific work with Philae, we rely on long and predictable contact times," says Ulamec. Once Philae can receive and execute extensive command sequences safely, store the measurement data and send it to the ground team, its 10 instruments will be operated again.
• June 24, 2015: Exposed water ice detected on Comet's surface. Using the high-resolution science OSIRIS camera on board ESA’s Rosetta spacecraft, scientists have identified more than a hundred patches of water ice a few meters in size on the surface of Comet 67P/Churyumov-Gerasimenko. A new study just published in the journal Astronomy & Astrophysics focuses on an analysis of bright patches of exposed ice on the comet’s surface. 194) 195)
- Based on observations of the gas emerging from comets, they are known to be rich in ices. As they move closer to the Sun along their orbits, their surfaces are warmed and the ices sublimate into gas, which streams away from the nucleus, dragging along dust particles embedded in the ice to form the coma and tail.
- But some of the comet’s dust also remains on the surface as the ice below sublimates, or falls back on to the nucleus elsewhere, coating it with a thin layer of dusty material and leaving very little ice directly exposed on the surface. These processes help to explain why Comet 67P/Churyumov-Gerasimenko and other comets seen in previous flyby missions are so dark.
- Despite this, Rosetta’s suite of instruments has already detected a variety of gases, including water vapor, carbon dioxide and carbon monoxide, thought to originate from frozen reservoirs below the surface.
- Now, using images taken with Rosetta’s OSIRIS narrow-angle camera last September, scientists have identified 120 regions on the surface of Comet 67P/Churyumov-Gerasimenko that are up to ten times brighter than the average surface brightness. Some of these bright features are found in clusters, while others appear isolated, and when observed at high resolution, many of them appear to be boulders displaying bright patches on their surfaces.
- The clusters of bright features, comprising a few tens of meter-sized boulders spread over several tens of meters, are typically found in debris fields at the base of cliffs. They are most likely the result of recent erosion or collapse of the cliff wall revealing fresher material from below the dust-covered surface.
- By contrast, some of the isolated bright objects are found in regions without any apparent relation to the surrounding terrain. These are thought to be objects lifted up from elsewhere on the comet during a period of cometary activity, but with insufficient velocity to escape the gravitational pull of the comet completely.
- In all cases, however, the bright patches were found in areas that receive relatively little solar energy, such as in the shadow of a cliff, and no significant changes were observed between images taken over a period of about a month. Furthermore, they were found to be bluer in color at visible wavelengths compared with the redder background, consistent with an icy component.
- “Water ice is the most plausible explanation for the occurrence and properties of these features,” says Antoine Pommerol of the University of Bern and lead author of the study. “At the time of our observations, the comet was far enough from the Sun such that the rate at which water ice would sublimate would have been less than 1 mm per hour of incident solar energy. By contrast, if carbon dioxide or carbon monoxide ice had been exposed, it would have rapidly sublimated when illuminated by the same amount of sunlight. Thus we would not expect to see that type of ice stable on the surface at this time.”
- The team also turned to laboratory experiments that tested the behavior of water ice mixed with different minerals under simulated solar illumination in order to gain more insights into the process. They found that after a few hours of sublimation, a dark dust mantle a few millimeters thick was formed. In some places this acted to completely conceal any visible traces of the ice below, but occasionally larger dust grains or chunks would lift from the surface and move elsewhere, exposing bright patches of water ice. “A 1 mm thick layer of dark dust is sufficient to hide the layers below from optical instruments,” confirms Holger Sierks, OSIRIS principal investigator at the Max Planck Institute for Solar System Research in Göttingen.
- The team also speculates about the timing of the formation of the icy patches. One hypothesis is that they were formed at the time of the last closest approach of the comet to the Sun, 6.5 years ago, with icy blocks ejected into permanently shadowed regions, preserving them for several years below the peak temperature needed for sublimation.
- Another idea is that even at relatively large distances from the Sun, carbon dioxide and carbon monoxide driven-activity could eject the icy blocks. In this scenario, it is assumed that the temperature was not yet high enough for water sublimation, such that the water-ice-rich components outlive any exposed carbon dioxide or carbon monoxide ice.
- “As the comet continues to approach perihelion, the increase in solar illumination onto the bright patches that were once in shadow should cause changes in their appearance, and we may expect to see new and even larger regions of exposed ice,” says Matt Taylor, ESA’s Rosetta project scientist. “Combining OSIRIS observations made pre- and post-perihelion with other instruments will provide valuable insight into what drives the formation and evolution of such regions.”
Figure 131: Ice on Comet 67P/Churyumov-Gerasimenko
• June 23, 2015: The adventure continues: ESA confirmed that its Rosetta mission will be extended until the end of September 2016, at which point the spacecraft will most likely be landed on the surface of Comet 67P/Churyumov-Gerasimenko.Rosetta’s nominal mission was originally funded until the end of December 2015, but at a meeting today, ESA’s Science Program Committee has given formal approval to continue the mission for an additional nine months. At that point, as the comet moves far away from the Sun again, there will no longer be enough solar power to run Rosetta’s set of scientific instrumentation efficiently. 196)
- “This is fantastic news for science,” says Matt Taylor, ESA’s Rosetta Project Scientist. “We’ll be able to monitor the decline in the comet’s activity as we move away from the Sun again, and we’ll have the opportunity to fly closer to the comet to continue collecting more unique data. By comparing detailed ‘before and after’ data, we’ll have a much better understanding of how comets evolve during their lifetimes.”
- Comet 67P/Churyumov-Gerasimenko will make its closest approach to the Sun on 13 August and Rosetta has been watching its activity increase over the last year. Continuing its study of the comet in the year following perihelion will give scientists a fuller picture of how a comet’s activity waxes and wanes along its orbit.
- As the activity diminishes post-perihelion, it should be possible to move the orbiter much closer to the comet’s nucleus again, to make a detailed survey of changes in the comet’s properties during its brief ‘summer’. In addition, there may be an opportunity to make a definitive visual identification of Philae. Although candidates have been seen in images acquired from a distance of 20 km, images taken from 10 km or less after perihelion could provide the most compelling confirmation.
- During the extended mission, the team will use the experience gained in operating Rosetta in the challenging cometary environment to carry out some new and potentially slightly riskier investigations, including flights across the night-side of the comet to observe the plasma, dust, and gas interactions in this region, and to collect dust samples ejected close to the nucleus.
- As the comet recedes from the Sun, the solar-powered spacecraft will no longer receive enough sunlight to operate efficiently and safely, equivalent to the situation in June 2011 when the spacecraft was put into hibernation for 31 months for the most distant leg of its journey out towards the orbit of Jupiter.
Figure 132: A Rosetta NAVCAM camera single frame image of Comet 67P/Churyumov-Gerasimenko, acquired on 5 June 2015 from a distance of 208 km from the comet center. The image has a resolution of 17.7 m/pixel and measures 18.1 km across (image credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0)
• June 19, 2015: The DLR team received data from the Philae lander for the third time on 19 June 2015. Between 13:20 and 13:39 UTC, Philae sent 185 data packets. "Among other things, we have received updated status information," says Michael Maibaum, a systems engineer at the DLR (Lander Control Center) in Cologne and Deputy Operations Manager. "At present, the lander is operating at a temperature of zero degrees Celsius, which means that the battery is now warm enough to store energy. This suggests that Philae will also be able to work during the comet's night, regardless of solar illumination." In the 19 minutes of transmission, the lander sent data recorded last week; from this, the engineers determined that the amount of sunlight has increased: "More solar panels were illuminated; at the end of contact, four of Philae's panels were receiving energy". There were a number of interruptions in the connection, but it was otherwise stable over a longer period for the first time. “The contact has confirmed that Philae is doing very well.” 197)
Figure 133: Photo of the LLC (Lander Control Center) at DLR (image credit: DLR)
• June 15, 2015: The receipt of signals from Rosetta's Philae lander on 13 June after 211 days of hibernation marked the start of intense activity. In coordination with its mission partners, ESA teams are working to juggle Rosetta's flight plan to help with renewed lander science investigations. 198)
- Since March 2015, when Philae's environmental conditions started to improve with higher surface temperatures and better illumination, the orbiter's receiver had been turned on periodically to listen for signals from the lander when the orbital geometry was thought to be optimum. On the evening of 13 June, a weak but solid radio link between Rosetta and the lander was finally established for 85 seconds. More than 300 'packets' (663 kbit) of lander housekeeping telemetry were received. This information had been stored on board at an as-yet-to-be determined time in the past, as much as several days to a few weeks, so does not necessarily reflect the lander's current status.
- "We are still examining the housekeeping information at the Lander Control Center in the DLR German Aerospace Center's establishment in Cologne, but we can already tell that all lander subsystems are working nominally, with no apparent degradation after more than half a year hiding out on the comet's frozen surface," says DLR's Stephan Ulamec, Philae Lander Project Manager.
- A second, smaller burst of lander data was received on Sunday, 14 June, at about 21:26 UTC, lasting just a few seconds. These data were confirmed to give the current status, showing the lander's internal temperature had already risen to –5ºC.
- Engineers at the Lander Control Center have determined that Philae is already being exposed to sufficient sunlight to heat it to an acceptable operating temperature and to generate electricity.
- "Power levels increase during the local 'comet day' – the part of the about-12 hour comet rotation when Philae is in sunlight – from 13 W at comet sunrise to above 24 W," notes ESA's Patrick Martin, Rosetta Mission Manager. "It needs at least 19 W to switch on the transmitter."
- The telemetry downloaded covered the lander's status for a full night–day cycle of the comet, which is helping ground teams to understand how the Sun is shining on the lander. The solar panels appear to be receiving power for over 135 minutes in each illumination period.
- The main task now for all the mission partners — ESA for Rosetta operations and DLR and France's CNES space agency for lander operations and science, respectively — is to determine how to optimize Rosetta's orbit so as to facilitate contact and enable new science investigations. It is believed that there is sufficient power now being generated to allow some science measurements during the time Philae is illuminated, with initial activities focusing on low-power measurements. This first phase would also likely include measurements that did not previously generate science in November.
- However, the mission teams first must establish a more robust link between Rosetta and Philae before uploading the first batch of science operations commands. The quality of the communication link is also possibly related to the trajectory Rosetta is flying and the orientation it adopts.
- Optimizing an orbit 305 million km away: Currently, Rosetta experiences two possible communication slots per 24 hours – once per 12-hour comet rotation. Until 23:35 UTC on Tuesday, 16 June, Rosetta will be flying an orbit set by already-uploaded commands on the terminator – the plane between comet day and night – moving out from about 200 km to 235 km altitude.
- This orbit is not optimized for lander communication, so longer periods of contact may not be possible until the trajectory has been changed. "With work done by the flight dynamics and operations team at ESOC and based on intense planning being conducted with the mission partners today, a new orbit will be devised that ensures optimum lander communications beginning with the next command upload later tonight," says Paolo Ferri, ESA's Head of Mission Operations.
- This new orbit will include an already-planned reduction of distance from the nucleus, down to 180 km versus 200 km, and 'nadir pointing' – continuously pointing Rosetta's communications unit at the comet. In the coming days, the orbiter may also be moved closer to the comet, without compromising the safety of the spacecraft, to help communications.
- The new orbit will be flown by Rosetta starting after 23:25 UTC on 16 June until 19 June, aiming to enable more and longer contacts with Philae, especially towards the end of this period. Establishing a regular and predictable pattern of contacts is a prerequisite for performing a complete assessment of the lander's status and for planning science operations. "If we manage to achieve and maintain a predictable contact pattern," continues Paolo Ferri, "the lander teams can devise a strategy for a new sequence of scientific operations.
- As a bonus, any operation of Philae's instruments up to or through perihelion on 13 August – the comet's closest point to the Sun along its orbit – will allow in-situ study of a comet during its peak activity.
• The Philae lander has reported back on 13 June 2015 at 20:28 (UTC), coming out of hibernation and sending the first data to Earth. More than 300 data packets have been analyzed by the team at the DLR ( German Aerospace Center) Lander Control Center: "Philae is doing very well – it has an operating temperature of minus 35º Celsius and has 24 W of power available," explains DLR’s Philae Project Manager, Stephan Ulamec. "The lander is ready for operations." Philae 'spoke' for 85 seconds with its team on ground in its first contact since it went into hibernation. The signals were also received at ESA/ESOC in Darmstadt, Germany. 199) 200)
- When analyzing the status data, it became clear that Philae also must have been awake earlier: "We have also received historical data – until now, however, the lander had not been able to contact us. "Now, the scientists are waiting for the next contact. In Philae's mass memory, there are still more than 8000 data packets, which will give the DLR team information on what happened to Philae in the past few days on comet Churyumov-Gerasimenko.
- Philae shut down on 15 November 2014 at 02:15 UTC, after being in operation on the comet for about 60 hours. Since 12 March 2015, the communication unit on the Rosetta orbiter has repeatedly been turned on to communicate with the lander and receive its reply.
• June 2, 2015: Rosetta's continued close study of Comet 67P/Churyumov–Gerasimenko has revealed an unexpected process at work, causing the rapid breakup of water and carbon dioxide molecules spewing from the comet’s surface. - One instrument, the Alice spectrograph provided by NASA, has been examining the chemical composition of the comet's atmosphere, or coma, at far-ultraviolet wavelengths. At these wavelengths, Alice allows scientists to detect some of the most abundant elements in the Universe such as hydrogen, oxygen, carbon and nitrogen. The spectrograph splits the comet's light into its various colors – its spectrum – from which scientists can identify the chemical composition of the coma gases. 201) 202)
- In a study, scientists from several institutions report the detections made by Alice from Rosetta’s first four months at the comet, when the spacecraft was between 10 km and 80 km from the center of the comet nucleus. The research team focused on the nature of 'plumes' of water and carbon dioxide gas erupting from the comet's surface, triggered by the warmth of the Sun. To do so, they looked at the emission from hydrogen and oxygen atoms resulting from broken water molecules, and similarly carbon atoms from carbon dioxide molecules, close to the comet nucleus. They discovered that the molecules seem to be broken up in a two-step process. 203)
- First, an ultraviolet photon from the Sun hits a water molecule in the comet’s coma and ionizes it, knocking out an energetic electron. This electron then hits another water molecule in the coma, breaking it apart into two hydrogen atoms and one oxygen, and energizing them in the process. These atoms then emit ultraviolet light that is detected at characteristic wavelengths by Alice.
- Similarly, it is the impact of an electron with a carbon dioxide molecule that results in its break-up into atoms and the observed carbon emissions. "Analysis of the relative intensities of observed atomic emissions allows us to determine that we are directly observing the ‘parent’ molecules that are being broken up by electrons in the immediate vicinity, about 1 km, of the comet’s nucleus where they are being produced," says Paul Feldman, professor of physics and astronomy at the Johns Hopkins University in Baltimore, and lead author of the paper discussing the results.
