New Horizons Mission
For decades after American astronomer Clyde Tombaugh discovered Pluto in 1930, this small world was considered an oddity. The other planets fit neatly into the known architecture of the solar system – four small, rocky bodies in the inner orbits and four gas giants in the outer orbits, with an asteroid belt in between. Distant Pluto was an icy stranger in a strange orbit. 1) 2) 3)
By the 1950s, some researchers, most notably Dutch-American astronomer Gerard Kuiper, had suggested that Pluto was not a lone oddity but the brightest of a vast collection of objects orbiting beyond Neptune. This concept, which became known as the Kuiper Belt, appeared in scientific literature for decades, but repeated searches for this myriad population of frosty worlds came up short.
In the late 1980s, scientists determined that only something like the Kuiper Belt could explain why short-period comets orbit so close to the plane of the solar system. This circumstantial evidence for a distant belt of bodies in the same region as Pluto drove observers back to their telescopes in search of undiscovered, faint objects. This time, though, they had technology on their side: telescopes with electronic light detectors made searches far more sensitive than work done previously with photographic plates.
In 1992, astronomers at the Mauna Kea Observatory in Hawaii discovered the first Kuiper Belt Object (KBO), which was about 10 times smaller and almost 10,000 times fainter than Pluto. Since then, observers have found more than 1,000 KBOs, with diameters ranging from 50 to 2,000 kilometers – and researchers estimate that the Kuiper Belt contains more than 100,000 objects larger than 100 kilometers across. In essence, the Kuiper Belt has turned out to be the big brother of the asteroid belt, with more mass and objects, and a greater supply of ancient, icy and organic material left over from the birth of the planets than imagined.
The Kuiper Belt's discovery made it clear that Pluto is not an anomalous body, but instead moves within a swarm of smaller bodies orbiting 5 billion kilometers (and beyond) from the Sun. Because this far-off region may hold important clues to the early development of the solar system, astronomers are very interested in learning more about Pluto, its moons and their Kuiper Belt cousins.
The region is too far to observe from Earth in any detail; even the Hubble Space Telescope shows only blurry patches of light and dark materials on Pluto's surface. And although the Pioneer, Voyager and Galileo spacecraft provided scientists with marvelous up-close images of Jupiter, Saturn, Uranus and Neptune, no space probe has ever visited Pluto-Charon or the Kuiper Belt.
Astronomical Archeology: Exploring the Kuiper Belt is an archeological dig into the earliest days of the solar system – a close-up look at the remnants of the ancient planet-building process that hold critical clues to the history of the outer solar system. Scientists will use New Horizons to sample the region, getting a valuable glimpse of the long-gone era of planetary formation.
Why are astronomers so interested in studying Pluto-Charon and the Kuiper Belt? For one, the size, shape and general nature of the Kuiper Belt appear to be much like the debris belts seen around other nearby stars. Additionally, when researchers used computer-modeling techniques to simulate the formation of the KBOs as the solar system was coalescing from a whirling disk of gas and dust, they found that the ancient Kuiper Belt must have been at least 10 times more massive than it is today to give rise to Pluto-Charon and the KBOs we see. In fact, there was once enough solid material to have formed another planet the size of Uranus or Neptune in the Kuiper Belt. And the same simulations revealed that large planets would have naturally grown from the KBOs in a very short time had nothing disturbed the region.
But something disrupted the Kuiper Belt at about the time Pluto formed. Was it Neptune's formation near the belt's inner boundary? Perhaps instead it was the gravitational influence of a large number of planetary embryos – rocky bodies thousands of kilometers across – moving rapidly through the Kuiper Belt after they were ejected by Uranus and Neptune from their own formation zones. Or maybe it was something else altogether. Whatever the cause, the Kuiper Belt lost most of its mass and the growth of bodies in the region suddenly stopped.
Scientific Priority: What little we do know about the Pluto-Charon system indicates that they are a scientific wonderland of their own. Charon has a diameter of about 1,200 km, more than half of Pluto's, the largest moon in the solar system compared to the planet it orbits. (In contrast, most satellites are but a few percent of their parent planet's diameter.) Because the two bodies are so close in size, and that they orbit about a center of mass that is outside Pluto's surface, Pluto-Charon is considered a double planet. No other planet in our solar system falls into this category, but astronomers have discovered many double asteroids and double KBOs. There is now little doubt that binary objects like Pluto-Charon are common in our solar system, and most likely in others. NASA's New Horizons mission will be the first trip to a binary world.
