Minimize GAIA (Global Astrometric Interferometer for Astrophysics)

Gaia Astrometry Mission

Spacecraft    Launch    Mission Status    Payload Module    Ground Segment    Big Data Archive    References

Gaia (mother Earth in Greek mythology) is an ESA cornerstone space astrometric mission, part of the Horizon 2000 Plus long-term scientific program, with the goal to compile a 3D space catalog of > 1000 million stars, or roughly 1% of the stars in our home galaxy, the Milky Way. Gaia will monitor each of its target stars about 70 times to a magnitude of G=20 over a period of 5 years. It will precisely chart their positions, distances, movements, and changes in brightness. It is expected to discover hundreds of thousands of new celestial objects, such as extra-solar planets and brown dwarfs, and observe hundreds of thousands of asteroids within our own Solar System. The mission will also study about 500,000 distant quasars and will provide stringent new tests of Albert Einstein’s General Theory of Relativity. 1) 2) 3) 4) 5)

Cataloguing the night sky is an essential part of astronomy. Before astronomers can investigate a celestial object, they must know where to find it. Without this knowledge, astronomers would wander helplessly in what Galileo once termed a ‘dark labyrinth’.

During the satellite’s expected lifetime of five years, Gaia will observe each star about 70 times, each time recording its brightness, color and, most importantly, its position. The precise measurement of a celestial object’s position is known as astrometry, and since humans first started studying the sky, astronomers have devoted much of their time to this art. However, Gaia will do so with extraordinary precision, far beyond the dreams of those ancient astronomers.

By comparing Gaia’s series of precise observations, today’s astronomers will soon be able to make precise measurements of the apparent movement of a star across the heavens, enabling them to determine its distance and motion through space. The resulting database will allow astronomers to trace the history of the Milky Way.

In the course of charting the sky, Gaia’s highly superior instruments are expected to uncover vast numbers of previously unknown celestial objects, as well as studying normal stars. Its expected haul includes asteroids in our Solar System, icy bodies in the outer Solar System, failed stars, infant stars, planets around other stars, far-distant stellar explosions, black holes in the process of feeding and giant black holes at the centers of other galaxies.

The primary mission objectives are:

• Measure the positions and velocity of approximately one billion stars in our Galaxy

• Determine their brightness, temperature, composition and motion through space

• Create a three-dimensional map of the Galaxy.

Additional discoveries expected:

- hundreds of thousands of asteroids and comets within our Solar System

- seven thousand planets beyond our Solar System

- tens of thousands of ‘failed’ stars, called brown dwarfs

- twenty thousand exploding stars, called supernovae

- hundreds of thousands of distant active galaxies, called quasars.

The Gaia objective is to provide a very accurate dynamical 3D map of our Galaxy by using global astrometry from space, complemented with multi-color multi-epoch photometric measurements. The aim is to produce a catalog complete for star magnitudes up to 20, which corresponds to more than one billion stars or about 1% of the stars of our Galaxy. The instrument sensitivity is such that distances beyond 20-100 kiloparsec (kpc) will be covered, therefore including the Galaxy bulge (8.5 kpc) and spiral arms. The measurements will not be limited to the Milky Way stars. These include the structure, dynamics and stellar population of the Magellanic Clouds, the space motions of Local Group Galaxies and studies of supernovae, galactic nuclei and quasars, the latter being used for materializing the inertial frame for Gaia measurements.

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Figure 1: Gaia measurements objectives (image credit: ESA, Airbus Defence and Space) 6)


Background: Gaia is ESA's second space mission dedicated to astrometry. It builds on the legacy of the successful Hipparcos mission (1989-1993). 7) Like Hipparcos, Gaia's observation strategy is based on detecting stellar positions in two fields of view separated by a 'basic angle', which for Gaia is 106.5º. This strategy allows astronomers to establish a coherent reference frame over the entire sky, yielding highly accurate measurements of stellar positions.

After a detailed concept and technology study during 1998–2000, Gaia was selected as a confirmed mission within ESA’s scientific program in October 2000. It was confirmed by ESA’s Science Program Committee following a re-evaluation of the science program in June 2002, and reconfirmed following another re-evaluation of the program in November 2003. The project entered Phase-B2/C/D in February 2006. As of the summer 2012, Gaia is in Phase-D (Qualification and Production) and will be launched in the second half of 2013. 8) 9) 10)

• In June 2013, ESA's billion-star surveyor, Gaia, has completed final preparations in Europe and is ready to depart for its launch site in French Guiana. The Gaia spacecraft arrived in Cayenne, French Guiana, on August 23, 2013 on board the Antonov 124 aircraft.

• On Oct. 23, 2013, ESA postponed the launch of the Gaia mission. The decision was taken due to a technical issue that was identified in another satellite already in orbit. The issue concerns components used in two transponders on Gaia that generate ‘timing signals’ for downlinking the science telemetry. To avoid potential problems, they will be replaced.

The transponders were removed from Gaia at Kourou and returned to Europe, where the potentially faulty components were replaced and verified. After the replacements have been made, the transponders will be refitted to Gaia and a final verification test made. As a consequence of these precautionary measures, it will not be possible to launch Gaia within the window that includes the previously targeted launch date of 20 November. The next available launch window is 17 December to 5 January 2014. 11)

• Update Oct. 20, 2013: The upcoming launch manifest of Arianespace has now been established. Gaia is scheduled for launch on 20 December.

• Update Nov. 22, 2013: The checks on the Gaia satellite are proceeding well, enabling the launch to take place on December 19, 2013 (Ref. 11).


Some astrometry basics:

The precise measurement of a celestial object’s position is known as astrometry, and since humans first started studying the sky, astronomers have devoted much of their time to this art. However, Gaia will do so with extraordinary precision, far beyond the dreams of those ancient astronomers (Ref. 21). 12)

By comparing Gaia’s series of precise observations, today’s astronomers will soon be able to make precise measurements of the apparent movement of a star across the heavens, enabling them to determine its distance and motion through space. The resulting database will allow astronomers to trace the history of the Milky Way.

In the course of charting the sky, Gaia’s highly superior instruments are expected to uncover vast numbers of previously unknown celestial objects, as well as studying normal stars. Its expected haul includes asteroids in our Solar System, icy bodies in the outer Solar System, failed stars, infant stars, planets around other stars, far-distant stellar explosions, black holes in the process of feeding and giant black holes at the centers of other galaxies. Gaia will be a discovery machine.

Stars as individuals and collectives:

To understand fully the physics of a star, its distance from Earth must be known. This is more difficult than it sounds because stars are so remote. Even the closest one is 40 trillion km away, and we cannot send spacecraft out to them to measure as they go. Nor can we bounce radar signals off them, which is the method used to measure distances within the Solar System. Instead, astronomers have developed other techniques for measuring and estimating distances.

The most reliable and only direct way to measure the distance of a star is by determining its 'parallax'. By obtaining extremely precise measurements of the positions of stars, Gaia will yield the parallax for one billion stars; more than 99% of these have never had their distances measured accurately. Gaia will also deliver accurate measurements of other important stellar parameters, including the brightness, temperature, composition and mass. The observations will cover many different types of stars and many different stages of stellar evolution.

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Figure 2: Distance to a star can be calculated with simple trigonometry from the measured parallax angle (1 a.u. is 1 Astronomical Unit, or 149.6 million km), image credit: ESA/Medialab

The principles of Gaia:

At its heart, Gaia is a space telescope – or rather, two space telescopes that work as one. These two telescopes use ten mirrors of various sizes and surface shapes to collect, focus and direct light to Gaia’s instruments for detection. The main instrument, an astrometer, precisely determines the positions of stars in the sky, while the photometer and spectrometer spread their light out into spectra for analysis.

Gaia’s telescopes point at two different portions of the sky, separated by a constant 106.5º. Each has a large primary mirror with a collecting area of about 0.7 m2. On Earth we are used to round telescope mirrors, but Gaia’s will be rectangular to make the most efficient use of the limited space within the spacecraft. These are not large mirrors by modern astronomical standards, but Gaia’s great advantage is that it will be observing from space, where there is no atmospheric disturbance to blur the images. A smaller telescope in space can yield more accurate results than a large telescope on Earth.

Gaia is just 3.5 m across, so three curved mirrors and three flat ones are used to focus and repeatedly fold the light beam over a total path of 35 m before the light hits the sensitive, custom-made detectors. Together, Gaia’s telescopes and detectors will be powerful enough to detect stars up to 400,000 times fainter than those visible to the naked eye.