- By comparison, from Earth or from Earth-orbiting space observatories such as the Hubble Space Telescope, the atomic constituents of comets can only be seen after their parent molecules, such as water and carbon dioxide, have been broken up by sunlight, hundreds to thousands of kilometers away from the nucleus of the comet. "The discovery we’re reporting is quite unexpected," says Alice Principal Investigator Alan Stern, an associate vice-president in the Space Science and Engineering Division of the Southwest Research Institute (SwRI). "It shows us the value of going to comets to observe them up close, since this discovery simply could not have been made from Earth or Earth orbit with any existing or planned observatory. And, it is fundamentally transforming our knowledge of comets."
- The results from Alice are supported by data obtained by other Rosetta instruments, in particular MIRO, ROSINA and VIRTIS, which are able to study the abundance of different coma constituents and their variation over time, and particle detecting instruments like RPC-IES. "These early results from Alice demonstrate how important it is to study a comet at different wavelengths and with different techniques, in order to probe various aspects of the comet environment," says ESA’s Rosetta project scientist Matt Taylor. "We’re actively watching how the comet evolves as it moves closer to the Sun along its orbit towards perihelion in August, seeing how the plumes become more active due to solar heating, and studying the effects of the comet’s interaction with the solar wind."
- Studying the properties of a comet can provide clues to the role that magnetic fields played in the formation of Solar System bodies almost 4.6 billion years ago. The infant Solar System was once nothing more than a swirling disc of gas and dust but, within a few million years, the Sun burst into life in the center of this turbulent disc, with the leftover material going into forming the asteroids, comets, moons and planets. — The dust contained an appreciable fraction of iron, some of it in the form of magnetite. Indeed, millimeter-sized grains of magnetic materials have been found in meteorites, indicating their presence in the early Solar System.
Figure 134: Rosetta and Philae investigate magnetic properties of Comet 67P/C-G (image credit: ESA)
- This leads scientists to believe that magnetic fields threading through the proto-planetary disc could have played an important role in moving material around as it started to clump together to form larger bodies. But it remains unclear as to how crucial magnetic fields were later on in this accretion process, as the building blocks grew to centimeters, meters and then tens of meters across, before gravity started to dominate when they grew to hundreds of meters and kilometers in scale. Some theories concerning the aggregation of magnetic and non-magnetic dust particles show that the resulting bigger objects could also remain magnetised, allowing them to also be influenced by the magnetic fields of the proto-planetary disc.
- Because comets contain some of the most pristine materials in the Solar System, they offer a natural laboratory for investigating whether or not these larger chunks could have remained magnetized. However, detecting the magnetic field of comets has proven difficult in previous missions, which have typically made rapid flybys, relatively far from comet nuclei. — It has taken the proximity of ESA’s Rosetta orbiter to Comet 67P/Churyumov-Gerasimenko, and the measurements made much closer to and at the surface by its lander Philae, to provide the first detailed investigation of the magnetic properties of a comet nucleus.
• Reconstructing Philae’s trajectory: Magnetic field data from ROMAP on Philae, combined with information from the CONSERT experiment that provided an estimate of the final landing region, timing information, images from Rosetta’s OSIRIS camera, assumptions about the gravity of the comet, and measurements of its shape, have been used to reconstruct the trajectory of the lander during its descent and subsequent landings on and bounces over the surface of Comet 67P/Churyumov-Gerasimenko on 12 November 2014. The times are as recorded by the spacecraft; the confirmation signals arrived on Earth 28 minutes later. 206)
- Initially, Philae was seen to rotate slowly during the descent to Agilkia. It landed and then bounced, rotating significantly faster as the momentum of the internal flywheel was transferred to the lander. It collided with a cliff 45 minutes later, then tumbled, flying above the surface for more than an hour longer, before bouncing once again and coming to a stop a few meters away, a few minutes later.
- The position of the first touchdown point at Agilkia is very well determined from direct images, but the locations of the possible cliff collision depends on the ballistic model used, while the general location marked for the subsequent second and third touchdowns at Abydos come from the CONSERT measurements. Thus, these latter positions represent preliminary and approximate locations only. - The heights above the surface assume a reference sphere centered on the center of mass of the comet and with a radius of 2393 m reaching first touchdown point.
Figure 135: Reconstructing Philae's trajectory across Comet 67P/Churyumov-Gerasimenko (ESA/Data: Auster et al. (2015)/Comet image: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)
- Philae’s magnetic field measuring instrument is the ROMAP (Rosetta Lander Magnetometer and Plasma Monitor), while Rosetta carries a magnetometer as part of the RPC-MAP (Rosetta Plasma Consortium suite of sensors). Changes in the magnetic field surrounding Rosetta allowed RPC-MAG to detect the moment when Philae was deployed in the morning of 12 November 2014.
- Then, by sensing periodic variations in the measured external magnetic field and motions in its boom arm, ROMAP was able to detect the touchdown events and determine the orientation of Philae over the following hours. Combined with information from the CONSERT experiment that provided an estimate of the final landing site location, timing information, images from Rosetta’s OSIRIS camera, assumptions about the gravity of the comet, and measurements of its shape, it was possible to determine Philae’s trajectory.
- The mission teams soon discovered that Philae not only touched down once at Agilkia, but also came into contact with the comet’s surface four times in fact – including a grazing collision with a surface feature that sent it tumbling towards the final touchdown point at Abydos. This complex trajectory turned out to be scientifically beneficial to the ROMAP team.
- “The unplanned flight across the surface actually meant we could collect precise magnetic field measurements with Philae at the four points we made contact with, and at a range of heights above the surface,” says Hans-Ulrich Auster, co-principal investigator of ROMAP and lead author of the results published in the journal Science and presented at the European Geosciences Union General Assembly in Vienna, Austria (April 12-17, 2015). 207)
- The multiple descents and ascents meant that the team could compare measurements made on the inward and outward journeys to and from each contact point, and as it flew across the surface. ROMAP measured a magnetic field during these sequences, but found that its strength did not depend on the height or location of Philae above the surface. This is not consistent with the nucleus itself being responsible for that field.
- “If the surface was magnetized, we would have expected to see a clear increase in the magnetic field readings as we got closer and closer to the surface,” explains Hans-Ulrich. “But this was not the case at any of the locations we visited, so we conclude that Comet 67P/Churyumov-Gerasimenko is a remarkably non-magnetic object.”
- Instead, the magnetic field that was measured was consistent with an external one, namely the influence of the solar wind interplanetary magnetic field near the comet nucleus. This conclusion is confirmed by the fact that variations in the field that were measured by Philae closely agree with those seen at the same time by Rosetta.
- “During Philae’s landing, Rosetta was about 17 km above the surface, and we could provide complementary magnetic field readings that rule out any local magnetic anomalies in the comet’s surface materials,” says Karl-Heinz Glassmeier, principal investigator of RPC-MAG on board the orbiter and a co-author of the Science paper.
- If large chunks of material on the surface of 67P/Churyumov-Gerasimenko were magnetized, ROMAP would have recorded additional variations in its signal as Philae flew over them. “If any material is magnetized, it must be on a scale of less than one meter, below the spatial resolution of our measurements. And if Comet 67P/Churyumov-Gerasimenko is representative of all cometary nuclei, then we suggest that magnetic forces are unlikely to have played a role in the accumulation of planetary building blocks greater than one meter in size,” concludes Hans-Ulrich.
- “It’s great to see the complementary nature of Rosetta and Philae’s measurements, working together to answer this simple, but important ‘yes-no’ question as to whether the comet is magnetized,” says Matt Taylor, ESA’s Rosetta project scientist.
Legend to Figure 136: Magnetic field data collected by Philae’s ROMAP instrument immediately before (top) and after (bottom) the cliff collision at 16:20 GMT on 12 November 2014 (onboard spacecraft time), between the first and second touchdowns. Height above the surface is plotted on the x-axis and magnetic field strength on the y-axis. Therefore time runs left-to-right for the ascent (lower) plot, but right-to-left for the descent (upper) plot.
The measurements (crosses) are compared with a hypothetical model (solid line) assuming a slightly magnetized surface. Also included is the strength of and variation in the external field, namely the influence of the solar wind interplanetary magnetic field near the comet nucleus.
At distances of 10 m or greater from the surface, the surface component would be very weak, leaving just the external field, as measured. But closer to the surface, the comet’s own field should increase and dominate. That is not seen, therefore the data suggest that at scales of greater than one meter (the resolution of the instrument), the comet is not magnetized.
• April 13, 2015: Four months from today, on 13 August, Comet 67P/Churyumov-Gerasimenko will reach perihelion – a moment that defines its closest point to the Sun along its orbit. For 67P/Churyumov-Gerasimenko, this takes place at a distance of about 185 million km from the Sun, between the orbits of Earth and Mars. The Rosetta spacecraft is along for the ride, and has been watching the gradual evolution of the comet since arriving in August 2014. 208)
- As the comet’s surface layers are gently warmed, frozen ices sublimate. The escaping gas carries streams of dust out into space, and together these slowly expand to create the comet’s fuzzy atmosphere, or coma. — As the comet continues to move closer to the Sun, the warming continues and activity rises, and pressure from the solar wind causes some of the materials to stream out into long tails, one made of gas, the other of dust. The comet’s coma will eventually span tens of thousands of kilometers, while the tails may extend hundreds of thousands of kilometers, and both will be visible through large telescopes on Earth.
- But it is Rosetta’s close study of the comet, from just a few tens of kilometers above its surface, which enables the source of the comet’s activity to be studied in great detail, providing context to the more distant ground-based observations.
- This spectacular montage of 18 images (Figure 137) shows off the comet’s activity from many different angles as seen between 31 January (top left) and 25 March (bottom right), when the spacecraft was at distances of about 30 to 100 km from the comet. At the same time, Comet 67P/Churyumov-Gerasimenko was at distances between 363 million and 300 million km from the Sun. — After perihelion, Rosetta will continue to follow the comet, watching how the activity subsides as it moves away from the Sun and back to the outer Solar System again.
- This leads scientists to believe that magnetic fields threading through the proto-planetary disc could have played an important role in moving material around as it started to clump together to form larger bodies.
• March 20, 2015: Perhaps it is still too cold for the Philae lander to wake up on Comet 67P/Churyumov-Gerasimenko. Maybe its power resources are not yet sufficient to send a signal to the team at DLR (German Aerospace Center) Lander Control Center. On 12 March 2015, the Rosetta orbiter began to send signals to the lander and listen for a response, but Philae has not yet reported back. "It was a very early attempt; we will repeat this process until we receive a response from Philae," says DLR Project Manager Stephan Ulamec. "We have to be patient." On 20 March 2015 at 05:00 CET (Central European Time), the communication unit on the Rosetta orbiter was switched off. Now, the DLR team is calculating when the next favorable alignment between the orbiter and the lander will occur, and will then listen again for a signal from Philae. The next chance to receive a signal from the lander is expected to occur during the first half of April. 209)
• March 19, 2015: ESA’s Rosetta spacecraft has made the first measurement of molecular nitrogen at a comet, providing clues about the temperature environment in which Comet 67P/Churyumov–Gerasimenko formed. The in situ detection of molecular nitrogen has long been sought at a comet. Nitrogen had only previously been detected bound up in other compounds, including hydrogen cyanide and ammonia, for example. 210)
- Its detection is particularly important since molecular nitrogen is thought to have been the most common type of nitrogen available when the Solar System was forming. In the colder outer regions, it likely provided the main source of nitrogen that was incorporated into the gas planets. It also dominates the dense atmosphere of Saturn’s moon, Titan, and is present in the atmospheres and surface ices on Pluto and Neptune’s moon Triton.
Legend to Figure 138: The graph shows the variation in the signals measured for molecular nitrogen (N2) and carbon monoxide (CO) by Rosetta’s ROSINA instrument. The signals vary as a function of time, comet rotation and position of the spacecraft above the comet. An average ratio of N2/CO = (5.70 ± 0.66) x 10–3 was determined for the period 17–23 October 2014. The minimum and maximum values measured were 1.7 x 10–3 and 1.6 x 10–2, respectively (note that the ratio cannot be derived directly from this graph – a correction factor accounting for the instrument sensitivity is applied).
By comparing the ratio of N2 to CO at the comet with that of the protosolar nebula, it was determined that the comet must have formed at low temperatures, consistent with a Kuiper Belt origin. The study also finds that Jupiter-family comets like Comet 67P/ Churyumov–Gerasimenko were unlikely the source of Earth’s nitrogen.
• On March 4, 2015, ESA posted the four NAVCAM images of Figure 139 from distances of 80-100 km, to show the current activity of the comet. While most of Rosetta’s NAVCAM images are taken for navigation purposes, these images were obtained to provide context in support of observations performed at the same time with the Alice ultraviolet (UV) imaging spectrograph on Rosetta. The four images show the nucleus at different orientations, providing a good overview of the comet’s activity over the time interval between 25 and 27 February 2015. 211)
Figure 139: Montage of four single-frame images of Comet 67P/C-G taken by Rosetta’s Navigation Camera (NAVCAM) on Feb.25 (top left), Feb. 26, (top right) and the two bottom pictures on Feb. 27, 2015. The images have been processed to bring out the details of the comet’s activity. The exposure time for each image is 2 s (image credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0)
• March 3, 2015: Images from the OSIRIS scientific imaging camera taken during the close flyby on February 14, 2015 have now been downlinked to Earth, revealing the surface of Comet 67P/C-G in unprecedented detail, and including the shadow of the spacecraft encircled in a wreath of light. During the flyby, Rosetta not only passed closer by the comet than ever before, but also passed through a unique observational geometry: for a short time the Sun, spacecraft, and comet were exactly aligned. In this geometry, surface structures cast almost no shadows, and therefore the reflection properties of the surface material can be discerned. 212)
Legend to Figure 140: Close-up view of a 228 x 228 m region on Comet 67P/Churyumov-Gerasimenko, as seen by the OSIRIS narrow-angle camera during Rosetta’s flyby at 12:39 UTC on 14 February 2015. The image was taken 6 km above the comet’s surface, and the image resolution is just 11 cm/pixel. Rosetta’s fuzzy shadow, measuring approximately 20 x 50 m, is seen at the bottom of the image.
• Feb. 17, 2015: The spacecraft is no longer orbiting the comet, it is now performing a series of flybys to continue its science. 213)
- The video of Ref. 213) explains the next stage of the Rosetta mission, the science that will be done during 2015 by the orbiter’s flybys, and assesses the possibility of the Philae lander’s reactivation from hibernation. So far Rosetta has only mapped about seventy percent of the surface because the comet’s orbit and rotation kept certain areas in darkness. — This year new regions will come into view alongside new activity on the surface. When the comet is at the peak of its activity in the summer, Rosetta’s instruments will be there to observe, measure and record a spectacular event.