Astronomers are eager to know how a system like Pluto and its moons could form. The prevailing theory is that Pluto collided with another large body in the distant past, and that much of the debris from this impact went into orbit around Pluto and eventually coalesced to form Charon. Because scientists believe that a similar collision led to the creation of Earth's moon, the study of Pluto and Charon could shed some light on that subject.
Researchers also want to understand why Pluto and Charon look so different. Observations from Earth and the Hubble Space Telescope indicate that Pluto has a highly reflective surface with distinct markings that indicate expansive polar caps. Charon's surface is far less reflective, with indistinct markings. And where Pluto has an atmosphere, Charon apparently does not. Is the sharp dichotomy between these two neighboring worlds a result of divergent evolution, perhaps owing to their different sizes and compositions, or is it a consequence of how they originally formed?
Further still, Pluto's density, size and surface composition are strikingly similar to those of Neptune's largest satellite, Triton. A great surprise of Voyager 2's exploration of the Neptune system was the discovery of ongoing volcanic activity on Triton. Will Pluto or other KBOs display such activity? Exploring Pluto and other KBOs will provide insight that guides us to a better understanding of these small worlds.
Yet another allure Pluto offers is its bizarre atmosphere. Although Pluto's atmosphere is about 50 times less dense than Mars' – which is, in turn, about 150 times less dense than Earth's – it offers unique insights into the workings of planetary atmospheres. Where Earth's atmosphere contains only one gas (water vapor) that regularly transitions between solid and gas, Pluto's atmosphere contains three: nitrogen, carbon monoxide and methane.
Furthermore, Pluto's surface temperature varies greatly because of the planet's eccentric orbit and polar tilt. Pluto reached its closest approach to the Sun in 1989. As the planet moves farther away and cools, most astronomers believe that the average surface temperature will eventually drop and that most of the atmosphere will freeze out on the surface. As a result of this, and because the planet is essentially tipped on its side, with its rotational north pole 28 degrees below the ecliptic plane, Pluto may have the most complex seasonal patterns of any planet in the solar system.
Figure 1: Pluto's orbit in the solar system: Owing to the great scientific interest in Pluto, and also in the ancient, icy Kuiper Belt of miniature planets, smaller worlds and comets, the U.S. National Academy of Sciences ranked a Pluto-Kuiper Belt mission its highest priority for a New Frontiers mission start in this decade. New Horizons is that mission (image credit: NASA)
Figure 2: Orbits of the Pluto system: This graphic shows the Pluto system as seen from Earth, planet sizes not to scale. The circular orbits look elliptical when projected onto the plane of the sky to mimic what one could see from the Hubble Space Telescope - which scientists used in 2005 to discover Pluto's two smaller satellites. The orbits of satellites P1 and P2 are likely to be essentially circular and in the plane of Pluto's equator - like Charon's orbit (image credit: NASA)
What's more, Pluto's atmosphere is thought to bleed into space at a rate much like a comet's. This extremely fast leakage, in which the thermal energy of typical molecules in the upper atmosphere is sufficient to escape the planet's gravity, is called hydrodynamic escape. Although we don't see hydrodynamic escape on any other planet today, it may have been responsible for the rapid loss of hydrogen from Earth's atmosphere early in our planet's history. In this way, hydrodynamic escape may have helped make Earth suitable for life. Pluto is the only planet in the solar system where we can study this process today.
Another important connection between Pluto and life on Earth is the likely presence of organic compounds (such as frozen methane) on Pluto's surface and water ice in the planet's interior. Recent observations of other KBOs show that they, too, most likely harbor large amounts of ice and organics. Billions of years ago such objects are thought to have routinely strayed into the inner part of the solar system and helped to seed the young Earth with the raw materials of life.
Given all these fascinating scientific motivations, it's easy to understand why the planetary research community wanted to send a spacecraft to Pluto and the Kuiper Belt. In July 2002, the National Research Council's Decadal Survey for Planetary Science ranked the reconnaissance of Pluto-Charon and the Kuiper Belt as its highest priority for a new planetary mission in this decade, citing the fundamental scientific importance of these bodies to advancing understanding of our solar system.