Gaia uses the global astrometry concept demonstrated by Hipparcos. The principle is to link stars with large angular distances in a network where each star is connected to a large number of other stars in every direction. The condition of closure of the network ensures the reduction of the position errors of all stars. This is achieved by the simultaneous observation of two fields of views separated by a very stable basic angle. The spacecraft is slowly rotating at a constant angular rate of 1º/min around a spin axis perpendicular to both fields of view, which describe a great circle on the sky in 6 hours. The spacecraft rotation axis makes an angle of 45º with the Sun direction (Figure 3). A slow precession around the Sun-to-Earth direction, with a 63.12 days period, enables to repeat the observation of sky objects with 86 transits on average over the 5 years of mission.

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Figure 3: Illustration of the sky scanning principle (image credit: ESA)

The resulting performance will enable a breakthrough in the astrometry field, as well regarding star position and velocity performance as for the number of objects observed.

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Figure 4: Gaia will improve the accuracy of astrometry measurements by several orders of magnitude compared with previous systems and observations (image credit: ESA)




Spacecraft:

Gaia is an exceptionally complex space observatory. ESA awarded Airbus Defence and Space (former Astrium SAS,Toulouse, France) the prime contract in May 2006 to develop and build the spacecraft. Together with the German and British branches of Astrium, more than 50 industrial subcontractor companies from across Europe are involved in building this discovery machine. The Gaia DPAC (Data Processing and Analysis Consortium) will process the raw data to be published in the largest stellar catalog ever made. 13) 14) 15) 16) 17) 18) 19) 20)

The Gaia spacecraft is composed of two sections: the Payload Module and the Service Module. The Payload Module is housed inside a protective dome and contains the two telescopes and the three science instruments. They are all mounted on a torus made of a ceramic material (silicon carbide). The extraordinary measurement accuracy required from Gaia calls for an extremely stable Payload Module that will barely move or deform once in space; this is achieved thanks to the extensive use of this material. 21)

Underneath the Payload Module, the Service Module contains electronic units to run the instruments, as well as the propulsion system, communications units and other essential components. These components are mounted on CFRP (CarbonFiber Reinforced Plastic) panels in a conical framework.

Finally, beneath the Service Module, a large sunshield keeps the spacecraft in shadow, maintaining the Payload Module at an almost constant temperature of around -110ºC, to allow the instruments to take their precise and sensitive readings. The sunshield measures about 10 m across, too large for the launch vehicle fairing, so it comprises a dozen folding panels that will be deployed after launch. Some of the solar array panels that are needed to generate power are fixed on the sunshield, with the rest on the bottom of the spacecraft.

The Gaia spacecraft configuration is driven by the required very high thermo-mechanical stability of the entire spacecraft. A low disturbance cold gas micro-propulsion is used for fine attitude control. The astrometric instrument is used for precise rate sensing in fine pointing operating mode.

Spacecraft launch mass

~2034 kg (dry mass = 1392 kg)

Spacecraft power

1.91 kW (EOL)

Spacecraft dimensions

Height = 3.5 m, deployed diameter = 10 m

Mission duration

5 years

Table 1: Parameters of the Gaia spacecraft

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Figure 5: Artist's rendition of the deployed Gaia spacecraft (image credit: ESA)


SVM (Service Module):

The SVM is generally referred to as 'the platform'. The SVM in turn is comprised of MSM (Mechanical Service Module) and an ESM (Electrical Service Module). 22) 23) 24)

MSM (Mechanical Service Module): The spacecraft main structure is of hexagonal conical shape. It is a sandwich panel structure with CFRP (Carbon Fiber Reinforced Plastic) face sheets, and a central cone supporting the propellant tanks. The MSM houses instruments needed for the basic control and operation of the satellite; this includes all mechanical, structural and thermal elements that support the instrument payload and spacecraft electronics. It also includes the chemical & micro propulsion systems, the deployable sunshield with solar arrays, the payload thermal tent and harness. The module consists of a central tube that is about 1.17 m long and hosts six radial panels to create a hexagonal spacecraft shape.

The service module also houses the communication subsystem, central computer and data handling subsystem, the high rate data telemetry, attitude control and star trackers. For telemetry and telecommand, low gain antenna uplink and downlink with a few kbit/s capacity are employed. The high gain antenna used for the science telemetry downlink will be used during each ground station visibility period of an average of about 8 hours per day.

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Figure 6: Photo of the SVM integration (image credit: EADS Astrium)

ESM (Electrical Service Module): The ESM design is driven by the science performance (attitude control laws with the hybridization of star tracker and payload measurements, high rate data telemetry, and regulated power bus for thermal stability). It houses the AOCS units, the communication subsystem, central computer and data handling subsystem, and the power subsystem.

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Figure 7: Diagram of the ESM (image credit: EADS Astrium)

AOCS (Attitude and Orbit Control Subsystem). The AOCS subsystem is characterized by:

- High precision 3-axis control

- The ASTRO (Astrometric) instrument is used for precise rate sensing during the fine pointing operational mode

- A high precision gyroscope is used for quick and efficient transitions during the fine pointing operational mode. Three FOGs (Fiber Optics Gyroscopes) use the interference of light to detect mechanical rotation. Each unit contains four closed-loop gyroscope channels to provide built-in redundancy.

- Rugged flight-proven initial acquisition and safe modes

- Three sun acquisition sensors plus one gyroscope provide spin-axis stabilization during the L2 transfer phase of the mission

- One large field of view star sensor plus use of the main instrument SM (Sky Mapper) for the 3-axis controlled operational phase.

Gaia AOCS architecture is based on a fully redundant set of equipment. Moving parts on board are strictly minimized (e.g. no reaction wheels, no mechanically steerable antenna). The data is downlinked through a novel electromagnetically steerable phased array antenna and attitude control is provided by a micro propulsion system that has its first flight use with Gaia. An atomic clock is used for precise time-stamping.

Two Autonomous Star Trackers are used in cold redundancy three FSS (Fine Sun Sensors) are used in hot redundancy through triple majority voting. Three Gyro packages provide coarse rate measurements where each gyro package comprises two fully independent co-aligned channels (i.e. a fiber optic gyroscope sensor plus associated electronics per channel), with the channels being used in cold redundancy. As mentioned above, the payload module provides very precise rate measurements when the spacecraft is operated in fine pointing science modes. 25)

For actuators, a bi-propellant Chemical Propulsion System (2 x 8 10 N thrusters used in cold redundancy) is used for orbit maintenance and attitude control in coarse AOCS modes (circa 350 kg MON + MMH).

The Micro-propulsion System (2 x 6 proportional micro-thrusters) can provide a range of 0 - 1000 µN at a resolution of 0.1 µN. The individual thrusters are driven by the micro-propulsion electronic, which is internally redundant and used in cold redundancy (circa 57 kg of GN2). The nominal science AOCS mode uses the cold gas micro-propulsion system.

C&DMS (Command & Data Management Subsystem). The C&DMS is characterized by:

- An ERC-32 based central computer and distinct input/output units for efficient software development

- Two segregated MIL-STD-1553 B data buses: one for the payload module and one for the service module

- SpaceWire data links for high-speed payload data

- FDIR architecture aiming at preserving payload integrity, with built-in autonomy for increased availability.

The PDHU (Payload Data Handling Unit) is, among other things, the 'hard-disk' of Gaia, responsible for temporary storage of science data received from the telescope before transmission back to Earth. It will receive thousands of compressed images per second from the observing system; this data will be sorted and stored. The individual star data objects will be prioritized based on the magnitude of the star. A complex file management system allows deletion of low-priority data in the event of data rates or volumes that exceed the capacity of the storage or transmission systems.

The solid-state storage subsystem of the PDHU has a capacity of 960 GB which, while not impressive by terrestrial standards, is extremely large for a space system. It uses a total of 240 SDRAM modules, each with a capacity of 4 GB, which populate six memory boards. The PDHU controller board is responsible for communication with the other spacecraft subsystems, file system management and the management of telemetry and telecommands. 26) 27)

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Figure 8: The PDHU (Payload Data Handling Unit), image credit: ESA

The PDHU communicates with the gigapixel focal plane over seven redundant 40 Mbit/s SpaceWire channels to acquire the scientific data coming from the seven VPUs (Video Processing Units) of the camera. The unit's controller sorts the incoming data according to star magnitude and manages deletion of low priority data should this become necessary. It sends data for transmission to Earth under the control of the CDMU (Command and Data Management Unit). The PDHU communicates with the CDMU via a MIL-STD-1553 data bus and delivers the science data over two 10 Mbit/s PacketWire channels.- The PDHU consumes only 26 W, has a mass of 14 kg, and occupies a volume of 2.3 liter.