• On 14 February 2015, Rosetta swooped over the surface of Comet 67P/Churyumov–Gerasimenko at a distance of just 6 km. The closest approach took place at 12:41 GMT over a region known as Imhotep, which is on the larger of the comet’s two lobes. 214)
- The image of Figure 141 reveals the contrasting terrains seen on this comet. Layered and fractured exposed surfaces contrast against expanses of smooth, dust-covered terrain. In some places, such as to the lower right of this image, the faint outline of raised near-circular objects with smooth surfaces can be seen. Elsewhere, boulders ranging in size from a few meters to a few tens of meters are scattered across the surface. The largest boulder, seen to the upper right, is named Cheops.
• January 26, 2015: ESA's Rosetta mission is providing unique insight into the life cycle of a comet's dusty surface, watching 67P/Churyumov-Gerasimenko as it sheds the dusty coat it has accumulated over the past four years. 215)
- The COSIMA (COmetary Secondary Ion Mass Analyzer) instrument is one of Rosetta's three dust analysis experiments. It started collecting, imaging and measuring the composition of dust particles shortly after the spacecraft arrived at the comet in August 2014.
- Results from the first analysis of its data are reported in the journal Nature on January 26, 2015. The study covers August to October, when the comet moved along its orbit between about 535 million km to 450 million km from the Sun. Rosetta spent most of this time orbiting the comet at distances of 30 km or less. 216)
Legend to Figure 142: Two examples of dust grains were collected by the COSIMA instrument in the period 25-31 October 2014. Both grains were collected at a distance of 10-20 km from the comet nucleus. Image (a) shows a dust particle,named by the COSIMA team as Eloi, that crumbled into a rubble pile when collected; (b) shows a dust particle that shattered, named Arvid.
For both grains, the image is shown twice under two different grazing illumination conditions: the top image is illuminated from the right, the bottom image from the left. The brightness is adjusted to emphasize the shadows, in order to determine the height of the dust grain. Eloi therefore reaches about 0.1 mm above the target plate; Arvid about 0.06 mm. The two small grains at the far right of image (b) are not part of the shattered cluster.
The fact that the grains broke apart so easily means their individual parts are not well glued together. If they contained ice they would not shatter; instead, the icy component would evaporate off the grain shortly after touching the collecting plate, leaving voids in what remained. By comparison, if a pure water-ice grain had struck the detector, then only a dark patch would have been seen.
These 'fluffy' grains are thought to originate from the dusty layer built up on the comet's surface since its last close approach to the Sun, and will soon be lost into the coma.
- The scientists looked at the way that many large dust grains broke apart when they were collected on the instrument's target plate, typically at low speeds of 1-10 m/s. The grains, which were originally at least 0.05 mm across, fragmented or shattered upon collection. — The fact that they broke apart so easily means that the individual parts were not well bound together. Moreover, if they had contained ice, they would not have shattered. Instead, the icy component would have evaporated off the grain shortly after touching the collecting plate, leaving voids in what remained.
• January 22, 2015: Rosetta is revealing its host comet as having a remarkable array of surface features and with many processes contributing to its activity, painting a complex picture of its evolution. In a special edition of the journal Science, initial results are presented from seven of Rosetta's 11 science instruments based on measurements made during the approach to and soon after arriving at Comet 67P/Churyumov–Gerasimenko in August 2014. 217)
- The familiar shape of the dual-lobed comet has now had many of its vital statistics measured: the small lobe measures 2.6 x 2.3 x 1.8 km and the large lobe 4.1 x 3.3 x 1.8 km. The total volume of the comet is 21.4 km3 and the Radio Science Instrument has measured its mass to be 10 billion tons, yielding a density of 470 kg/m3.
- By assuming an overall composition dominated by water ice and dust with a density of 1500-2000 kg/m3, the Rosetta scientists show that the comet has a very high porosity of 70-80%, with the interior structure likely comprising weakly bonded ice-dust clumps with small void spaces between them.
Legend to Figure 143: The 19 regions identified on Comet 67P/Churyumov–Gerasimenko are separated by distinct geomorphological boundaries. Following the ancient Egyptian theme of the Rosetta mission, they are named for Egyptian deities. They are grouped according to the type of terrain dominant within each region. Five basic categories of terrain type have been determined: dust-covered (Ma’at, Ash and Babi); brittle materials with pits and circular structures (Seth); large-scale depressions (Hatmehit, Nut and Aten); smooth terrains (Hapi, Imhotep and Anubis), and exposed, more consolidated (‘rock-like’) surfaces (Maftet, Bastet, Serqet, Hathor, Anuket, Khepry, Aker, Atum and Apis).
- The OSIRIS scientific camera, has imaged some 70% of the surface to date: the remaining unseen area lies in the southern hemisphere that has not yet been fully illuminated since Rosetta’s arrival.
- Much of the northern hemisphere is covered in dust. As the comet is heated, ice turns directly into gas that escapes to form the atmosphere or coma. Dust is dragged along with the gas at slower speeds, and particles that are not travelling fast enough to overcome the weak gravity fall back to the surface instead.
Figure 144: Summary of the comet's vital statistics as determined by Rosetta’s instruments during the first few months of its comet encounter (image credit: ESA)
The full range of values are presented and discussed in a series of papers published in the 23 January 2015 issue of the journal Science, Vol, 347, Issue 6220 (http://www.sciencemag.org/content/347/6220.toc#SpecialIssue). The following papers are listed:
1) H. Sierks, C. Barbieri, P. L. Lamy, et al., “On the nucleus structure and activity of comet 67P/Churyumov-Gerasimenko.”
2) A. Rotundi, H. Sierks, V. Della Corte, et al., “Dust measurements in the coma of comet 67P/Churyumov-Gerasimenko inbound to the Sun.”
3) F. Capaccioni, A. Coradini, G. Filacchione, et al., “The organic-rich surface of comet 67P/Churyumov-Gerasimenko as seen by VIRTIS/Rosetta.”
4) H. Nilsson, G. Stenberg-Wieser, E. Behar, “Birth of a comet magnetosphere: A spring of water ions.”
5) N. Thomas, H. Sierks, C. Barbieri, et al., “The morphological diversity of comet 67P/ Churyumov-Gerasimenko.”
6) K. Altwegg, H. Balsiger, A. Bar-Nun, et al., “67P/Churyumov-Gerasimenko, a Jupiter family comet with a high D/H ratio.”
7) M. Hässig, K. Altwegg, H. Balsiger, et al., “Time variability and heterogeneity in the coma of 67P/Churyumov-Gerasimenko.”
8) S. Gulkis, M. Allen, P. von
Allmen, et al., “Subsurface properties and early activity of
• Dec. 10, 2014: ESA’s Rosetta spacecraft has found the water vapor from its target comet to be significantly different to that found on Earth. The discovery fuels the debate on the origin of our planet’s oceans. The measurements were made in the month following the spacecraft’s arrival at Comet 67P/Churyumov–Gerasimenko on 6 August. It is one of the most anticipated early results of the mission, because the origin of Earth’s water is still an open question. 218) 219) 220) 221)
- One of the leading hypotheses on Earth’s formation is that it was so hot when it formed 4.6 billion years ago that any original water content should have boiled off. But, today, two thirds of the surface is covered in water, so where did it come from? In this scenario, it should have been delivered after our planet had cooled down, most likely from collisions with comets and asteroids. The relative contribution of each class of object to our planet’s water supply is, however, still debated.
- The key to determining where the water originated is in its ‘flavor’, in this case the proportion of deuterium – a form of hydrogen with an additional neutron – to normal hydrogen. This proportion is an important indicator of the formation and early evolution of the Solar System, with theoretical simulations showing that it should change with distance from the Sun and with time in the first few million years.
- One key goal is to compare the value for different kinds of object with that measured for Earth’s oceans, in order to determine how much each type of object may have contributed to Earth’s water.
- Comets in particular are unique tools for probing the early Solar System: they harbor material left over from the protoplanetary disc out of which the planets formed, and therefore should reflect the primordial composition of their places of origin. - But thanks to the dynamics of the early Solar System, this is not a straightforward process. Long-period comets that hail from the distant Oort cloud originally formed in Uranus–Neptune region, far enough from the Sun that water ice could survive. They were later scattered to the Solar System’s far outer reaches as a result of gravitational interactions with the gas giant planets as they settled in their orbits.
- Conversely, Jupiter-family comets like Rosetta’s comet were thought to have formed further out, in the Kuiper Belt beyond Neptune. Occasionally these bodies are disrupted from this location and sent towards the inner Solar System, where their orbits become controlled by the gravitational influence of Jupiter.
- Indeed, Rosetta’s comet now travels around the Sun between the orbits of Earth and Mars at its closest and just beyond Jupiter at its furthest, with a period of about 6.5 years.
Figure 145: First measurements of comet’s water ratio (image credit: Spacecraft: ESA/ATG medialab; Comet: ESA/Rosetta/NavCam; Data: Altwegg et al. 2014) 222)
Legend to Figure 145: Rosetta’s measurement of the deuterium-to-hydrogen ratio (D/H) measured in the water vapor around Comet 67P/Churyumov–Gerasimenko. The measurements were made using ROSINA’s DFMS double focusing mass spectrometer between 8 August and 5 September 2014.
Deuterium is an isotope of hydrogen with an added neutron. The ratio of deuterium to hydrogen in water is a key diagnostic to determining where in the Solar System an object originated and in what proportion asteroids and/or comets contributed to Earth’s oceans.
Figure 145 displays the different values of D/H in water observed in various bodies in the Solar System. The data points are grouped by color as planets and moons (blue), chondritic meteorites from the Asteroid Belt (grey), comets originating from the Oort cloud (purple) and Jupiter family comets (pink). Rosetta’s Jupiter-family comet is highlighted in yellow. Diamonds represent data obtained in situ; circles represent data obtained by astronomical methods. The lower part of the graph shows the value of D/H measured in molecular hydrogen in the atmosphere of the giant planets of the Solar System (Jupiter, Saturn, Uranus, Neptune) and an estimate of the typical value in molecular hydrogen for the protosolar nebula, from which all objects in our Solar System formed.
The ratio for Earth’s oceans is 1.56 x10–4 (shown as the blue horizontal line in the upper part of the graph). The value for Comet 67P/Churyumov–Gerasimenko is found to be 5.3 x 10–4, more than three times greater than for Earth’s oceans. The discovery fuels the debate on the origin of Earth’s oceans and whether asteroids or comets played the bigger role in delivering water.
Table 17: Some background on previous measurements of the D/H ratio (Ref. 219)
Legend to Figure 146: Deuterium is an isotope of hydrogen with an added neutron. The ratio of deuterium to hydrogen in water is a key diagnostic to determining where in the Solar System an object originated and in what proportion asteroids and/or comets contributed to Earth’s oceans.
The data points are grouped by color as planets and moons (blue), chondritic meteorites from the Asteroid Belt (grey), comets originating from the Oort cloud (purple) and Jupiter family comets (pink). Rosetta’s Jupiter-family comet is highlighted in yellow. Diamonds represent data obtained in situ; circles represent data obtained by astronomical methods. The lower part of the graph shows the value D/H measured in molecular hydrogen in the atmosphere of the giant planets of the Solar System (Jupiter, Saturn, Uranus, Neptune) and an estimate of the typical value in molecular hydrogen for the protosolar nebula, from which all objects in our Solar System formed.
The horizontal blue line shows the value of the ratio in Earth's oceans, which has been determined to be 1.56 x 10–4. Rosetta’s ROSINA instrument measured the water vapor emanating from Comet 67P/Churyumov–Gerasimenko and found it to be 5.3 x 10–4, more than three times greater than for Earth’s oceans.
The discovery fuels the debate on the origin of Earth’s oceans and whether asteroids or comets played the bigger role in delivering water.
Figure 147: Kuiper Belt and Oort Cloud in context (image credit: ESA) 224)
Legend to Figure 147: The illustration shows the two main reservoirs of comets in the Solar System: the Kuiper Belt, at a distance of 30–50 astronomical units (AU: the Earth–Sun distance) from the Sun, and the Oort Cloud, which may extend up to 50 000–100 000 AU from the Sun.
Halley’s comet is thought to originate from the Oort Cloud, while 67P/Churyumov–Gerasimenko, the focus of ESA’s Rosetta mission, hails from the Kuiper Belt. It is now in a 6.5-year orbit around the Sun between the orbits of Earth and Mars at its closest and just beyond Jupiter at its furthest.
• Nov. 28, 2014: Data collected by ROMAP (Rosetta Lander Magnetometer and Plasma Monitor) onboard Philae, is being used to help reconstruct the trajectory of the lander to its final landing site on Comet 67P/Churyumov-Gerasimenko. The scientists have now been able to use ROMAP data to reconstruct the chain of events that took place on 12 November as follows: 225)
- Separation was confirmed as a decay in the magnetic field perturbation as the distance between Philae and the orbiter increased; at this point the lander was spinning at a rate of about 1 rotation per 5 minutes.
- The ROMAP boom was deployed successfully and a magnetic field decay was measured corresponding to the increased distance of the ROMAP sensor with respect to its original position on the lander.
- During the seven-hour descent, all measurements were nominal, and ROMAP recorded the first touchdown at 15:34:04 GMT spacecraft time (the signal arrived on Earth just over 28 minutes later, and was confirmed at 16:03 GMT).
- After the first touchdown, the spin rate started increasing. As the lander bounced off the surface, the control electronics of the flywheel were turned off and during the following 40 minutes of flight, the flywheel transferred its angular momentum to Philae. After this time, the lander was spinning at a rate of about 1 rotation per 13 seconds.
- At 16:20 GMT spacecraft time the lander is thought to have collided with a surface feature, a crater rim, for example.
“It was not a touchdown like the first one, because there was no signature of a vertical deceleration due to a slight dipping of the magnetometer boom as measured during the first and also the final touchdown,” according to Hans-Ulrich Auster, the PI of ROMAP at the TU Braunschweig. It is assumed that Philae probably touched a surface with one leg only – perhaps grazing a crater rim – and after that the lander was tumbling. No simple rotation about the lander’s z-axis could be seen.
- Following this event, the main rotation period had decreased slightly to 1 rotation per 24 seconds.
- At 17:25:26 GMT, Philae touched the surface again, initially with just one foot but then all three, giving the characteristic touchdown signal.
- At 17:31:17 GMT, after travelling probably a few more meters, Philae found its final parking position on three feet.
Despite this bumpy start to its life on the comet, all of Philae’s instruments were operated in the following two days. — The search for Philae’s final landing spot is still on, and the ROMAP data are being used with other instrument data from both Philae and Rosetta to try to reconstruct the lander’s full trajectory and to identify its current location. Also, a comprehensive scientific analysis of the data from all instruments is underway.
• Nov. 19, 2014: With the Philae lander’s mission complete, Rosetta will now continue its own extraordinary exploration, orbiting Comet 67P/Churymov–Gerasimenko during the coming year as the enigmatic body arcs ever closer to our Sun. The Rosetta spacecraft is in excellent condition, with all of its systems and instruments performing as expected. 226)
- Rosetta will become the first spacecraft to witness at close quarters the development of a comet’s coma and the subsequent tail streaming for millions of kilometers into space. Rosetta will then have to stay further from the comet to avoid the coma affecting its orbit. - In addition, as the comet nears the Sun, illumination on its surface is expected to increase. This may provide sufficient sunlight for the DLR-operated Philae lander, now in hibernation, to reactivate, although this is far from certain.