Core Science Goals: New Horizons' core science goals reflect what the science community has wanted to learn about Pluto for the past two decades. The craft will map the surfaces of Pluto and Charon with an average resolution of one kilometer (in contrast, the Hubble Space Telescope cannot do better than about 500- kilometer resolution when it views Pluto and Charon). It will map the surface composition across the various geological provinces of the two bodies. And it will determine the composition, structure and escape rate of Pluto's atmosphere. NASA has also outlined a list of lower priorities, including the measurement of surface temperatures and the search for additional satellites or rings around Pluto.
New Horizons will begin its study of the Pluto system five months before the closest approach to the planet. Once the craft is about 100 million km from Pluto – about three months before closest approach – its images of the planet will be better than those from the Hubble Space Telescope.
In the weeks leading up to closest approach, the mission team will be able to map Pluto-Charon in increasing detail and observe phenomena such as Pluto's weather by comparing the images of the planet over time. It will take high-resolution views of Pluto and its moons to decide which geological features are worthy of intensive scrutiny. The highest-resolution images will be near Landsat-class in quality, with resolution in the tens of meters.
During closest approach, New Horizons' imagers will map the entire sunlit faces of Pluto and Charon and also map their outer surface compositions. The team hasn't yet determined exactly how close New Horizons will come to Pluto; pre-launch planning is in the range of 10,000 kilometers.
Once the spacecraft passes Pluto, it will turn around and map the planet's night side, which will be softly illuminated by the reflected moonlight from Charon. And the spacecraft's antenna will receive a powerful radio beam from Earth, aimed so that it passes through Pluto's atmosphere. By measuring the effects of atmospheric refraction on the radio beam as it travels to the spacecraft, and similar effects on ultraviolet sunlight passing through the atmosphere, scientists will be able to plot the temperature and density profile of the atmosphere down to the surface.
New Horizons will also sample the density and composition of material escaping from Pluto's atmosphere, map surface temperatures across Pluto and Charon, study Pluto's ionosphere, refine the radii and masses of Pluto and its moons, search for dust particles in the Pluto system and search for rings and additional moons – among other studies.
After the Pluto-Charon encounter, New Horizons will maneuver to begin a series of what the team hopes could be one to two encounters with other Kuiper Belt Objects over the following five to seven years. Funding that extended mission will require NASA approval.
The first exploration of the Pluto-Charon system and the Kuiper Belt will inspire and excite the scientific community and the public. New Horizons will provide invaluable insights into the origin of the outer solar system and the ancient outer solar nebula, the origin and evolution of planet–satellite systems presumably formed by giant impacts, and the comparative geology, geochemistry, tidal evolution, atmospheres and volatile transport mechanics of icy worlds.
NASA's New Frontiers Program
With the New Frontiers Program, NASA aims to explore the solar system with frequent, medium-class, scientifically focused spacecraft missions. NASA established the program in 2003 while building on the innovative approaches used in its Discovery and Explorer programs – providing a way to identify and select missions too challenging within Discovery's cost and time constraints.
New Frontiers missions will tackle specific exploration goals identified as top priorities in the landmark 2002 National Research Council study, New Frontiers in the Solar System: An Integrated Exploration Strategy. Also known as the "Decadal Survey," the study was conducted by the Space Studies Board of the National Research Council at NASA's request. In doing so, NASA sought to examine the big picture of solar system exploration, survey the current knowledge of our solar system, compile the scientific questions that should guide solar system exploration in the next decade, and list (in order) the most promising avenues for flight investigations and supporting ground-based activities. - The high-priority scientific goals identified by the study related to the exploration of Pluto and the Kuiper Belt, Venus, Jupiter, the south pole of the Moon (including the Aitken Basin) and comets.
Open Competition: New Frontiers missions start as proposals – sent to NASA after an open announcement – and are chosen through a competitive peer review process. A principal investigator (PI), typically affiliated with a university or research institution, leads each mission. The PI selects team members from industry, small businesses, government laboratories and universities to develop the science objectives and instrument payload. The PI is responsible for the overall success of the project by assuring it will meet all cost, schedule and performance objectives.
Principal Investigator: Dr. Alan Stern, SwRI (Southwest Research Institute), Boulder CO.
Project Scientist: Dr. Hal Weaver, JHU/APL (Johns Hopkins University/Applied Physics Laboratory), Laurel, MD.
Deputy Project Scientist: Dr. Leslie Young, SwRI (Southwest Research Institute), Boulder, CO.