EPS (Electrical Power Subsystem): The spacecraft is equipped with a 12.8 m2 high-efficiency triple-junction GaAs (Gallium-Arsenide) cell solar array, of which 7.3 m2 is in the form of a fixed solar array and 5.5 m2 is covered by 6 panels mechanically linked to deployable sunshield assembly.

For the launch, the deployable sunshield is folded against the payload module. After separation from the launch vehicle, it is deployed around the fixed solar array, in the same plane. During LEOP (Launch and Early Operations Phase), power is supplied by a 60 Ah mass-efficient Lithium-ion battery.

Optimum power supply during all phases of the mission is ensured by a PCDU (Power Control and Distribution Unit) with maximum power point tracking. The PCDU performs power management by generating a 28 V primary power bus that supplies power to all spacecraft subsystems. It also controls the battery state of charge and generates pyrotechnic commands as well as heater actuation as commanded by the C&DMS (Command & Data Management Subsystem).

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Figure 9: Photo of the battery (image credit: ABSL)

Propulsion: After injection into the L2 transfer orbit by the Soyuz-Fregat launcher, a chemical bi-propellant propulsion system (8 x 10 N) is used for the transfer phase. It will cover attitude acquisition, spin control, mid-course corrections, L2 orbit injection, and safe mode.

After arriving at L2, one redundant set of micro-propulsion thrusters will control the spin and precession motion of the spacecraft. Regular orbit maintenance will be performed by using the chemical propulsion thrusters. - The spacecraft uses a cold gas micropropulsion system for fine attitude control.

CPS (Chemical Propulsion Subsystem): CPS is a bi-propellant system using two tanks of Herschel/Planck heritage filled with with a total of ~400 kg of propellant featuring a blowdown ratio of 4:3. Use of monomethylhydrazine as fuel and nitrogen tetroxide as oxidizer. The 10 N thrusters are manufactured by Astrium consisting of a platinum alloy combustion chamber and nozzle that tolerates the operational temperature of 1,500°C. The thruster can be operated in a thrust range of 6 to 12.5 N with a nominal thrust of 10 N which generates a specific impulse of 291 seconds.

MPS (Micro Propulsion Subsystem): The MPS is being used for fine attitude pointing and spin rate management. A total of 12 cold gas thrusters are installed on the spacecraft being grouped in three clusters each featuring four cold gas thrusters. The thruster system uses high-pressure nitrogen propellant to provide very small impulses with a thrust range of 1 - 500 µN. The system uses two nitrogen tanks, each containing 28.5 kg of N2, stored at a pressure of 310 bar (Ref. 19). The CG-MPS (Cold Gas-Micro Propulsion Subsystem) was developed by TAS-I and Selex ES S.p.A., Italy. 28)

RF communications: All communication with the Gaia spacecraft is done using the X-band. For TT&C (Tracking Telemetry and Command), a low gain antenna uplink and downlink with a few kbit/s capacity and an omnidirectional coverage are employed. The science telemetry X-band downlink is based on a set of electronically-scanned phased array antennae accommodated on the service module bottom panel. This high gain antenna is used during each ground station visibility period of about 8 hours per day.

The X-band payload downlink rate is 10 Mbit/s from L2. To achieve this, Gaia uses a specially designed on-board phased array antenna to beam the payload data to Earth (a conventional steerable antenna would disturbed the very precise measurements).

Gaia is equipped with a total of three communication antennas – two LGAs (Low Gain Antennas) and a single X-band Medium Gain Phased Array Antenna. One LGA is located pointing in the +X direction while the other points to –X being located on the Thermal Tent and the base of the spacecraft, respectively. The two LGAs build an omni-direction communications system for housekeeping telemetry downlink and command uplink with data rates of a few kbit/s.

The MGA (Medium Gain Antenna) is located on the base of the Payload Module, protruding the DSA(Deployable Sunshield Assembly) . This directional antenna can achieve data rates of up to 10 Mbit/s for science data and telemetry downlink and telecommand reception.

Demodulation of the uplink signal is completed by the transponder units before the data flow is passed on to the CDMU (Command & Data Management Unit). The downlink data is encoded by the CDMU and modulated in X-band within the transponders before being amplified by the SSPA (Solid State Power Amplifier). The signal is combined in the phased array of the active antenna in order to orient the beam towards the Earth.

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Figure 10: Photo of the phased array antenna (image credit: EADS Astrium)

CCSDS Image Data Compression ASIC: In order to transmit all the data generated on board, a particularly challenging compression factor averaging 2.8 was necessary. Unfortunately the standard suite of algorithms was not able to reach this target, because of the peculiarities of Gaia imagery, which include ‘outliers’, such as bright stars and planets, and which are marred by the momentary ‘hot pixels’ due to cosmic rays in deep space. Instead, with the support of ESA compression experts, industry developed an ad hoc solution, enabling all Gaia mission data to reach their home planet. 29)

CWICOM (CCSDS Wavelet Image COMpression ASIC) is a very high-performance image compression ASIC that implements the CCSDS 122.0 wavelet-based image compression standard, to output compressed data according to the CCSDS output source packet protocol standard. This integrated circuit was developed by Airbus DS through an ESA contract.

CWICOM offers dynamic, large compressed-rate range and high-speed image compression potentially relevant for compression of any 2D image with bi-dimensional data correlation (such as a hyperspectral data cube). Its highly optimized internal architecture allows lossless and lossy image compression at very high data rates (up to 60 Mpixels/s) without any external memory by taking advantage of its on-chip memory – almost 5 Mbit of embedded internal memory).

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Figure 11: The CWICOM ASIC is a customized microchip for imaging data compression (image credit: Airbus DS, ESA)

CWICOM is implemented using the largest matrix of the Atmel ATC18RHA ASIC family, and is provided within a standard surface mount package (CQFP 256). CWICOM offers a low-power, cost-effective and highly integrated solution for any image compression application, performing CCSDS image compression treatments without requiring any external memory. The simplicity of such a standalone implementation is achieved thanks to a very efficient internal embedded memory organization that removes any need for extra memory chip procurement and the potential obsolescence threatened by being bound to a specific external memory interface.

Change in data compression implementation:

In order to transmit all the data generated on board, a particularly challenging compression factor averaging 2.8 was necessary. Unfortunately the standard suite of algorithms [which includes CCSDS standards, including the 122.0 implemented by CWICOM] was not able to reach this target, because of the peculiarities of Gaia imagery, which include ‘outliers’, such as bright stars and planets, and which are marred by the momentary ‘hot pixels’ due to cosmic rays in deep space. Instead, with the support of ESA compression experts, industry developed an ad hoc solution,enabling all Gaia mission data to reach their home planet.

Instead of CWICOM, Gaia applies a tailored data compression algorithm (using a heavily tailored pre-processing stage followed by a variant of the Rice coder), using a software implementation running on VPUs (Video Processing Units). The GOCA (Gaia Optimum Compression Algorithm) project was entrusted by ESA to GTD System & Software Engineering (project prime) and IEEC (Institut d'Estudis Espacials de Catalunya), scientific partner), aimed in providing a deep understanding of the Gaia compression problem and offering a complete data compression system, both at an algorithm and implementation level. The main objectives of GOCA not only encompassed the review and evaluation of the already proposed compression scheme but the design of new algorithms for the mission. 30) 31) 32)

The CCDs in the focal plane are commanded by video-processing units (VPUs). Gaia has seven identical VPUs, each one dealing with a dedicated row of CCDs. Each CCD row, contains in order, two SM CCDs (one for each telescope), 9 AF CCDs, 1 BP CCD, 1 RP CCD, and 3 RVS CCDs (the latter only for four of the seven CCD rows). The VPUs run seven identical instances of the video-processing algorithms (VPAs), not necessarily with exactly the same parameter settings though. This (mix of some hardware and mostly) software is responsible for object detection (after local background subtraction), object windowing (see below), window conflict resolution, data binning, data prioritization, science-packet generation, data compression, etc.) 33)

TCS (Thermal Control Subsystem): A deployable sunshield with optimal thermoelastic behavior, made of multi-layer insulation sheets, is attached to the service module and folded against the payload module for the launch. After separation of the Gaia spacecraft from the launch vehicle, the Sun shield is deployed around the fixed solar array, in the same plane. - A thermal tent covers the payload, offering extra protection against micrometeoroids and radiation.

The very high stability thermal control is mostly passive and is achieved through optical surface reflector material, multilayer insulation sheets on the outer faces of the service module, and a black painted cavity, supplemented by heaters where required. Thermal stability is guaranteed by a constant solar aspect angle and the avoidance (as far as possible) of any equipment switch-ON/OFF cycles during nominal operation.