- Early next year, Rosetta will be switched into a mode that allows it to listen periodically for beacon signals from the surface.
• Nov. 17, 2014: The mosaic series in Figure 148 show the breathtaking journey of Rosetta’s Philae lander as it approached and then rebounded from its first touchdown on Comet 67P/Churyumov–Gerasimenko on November 12, 2014. The images were taken with Rosetta’s OSIRIS narrow-angle camera when the spacecraft was 17.5 km from the comet center, or roughly 15.5 km from the surface. They have a resolution of 28 cm/pixel and the enlarged insets are 17 m x 17 m. 227) 228) 229)
Figure 148: Mosaic of Rosetta’s OSIRIS narrow-angle camera on Nov. 12 spotting Philae drifting across the comet (image credit: ESA, Rosetta,MPS for OSIRIS Team MPS, UPD, LAM, IAA, SSO, INTA, UPM, DASP, IDA)
Legend to Figure 148: The mosaic comprises a series of images captured by Rosetta’s OSIRIS camera over a 30 minute period spanning the first touchdown. The time of each of image is marked on the corresponding insets and is in GMT. A comparison of the touchdown area shortly before and after first contact with the surface is also provided.
From left to right, the images show Philae descending towards and across the comet before touchdown. The image taken after touchdown, at 15:43 GMT, confirms that the lander was moving east, as first suggested by the data returned by the CONSERT experiment, and at a speed of about 0.5 m/s.
The final location of Philae is still not known, but after touching down and bouncing again at 17:25 GMT, it reached there at 17:32 GMT. The imaging team is confident that combining the CONSERT ranging data with OSIRIS and NAVCAM images from the orbiter and images from near the surface and on it from Philae’s ROLIS and CIVA cameras will soon reveal the lander’s whereabouts (Ref. 227).
Before going into hibernation at 00:36 GMT ( 01:36 CET) on 15 November 2014, the Philae lander was able to conduct some work using power supplied by its primary battery. With its 10 instruments, the mini laboratory sniffed the atmosphere, drilled, hammered and studied Comet 67P/ Churyumov-Gerasimenko while over 500 million km from Earth (Ref. 228).
• Nov. 15, 2014: Rosetta’s lander Philae has completed its primary science mission after nearly 57 hours on Comet 67P/Churyumov–Gerasimenko. 230)
- After being out of communication visibility with the lander since 09:58 GMT / 10:58 CET on Nov. 14 (Friday), Rosetta regained contact with Philae at 22:19 GMT /23:19 CET last night (Nov. 14). The signal was initially intermittent, but quickly stabilized and remained very good until 00:36 GMT / 01:36 CET this morning (Nov. 15). — In this period, the lander returned all of its housekeeping data, as well as science data from the targeted instruments, including ROLIS, COSAC, Ptolemy, SD2 and CONSERT. This completed the measurements planned for the final block of experiments on the surface.
- In addition, the lander’s body was lifted by about 4 cm and rotated about 35° in an attempt to receive more solar energy. But as the last science data fed back to Earth, Philae’s power rapidly depleted. — “It has been a huge success, the whole team is delighted,” said Stephan Ulamec, lander manager at the DLR German Aerospace Agency, who monitored Philae’s progress from ESOC in Darmstadt, Germany, this week. - “We still hope that at a later stage of the mission, perhaps when we are nearer to the Sun, that we might have enough solar illumination to wake up the lander and re-establish communication, ” added Stephan.
- From now on, no contact will be possible unless sufficient sunlight falls on the solar panels to generate enough power to wake it up. The possibility that this may happen later in the mission was boosted when mission controllers sent commands to rotate the lander’s main body with its fixed solar panels. This should have exposed more panel area to sunlight.
- Meanwhile, the Rosetta orbiter has been moving back into a 30 km orbit around the comet. t will return to a 20 km orbit on Dec. 6, 2014 and continue its mission to study the body in great detail as the comet becomes more active, en route to its closest encounter with the Sun on August 13, 2015. Over the coming months, Rosetta will start to fly in more distant ‘unbound’ orbits, while performing a series of daring flybys past the comet, some within just 8 km of its center.
Data collected by the orbiter will allow scientists to watch the short- and long-term changes that take place on the comet, helping to answer some of the biggest and most important questions regarding the history of our Solar System. How did it form and evolve? How do comets work? What role did comets play in the evolution of the planets, of water on the Earth, and perhaps even of life on our home world.
“The data collected by Philae and Rosetta is set to make this mission a game-changer in cometary science,” says Matt Taylor, ESA’s Rosetta project scientist. Fred Jansen, ESA’s Rosetta mission manager, says, “At the end of this amazing rollercoaster week, we look back on a successful first-ever soft-landing on a comet. This was a truly historic moment for ESA and its partners. We now look forward to many more months of exciting Rosetta science and possibly a return of Philae from hibernation at some point in time.”
Figure 149: Labelled trajectory of Rosetta’s orbit, focusing on the maneuvers after 12 November 2014 (image credit: ESA, Ref. 230)
• Nov.14, 2014 (update information): Although the Philae touchdown was confirmed at ESOC to have occurred at 16:03 GMT/17:03 CET on 12 November, scientists, flight dynamics specialists and engineers from all mission participants have been studying the first data returned from the lander. These revealed the astonishing conclusion that the lander did not just touch down on Comet 67P/Churyumov–Gerasimenko once, but three times. 231)
- The harpoons did not fire and Philae appeared to be rotating after the first touchdown, which indicated that it had lifted from the surface again. Stephan Ulamec, Philae manager at the DLR German Aerospace Center, reported that it touched the surface at 15:34, 17:25 and 17:32 GMT (comet time – it takes over 28 minutes for the signal to reach Earth, via Rosetta). The information was provided by several of the scientific instruments, including the ROMAP magnetic field analyzer, the MUPUS thermal mapper, and the sensors in the landing gear that were pushed in on the first impact.
- The first touchdown was inside the predicted landing ellipse, confirmed using the lander’s downwards-looking ROLIS descent camera in combination with the orbiter’s OSIRIS images to match features.
- But then the lander lifted from the surface again – for 1 hour 50 minutes. During that time, it travelled about 1 km upward at a speed of 38 cm/s. It then made a smaller second hop, travelling at about 3 cm/s, and landing in its final resting place seven minutes later.
- The touchdown signal generated on first touchdown induced the instruments to ‘think’ that Philae had landed, triggering the next sequence of experiments. Now those data are being used to interpret the bounces.
- The lander remains unanchored to the surface at an as yet undetermined orientation. The science instruments are running and are delivering images and data, helping the team to learn more about the final landing site.
- The descent camera revealed that the surface is covered by dust and debris ranging in size from mm to m. Meanwhile, Philae’s CIVA camera returned a panoramic image that on first impressions suggests the lander is close to a rocky wall, and perhaps has one of its three feet in open space.
- After discussions as to whether to activate those science instruments that may cause the position of Philae to shift, MUPUS and APXS have both been deployed.
- The primary battery enabling the core science goals of the lander may run out some time in the next 24 hours. As for the secondary battery, charged by solar panels on Philae, with only 1.5 hours of sunlight available to the lander each day, there is an impact on the energy budget to conduct science for a longer period of time. The original landing site offered nearly seven hours of illumination per 12.4 hour comet day.
• Nov. 13, 2014: Rosetta’s lander Philae is safely on the surface of Comet 67P/Churyumov-Gerasimenko (Figure 151). One of the lander’s three feet can be seen in the foreground. The image is a two-image mosaic of CIVA (Comet Nucleus Infrared and Visible Analyzer), a group of six identical micro-cameras. 232) 233)
- Philae landed nearly vertically on its side with one leg up in outer space. Philae settled into its final landing spot after a harrowing first bounce that sent it flying as high as a kilometer above the comet’s surface. After hovering for two hours, it landed a second time only to bounce back up again a short distance – this time 3 cm. Seven minutes later it made its third and final landing. Incredibly, the little craft still functions after trampolining for hours!
- Despite its awkward stance, Philae continues to do a surprising amount of good science. Scientists are still hoping to come up with a solution to better orientate the lander. Their time is probably limited. The craft landed in the shadow of a cliff, blocking sunlight to the solar panels used to charge its battery. Philae receives only 1.5 hours instead of the planned 6-7 hours of sunlight each day.
Figure 150: Stephan Ulamec, Philae Lander manager, describes how Philae first landed less than 100 m from the planned Agilkia site (red square). Without functioning harpoons and thrusters to fix it to the ground there, it rebounded and shot a kilometer above the comet. Right now, it’s somewhere in the blue diamond (image credit: ESA)
• Nov. 12, 2014: The following statement is from John Grunsfeld, astronaut and associate administrator for NASA’s Science Mission Directorate in Washington, about the successful comet landing by the European Space Agency’s Rosetta spacecraft: 234)
- “We congratulate ESA on their successful landing on a comet today. This achievement represents a breakthrough moment in the exploration of our solar system and a milestone for international cooperation. We are proud to be a part of this historic day and look forward to receiving valuable data from the three NASA instruments on board Rosetta that will map the comet’s nucleus and examine it for signs of water.
- “The data collected by Rosetta will provide the scientific community, and the world, with a treasure-trove of data. Small bodies in our solar system like comets and asteroids help us understand how the solar system formed and provide opportunities to advance exploration. We look forward to building on Rosetta's success exploring our solar system through our studies of near earth asteroids and NASA's upcoming asteroid sample return mission OSIRIS-REx. It’s a great day for space exploration."
• Nov. 12, 2014: Philae has landed! ESA’s Rosetta mission has soft-landed its Philae probe on a comet, the first time in history that such an extraordinary feat has been achieved. After a tense wait during the seven-hour descent to the surface of Comet 67P/Churyumov–Gerasimenko, the signal confirming the successful touchdown arrived on Earth at 16:03 GMT (17:03 CET). 235)
- The confirmation was relayed via the Rosetta orbiter to Earth and picked up simultaneously by ESA’s ground station in Malargüe, Argentina and NASA’s station in Madrid, Spain. The signal was immediately confirmed at ESA/ESOC, in Darmstadt, and DLR’s Lander Control Center in Cologne, both in Germany. The first data from the lander’s instruments were transmitted to the Philae Science, Operations and Navigation Center at France’s CNES space agency in Toulouse.
- Touchdown was planned to take place at a speed of around 1 m/s, with the three-legged landing gear absorbing the impact to prevent rebound, and an ice screw in each foot driving into the surface.
- Over the next 2.5 days, the lander will conduct its primary science mission, assuming that its main battery remains in good health. An extended science phase using the rechargeable secondary battery may be possible, assuming the Sun illumination conditions allow and dust settling on the solar panels does not prevent it. This extended phase could last until March 2015, after which conditions inside the lander are expected to be too hot for it to continue operating.
- Science highlights from the primary phase will include a full panoramic view of the landing site, including a section in 3D, high-resolution images of the surface immediately underneath the lander, on-the-spot analysis of the composition of the comet’s surface materials, and a drill that will take samples from a depth of 23 cm and feed them to an onboard laboratory for analysis.
- The lander will also measure the electrical and mechanical characteristics of the surface. In addition, low-frequency radio signals will be beamed between Philae and the orbiter through the nucleus to probe the internal structure. The detailed surface measurements that Philae makes at its landing site will complement and calibrate the extensive remote observations made by the orbiter covering the whole comet.
- While Philae begins its close-up study of the comet, Rosetta must maneuver from its post-separation path back into an orbit around the comet, eventually returning to a 20 km orbit on December 6, 2014. The plans of the ground-breaking Rosetta mission are to follow the comet around the Sun for 13 months, watching as its activity changes and its surface evolves.
- Next year, as the comet grows more active, Rosetta will need to step further back and fly unbound ‘orbits’, but dipping in briefly with daring flybys, some of which will bring it within just 8 km of the comet center.
- The comet will reach its closest distance to the Sun on 13 August 2015 at about 185 million km, roughly between the orbits of Earth and Mars. Rosetta will follow it throughout the remainder of 2015, as they head away from the Sun and activity begins to subside.
• Nov. 12, 2014 [at 10:02 CET (Central European Time)]: The Philae lander has separated from the Rosetta orbiter, and is now on its way to becoming the first spacecraft to touch down on a comet. Separation was confirmed at ESA/ESOC at 09:03 GMT. It takes the radio signals from the transmitter on Rosetta 28 minutes and 20 seconds to reach Earth, so separation actually occurred in space at 08:35 GMT. 236)
- The first signal from Philae is expected in around two hours, when the lander establishes a communication link with Rosetta. Philae cannot send its data to Earth directly – it must do it via Rosetta. Once the link has been established, the lander will relay via Rosetta a status report of its health, along with the first science data. This will include images taken of the orbiter shortly after separation.
- The descent to the surface of Comet 67P/Churyumov–Gerasimenko will take around seven hours, during which the lander will take measurements of the environment around the comet. It will also take images of the final moments of descent.
- Confirmation of a successful touchdown is expected in a one-hour window centered on 17:02 GMT. The first image from the surface is expected some two hours later.
Figure 152: Infographic to summarize the measurements carried out by Rosetta’s lander, Philae, during its seven-hour descent to Comet 67P/Churyumov–Gerasimenko and immediately after touchdown (image credit: ESA/ATG medialab)
Figure 153: Rosetta’s lander Philae took this parting shot of its mothership shortly after separation. The image was taken with the lander’s CIVA-P imaging system and captures one of Rosetta's 14 m long solar arrays. It was stored onboard the lander until the radio link was established with Rosetta around two hours after separation, and then relayed to Earth (image credit: ESA, Rosetta, Philae, CIVA). 237)
Figure 154: Rosetta’s OSIRIS narrow-angle camera captured this parting shot of the Philae lander after separation (image credit: ESA, Rosetta,MPS for OSIRIS Team MPS, UPD, LAM, IAA, SSO, INTA, UPM, DASP, IDA) 238)
Figure 155: First photo released of Comet 67P/C-G taken by the ROLIS camera of Philae during its descent on Nov. 12 at 14:38:41 UTC from a distance of ~3 km from the surface. The landing site is imaged with a resolution of about 3 m per pixel (image credit: DLR) 239)
Legend to Figure 155: The ROLIS camera is a down-looking imager that acquires images during the descent and doubles as a multispectral close-up camera after the landing. The aim of the ROLIS experiment is to study the texture and microstructure of the comet's surface. In the upper right corner a segment of the Philae landing gear is visible.
• November 11, 2014: ESA’s comet-chasing Rosetta mission spent much of the second half of October orbiting Comet 67P/Churyumov–Gerasimenko at less than 10 km from its surface. This selection of previously unpublished ‘beauty shots’, taken by Rosetta’s navigation camera, presents the varied and dramatic terrain of this mysterious world from this close orbit phase of the mission. 240)
- Some light contrast enhancements have been made to emphasize certain features and to bring out features in the shadowed areas. In reality, the comet is extremely dark — blacker than coal. The images, taken in black-and-white, are grey-scaled according to the relative brightness of the features observed, which depends on local illumination conditions, surface characteristics and composition of the given area. Some slight vignetting can also be seen in the corners of some images.
Legend to Figure 156: Pits like these are thought to be where gas vents into space from the porous subsurface, carrying with it dusty grains of comet material. Scientists are keen to learn the role of this pit – and others – in the development of the comet’s activity, as it gets ever closer to the Sun.
This single-frame NAVCAM image measures 1024 x 1024 pixels. It was captured from a distance of 9.9 km from the center of the comet (about 7.7 km from the surface) at 02:22 GMT on 15 October 2014. At this distance, the image resolution is 84.6 cm/pixel and the size of the image is 866 x 866 m.
Legend to Figure 157: This image of comet 67P/Churyumov-Gerasimenko was obtained on October 30, 2014 by the OSIRIS scientific imaging system on the Rosetta spacecraft. The right half is obscured by darkness. The image was taken from a distance of approximately 30 km.
- The landing site, previously known as ‘Site J’, is named for Agilkia Island, an island on the Nile River in the south of Egypt. A complex of Ancient Egyptian buildings, including the famous Temple of Isis, was moved to Agilkia from the island of Philae when the latter was flooded during the building of the Aswan dams last century.
- The name was selected by a jury comprising members of the Philae Lander Steering Committee as part of a public competition run 16–22 October by ESA and the German, French and Italian space agencies.
- Agilkia was one of the most popular entries – it was proposed by over 150 participants. The committee selected Alexandre Brouste from France as the overall winner. As a prize, Mr. Brouste will be invited to ESA’s Space Operations Control Centre in Darmstadt, Germany, to follow the landing live.
- Although perhaps not quite as complicated as navigating Rosetta and Philae towards the comet, the task of choosing a name was by no means simple. More than 8000 entries from 135 countries were received in one week, showing great creativity and cultural diversity.
- The entries covered a tremendous range of themes, from abstract concepts to the names of places on Earth. As with the winning entry, many suggestions echoed the Egyptian origins of Rosetta and Philae, named in recognition of milestones in decoding hieroglyphics, the sacred writing system of ancient Egypt.
Figure 158: The Philae landing site Agilkia is located on the 'head' of Comet 67P/Churyumov-Gerasimenko (image credit: ESA, DLR)
• October 20-24, 2014: In the “Our week through the lens” series, ESA released an image of Comet 67P/Churyumov-Gerasimenko (Figure 159), showing jets of cometary activity along almost the entire body of the comet. 245) 246)
• October 15, 2014: ESA has given the green light for its Rosetta mission to deliver its lander, Philae, to the primary site on 67P/Churyumov–Gerasimenko on 12 November, in the first-ever attempt at a soft touchdown on a comet. 247)
- Philae’s landing site, currently known as Site J and located on the smaller of the comet’s two ‘lobes’, was confirmed on 14 October following a comprehensive readiness review. — Since the arrival, the mission has been conducting an unprecedented survey and scientific analysis of the comet, a remnant of the early phases of the Solar System’s 4.6 billion-year history.
- At the same time, Rosetta has been moving closer to the comet: starting at 100 km on 6 August, it is now just 10 km from the center of the 4 km-wide body. This allowed a more detailed look at the primary and backup landing sites in order to complete a hazard assessment, including a detailed boulder census.
- The decision that the mission is ‘Go’ for Site J also confirms the timeline of events leading up to the landing. Rosetta will release Philae at 08:35 GMT/09:35 CET on 12 November at a distance of approximately 22.5 km from the center of the comet. Landing will be about seven hours later at around 15:30 GMT/16:30 CET (Central European Time).
- With a one-way signal travel time between Rosetta and Earth on 12 November of 28 minutes 20 seconds, that means that confirmation of separation will arrive on Earth ground stations at 09:03 GMT/10:03 CET and of touchdown at around 16:00 GMT/17:00 CET.
Figure 160: Philae’s primary landing site – mosaic (image credit: ESA, Rosetta,MPS for OSIRIS Team MPS, UPD, LAM, IAA, SSO, INTA, UPM, DASP, IDA)
• October 10, 2014: The OSIRIS camera on board Europe’s Rosetta spacecraft has caught a spectacular glimpse of one of the many boulders that cover the surface of comet 67P/Churyumov-Gerasimenko. With a maximum extension of 45 m, it is one of the larger structures of this kind on the comet and stands out among a group of boulders located on the lower side of 67P’s larger lobe. Since this cluster of boulders reminded the scientists of the pyramids of Giza, the boulder has been named Cheops after the largest pyramid within the Giza Necropolis of Egypt. The boulder-like structures that Rosetta has revealed on the surface of 67P in the past months are one of the comet’s most striking and mysterious features. 248)
The large boulder now dubbed Cheops was seen for the first time in images obtained in early August upon Rosetta’s arrival at the comet (Figure 172). In the past weeks as Rosetta has navigated closer and closer to the comet’s surface, OSIRIS imaged the unique structure again – but this time with a much higher resolution of 50 cm/pixel. The image of Figure 161 was acquired on Sept. 19, 2014 from a distance of 28.5 km. 249)
Figure 161: Close-up of the boulder Cheops as it casts a long shadow on the surface of comet 67P/Churyumov-Gerasimenko, (image credit: ESA, Rosetta,MPS for OSIRIS Team MPS, UPD, LAM, IAA, SSO, INTA, UPM, DASP, IDA)
• On October 8, 2014, ESA reports of grooves found on asteroid Lutetia when Rosetta flew past Lutetia at a distance of 3168 km in July 2010. The spacecraft took images of the 100 km-wide asteroid for about two hours during the flyby, revealing numerous impact craters and hundreds of grooves all over the surface. 250)
Impact craters are commonly seen on all Solar System worlds with solid surfaces, recording an intense history of collisions between bodies. However, grooves are much less prevalent. To date, they have been discovered by visiting spacecraft only on the martian moon Phobos and the asteroids Eros and Vesta.
Figure 162: Tracing Lutetia's grooves,”(image credit: ESA)
• Oct. 3, 2014: Comet 67P/Churyumov-Gerasimenko's dimensions, as measured from images taken by Rosetta's OSIRIS imaging system. The images shown in the graphic were taken by Rosetta's navigation camera on August 19, 2014. - The larger lobe of the comet measures 4.1 x 3.2 x 1.3 km, while the smaller lobe is 2.5 x 2.5 x 2.0 km. 251)
- One of the key things is the so-called “shape model”, meaning a 3D model of the comet based on images from the OSIRIS and NAVCAM cameras. Because roughly 30% of the ‘dark side’ of 67P/C-G has not been resolved and analyzed fully yet, the shape model is very incomplete over those regions. As a result, some of the derived parameters for the comet are only best estimates at present. These include the volume and the global density, the latter depending on the mass and the volume. 252)
Figure 163: Measuring Comet 67P/C-G (image credit: ESA, Rosetta/OSIRIS; Dimensions: ESA, Rosetta, MPS for OSIRIS Team MPS, UPD, LAM, IAA, SSO, INTA, UPM, DASP, IDA)
• September 26, 2014: The Rosetta mission will deploy its lander, Philae, to the surface of Comet 67P/Churyumov-Gerasimenko on November 12, 2014. Philae's landing site, currently known as Site J, is located on the smaller of the comet's two 'lobes', with a backup site on the larger lobe. The sites were selected just six weeks after Rosetta arrived at the comet on 6 August, following its 10-year journey through the Solar System. 253)
- The primary landing site was chosen from five candidates during the Landing Site Selection Group meeting held on 13–14 September 2014.
Figure 164: Rosetta's NAVCAM camera took this image of Comet 67P/Churyumov-Gerasimenko on 21 September, from a distance of 27.8 km from the comet center. The image covers an area of about 2 km x 1.9 km and focuses on the smaller of the two comet lobes. The primary landing site J is 'above' the distinctive depression in this view (image credit: ESA, Rosetta). 254)
Figure 165: Site J is located on the head of Comet 67P/Churyumov–Gerasimenko. An inset showing a close up of the landing site is also shown (image credit: ESA, Rosetta, MPS for OSIRIS Team MPS, UPD, LAM, IAA SSO, INTA, UPM, DASP, IDA)
Legend to Figure 165: The inset image was taken by Rosetta's OSIRIS narrow-angle camera on 20 August 2014 from a distance of about 67 km. The image scale is 1.2 m/pixel. The background image was taken on 16 August from a distance of about 100 km. The comet nucleus is about 4 km across.
• Sept. 5, 2014: After examining the images of the comet obtained on August 6, members of the Rosetta team at ESA couldn’t help but notice that Comet 67P/C-G appeared to be a very oddly shaped object. Its peculiar shape led them to nickname the comet the “rubber duck”. As the team continues to study Comet 67P/C-G, the scientists are looking to get a better understanding of its surface properties. 255)
- Orbit: Rosetta is in an orbit about the comet at an avearge distance of ~30 km. The team is planning to occasionally lower the spacecraft’s orbit to about 10 km above the comet’s surface or possibly even lower when the Rosetta’s attached Philae lander is deployed in November.
- An ideal landing site for Philae is that would be about 1 km2 in size and able to provide enough sunlight to charge the probe’s battery. - Since the comet’s gravity is so low, the probe will most likely bounce when it first touches down, so ESA engineers have equipped it with two harpoons and some ice screws to keep the probe steady and attached to 67P/C-G’s surface.
• August 25, 2014: Using detailed information collected by ESA’s Rosetta spacecraft during its first two weeks at Comet 67P/Churyumov-Gerasimenko, five locations have been identified as candidate sites to set down the Philae lander in November – the first time a landing on a comet has ever been attempted. 256)
The approximate locations of the five regions are marked on these OSIRIS narrow-angle camera images of Figure 166 taken on 16 August from a distance of about 100 km. The comet nucleus is about 4 km across.
The sites were assigned a letter from an original pre-selection of 10 possible sites identified A through J. The lettering scheme does not signify any ranking. Three sites (B, I and J) are located on the smaller of the two lobes of the comet and two sites (A and C) are located on the larger lobe.
The landing of Philae is expected to take place in mid-November when the comet is about 450 million km from the Sun, before activity on the comet reaches levels that might jeopardise the safe and accurate deployment of Philae to the comet’s surface, and before surface material is modified by this activity.
The comet is on a 6.5 year orbit around the Sun and today is 522 million km from it. At their closest approach on 13 August 2015, just under a year from now, the comet and Rosetta will be 185 million km from the Sun, meaning an eightfold increase in the light received from the Sun.
While Rosetta and its scientific instruments will watch how the comet evolves as heating by the Sun increases, observing how its coma develops and how the surface changes over time, the lander Philae and its instruments will be tasked with making complementary in situ measurements at the comet’s surface. The lander and orbiter will also work together using the CONSERT experiment to send and detect radio waves through the comet’s interior, in order to characterise its internal structure.
• Comet characterization (Ref. 261): This phase was mainly driven by the technically challenging and time critical need for the operations team to develop engineering models of the comet such that the proper orbit phase and the landing site selection process could start.
For the team to design and plan the next phase it was necessary to have a pretty accurate model of the gravity field and of the comet attitude with an associated reference frame. In total absence of further information about the comet, the first step of the process was to catalog the so-called landmarks i.e. evident features of the comet’s surface that could easily be recognized in the images and used as navigation references. The position of these landmarks in subsequent images and the traditional radiometric data (ranging and Doppler) were then fed into the orbit determination system which essentially consisted of an estimator of:
- Spacecraft position and velocity
- Comet position and velocity
- Comet spin axis
- Comet attitude evolution (thus rotation period)
- Comet gravity potential (thus mass) and position of the center of mass
- Comet shape.
Due to the extremely large inaccuracy in the a priori knowledge of the comet’s gravity potential it was not possible to inject the spacecraft onto a proper captured orbit. Therefore all these measurements were collected from a so-called “pyramid orbit” consisting of a sequence of three portions of hyperbolic arcs flown in front of the comet, on the illuminated side (Figure 167), at a distance varying between 115 and 90 km. In this way the spacecraft was not captured by the gravity but would still be in a position to have its orbit affected by it and have the possibility to observe the comet from different angles.
This triangular trajectory was flown for 10 days and, after a transfer of 7 days, another triangle was flown at distances ranging from 70 to 50 km (Figure 168).
The timeline of events reported in Table 18 shows the time pressure the operations team was encountering during this phase (major events in bold italics). This was mainly due to the need of starting the landing site selection process as soon as possible to meet the delivery deadline of mid November 2014. During this phase the conceived mission planning concept revealed to be extremely robust and was key to the success. At the same time the full complement of scientific instruments was performing uninterrupted scientific measurements.
The estimation phase of cometary parameters and the development of the relevant models was conducted according to plan with the first full operational set released on 14 August at the end of the 2nd segment of the first triangle. At this stage also the shape model of the comet was available with an accuracy level beyond the expectations. The only estimation process that took longer to converge was the one determining the position of the center of mass of the comet. The comet was found – unexpectedly - not to be nutating at all and this had the consequence that the position of the center of mass could only be estimated from the spacecraft orbital reconstruction without any aid from the optical images.
This phase also marked the transition from far approach optical navigation, where images were used to compute the position of the comet with respect to fixed stars, to proximity navigation, where the spacecraft position with respect to the comet is determined by the position of the landmarks in the images. Due to the active environment around the comet, where rather unpredictable aerodynamic forces act on the spacecraft, the proximity optical navigation method will constitute the basis for navigation throughout the mission. This imposes specific constraints for operations planning (e.g. regular image taking sessions capable of coping with pointing uncertainties) and execution (e.g. availability of recent optical navigation data for orbit determination and commands generation sessions) that have to be considered by the operations team.
The models developed at this stage were accurate enough to kick-off the next two steps of the mission:
- the design of the orbital phase
- the landing site selection process.
Orbit operations and preparation for landing:
The whole orbiting plan for the phase immediately following the comet characterization (i.e. the 2 pyramid orbits) was fully dependant on the actual comet parameters, mainly spin axis and mass. With these parameters resolved, it was possible for the operations team to plan and release the operational orbits. These include an initial orbital phase at ca. 29 km radius with orbital plane tilted 60º away from the Sun direction then followed by a phase where the orbit radius is reduced to ca. 19 km and the orbital plane is moved to the terminator plane (Figure 169).
A further reduction of the orbit radius will be conducted only if allowed by the dynamical stability of the comet environment. This strategy was necessary to combine the wish of the scientists to have good observation conditions with the need to minimize the spacecraft surface exposed to the – mostly radial - gas flow coming from the comet. The terminator plane, being perpendicular to the Sun direction, ensures that only the edge of the solar arrays is exposed to the gas flow, thus minimizing the aerodynamic forces acting on the spacecraft.