DSA (Deployable Sunshield Assembly): The bottom floor of the SVM is a dodecagonal-shaped panel to comply with the 12 frames of the DSA The main structure consists of carbon-fiber reinforced plastic face sheets.

DSA is folded up during launch and is deployed early in the flight. It is required to shade the payload unit and protect it from direct sunlight that could compromise instrument accuracy. Keeping the instrument at a constant temperature prevents expansion and contraction during temperature variations which would alter the instrument geometry ever so slightly with a large effect on data quality. The DSA is 10 m in diameter.

The DSA is an umbrella-type structure that consists of MLI (Multilayer Insulation) as the primary shield material and six rigid deployment booms as well as six secondary stiffeners. These booms have a single articulation on the base of the Service Module for easy deployment in the radial direction by a spring system. Spacing cables link the booms to the others to ensure a synchronized deployment sequence. The booms and strings are located on the cold side of the cover to limit thermoelastic flexing.

Attached to the DSA are six rectangular solar panels (with triple-junction solar GaAs cells) that are constantly facing the sun once the shield is deployed. They provide 1910 W of EOL (End of Life) power.

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Figure 12: Photo of Gaia's DSA deployment (image credit: Astrium SAS)

Legend to Figure 12: The DSA during deployment testing at Astrium Toulouse. Since the DSA will operate in microgravity, it is not designed to support its own weight in the one-g environment at Earth's surface. During deployment testing, the DSA panels are attached to a system of support cables and counterweights that bears their weight, preventing damage and providing a realistic test environment. The flight model thermal tent is visible inside the deploying sunshield and the mechanically representative dummy payload can be seen through the aperture in the tent.

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Figure 13: Photo of the Gaia SVM in the EMC chamber at Intespace, Toulouse, during launcher EMC compatibility testing (image credit: Astrium SAS)

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Figure 14: Exploded view of the Gaia spacecraft (image credit: EADS Astrium)

Propulsion systems

- 8 x 10 N bipropellant thrusters for coarse attitude control, orbit injection and trajectory corrections
- Low disturbance cold gas micro-propulsion for fine attitude control

Command and data management system

- ERC 32 based central computer with embedded solid state mass memory, complemented by a distinct input / output management unit
- Two segregated MIL-STD-1553B data bus, one for Payload Module and one for Service Module
- SpaceWire data links for high speed payload data

3-axes AOCS (Attitude and Orbit Control Subsystem)

- Payload instrument used for precise rate sensing in science mode
- High precision gyroscope to support transition to fine science pointing with instrument in the loop
- 3-axes autonomous star sensor for inertial attitude measurement
- Fine sun sensor for independent attitude monitoring

TT&C (Telemetry, Tracking and Command Subsystem)

- All X-band system
- Low Gain Antenna for spacecraft commanding, used also for spacecraft telemetry during critical phases
- Electrically steerable high gain antenna with built in solid state amplifiers for science downlink up to 8.7 Mbit/s from L2 orbit without mechanical disturbances

EPS (Electrical Power Subsystem)

- 60 Ah Li-ion battery
- 13 m2 triple junction cell solar array
- Power control and distribution with maximum power point tracking system

Payload data handling

- 7 Video Processing Units in parallel, each provided with 1 GIPS (Giga Instruction/s) processing capability
- 1 Terabit mass memory
- Atomic rubidium clock for accurate sequencing of the focal plane and star measurement dating

TCS (Thermal Control Subsystem)

- Fully passive thermal control
- Thermal tent for payload instrument, providing extra protection against space environment
- 100 m2 sunshield featuring a 0.1º flatness

Structure

- All Silicium Carbide instrument: structure & mirrors
- CFRP (Carbon Fiber Reinforced Plastic) SVM structure

Table 2: Summary of spacecraft subsystems (Ref. 6)

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Figure 15: Alternate exploded view of the Gaia spacecraft elements (image credit: EADS Astrium)

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Figure 16: The Gaia flight model spacecraft undergoing final electrical tests at Astrium Toulouse in June 2013 (image credit: EADS Astrium)

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Figure 17: Photo of the Gaia spacecraft in Nov. 2013 with an Astrium AIT engineer installing the transponders at the launch site (image credit: ESA)

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Figure 18: Photo of the Gaia spacecraft, tucked up inside the Soyuz fairing, ready to be mated with the Soyuz lower stages (image credit: ESA, M. Pedoussaut) 34)


Launch: The Gaia spacecraft was launched on December 19, 2013 (09:12:19 UTC) from Kourou by Arianespace, Europe’s Spaceport in French Guiana. The launch vehicle was a Soyuz-STB with a Fregat-MT upper stage The launch is designated as Soyuz flight VS06. 35) 36) 37)

- About ten minutes later, after separation of the first three stages, the Fregat upper stage ignited, delivering Gaia into a temporary parking orbit at an altitude of 175 km.

- A second firing of the Fregat 11 minutes later took Gaia into its transfer orbit, followed by separation from the upper stage 42 minutes after liftoff. Ground telemetry and attitude control were established by controllers at ESOC (European Space Operation Centre) in Darmstadt, Germany, and the spacecraft began activating its systems.

- The sunshield, which keeps Gaia at its working temperature and carries solar cells to power the satellite, was deployed in a 10 minute automatic sequence, completed around 88 minutes after launch. Gaia is now en route towards L2 (Ref. 35).

Orbit: Large Lissajous orbits around L2 (Lagrangian Point 2), about 1.5 million km from Earth. L2 offers a stable thermal environment because the sunshield will protect Gaia from the Sun, Earth and Moon simultaneously, allowing the satellite to keep cool and enjoy a clear view of the Universe from the other side. In addition, L2 provides a moderate radiation environment, which benefits the longevity of the instrument detectors.

• The critical LEOP (Launch and Early Orbit Phase) will last approximately four days. In this phase, Gaia will perform the first activations – transmitter switch ON, priming of the chemical thrusters, first attitude control and finding of the sun position – followed by the sun shield deployment. Engineers on ground will perform orbit determination, then prepare and execute the critical 'Day 2' maneuver to inject Gaia into its final transfer trajectory toward the L2 Lagrange point (Ref. 175).

• LEOP will be followed by the transfer cruise phase, lasting up to 30 days, an L2 orbit injection maneuver, then the in-orbit commissioning phase, during which all operations to prepare for the routine operational phase are performed. In particular, the scientific FPA (Focal Plane Assembly) and related avionics will be thoroughly tested and calibrated. The commissioning phase is expected to last four months.

• The insertion into the final 300,000 x 200,000 km Lissajous orbit around L2 was performed one month after launch.

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Figure 19: Gaia mission scenario, from launch to in-orbit operations (image credit: ESA)




Mission status:

• January 21, 2020: A 500-day global observation campaign spearheaded more than three years ago by ESA’s galaxy-mapping powerhouse Gaia has provided unprecedented insights into the binary system of stars that caused an unusual brightening of an even more distant star. 38)

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Figure 20: Artist's impression of the binary stellar system discovered in the Gaia16aye microlensing event, its gravity bending the fabric of spacetime and distorting the path of light rays coming from an even more distant star (image credit: M. Rębisz) 39)

- The brightening of the star, located in the Cygnus constellation, was first spotted in August 2016 by the Gaia Photometric Science Alerts program.

- This system, maintained by the Institute of Astronomy at the University of Cambridge, UK, scans daily the huge amount of data coming from Gaia and alerts astronomers to the appearance of new sources or unusual brightness variations in known ones, so that they can quickly point other ground and space-based telescopes to study them in detail. The phenomena may include supernova explosions and other stellar outbursts.

- In this particular instance, follow-up observations performed with more than 50 telescopes worldwide revealed that the source – since then named Gaia16aye after Ayers Rock, the famous landmark in Australia – was behaving in a rather strange way.

- “We saw the star getting brighter and brighter and then, within one day, its brightness suddenly dropped,” says Lukasz Wyrzykowski from the Astronomical Observatory at the University of Warsaw, Poland, who is one of the scientists behind the Gaia Photometric Science Alert program.

- “This was a very unusual behavior. Hardly any type of supernova or other star does this.”