This orbital phase is considered as most valuable by the vast majority of the scientists in charge of Rosetta’s on-board instruments because it might be the only opportunity to fly orbits so close to the surface, at least in this initial comet operations phase. In any case, with the decrease of the heliocentric distance the comet is expected to increase its activity and will force the operations team to fly at larger distances from the comet nucleus.
• On 6 August 2014, the Rosetta mission achieved a significant milestone by becoming the first mission to rendezvous with a comet. During the coming months, Rosetta will orbit the comet, deploy the Philae lander (in November, 2014), and accompany the comet through perihelion (August 2015) until the nominal end of the mission. 257)
- After a decade-long journey chasing its target, ESA’s Rosetta has today become the first spacecraft to rendezvous with a comet, opening a new chapter in Solar System exploration. 258)
Comet 67P/Churyumov–Gerasimenko and Rosetta now lie 405 million km from Earth, about half way between the orbits of Jupiter and Mars, rushing towards the inner Solar System at nearly 55 000 km/hr.
The comet is in an elliptical 6.5 year orbit that takes it from beyond Jupiter at its furthest point, to between the orbits of Mars and Earth at its closest to the Sun. Rosetta will accompany it for over a year as they swing around the Sun and back out towards Jupiter again.
Figure 170: Rosetta's OSIRIS narrow-angle camera image of Comet 67P/Churyumov–Gerasimenko on August 3, 2014 from a distance of 285 km, image resolution = 5.3 m (image credit: ESA, Rosetta, MPS for OSIRIS Team MPS, UPD, LAM, IAA SSO, INTA, UPM, DASP, IDA)
- The comet began to reveal its personality while Rosetta was on its approach. Images taken by the OSIRIS camera between late April and early June showed that its activity was variable. The comet’s ‘coma’ – an extended envelope of gas and dust – became rapidly brighter and then died down again over the course of those six weeks.
In the same period, first measurements from the MIRO (Microwave Instrument) on the Rosetta Orbiter suggested that the comet was emitting water vapor into space at about 300 ml/s (milliliter).
Meanwhile, VIRTIS (Visible and Infrared Thermal Imaging Spectrometer) measured the comet’s average temperature to be about –70ºC, indicating that the surface is predominantly dark and dusty rather than clean and icy.
Then, stunning images taken from a distance of about 12 000 km began to reveal that the nucleus comprises two distinct segments joined by a ‘neck’, giving it a duck-like appearance. Subsequent images showed more and more detail (Figures 170 and 171) — the most recent, highest-resolution image was downloaded from the spacecraft on August 8 (Figure 172).
Figure 171: Comet on 3 August 2014 the by the OSIRIS narraow-angle camera from a distance of 285 km, image resolution = 5.3 m (image credit: ESA, Rosetta, MPS for OSIRIS Team MPS, UPD, LAM, IAA SSO, INTA, UPM, DASP, IDA)
Figure 172: Close-up detail of comet 67P/Churyumov-Gerasimenko (image credit: ESA, Rosetta, MPS for OSIRIS Team MPS, UPD, LAM, IAA SSO, INTA, UPM, DASP, IDA) 259)
Legend to Figure 172: Stunning close up detail focusing on a smooth region on the ‘base’ of the ‘body’ section of comet 67P/Churyumov-Gerasimenko. The image was taken by Rosetta’s OSIRIS narrow-angle camera and downloaded today, 6 August. The image clearly shows a range of features, including boulders, craters and steep cliffs. - The image was taken from a distance of 130 km and the image resolution is 2.4 m.
• August 2, 2014: An image of the comet's activity was acquired (Figure 173) with the OSIRIS camera from a distance of 550 km. The exposure time of the image was 330 seconds and the comet nucleus is saturated to bring out the detail of the comet activity.
• During May - August 2014, Rosetta was executing a series of 10 OCMs (Orbit Correction Maneuvers) to line itself up for arrival at comet 67P on August 6 – approximately one burn every two weeks in May and June, and one per week in July (see also Table 19). 260)
- The first, on 7 May, was quite small, achieving a ΔV of just 20 m/s (with respect to the comet).
- The second was carried out on 21 May, referred to as Big Burn, it lasted for 7 hrs and 16minutes, used 218 kg of propellant and delivered a total ΔV of 289.59 m/s. During the OCM, teams at ESOC monitored parameters such as temperature and pressure of the four thrusters.
- Big Burn2 was on June 4 resulting in a ΔV of 269.5 m/s. At this event, Rosetta was 425, 250 km from the comet, approaching at a relative speed of 463 m/s. The one-way radio signal time to Earth was 25 min:56 seconds.
• Following the January 2014 wake-up of the Rosetta spacecraft after the hibernation period, Rosetta conducted the delicate approach phase during which it slowly discovered its unexpected irregular shape. In order to complete the rendezvous as planned Rosetta had to reduce the miss- distance to a few tens of km and the relative velocity down to ca. 1 m/s. This could have been possible by imparting to the spacecraft a n acceleration of ca. 775 m/s at the right time on June 3, resulting in the two objects flying on the same heliocentric orbit. - However, the uncertainties in the knowledge of the orbit of the comet (ca. 10000 km on the position) were such that it was not possible to conduct this operation without further information. For this reason an optical navigation campaign was designed and conducted such that by means of images taken by the spacecraft the flight controllers at the Mission Control Center at ESOC were in a position to reconstruct the relative trajectories of the two bodies with incremental accuracy. Figure 174 shows how Rosetta was flying on the illuminated side of the comet, a mandatory configuration for the conduct of the optical navigation campaign. 261)
The rendezvous maneuver was split into a sequence of 10 single maneuvers spread over a period of ca. 3 months, during which the parameters of the relative trajectory were resolved to the accuracy required for a successful orbit insertion. Table 19 reports the details of the maneuvers as conducted. The last four columns report respectively:
- the date of the comet flyby in case the manoeuvre would not have been executed before the relevant flyby date
- the distance of the flyby as determined with the orbit reconstruction process
- the 3σ uncertainty associated to this distance
- the time margin the operations team had to complete the manoeuvre before the spacecraft would fly-past the comet and find itself to have to come back on the night side of the comet with limited optical navigation capabilities.
The optical navigation campaign was conducted with images initially taken by the OSIRIS/NAC and later by the spacecraft Navigation Camera (NAVCAM). The very first set of images acquired with the NAC ( Figure 175) at the end of March 2014, when the comet was at ca. 4.8 million km from the spacecraft, allowed the operations team to determine that the comet was ca. 2000 km away from its expected position i.e. well within the expected uncertainty.
• After the successful wakeup, Rosetta and Philae underwent a post hibernation commissioning. The orbiter instruments (like e.g. the OSIRIS cameras, VIRTIS, MIRO, Alice and ROSINA) characterize the target comet and its environment to allow landing site selection and the definition of a separation, descent and landing (SDL) strategy for the Lander (Ref. 63).
- The first switch-on of the Philae lander took place on March 28, 2014, when an updated software for the CDMS (Central Data Management System) was uploaded. This activity was followed by three commissioning blocks, where all lander subsystems and instruments were activated, EEPROMs have been refreshed and in some cases new software was uploaded. No major degradation has been observed, the lander was found to be in a state very similar as during the checkouts before entering hibernation.
- The lander was switched on for several further occasions, before the actual SDL (Separation, Descent and Landing) sequence will be initiated in November. The so-colled PDCS (Pre-Delivery Calibration and Science) phase includes background measurements and “sniffing” of the mass spectrometers (PTOLEMY and COSAC), calibration of the CIVA cameras as well as imaging of the comet nucleus, parallel operations of ROMAP with RPC (magnetic field, ion environment) and activation of CONSERT. The solar generator performance has been verified and the secondary batteries are cycled for capacity degradation measurement.
• January 20, 2014 : ESA's Rosetta orbiter woke up from a 31 month hibernation period at 18:18 GMT to begin an ambitious year of operations to become the first craft to rendezvous with a comet, follow it as it makes its close approach to the Sun and deploy the Philae lander onto its surface. 262)
- Operating on solar energy alone, Rosetta was placed into a deep space slumber in June 2011 as it cruised out to a distance of nearly 800 million km from the warmth of the Sun, beyond the orbit of Jupiter. Now, as Rosetta’s orbit has brought it back to within ‘only’ 673 million km from the Sun, there is enough solar energy to power the spacecraft fully again. - Thus today, still about 9 million km from the comet, Rosetta’s pre-programmed internal ‘alarm clock’ woke up the spacecraft. After warming up its key navigation instruments, coming out of a stabilising spin, and aiming its main radio antenna at Earth, Rosetta sent a signal to let mission operators know it had survived the most distant part of its journey.
- The signal was received by both of NASA’s Goldstone and Canberra ground stations during the first window of opportunity the spacecraft had to communicate with Earth. It was immediately confirmed at ESOC in Darmstadt and the successful wake-up announced.
• June 8, 2011: The final command placing ESA's Rosetta comet-chaser into deep-space hibernation was sent earlier today. With virtually all systems shut down, the probe will now coast for 31 months until waking up in 2014 for arrival at its comet destination. 263)
The event marks the end of the hugely successful first phase of Rosetta's ten-year cruise and the start of a long, dark hibernation during which all instruments and almost all control systems will be silent. The deep sleep is made necessary by the craft's enormous distance from the Sun and the weakness of the sunlight falling on its solar panels, which cannot produce enough electricity to power the probe fully.
Only the computer and several heaters will remain active. These will be automatically controlled to ensure that the entire satellite doesn't freeze as its orbit takes it from 660 million km from the Sun out to 790 million km and back between now and 2014.
• July 10, 2010: Asteroid Lutetia has been revealed as a battered world of many craters. ESA’s Rosetta mission has returned the first close-up images of the asteroid showing it is most probably a primitive survivor from the violent birth of the Solar System. The flyby was a spectacular success with Rosetta performing faultlessly. Closest approach took place at 18:10 CEST (Central European Standard Time), at a distance of 3162 km. 264)
- Lutetia is one of the largest objects orbiting within the main asteroid belt between Mars and Jupiter. Rosetta's encounter revealed an intriguing object which has survived since the birth of the planets, some 4.5 billion years ago. 265)
Discovered in 1852, Lutetia was among the first objects to be classified as an M-type (metallic) asteroid, but radar observations revealed an unusually low albedo, or reflectivity, that was inconsistent with a metallic surface. Meanwhile, spectra obtained at visible and infrared wavelengths found similarities with meteorites known as enstatite chondrites and with carbonaceous chondrite meteorites, typically associated with C-type asteroids.
Figure 176: OSIRIS camera image of Lutetia at closest approach (image credit: ESA)
• Sept. 5, 2008: The Rosetta control room at ESA/ESOC received the first radio signal after closest approach to asteroid (2867) Steins at 22:14 CEST, confirming a smooth fly-by. The closest approach was at a distance of 800 km. Rosetta’s relative speed with respect to asteroid Steins was 8.6 km/s, or about 31 000 km/h. 266)
- The first images from Rosetta’s OSIRIS imaging system and VIRTIS infrared spectrometer were derived from raw data this morning and have delivered spectacular results. 267)
Figure 177: Anaglyph image of Steins in 3D acquired at about the closest approach (image credit: ESA, MPS for OSIRIS Team MPS,UPD, LAM, IAA, RSSD,INTA,UPM, DASP, IDA)
- The observations by OSIRIS and VIRTIS on Rosetta brought new information that could not have been gained from the ground. The dimensions of Steins were found to be 6.67 x 5.81 x 4.47 km3. Rosetta scientists believe that Steins was part of a larger differentiated object that had broken up. It was later struck by other objects, creating impact craters. However, the interior is thought to be a rubble pile and the asteroid will eventually break up. 268)
Figure 178: Artist's view of the Rosetta spacecraft on its way to Comet 67P/Churyumov–Gerasimenko (image credit: DLR) 269)
• On Feb. 25, 2007, the Rosetta spacecraft encountered planet Mars for a gravity assit. The image of Figure 179 was taken just four minutes before the spacecraft reached closest approach, about 1,000 km from the planet’s surface. An area close to the Syrtis region is visible on the planet’s disk (Ref. 58).
• After launch on March 2, 2004, the Rosetta spacecraft was first inserted into a parking orbit, before being sent on its way towards the outer Solar System. 270)
Unfortunately, no existing rocket, not even the powerful European-built Ariane-5, has the capability to send such a large spacecraft directly to Comet 67P/Churyumov-Gerasimenko. Instead, Rosetta will bounce around the inner Solar System like a ‘cosmic billiard ball’, circling the Sun almost four times during its ten-year trek to Comet 67P/Churyumov-Gerasimenko.
Along this roundabout route, Rosetta will enter the asteroid belt twice and gain velocity from gravitational ‘kicks’ provided by close flybys of Mars (2007) and Earth (2005, 2007 and 2009).
Results from NASA's Hubble Space Telescope played a major role in preparing ESA's ambitious Rosetta mission for its new target, comet 67P/Churyumov-Gerasimenko (67P/C-G). For the first time in history, Rosetta will land a probe on a comet and study its origin. Hubble precisely measured the size, shape, and rotational period of comet 67P/C-G.
Hubble's observations revealed that comet 67P/C-G is approximately a three-by-two mile, football-shaped object on which it is possible to land. Mission scientists were concerned that the solid nucleus could be nearly 3.6 miles (6 km) across. The higher gravity on a comet that size might make a soft landing more difficult.