Figure 21: This animation shows a zoomed-in view into the star 2MASS19400112+3007533, located in the Cygnus constellation. Following the detection of a sudden brightening of this star by ESA’s Gaia satellite in August 2016, the source is also referred as Gaia16aye after Ayers Rock, the famous landmark in Australia. In the beginning, the animation shows a large portion of the Galactic plane, based on data from the Mellinger survey and spanning about 120 degrees across; then, the view moves to a smaller portion of the sky, around half a degree across, from the Digital Sky Survey; finally, an even smaller field of around 1 arcminute across is shown, centered on the star and based on the Pan-STARRS1 survey. The sudden brightening of Gaia16aye was first identified as part of the Gaia Photometric Science Alerts program, a system that scans daily the huge amount of data coming from Gaia and alerts astronomers to the appearance of new sources or unusual brightness variations in known ones, so that they can quickly point other ground and space-based telescopes to study them in detail (video credit: Mellinger/Digital Sky Survey/Pan-STARRS1; Wyrzykowski et al.)

- Lukasz and collaborators soon realized that this brightening was caused by gravitational microlensing – an effect predicted by Einstein’s theory of general relativity, caused by the bending of spacetime in the vicinity of very massive objects, like stars or black holes.

- When such a massive object, which may be too faint to be observed from Earth, passes in front of another, more distant source of light, its gravity bends the fabric of spacetime in its vicinity. This distorts the path of light rays coming from the background source – essentially behaving like a giant magnifying glass. — Gaia16aye is the second micro-lensing event detected by ESA’s star surveyor. However, the astronomers noticed it behaved strangely even for this type of event.

- ”If you have a single lens, caused by a single object, there would be just a small, steady rise in brightness and then there would be a smooth decline as the lens passes in front of the distant source and then moves away,” says Lukasz.

- “In this case, not only did the star brightness drop sharply rather than smoothly, but after a couple of weeks it brightened up again, which is very unusual. Over the 500 days of observation, we have seen it brighten up and decline five times.”

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Figure 22: This graph shows the variation of brightness of the a distant star caused by a microlensing event, referred to as Gaia16aye, as a foreground massive object – a binary system of stars – passed across the distant star's line of sight. The brightness is indicated on the vertical axis in terms of the astronomical magnitude, with smaller values (towards the top) indicating higher brightness; time is indicated on the horizontal axis (image credit: Adapted from Wyrzykowski et al. 2019)

- This sudden and sharp drop in brightness suggested that the gravitational lens causing the brightening must consist of a binary system – a pair of stars, or other celestial objects, bound to one another by mutual gravity.

- The combined gravitational fields of the two objects produce a lens with a rather intricate network of high magnification regions. When a background source passes through such regions on the plane of the sky, it lights up, and then dims immediately upon exiting it.

- From the pattern of subsequent brightenings and dimmings, the astronomers were able to deduce that the binary system was rotating at a rather fast pace.

- “The rotation was fast enough and the overall micro-lensing event slow enough that the background star entered the high magnification region, left it and then entered it again,” says Lukasz.

- The long period of observations, which lasted until the end of 2017, and the extensive participation of ground-based telescopes from around the globe enabled the astronomers to gather a large amount of data – almost 25,000 individual data points.

- In addition, the team also made use of dozens of observations of this star collected by Gaia as it kept scanning the sky over the months. These data have undergone preliminary calibration and were made public as part of the Gaia Science Alerts program.

- From this data set, Lukasz and his colleagues were able to learn a great deal of detail about the binary system of stars.

- “We don’t see this binary system at all, but from only seeing the effects that it created by acting as a lens on a background star, we were able to tell everything about it,” says co-author Przemek Mróz, who was a PhD student at the University of Warsaw when the campaign started, and is currently a postdoctoral scholar at the California Institute of Technology.

- “We could determine the rotational period of the system, the masses of its components, their separation, the shape of their orbits – basically everything – without seeing the light of the binary components.”

- The pair consists of two rather small stars, with 0.57 and 0.36 times the mass of our Sun, respectively. Separated by roughly twice the Earth-Sun distance, the stars orbit around their mutual center of mass in less than three years.

- “If it wasn’t for Gaia scanning the whole sky and then sending the alerts straight away, we would never have known about this microlensing event,” says co-author Simon Hodgkin from the University of Cambridge, who leads the Gaia Science Alerts program. - “Maybe we would have found it later, but then it might have been too late.”

- The detailed understanding of the binary system relied on the extensive observation campaign and on the broad international involvement that the Gaia16aye event attracted.

- “We acknowledge the professional astronomers, amateur astronomers and volunteers from all around the globe who have been observing this event: without the dedication of all those people we wouldn’t have been able to obtain such results,” says Lukasz.

- “Microlensing events like this can shed light on celestial objects that we would otherwise not be able to see,” says Timo Prusti, Gaia Project Scientist at ESA. “We are delighted that Gaia’s detection triggered the observation campaign that made this result possible.” 40)

• January 7, 2020: Astronomers at Harvard University have discovered a monolithic, wave-shaped gaseous structure — the largest ever seen in our galaxy — made up of interconnected stellar nurseries. Dubbed the “Radcliffe Wave” in honor of the collaboration’s home base, the Radcliffe Institute for Advanced Study, the discovery transforms a 150-year-old vision of nearby stellar nurseries as an expanding ring into one featuring an undulating, star-forming filament that reaches trillions of miles above and below the galactic disk. 41)

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Figure 23: In this illustration, the "Radcliffe Wave" data is overlaid on an image of the Milky Way galaxy (image from the WorldWide Telescope, courtesy of Alyssa Goodman)

- The work, published in Nature, was enabled by a new analysis of data from the European Space Agency’s Gaia spacecraft, launched in 2013 with the mission of precisely measuring the position, distance, and motion of the stars. The research team’s innovative approach combined the super-accurate data from Gaia with other measurements to construct a detailed, 3D map of interstellar matter in the Milky Way, and noticed an unexpected pattern in the spiral arm closest to Earth. 42)

- The researchers discovered a long, thin structure, about 9,000 light-years long and 400 light-years wide, with a wave-like shape, cresting 500 light-years above and below the mid-plane of our galaxy’s disk. The Wave includes many of the stellar nurseries that were thought to form part of “Gould’s Belt,” a band of star-forming regions believed to be oriented in a ring around the sun.

- “No astronomer expected that we live next to a giant, wave-like collection of gas — or that it forms the local arm of the Milky Way,” said Alyssa Goodman, the Robert Wheeler Willson Professor of Applied Astronomy, research associate at the Smithsonian Institution, and co-director of the Science Program at the Radcliffe Institute for Advanced Study. “We were completely shocked when we first realized how long and straight the Radcliffe Wave is, looking down on it from above in 3D — but how sinusoidal it is when viewed from Earth. The Wave’s very existence is forcing us to rethink our understanding of the Milky Way’s 3D structure.”

- “Gould and Herschel both observed bright stars forming in an arc projected on the sky, so for a long time, people have been trying to figure out if these molecular clouds actually form a ring in 3D,” said João Alves, a professor of physics and astronomy at the University of Vienna and 2018‒2019 Radcliffe Fellow. “Instead, what we’ve observed is the largest coherent gas structure we know of in the galaxy, organized not in a ring but in a massive, undulating filament. The sun lies only 500 light-years from the Wave at its closest point. It’s been right in front of our eyes all the time, but we couldn’t see it until now.”

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Figure 24: “No astronomer expected that we live next to a giant, wave-like collection of gas — or that it forms the local arm of the Milky Way,” said Harvard Professor Alyssa Goodman (left), standing with graduate student Catherine Zucker, a key member of the team (image credit: Kris Snibbe/Harvard Staff Photographer)

- The new, 3D map shows our galactic neighborhood in a new light, giving researchers a revised view of the Milky Way and opening the door to other major discoveries.

- “We don’t know what causes this shape, but it could be like a ripple in a pond, as if something extraordinarily massive landed in our galaxy,” said Alves. “What we do know is that our sun interacts with this structure. It passed by a festival of supernovae as it crossed Orion 13 million years ago, and in another 13 million years it will cross the structure again, sort of like we are ‘surfing the wave.’”

- Disentangling structures in the “dusty” galactic neighborhood within which we sit is a longstanding challenge in astronomy. In earlier studies, the research group of Douglas Finkbeiner, professor of astronomy and physics at Harvard, pioneered advanced statistical techniques to map the 3D distribution of dust using vast surveys of stars’ colors. Armed with new data from Gaia, Harvard graduate students Catherine Zucker and Joshua Speagle recently augmented these techniques, dramatically improving astronomers’ ability to measure distances to star-forming regions. That work, led by Zucker, is published in the Astrophysical Journal.