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140) ”Say goodbye to Philae,” DLR, July 26, 2016, URL: http://www.dlr.de
141) ”CometWatch 9 July,” ESA, July 15, 2016, URL: http://blogs.esa.int
142) ”Rosetta finale set for 30 September,” ESA, June 30, 2016, URL: http://sci.esa.int
143) ”Rosetta's comet contains ingredients for life,” ESA, May 27, 2016, URL: http://sci.esa.int
144) Kathrin Altwegg, Hans Balsiger, Akiva Bar-Nun, Jean-Jacques Berthelier, Andre Bieler, Peter Bochsler, Christelle Briois, Ursina Calmonte, Michael R. Combi, Hervé Cottin, Johan De Keyser, Frederik Dhooghe, Bjorn Fiethe, Stephen A. Fuselier, Sébastien Gasc, Tamas I. Gombosi, Kenneth C. Hansen, Myrtha Haessig, Annette Jäckel, Ernest Kopp, Axel Korth, Lena Le Roy, Urs Mall, Bernard Marty, Olivier Mousis, Tobias Owen, Henri Rème, Martin Rubin, Thierry Sémon, Chia-Yu Tzou, James Hunter Waite, Peter Wurz, ”Prebiotic chemicals—amino acid and phosphorus—in the coma of comet 67P/Churyumov-Gerasimenko,” Science Advances, 27 May 2016, Vol. 2, No 5, e1600285, doi: 10.1126/sciadv.1600285, URL:http://advances.sciencemag.org/content/advances/2/5/e1600285.full.pdf
145) ”Around Anuket,” ESA, Space Science image of the week, April 18, 2016, URL: http://www.esa.int/spaceinimages/Images/2016/04/Around_Anuket
146) ”The color-changing comet,” ESA, April 7, 2016, URL: http://www.esa.int/spaceinimages/Images/2016/04/The_colour-changing_comet
147) Gianrico Filacchione, Fabrizio Capaccioni, Mauro Ciarniello, Andrea Raponi, Federico Tosi, Maria Cristina De Sanctis, Stéphane Erard, Dominique Bockelée Morvan, Cedric Leyrat, Gabriele Arnold, Bernard Schmitt, Eric Quirico, Giuseppe Piccioni, Alessandra Migliorini, Maria Teresa Capria, Ernesto Palomba, Priscilla Cerroni, Andrea Longobardo, Antonella Barucci, Sonia Fornasier, Robert W. Carlson, Ralf Jaumann, Katrin Stephan, Lyuba V. Moroz, David Kappel, Batiste Rousseau, Sergio Fonti, Francesca Mancarella, Daniela Despan, Mathilde Faure, ”The global surface composition of 67P/CG nucleus by Rosetta/VIRTIS. (I) Prelanding mission phase,” Ikarus, Available online 16 March 2016, doi:10.1016/j.icarus.2016.02.055, URL: http://tinyurl.com/hsvnjv8
148) C. Goetz, C. Koenders, I. Richter, K. Altwegg, J. Burch, C. Carr, E. Cupido, A. Eriksson, C. Güttler, P. Henri, P. Mokashi, Z. Nemeth, H. Nilsson, M. Rubin, H. Sierks, B. Tsurutani, C. Vallat, M. Volwerk, K.-H. Glassmeier, ”First detection of a diamagnetic cavity at comet 67P/Churyumov-Gerasimenko,” Astronomy and Astrophysics, Vol. 588, April 2016, http://dx.doi.org/10.1051/0004-6361/201527728, published online: March 11, 2016
149) ”Rosetta finds magnetic field-free bubble at comet,” ESA, March 11, 2016, URL: http://www.esa.int/Our_Activities/Space_Science
150) ”Rosetta’s lander faces eternal hibernation,” ESA, Feb. 12, 2016, URL: http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_s_lander_faces_eternal_hibernation
151) ”A slow farewell – Time to say goodbye to Philae,” DLR, Feb. 12, 2016, URL: http://www.dlr.de
152) ”Inside Rosetta's Comet,” ESA, Feb. 4, 2016, URL: http://www.esa.int/Our_Activities/Space_Science/Rosetta/Inside_Rosetta_s_comet
153) M. Pätzold, T. Andert, M. Hahn, S. W. Asmar, J.-P. Barriot, M. K. Bird, B. Häusler, K. Peter, S. Tellmann, E. Grün, P. R. Weissman, H. Sierks, L. Jorda, R. Gaskell, F. Preusker, F. Scholten, ”A homogeneous nucleus for comet 67P/Churyumov–Gerasimenko from its gravity field,” Nature Letter, Vol. 530, Feb. 4, 2016, pp.63-65, doi:10.1038/nature16535
154) ”Exposed ice on Rosetta's comet confirmed as water,” ESA, Jan. 16, 2016, URL: http://www.esa.int/Our_Activities/Space_Science
155) G. Filacchione, M. C. De Sanctis, F. Capaccioni, A. Raponi, F. Tosi, M. Ciarniello, P. Cerroni, G. Piccioni, M. T. Capria, E. Palomba, G. Bellucci, S. Erard, D. Bockelee-Morvan, C. Leyrat, G. Arnold, M. A. Barucci, M. Fulchignoni, B. Schmitt, E. Quirico, R. Jaumann, K. Stephan, A. Longobardo, V. Mennella, A. Migliorini, E. Ammannito, et al., ”Exposed water ice on the nucleus of comet 67P/Churyumov–Gerasimenko,” Nature Letter, Jan. 13, 2016, doi:10.1038/nature16190
157) ”New comet shape model,” ESA, Nov. 30, 2015: URL: http://blowsiest
158) ”From one comet landing to another: planning Rosetta’s grand finale,” ESA Rosetta blog, Nov. 12, 2015, URL: http://blogs.esa.int/rosetta/2015/11/12
159) ”Rosetta and Philae: one year since landing on a comet,” ESA, Nov. 12, 2015, URL: http://www.esa.int/Our_Activities/Space_Science/Rosetta
160) ”The Philae lander – three 'feet' on the ground, and all set to go,” DLR, Nov. 12, 2015, URL: http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-15756/year-all/#/gallery/21163
161) Elisabet Canalias, Thierry Martin, Alejandro Blazquez, Eric Jurado, Thierry Ceolin, Romain Garmier, Julien Lauren-Varin, Alex Torres, Emile Remetean, Jens Biele, Laurent Jorda, Philippe Lamy, Jean-Baptiste Vincent, Holger Sierks,Vladimir Zakharov, Jean-François Crifo, Alexander Rodionov, Wlodek Kofman, Yves Rogez, Alain Herique, Philip Heinisch, Hans-Ulrich Auster, J. P. Bibring, ”Philae’s Flight Dynamics challenges: report of the landing on a comet,”Proceedings of the 66th International Astronautical Congress (IAC 2015), Jerusalem, Israel, Oct.12-16, 2015, paper: IAC-15-C1.1.1
162) Ulrich Herfort, Carlos M. Casas, ”Trajectory Preparation for the Approach of Spacecraft Rosetta to Comet 67p Churyomov-Gerasimenko,” Proceedings of the 25th International Symposium on Space Flight Dynamics, Munich, Germany, Oct. 19-23, 2015, URL: http://issfd.org/2015/files/downloads/papers/074_Herfort.pdf
163) T. Morley, F. Budnik, B. Godard, P. Muñoz, V. Janarthanan, “Rosetta Navigation from Reactivation until Arrival at Comet 67P/Churyumov-Gerasimenko,” Proceedings of the 25th International Symposium on Space Flight Dynamics, Munich, Germany, Oct. 19-23, 2015, URL: http://issfd.org/2015/files/downloads/papers/019_Morley.pdf
164) Bernard Godard, Frank Budnik, Pablo Muñoz, Trevor Morley, Vishnu Janarthanan, “Orbit Determination of Rosetta around Comet 67P/Churyumov-Gerasimenko,” Proceedings of the 25th International Symposium on Space Flight Dynamics, Munich, Germany, Oct. 19-23, 2015, URL: http://issfd.org/2015/files/downloads/papers/124_Godard.pdf
165) F. Castellini, D. Antal-Wokes, R. Pardo de Santayana, K. Vantournhout, “Far Approach Optical Navigation and Comet Photometry for the Rosetta Mission,” Proceedings of the 25th International Symposium on Space Flight Dynamics, Munich, Germany, Oct. 19-23, 2015, URL: http://issfd.org/2015/files/downloads/papers/146_Castellini.pdf
166) R. Pardo de Santayana, M. Lauer, “Optical Measurements for Rosetta Navigation near the Comet,” Proceedings of the 25th International Symposium on Space Flight Dynamics, Munich, Germany, Oct. 19-23, 2015, URL: http://issfd.org/2015/files/downloads/papers/062_Pardo.pdf
167) ”First detection of molecular oxygen at a comet,” ESA, Oct. 28, 2015, URL: http://www.esa.int/Our_Activities/Space_Science
168) A. Bieler, K. Altwegg, H. Balsiger, A. Bar-Nun, J.-J. Berthelier, P. Bochsler, C. Briois, U. Calmonte, M. Combi, J. De Keyser, E. F. van Dishoeck, B. Fiethe, S. A. Fuselier, S. Gasc, T. I. Gombosi, K. C. Hansen, M. Hässig, A. Jäckel, E. Kopp, A. Korth, L. Le Roy, U. Mall, R. Maggiolo, B. Marty, O. Mousis et al., ”Abundant molecular oxygen in the coma of comet 67P/Churyumov–Gerasimenko,” Nature Letter, Vol. 526, pp: 678–681, 29 October 2015, doi:10.1038/nature15707
169) Pablo Muñoz, Frank Budnik, Vicente Companys, Bernard Godard, Carlos M. Casas, Trevor Morley, Vishnu Janarthanan, “Rosetta navigation during lander delivery phase and reconstruction of Philae descent trajectory and rebound,” Proceedings of the 25th International Symposium on Space Flight Dynamics, Munich, Germany, Oct. 19-23, 2015, URL: http://issfd.org/2015/files/downloads/papers/148_Munoz.pdf
170) Thierry Martin, Alejandro Blazquez, Elisabet Canalias, Eric Jurado, Julien Lauren-Varin, Thierry Ceolin, Romain Garmier, Jens Biele, Laurent Jorda, Jean-Baptiste Vincent, Vladimir Zakharov, Jean-François Crifo, Alexander Rodionov, ”Flight Dynamics Analysis for Philae Landing Site Selection,” Proceedings of the 25th International Symposium on Space Flight Dynamics, Munich, Germany, Oct. 19-23, 2015, URL: http://issfd.org/2015/files/downloads/papers/047_Martin.pdf
171) D. S. Antal-Wokes, et al., “Rosetta: Imaging Tools, Practical Challenges and Evolution of Optical Navigation Around a Comet”, AAS/AIAA Astrodynamics Specialist Conference, Vail, CO, USA, 2015
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Elisabet Canalias, Eric Jurado, Emile Remetean, Julien Lauren-Varin,
Benoit Dolives, Alain Herique, Yves Roger, Wlodek Kofman, Pascal Puget,
Pierre Pasquero, Sonia Zine, Laurent Jorda, P. Heinish, ”Philae
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173) ”How Rosetta's Comet got its shape,” ESA, Sept. 28, 2015, URL: http://www.esa.int/Our_Activities/Space_Science/Rosetta/How_Rosetta_s_comet_got_its_shape
174) Matteo Massironi, Emanuele Simioni, Francesco Marzari, Gabriele Cremonese, Lorenza Giacomini, Maurizio Pajola, Laurent Jorda, Giampiero Naletto, Stephen Lowry, Mohamed Ramy El-Maarry, Frank Preusker, Frank Scholten, Holger Sierks, Cesare Barbieri, Philippe Lamy, Rafael Rodrigo, Detlef Koschny, Hans Rickman, Horst Uwe Keller, Michael F. A’Hearn, Jessica Agarwal, Anne-Thérèse Auger, M. Antonella Barucci, Jean-Loup Bertaux, Ivano Bertini, et al., ”Two independent and primitive envelopes of the bilobate nucleus of comet 67P,” Nature, Letter, published online Sept. 28, 2015, doi:10.1038/nature15511, URL: http://www.nature.com/nature/journal/vaop/ncurrent/full/nature15511.html
”ESA's Rosetta spacecraft has provided evidence for a daily
water-ice cycle on and near the surface of comets,” ESA, Sept.