- “We suspected there might be larger structures that we just couldn’t put in context. So, to create an accurate map of our solar neighborhood, we combined observations from space telescopes like Gaia with astrostatistics, data visualization, and numerical simulations,” explained Zucker, a National Science Foundation graduate fellow and a Ph.D. candidate in the Department of Astronomy at Harvard’s Graduate School of Arts and Sciences.

- “The sun lies only 500 light-years from the Wave at its closest point. It’s been right in front of our eyes all the time, but we couldn’t see it until now,” according to João Alves, Radcliffe Fellow 2018-19.

- Zucker played a key role in compiling the largest-ever catalog of accurate distances to local stellar nurseries — the basis for the 3D map used in the study. She has set herself the goal of painting a new picture of the Milky Way, near and far.

- “We pulled this team together so we could go beyond processing and tabulating the data to actively visualizing it — not just for ourselves but for everyone. Now, we can literally see the Milky Way with new eyes,” she said.

- “Studying stellar births is complicated by imperfect data. We risk getting the details wrong, because if you’re confused about distance, you’re confused about size,” said Finkbeiner.

- Goodman agreed, “All of the stars in the universe, including our sun, are formed in dynamic, collapsing, clouds of gas and dust. But determining how much mass the clouds have, how large they are, has been difficult, because these properties depend on how far away the cloud is.”

- According to Goodman, scientists have been studying dense clouds of gas and dust between the stars for more than 100 years, zooming in on these regions with ever-higher resolution. Before Gaia, there was no data set expansive enough to reveal the galaxy’s structure on large scales. Since its launch in 2013, the space observatory has enabled measurements of the distances to one billion stars in the Milky Way.

Figure 25: As part of the 2018–2019 Fellows’ Presentation Series at the Radcliffe Institute for Advanced Study, the astrophysicist João Alves RI ’19 explains how an exhibition by the artist Anna Von Mertens helped guide him to the “Radcliffe wave” findings published in Nature in January 2020 (video credit: Radcliffe Institute for Advanced Study)

- The flood of data from Gaia served as the perfect testbed for innovative, new statistical methods that reveal the shape of local stellar nurseries and their connection to the Milky Way’s galactic structure. Alves came to Radcliffe to work with Zucker and Goodman, as they anticipated the flood of data from Gaia would enhance the Finkbeiner group’s “3D Dust Mapping” technology enough to reveal the distances of local stellar nurseries. But they had no idea they would find the Radcliffe Wave.

- The Finkbeiner, Alves, and Goodman groups collaborated closely on this data-science effort. The Finkbeiner group developed the statistical framework needed to infer the 3D distribution of the dust clouds; the Alves group contributed deep expertise on stars, star formation, and Gaia; and the Goodman group developed the 3D visualizations and analytic framework, called “glue,” that allowed the Radcliffe Wave to be seen, explored, and quantitatively described.

- This study was supported by the NSF Graduate Research Fellowship Program (grant no. 1650114, AST-1614941), the Harvard Data Science Initiative, NASA through ADAP (grant no. NNH17AE75I), and a Hubble Fellowship (grant HST-HF2-51367.001-A) awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555.

• December 11, 2019: A new study on the kinematics and chemical composition of a sample of stars in the vicinity of the sun, led by Dr. Daniela Carollo, researcher of the Italian National Institute for Astrophysics (INAF) of Turin, Italy, has revealed that the stars that make up the thick disk of the Milky Way belong to two distinct stellar populations with different characteristics and not to a single one, as has been thought for more than two decades. 43)

- The new thick disk component, called the metal-weak thick disk (MWTD) or metal-poor thick disk, differs from the canonical one in the speed of rotation around the galactic center and its chemical composition. Indeed, stars that make up the TD have a rotational speed of about 180 km per second, while those of the MWTD rotate more slowly, at about 150 km per second. Stars belonging to the MWTD are also two times more metal-poor than those of the TD and have higher energy, a property that allows them to reach greater heights from the galactic plane.

- "It was almost 30 years that astronomers tried to solve this puzzle," said Dr. Carollo, scientist at the Astrophysical Observatory of Turin the first author of the article reporting the discovery, just published in The Astrophysical Journal. "In fact, it was thought that the MWTD was nothing but an extension of the thick disk and not an independent population with different astrophysical origins." 44)

- The accurate parameters provided by the ESA Gaia mission (positions, distances and intrinsic motion of the stars), and the chemical information on a sample of 40,000 stars of the Sloan Digital Sky Survey (SDSS), allowed the team to distinguish the MWTD in a diagram showing the angular momenta combined with the chemistry.

- "The angular momenta are quantities that are conserved during the formation and subsequent evolution of a physical a system like our galaxy," explains Dr. Carollo. "Thus, in an accurate diagram of the angular momenta, the stars brought into the galaxy by the same progenitor, as for example from a previous fusion of a satellite galaxy, will have similar angular momenta and will tend to cluster in the diagram."

- The TD and MWTD form two distinct groups in the diagram, as well as in their chemistry. In astronomy, the chemical elements heavier than hydrogen and helium, which were formed during the Big Bang, are defined as metals. These heavier chemical elements were produced during the nucleosynthesis of massive stars that exploded as supernovae.

- A particular group of light elements such as magnesium and titanium, when compared to heavier elements, such as iron, provide a fundamental parameter that allows scientists to distinguish populations of old stars from those of younger stars. The MWTD not only possesses stars poorer in iron, but those stars are also richer in elements of the magnesium and titanium group (alpha elements) that suggests an antecedent formation to the TD.

- These important differences between the TD and the MWTD, namely the kinematics and the chemistry of their stars, suggest that the two disks had a different origin during the galaxy formation process.

- But how did a second thick disk form in the Milky Way? The hypotheses are manifold: the MWTD could be older than the TD and its stars could have been energized by a merger of a dwarf satellite galaxy with the Milky Way, during its initial formation stage. Subsequently, the fusion of a second satellite galaxy would have given rise to the TD.

- Another possibility is that the MWTD stars had originally formed in an area closer to the center of the primordial galaxy and subsequently been transported to larger distances, closer to where the sun is located now, by internal phenomena such as instabilities of the central bar or the galaxy's spiral arms formation. Or an ancient satellite galaxy of mass similar to the Small Magellanic Cloud merged with the primordial galaxy and its stars began to spin around the galactic center due to the mutual gravitational interaction.

- All these hypotheses can be tested through theoretical models and simulations of Milky Way-like galaxy formation.

• October 24, 2019: The Gaia Data Release 2 catalog has been used extensively by astronomers across the world. About 3 to 4 papers appear per day based on the Gaia DR2 catalog, touching many different topics. 45)

Figure 26: Launched in December 2013, the Gaia mission is revolutionizing our understanding of the Milky Way. The space telescope is mapping our galaxy in unprecedented detail – measuring the position, movement and distance of stars. At a meeting in Groningen in the Netherlands, scientists have been discussing the challenge of processing and visualizing Gaia data. (video credit: ESA)

• August 28, 2019: Rather than leaving home young, as expected, stellar ‘siblings’ prefer to stick together in long-lasting, string-like groups, finds a new study of data from ESA’s Gaia spacecraft. 46)

- Exploring the distribution and past history of the starry residents of our galaxy is especially challenging as it requires astronomers to determine the ages of stars. This is not at all trivial, as ‘average’ stars of a similar mass but different ages look very much alike.

- To figure out when a star formed, astronomers must instead look at populations of stars thought to have formed at the same time – but knowing which stars are siblings poses a further challenge, since stars do not necessarily hang out long in the stellar cradles where they formed.