23, 2016, URL: http://sci.esa.int
176) M. C. De Sanctis, F. Capaccioni, M. Ciarniello, G. Filacchione, M. Formisano, S. Mottola, A. Raponi, F. Tosi, D. Bockelée-Morvan, S. Erard, C. Leyrat, B. Schmitt, E. Ammannito, G. Arnold, M. A. Barucci, M. Combi, M. T. Capria, P. Cerroni, W.-H. Ip, E. Kuehrt, T. B. McCord, E. Palomba, P. Beck, E. Quirico & The VIRTIS Team, ”The diurnal cycle of water ice on comet 67P/Churyumov–Gerasimenko,” Nature, Vol. 525, pp: 500–503, 24 September 2015, doi:10.1038/nature14869
177) ”The water-ice cycle of Rosetta's comet,” ESA, Sept. 23, 2015, URL: http://sci.esa.int
178) “Rosetta Mission: Ptolemy sniffs next piece of the comet puzzle,” OU (Open University), Sept. 15, 2015, URL: http://www3.open.ac.uk/media/fullstory.aspx?id=29376
179) Andrew Morse, Olivier Mousis, Simon Sheridan, Geraint Morgan, Dan Andrews, Simeon Barber, Ian Wright, “Low CO/CO2 ratios of comet 67P measured at the Abydos landing site by the Ptolemy mass spectrometer,” Astronomy & Astrophysics, 2015, DOI: 10.1051/0004-6361/201526624
180) “Rosetta's big day in the Sun,” ESA, August 12, 2015, URL: http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_s_big_day_in_the_Sun
181) “Comet's firework display ahead of perihelion,” ESA, Aug. 11, 2015, URL: http://www.esa.int/Our_Activities/Space_Science
182) “Comet on 6 August 2014 and 6 August 2015,” ESA Space Science Image of the week, August 10, 2015, URL: http://www.esa.int/spaceinimages/Images
183) “Celebrating a year at the comet,” ESA, August 6, 2015, URL: http://www.esa.int/Our_Activities/Space_Science/Rosetta/Celebrating_a_year_at_the_comet
184) “Science on the surface of a comet,” ESA, July 30, 2015, URL: http://sci.esa.int
185) J.-P. Bibring, M. G. G. T. Taylor, C. Alexander, U. Auster, J. Biele, A. Ercoli Finzi, F. Goesmann, G. Klingelhoefer, W. Kofman, S. Mottola, K. J. Seidensticker, T. Spohn, I. Wright , “Philae’s First Days On The Comet,” Science, Vol. 349, No 6247, p. 493, July 31, 2015, URL: http://www.sciencemag.org/content/349/6247/493.full
186) ”Philae's adapted first science sequence,” ESA, July 30,2015, URL: http://sci.esa.int
187) “Headache for Philae,” DLR News, July 20, 2015, URL: http://www.dlr.de
188) “Rosetta: Preparing for Perihelion,” ESA, July 13, 2015, URL: http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_preparing_for_perihelion
189) “New communication with Philae – commands executed successfully,” DLR, July 10, 2015, URL: http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-14156/year-all/#/gallery/17198
190) “Comet sinkholes generate jets,” ESA, July 1, 2015, URL: http://www.esa.int/Our_Activities/Space_Science/Rosetta/Comet_sinkholes_generate_jets
191) Jean-Baptiste Vincent, Dennis Bodewits, Sébastien Besse, Holger Sierks, Cesare Barbieri, Philippe Lamy, Rafael Rodrigo, Detlef Koschny, Hans Rickman, Horst Uwe Keller, Jessica Agarwal, Michael F. A'Hearn, Anne-Thérèse Auger, M. Antonella Barucci, Jean-Loup Bertaux, Ivano Bertini, Claire Capanna, Gabriele Cremonese, Vania Da Deppo, Björn Davidsson, Stefano Debei, Mariolino De Cecco, Mohamed Ramy El-Maarry, Francesca Ferri, Sonia Fornasier, et al.,“Large heterogeneities in comet 67P as revealed by active pits from sinkhole collapse,” Nature Letter, Vol. 523, pp: 63–66, published online 01 July 2015, doi:10.1038/nature14564
192) “Active pits on comet,” ESA, URL: http://www.esa.int/spaceinimages/Images/2015/07/Active_pits_on_comet
193) “Contact with Philae still irregular and unstable,” DLR, June 26, 2015, URL: http://www.dlr.de
194) “Exposed water ice detected on Comet's surface,” ESA, June 24, 2015, URL: http://www.esa.int/Our_Activities/Space_Science
195) A. Pommerol, N. Thomas, M. R. El-Maarry, M. Pajola, O. Groussin, A.-T. Auger, N. Oklay, S. Fornasier, C. Feller, B. Davidsson, A. Gracia-Berná, B. Jost, R. Marschall, O. Poch, M. A. Barucci, J. -L. Bertaux, F. La Forgia, H. U. Keller; 10, E. Kührt, S. C. Lowry, S. Mottola, G. Naletto, H. Sierks, C. Barbieri, P. L. Lamy, R. Rodrigo, D. Koschny, H. Rickman, J. Agarwal, M. F. A’Hearn, I. Bertini, S. Boudreault, G. Cremonese, V. Da Deppo, M. De Cecco, S. Debei, C. Güttler, M. Fulle, P. J. Gutierrez, S. F. Hviid, W. -H. Ip, L. Jorda, J. Knollenberg, G. Kovacs, J. -R. Kramm, E. Küppers, L. Lara, M. Lazzarin, J. L. Lopez Moreno, F. Marzari, H. Michalik, F. Preusker, F. Scholten, C. Tubiana, J. -B. Vincent, “OSIRIS observations of meter-sized exposures of H2O ice at the surface of 67P/Churyumov-Gerasimenko and interpretation using laboratory experiments,” Astronomy & Astrophysics, Accepted 15 May 2015, URL: http://www.aanda.org/articles/aa/pdf/forth/aa25977-15.pdf
196) “Rosetta mission extended,” ESA, June 23, 2015, URL: http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_mission_extended
197) “Lander Control Center in contact with Philae once again,” DLR, June 19, 2015, URL: http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-13980/year-all/#/gallery/17198
198) “Philae wake-up kicks off intense planning,” ESA, June 15, 2016, URL: http://sci.esa.int
199) Manuela Braun, Sephan Ulamec, “Lander Philae is awake – 'Hello' from space,” DLR, June 14, 2015, URL: http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-13900/year-all/#/gallery/2563
200) “Rosetta's Lander Philae wakes up from hibernation,” ESA, June 14, 2015, URL: http://www.esa.int/Our_Activities/Space_Science
201) “Ultraviolet study reveals surprises in comet coma,” ESA, June 2, 2015, URL: http://sci.esa.int
DC Agle, Dwayne Brown, Joe Fohn, Markus Bauer, “NASA Instrument
on Rosetta Makes Comet Atmosphere Discovery,” NASA, June 2, 2015,
203) Paul D. Feldman, Michael F. A'Hearn, Jean-Loup Bertaux, Lori M. Feaga, Joel Wm. Parker, Eric Schindhelm, Andrew J. Steffl , S. Alan Stern, Harold A. Weaver, Holger Sierks, Jean-Baptiste Vincent, “Measurements of the Near-Nucleus Coma of Comet 67P/Churyumov-Gerasimenko with the Alice Far-Ultraviolet Spectrograph on Rosetta,” accepted for publication in Astronomy and Astrophysics, arXiv:1506.01203 [astro-ph.EP], DOI: 10.1051/0004-6361/201525925
204) “Rosetta and Philae find comet not magnetized,” ESA, April 14, 2015, URL: http://www.esa.int
205) “Rosetta and Philae find comet not magnetized,” ESA, April 14, 2015, URL: http://sci.esa.int/rosetta/55788-rosetta-and-philae-find-comet-not-magnetised/
207) Hans-Ulrich Auster, Istvan Apathy, Gerhard Berghofer, Karl-Heinz Fornacon, Anatoli Remizov, Chris Carr, Carsten Güttler, Gerhard Haerendel, Philip Heinisch, David Hercik, Martin Hilchenbach, Ekkehard Kührt, Werner Magnes, Uwe Motschmann, Ingo Richter, Christopher T. Russell, Anita Przyklenk, Konrad Schwingenschuh, Holger Sierks, Karl-Heinz Glassmeier, “The non-magnetic nucleus of Comet 67P/Churyumov-Gerasimenko," Science Express, April 14, 2015, DOI: 10.1126/science.aaa5102
208) “Comet activity 31 January – 25 March 2015,” ESA, April 13, 2015, URL: http://www.esa.int/spaceinimages/Images/2015/04/Comet_activity_31_January_25_March_2015
209) “Waiting patiently for Philae,” DLR, March 20, 2015, URL: http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-13123/year-all/#/gallery/2564
210) “Rosetta makes first detection of molecular nitrogen at a comet,” ESA, March 19, 2015, URL: http://www.esa.int/Our_Activities/Space_Science/Rosetta
211) “All-round activity – CometWatch 25-26-27 February, “ ESA, March 4, 2015, URL: http://blogs.esa.int/rosetta/2015/03/04/all-round-activity-cometwatch-25-26-27-february/
212) “Comet flyby: OSIRIS catches glimpse of Rosetta’s shadow,” ESA, March 3, 2015, URL: http://blogs.esa.int/rosetta/2015/03/03/comet-flyby-osiris-catches-glimpse-of-rosettas-shadow/
213) “Rosetta's closest encounter,” ESA, Feb. 17, 2015, URL: http://www.esa.int/spaceinvideos/Videos/2015/02/Rosetta_s_closest_encounter
214) “Comet on 14 February from 8.7 km,” ESA, Feb. 16, 2015, URL: http://www.esa.int/spaceinimages/Images/2015/Comet_on_14_February_from_8.7_km
215) “Rosetta watches comet shed its dusty coat,” ESA, January 26, 2015, URL: http://sci.esa.int/rosetta/55341-rosetta-watches-comet-shed-its-dusty-coat/
216) Rita Schulz, Martin Hilchenbach, Yves Langevin, Jochen Kissel, Johan Silen, Christelle Briois, Cecile Engrand, Klaus Hornung, Donia Baklouti, Anaïs Bardyn, Hervé Cottin, Henning Fischer, Nicolas Fray, Marie Godard, Harry Lehto, Léna Le Roy, Sihane Merouane, François-Régis Orthous-Daunay, John Paquette, Jouni Rynö, Sandra Siljeström, Oliver Stenzel, Laurent Thirkell, Kurt Varmuza, Boris Zaprudin, “Comet 67P/Churyumov-Gerasimenko sheds dust coat accumulated over the past four years,” Nature, Published online 26 January 2015 , doi:10.1038/nature14159
217) “Getting to know Rosetta's comet,” ESA, January 22, 2015, URL: http://sci.esa.int
218) “Rosetta fuels debate on origin of Earth’s oceans,” ESA, Dec. 10, 2014, URL: http://www.esa.int/Our_Activities/Space_Science
219) K. Altwegg, H. Balsiger, A. Bar-Nun, J. J. Berthelier, A. Bieler, P. Bochsler, C. Briois, U. Calmonte, M. Combi, J. De Keyser, P. Eberhardt, B. Fiethe, S. Fuselier, S. Gasc, T. I. Gombosi, K. C. Hansen, M. Hässig, A. Jäckel, E. Kopp, A. Korth, L. LeRoy, U. Mall, B. Marty, O. Mousis, E. Neefs, T. Owen, H. Rème, M. Rubin, T. Sémon, C.-Y. Tzou, H. Waite, P. Wurz, “67P/Churyumov-Gerasimenko, a Jupiter family comet with a high D/H ratio,” Science, December 10, 2014, Science DOI: 10.1126/science.1261952
220) “Rosetta fuels debate on origin of Earth’s oceans,” UKSA, Dec. 11, 2014, URL: https://www.gov.uk/government/news/rosetta-fuels-debate-on-origin-of-earths-oceans
Emily Baldwin, Carl Walker, “Living with a Comet, Rosetta in
ictures, August-November 2014,” ESA Bulletin, No 160, November
2014, pp:2-13, URL: http://esamultimedia.esa.int
222) “First measurements of comet’s water ratio,” ESA, Dec. 10, 2014, URL: http://www.esa.int/spaceinimages/Images/2014/12/First_measurements_of_comet_s_water_ratio
223) “Deuterium-to-hydrogen in the Solar System,” ESA, Dec. 10, 2014, URL: http://www.esa.int/spaceinimages/Images/2014/12/Deuterium-to-hydrogen_in_the_Solar_System
224) “Kuiper Belt and Oort Cloud in context,” ESA, Dec. 10, 2014, URL: http://www.esa.int/spaceinimages/Images/2014/12/Kuiper_Belt_and_Oort_Cloud_in_context
225) “Did Philae graze a crater rim during its first bounce?,” ESA, Nov. 28, 2014, URL: http://blogs.esa.int/rosetta/2014/11/28/did-philae-graze-a-crater-rim-during-its-first-bounce/
226) “Rosetta continuous into its full science phase,” ESA, Nov. 19, 2014, URL: http://www.esa.int/Our_Activities/Operations/Rosetta_continues_into_its_full_science_phase
227) “OSIRIS spots Philae drifting across the comet,” ESA, Nov. 17, 2014, URL: http://www.esa.int/spaceinimages/Images/2014/11/OSIRIS_spots_Philae_drifting_across_the_comet
228) “Churyumov-Gerasimenko – hard ice and organic molecules,” DLR News, Nov. 17, 2014, URL: http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-12176/year-all/#/gallery/17219
229) “Philae finds hard ice and organic molecules,” UKSA, Nov. 18, 2014, URL: https://www.gov.uk/government/news/philae-finds-hard-ice-and-organic-molecules
230) “Pioneering Philae completes main mission before hibernation,” ESA, Nov. 15, 2014, URL: http://www.esa.int/Our_Activities/Space_Science
231) “Three touchdowns for Rosetta's lander,” ESA, Nov. 14, 2014, URL: http://www.esa.int/Our_Activities/Space_Science/Rosetta/Three_touchdowns_for_Rosetta_s_lander
232) “Welcome to a comet,” ESA, Nov. 13, 2014, URL: http://www.esa.int/spaceinimages/Images/2014/11/Welcome_to_a_comet
233) “Comet media briefing replay,” ESA, Nov. 13, 2014, URL: http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_media_briefing_replay
234) Dwayne Brown, ”NASA Statement on Successful Rosetta Comet Landing,” NASA Release 14-315, Nov. 12, 2014, URL: https://www.nasa.gov/press/2014/
235) “Touchdown! Rosetta's Philae Probe lands on Comet,” ESA, Nov. 12, 2014, URL: http://www.esa.int/Our_Activities/Space_Science
236) Rosetta and Philae separation confirmed,” ESA, Nov. 12, 2014, URL: http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_and_Philae_separation_confirmed
237) “Farewell Rosetta,” ESA, image released on Nov. 12, 2014 at 14:50 CET, URL: http://www.esa.int/spaceinimages/Images/2014/11/Farewell_Rosetta
238) “Farewell Philae - narrow-angle view,” ESA, image released on Nov. 12, 2014 at 15:59 CET, URL: http://www.esa.int/spaceinimages/Images/2014/11/Farewell_Philae_-_narrow-angle_view
239) “ROLIS-camera takes closest picture of Churyumov-Gerasimenko,” DLR, Nov. 12, 2014, URL: http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-12091/#/gallery/17139
240) “Top 10 at 10 km,” ESA, Nov. 11, 2014, URL: http://www.esa.int/Our_Activities/Space_Science/Rosetta/Highlights/Top_10_at_10_km
241) “Farewell 'J', Hello Agilkia,” ESA, Nov. 4, 2014, URL: http://www.esa.int/Our_Activities/Space_Science/Rosetta/Farewell_J_hello_Agilkia
242) “Philae landing site J is now named 'Agilkia',” DLR, Nov. 4, 2014, URL: http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-11960/year-all/#/gallery/16768
243) “Once upon a time... preparing for comet landing,” ESA, Nov. 3, 2014, URL: http://www.esa.int/spaceinvideos/Videos/2014/11/Once_upon_a_time_preparing_for_comet_landing
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247) “ESA confirms the primary landing site for Rosetta,” ESA, Oct. 15, 2014: http://www.esa.int/Our_Activities/Space_Science
248) “Close-up of boulder Cheops,” UKSA, Oct. 10, 2014, URL: https://www.gov.uk
249) “Bolder Cheops,” ESA, Our Week in images, Oct. 6-10, 2014, URL: http://www.esa.int/Highlights/Week_In_Images_06_10_October_2014
250) “LutetiaÄs dark side hosts hidden crater,” ESA, Oct. 8, 2014, URL: http://www.esa.int/Our_Activities/Space_Science/Rosetta/Lutetia_s_dark_side_hosts_hidden_crater
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253) “Rosetta to deploy Lander on 12 November,” ESA, Sept. 26, 2014, URL: http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_to_deploy_lander_on_12_November
255) “ESA’s Rosetta Probe Gets Close to Duck-Shaped Comet 67P/C-K,” ESA, Sept. 5, 2014, URL: http://blogs.voanews.com/science-world/2014/09/05
256) “Rosetta: Landing site search narrows,” ESA, August 25, 2014, URL: http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_Landing_site_search_narrows
258) “Rosetta arrives at comet destination,” ESA, August 6, 2014, URL: http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_arrives_at_comet_destination
259) “Comet details,” ESA, Aug. 6, 2014, URL: http://www.esa.int/spaceinimages/Images/2014/08/Comet_details
260) “The Big Burns – Part 2,” ESA, June 3, 2014, URL: http://blogs.esa.int
261) Andrea Accomazzo, Paolo Ferri , Sylvain Lodiot, Jose-Luis Pellon-Bailon, Armelle Hubault, Roberto Porta, Jakub Urbanek, Ritchie Kay , Matthias Eiblmaier, Tiago Francisco, “Rosetta operations at the comet,” Proceedings of the 65th International Astronautical Congress (IAC 2014), Toronto, Canada, Sept. 29-Oct. 3, 2014, paper: IAC-14-A3,4.1
262) “ESA's sleeping beauty wakes up from deep space hibernation,” ESA, Jan. 20. 2014, URL: http://www.esa.int/Our_Activities/Space_Science/Rosetta
263) “Rosetta comet probe enters hibernation in deep space,” ESA, June 8, 2011, URL: http://www.esa.int/Our_Activities/Operations/
264) “Rosetta triumphs at asteroid Lutetia,” ESA, July 10, 2010, URL: http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_triumphs_at_asteroid_Lutetia
265) “Rosetta reveals mysterious Lutetia,” ESA, Oct. 21, 2011, ESA, URL: http://sci.esa.int
266) “Rosetta Steins Flyby confirmed,” ESA Sept. 5, 2008, URL: http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_Steins_fly-by_confirmed
267) “Steins: A diamond in the sky,” ESA, Sept. 6, 2008, URL: http://www.esa.int/Our_Activities/Space_Science/Rosetta/Steins_A_diamond_in_the_sky
270) “The long trek,” ESA, URL: http://www.esa.int/Our_Activities/Space_Science/Rosetta/The_long_trek
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (email@example.com).