Figure 27: Gaia tracing starry strings in the Milky Way . This simulated video shows ESA’s Gaia spacecraft as it traces the structure and star formation activity of a large patch of space surrounding the Solar System. Gaia launched in 2013, and is on a mission to chart a three-dimensional map of our galaxy, pinpointing the locations, motions, and dynamics of roughly one percent of the stars within the Milky Way – along with additional information about many of these stars. - The video begins with a view of Gaia set against the bright plane of the Milky Way, which cuts horizontally across the frame. Different colored patches – each representing a different stellar ‘family’ observed by Gaia – then come into view, with yellows, greens, blues, purples and reds gradually filling up the region and creating a rainbow patchwork effect. Each family is identified with a different color and comprises a population of stars that formed at the same time. - Gaia then disappears from view, and the perspective zooms out to show the wider three-dimensional structure of the colorful star populations, along with their future paths through the galaxy based on Gaia’s measurements of proper motions (the motions have been speeded up for illustration purposes, with each second corresponding to 158730 years), video credit: ESA/Gaia/DPAC; Data: M. Kounkel & K. Covey (2019); Animation: S. Jordan / T. Sagristá / Gaia Sky (http://www.zah.uni-heidelberg.de/gaia/outreach/gaiasky) – CC BY-SA 3.0 IGO

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Figure 28: This diagram shows a face-on view of stellar ‘families’ – clusters (dots) and co-moving groups (thick lines) of stars – within about 3000 light-years from the Sun, which is located at the center of the image. The diagram is based on data from the second data release of ESA’s Gaia mission. Each family is identified with a different color and comprises a population of stars that formed at the same time. Purple hues represent the oldest stellar populations, which formed around 1 billion years ago; blue and green hues represent intermediate ages, with stars that formed hundreds of millions of years ago; orange and red hues show the youngest stellar populations, which formed less than a hundred million years ago. Thin lines show the predicted velocities of each group of stars over the next 5 million years, based on Gaia’s measurements. The lack of structures at the center is an artefact of the method used to trace individual populations, not due to a physical bubble (image credit: M. Kounkel & K. Covey (2019))

- “To identify which stars formed together, we look for stars moving similarly, as all of the stars that formed within the same cloud or cluster would move in a similar way,” says Marina Kounkel of Western Washington University, USA, and lead author of the new study. The study uses data from Gaia’s second release (DR2), provided in April 2018. 47)

- “We knew of a few such ‘co-moving’ star groups near the Solar System, but Gaia enabled us to explore the Milky Way in great detail out to far greater distances, revealing many more of these groups.”

- Marina used data from Gaia’s second release to trace the structure and star formation activity of a large patch of space surrounding the Solar System, and to explore how this changed over time. This data release, provided in April 2018, lists the motions and positions of over one billion stars with unprecedented precision.

- The analysis of the Gaia data, relying on a machine learning algorithm, uncovered nearly 2000 previously unidentified clusters and co-moving groups of stars up to about 3000 light years from us – roughly 750 times the distance to Proxima Centauri, the nearest star to the Sun. The study also determined the ages for hundreds of thousands of stars, making it possible to track stellar ‘families’ and uncover their surprising arrangements.

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Figure 29: This image shows a view of stellar ‘families’ – clusters and co-moving groups of stars in the Milky Way – identified using data from the second data release of ESA’s Gaia mission. Families younger than 30 million years are highlighted in orange, on top of an all-sky view based on Gaia observations [image credit: ESA/Gaia/DPAC; Data: M. Kounkel & K. Covey (2019)]

- “Around half of these stars are found in long, string-like configurations that mirror features present within their giant birth clouds,” adds Marina.

- “We generally thought young stars would leave their birth sites just a few million years after they form, completely losing ties with their original family – but it seems that stars can stay close to their siblings for as long as a few billion years.”

- The strings also appear to be oriented in particular ways with respect to our galaxy’s spiral arms – something that depends upon the ages of the stars within a string. This is especially evident for the youngest strings, comprising stars younger than 100 million years, which tend to be oriented at right angles to the spiral arm nearest to our Solar System.

- The astronomers suspect that the older strings of stars must have been perpendicular to the spiral arms that existed when these stars formed, which have now been reshuffled over the past billion years.

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Figure 30: This diagram shows an edge-on view of stellar ‘families’ – clusters (dots) and co-moving groups (thick lines) of stars – within about 3000 light-years from the Sun, which is located at the center of the image. The diagram is based on data from the second data release of ESA’s Gaia mission [image credit: M. Kounkel & K. Covey (2019)]

- “The proximity and orientation of the youngest strings to the Milky Way’s present-day spiral arms shows that older strings are an important ‘fossil record’ of our galaxy’s spiral structure,” says co-author Kevin Covey, also of Western Washington University, USA.

- “The nature of spiral arms is still debated, with the verdict on them being stable or dynamic structures not settled yet. Studying these older strings will help us understand if the arms are mostly static, or if they move or dissipate and re-form over the course of a few hundred million years – roughly the time it takes for the Sun to orbit around the galactic center a couple of times.”

• July 25, 2019: On 31 March 2017, Jupiter’s moon Europa passed in front of a background star – a rare event that was captured for the first time by ground-based telescopes thanks to data provided by ESA’s Gaia spacecraft. 48)

- Previously, observatories had only managed to watch two of Jupiter’s other moons – Io and Ganymede – during such an event.

- Gaia has been operating in space since late 2013. The mission aims to produce a three-dimensional map of our Galaxy, and characterize the myriad stars that call the Milky Way home. It has been immensely successful so far, revealing the locations and motions of over one billion stars.

- Knowing the precise locations of the stars we see in the sky allows scientists to predict when various bodies in the Solar System will appear to pass in front of a background star from a given vantage point: an event known as a stellar occultation.

- Gaia is no stranger to such events – the spacecraft helped astronomers make unique observations of Neptune’s moon Triton as it passed in front of a distant star in 2017, revealing more about the moon’s atmosphere and properties.

- Occultations are hugely valuable; they enable measurements of the characteristics of the foreground body (size, shape, position, and more), and can reveal structures like rings, jets, and atmospheres. Such measurements can be made from the ground – something that Bruno Morgado of the Brazilian National Observatory and LIneA, Brazil, and colleagues took advantage of to explore Jupiter’s moon Europa.

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Figure 31: Jupiter's largest moons. This 'family portrait' shows a composite of images of Jupiter, including it's Great Red Spot, and its four largest moons. From top to bottom, the moons are Io, Europa, Ganymede and Callisto. Europa is almost the same size as Earth's moon, while Ganymede, the largest moon in the Solar System, is larger than planet Mercury. - While Io is a volcanically active world, Europa, Ganymede and Callisto are icy, and may have oceans of liquid water under their crusts. Europa in particular may even harbor a habitable environment. Jupiter and its large icy moons will provide a key focus for ESA's Juice mission. The spacecraft will tour the Jovian system for about three-and-a-half years, including flybys of the moons. It will also enter orbit around Ganymede, the first time any moon beyond our own has been orbited by a spacecraft. The images of Jupiter, Io, Europa and Ganymede were taken by NASA's Galileo probe in 1996, while the Callisto image is from the 1979 flyby of Voyager (image via NASA Photojournal)

- “We used data from Gaia’s first data release to forecast that, from our viewpoint in South America, Europa would pass in front of a bright background star in March 2017 – and to predict the best location from which to observe this occultation,” said Bruno, lead researcher of a new paper reporting the findings from the 2017 occultation. Gaia’s first data release was provided in September 2016. 49)

- “This gave us a wonderful opportunity to explore Europa, as the technique offers an accuracy comparable to that of images obtained by space probes.”

- The Gaia data showed that the event would be visible from a thick band slicing from north-west to south-east across South America. Three observatories located in Brazil and Chile were able to capture data – a total of eight sites attempted, but many experienced poor weather conditions.

- In-keeping with previous measurements, the observations refined Europa’s radius to 1561.2 km, precisely determined Europa’s position in space and in relation to its host planet, Jupiter, and characterized the moon’s shape. Rather than being exactly spherical, Europa is known to be an ellipsoid. The observations show the moon to measure 1562 km when measured across in one direction (the so-called apparent ‘semi-major’ axis), and 1560.4 km when measured across the other (the apparent ‘semi-minor’ axis).

- “It’s likely that we’ll be able to observe far more occultations like this by Jupiter’s moons in 2019 and 2020,” adds Bruno. “Jupiter is passing through a patch of sky that has the galactic center in the background, making it drastically more likely that its moons will pass in front of bright background stars. This would really help us to pin down their three-dimensional shapes and positions – not only for Jupiter's four largest moons, but for smaller, more irregularly-shaped ones, too.”

- Using Gaia’s second data release, provided in April 2018, the scientists predict the dates of further occultations of bright stars by Europa, Io, Ganymede and Callisto in coming years, and list a total of 10 events through 2019 and 2021. Future events comprise stellar occultations by Europa (22 June 2020), Callisto (20 June 2020, 4 May 2021), Io (9 and 21 September 2019, 2 April 2021), and Ganymede (25 April 2021).

- Three have already taken place in 2019, two of which – stellar occultations by Europa (4 June) and Callisto (5 June) – were also observed by the researchers, and for which the data are still under analysis.

- The upcoming occultations will be observable even with amateur telescopes as small as 20 cm from various regions around the world. The favorable position of Jupiter, with the galactic plane in the background, will only occur again in 2031.

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Figure 32: Upcoming stellar occultations by Jupiter’s four largest moons. Astronomers can learn a great deal about a celestial body by observing it as it moves in front of a bright background star: an alignment known as a stellar occultation. Such events are unusual for Jupiter’s moons. In fact, until recently, only two of the gas giant’s moons – Io and Ganymede – had been observed during stellar occultations. Now, a study presents observations of another of Jupiter’s moons, Europa, as it obscured a bright star on 31 March 2017. This event allowed the astronomers to better characterize Europa’s size, position in space and in relation to Jupiter, and three-dimensional shape; they used precise data from Gaia’s first data release, provided in September 2016, on stellar positions to determine the best location from which to observe the event, and subsequently gathered data from three observatories in Brazil and Chile [image credit: ESA/Gaia/DPAC; Bruno Morgado (Brazilian National Observatory/LIneA, Brazil) et al (2019)]

- “Stellar occultation studies allow us to learn about moons in the Solar System from afar, and are also relevant for future missions that will visit these worlds,” says Timo Prusti, ESA Gaia Project Scientist. “As this result shows, Gaia is a hugely versatile mission: it not only advances our knowledge of stars, but also of the Solar System more widely.”

- An accurate knowledge of Europa’s orbit will help to prepare space missions targeting the Jovian system such as ESA’s JUICE (JUpiter ICy moons Explorer) and NASA’s Europa Clipper, both of which are scheduled for launch in the next decade.

Figure 33: Juice's Europa flyby. ESA’s Jupiter Icy Moons Explorer, Juice, is set to embark on a seven-year cruise to Jupiter starting May 2022. The mission will investigate the emergence of habitable worlds around gas giants and the Jupiter system as an archetype for the numerous giant planets now known to orbit other stars. During the tour of the Jovian system, Juice will make two flybys of Europa, which has strong evidence for an ocean of liquid water under its icy shell. Juice will look at the moon’s active zones, its surface composition and geology, search for pockets of liquid water under the surface and study the plasma environment around Europa (image credit: ESA)

- “These kinds of observations are hugely exciting,” says Olivier Witasse, ESA’s Juice Project Scientist. “Juice will reach Jupiter in 2029; having the best possible knowledge of the positions of the system’s moons will help us to prepare for the mission navigation and future data analysis, and plan all of the science we intend to do.

- “This science depends upon us knowing things such as accurate moon trajectories and understanding how close a spacecraft will come to a given body, so the better our knowledge, the better this planning – and the subsequent data analysis – will be.”

• July 16, 2019: The first direct measurement of the bar-shaped collection of stars at the center of our Milky Way galaxy has been made by combining data from ESA’s Gaia mission with complementary observations from ground- and space-based telescopes. 50)

Figure 34: Revealing the galactic bar. This color chart shows the distribution of 150 million stars in the Milky Way probed using data from the second release of ESA’s Gaia mission in combination with infrared and optical surveys, with orange/yellow hues indicating a greater density of stars. Most of these stars are red giants. While the majority of charted stars are located closer to the Sun (the larger orange/yellow blob in the lower part of the image), a large and elongated feature populated by many stars is also visible in the central region of the galaxy: this is the first geometric indication of the galactic bar. The distances to the stars shown in this chart, along with their surface temperature and extinction – a measure of how much dust there is between us and the stars – were estimated using the StarHorse computer code [video credit: Data: ESA/Gaia/DPAC, A. Khalatyan(AIP) & StarHorse team; Galaxy map: NASA/JPL-Caltech/R. Hurt (SSC/Caltech)]

- The second release of data from ESA’s Gaia star-mapping satellite, published in 2018, has been revolutionizing many fields of astronomy. The unprecedented catalog contains the brightnesses, positions, distance indicators and motions across the sky for more than one billion stars in our Milky Way galaxy, along with information about other celestial bodies.

- As impressive as this dataset sounds, this is really just the beginning. While the second release is based on the first 22 months of Gaia’s surveys, the satellite has been scanning the sky for five years and has many years ahead. New data releases planned in the coming years will steadily improve measurements as well as provide extra information that will enable us to chart our home galaxy and delve into its history like never before.

- Meanwhile, a team of astronomers have combined the latest Gaia data with infrared and optical observations performed from ground and space to provide a preview of what future releases of ESA’s stellar surveyor will reveal.

- “We looked in particular at two of the stellar parameters contained in the Gaia data: the surface temperature of stars and the ‘extinction’, which is basically a measure of how much dust there is between us and the stars, obscuring their light and making it appear redder,” says Friedrich Anders from University of Barcelona, Spain, lead author of the new study. “These two parameters are interconnected, but we can estimate them independently by adding extra information obtained by peering through the dust with infrared observations.”

- The team combined the second Gaia data release with several infrared surveys using a computer code called StarHorse, developed by co-author Anna Queiroz and collaborators. The code compares the observations with stellar models to determine the surface temperature of stars, the extinction and an improved estimate of the distance to the stars.

- As a result, the astronomers obtained a much better determination of the distances to about 150 million stars – in some cases, the improvement is up to 20% or more. This enabled them to trace the distribution of stars across the Milky Way to much greater distances than possible with the original Gaia data alone.

- “With the second Gaia data release, we could probe a radius around the Sun of about 6500 light years, but with our new catalog, we can extend this ‘Gaia sphere’ by three or four times, reaching out to the center of the Milky Way,” explains co-author Cristina Chiappini from Leibniz Institute for Astrophysics Potsdam, Germany, where the project was coordinated.

- There, at the center of our galaxy, the data clearly reveals a large, elongated feature in the three-dimensional distribution of stars: the galactic bar.

- “We know the Milky Way has a bar, like other barred spiral galaxies, but so far we only had indirect indications from the motions of stars and gas, or from star counts in infrared surveys. This is the first time that we see the galactic bar in 3D space, based on geometric measurements of stellar distances,” says Friedrich.

- “Ultimately, we are interested in galactic archeology: we want to reconstruct how the Milky Way formed and evolved, and to do so we have to understand the history of each and every one of its components,” adds Cristina.

- “It is still unclear how the bar – a large amount of stars and gas rotating rigidly around the center of the galaxy – formed, but with Gaia and other upcoming surveys in the next years we are certainly on the right path to figure it out.”

- The team is looking forward to the next data release from the Apache Point Observatory Galaxy Evolution Experiment (APOGEE-2), as well as upcoming facilities such as the 4-meter Multi-Object Survey Telescope (4MOST) at the European Southern Observatory in Chile and the WEAVE (WHT Enhanced Area Velocity Explorer) survey at the William Herschel Telescope (WHT) in La Palma, Canary Islands.

- The third Gaia data release, currently planned for 2021, will include greatly improved distance determinations for a much larger number of stars, and is expected to enable progress in our understanding of the complex region at the center of the Milky Way.

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Figure 35: Revealing the galactic bar. This color chart shows the distribution of 150 million stars in the Milky Way probed using data from the second release of ESA’s Gaia mission in combination with infrared and optical surveys, with orange/yellow hues indicating a greater density of stars. Most of these stars are red giants. The distribution is superimposed on an artistic top view of our galaxy. - While the majority of charted stars are located closer to the Sun (the larger orange/yellow blob in the lower part of the image), a large and elongated feature populated by many stars is also visible in the central region of the galaxy: this is the first geometric indication of the galactic bar. - The distances to the stars shown in this chart, along with their surface temperature and extinction – a measure of how much dust there is between us and the stars – were estimated using the StarHorse computer code [image credit: Data: ESA/Gaia/DPAC, A. Khalatyan (AIP) & StarHorse team; Galaxy map: NASA/JPL-Caltech/R. Hurt (SSC/Caltech)]

- “With this study, we can enjoy a taster of the improvements in our knowledge of the Milky Way that can be expected from Gaia measurements in the third data release,” explains co-author Anthony Brown of Leiden University, The Netherlands, and chair of the Gaia Data Processing and Analysis Consortium Executive.

- “We are revealing features in the Milky Way that we could not see otherwise: this is the power of Gaia, which is enhanced even further in combination with complementary surveys,” concludes Timo Prusti, Gaia project scientist at ESA.

- “Photo-astrometric distances, extinctions, and astrophysical parameters for Gaia DR2 stars brighter than G=18” by F. Anders et al. is published in Astronomy & Astrophysics. 51)

- The study combines data from Gaia’s second release with the Pan-STARRS1 survey conducted with the first Pan-STARRS telescope in Hawaii, US; the Two Micron All Sky Survey (2MASS) conducted with telescopes in the US and Chile; the AllWISE survey from NASA’s Wide-field Infrared Survey Explorer (WISE).

- The computations were conducted at the cluster facility of the Leibniz Institute for Astrophysics Potsdam, Germany.
The new catalogue is available here: https://data.aip.de/projects/starhorse2019.html