Missions: With its mission plan and management structure already closely aligned to the program's goals, New Horizons became the first New Frontiers mission when the program was established. The second New Frontiers mission is Juno, scheduled to launch in 2011 and conduct an in-depth study of Jupiter. Juno plans to place a spacecraft in a polar orbit around the giant planet to look for an ice-rock core, determine how much water and ammonia exists in the atmosphere, study convection and deep wind profiles in the atmosphere, examine the origin of the Jovian magnetic field, and explore the polar magnetosphere.
NASA's Discovery and New Frontiers Program Office at Marshall Space Flight Center in Huntsville, Ala., assists the Science Mission Directorate at NASA Headquarters with program management, technology planning, systems assessment, flight assurance and public outreach. The Marshall Center assures the availability of the technical expertise to quickly assess needs and manage the support structure to provide oversight to these missions.
Note: New Horizons was proposed to AO-OSS-01, NASA's Jan. 20, 2001, request for flyby mission proposals to Pluto-Charon and the Kuiper Belt. New Horizons was one of two proposals chosen for further concept study in June 2001, and NASA selected New Horizons as its Pluto mission on Nov. 29, 2001. Led by Principal Investigator (PI) Alan Stern of the Southwest Research Institute's Space Studies Department, Boulder, CO, the mission team included major partners at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md.; Stanford University, Palo Alto, Calif.; Ball Aerospace Corp., Boulder; NASA Goddard Space Flight Center, Greenbelt, Md.; and the Jet Propulsion Laboratory, Pasadena, Calif. — New Horizons is the first-ever PI-led mission to the outer planets and the first mission of the New Frontiers Program.
Designed and integrated at the JHU/APL (Johns Hopkins University /Applied Physics Laboratory ) in Laurel, Md. – with contributions from companies and institutions around the world – the New Horizons spacecraft is a robust, lightweight observatory designed to withstand the long, difficult journey from the launch pad on Earth to the solar system's coldest, darkest frontiers.
The New Horizons science payload was developed under direction of SwRI ( Southwest Research Institute), with instrument contributions from SwRI, APL, NASA's Goddard Space Flight Center, the University of Colorado, Stanford University and Ball Aerospace Corporation.
Fully fueled, the agile, the minisatellite has a mass of 478 kg. Designed to operate on a limited power source – a single RTG (Radioisotope Thermoelectric Generator) – New Horizons needs less power than a pair of 100 W light bulbs to complete its mission at Pluto.
On average, each of the seven science instruments uses between 2 and 10 W – about the power of a night light – when turned on. The instruments send their data to one of two onboard solid state memory banks, where data is recorded before later playback to Earth. During normal operations, the spacecraft communicates with Earth through its 2.1 m wide high-gain antenna. Smaller antennas provide backup and near-Earth communications. And when the spacecraft hibernates through long stretches of its voyage, its computer is programmed to monitor its systems and report status back home with a specially coded, low-energy beacon signal.
The spacecraft's "thermos bottle" design retains heat and keeps the spacecraft operating at room temperature without large, excess heaters. Aside from protective covers on five instruments, New Horizons has no deployable mechanisms or scanning platforms. It does have backup devices for all major electronics, its star-tracking navigation cameras and data recorders.
Figure 3: Illustration of the New Horizons spacecraft (image credit: JHU/APL, NASA, SwRI)
New Horizons will operate in a spin-stabilized mode after launch, during early operations and while cruising between planets, and in a three-axis "pointing" mode that allows for pointing or scanning instruments during planetary encounters. There are no reaction wheels on the spacecraft; small thrusters in the propulsion system handle pointing, spinning and course corrections. The spacecraft navigates using onboard gyros, star trackers and Sun sensors.
The spacecraft's high-gain antenna dish is linked to advanced electronics and shaped to receive even the faintest radio signals from home – a necessity when the mission's main target is more than 5 billion kilometers from Earth and round-trip transmission time is nine hours.
Spacecraft Systems and Components
Structure: New Horizons' primary structure includes an aluminum central cylinder that supports honeycomb panels, serves as the payload adapter fitting that connects the spacecraft to the launch vehicle, supports the interface between the spacecraft and its power source, and houses the propellant tank. Keeping mass down, the panels surrounding the central cylinder feature an aluminum honeycomb core with ultra-thin aluminum face sheets (about as thick as two pieces of paper). To keep it perfectly balanced for spinning operations, the spacecraft is weighed and then balanced with additional weights just before mounting on the launch vehicle.
Command and Data Handling: The command and data handling system – a radiation-hardened 12 MHz Mongoose V processor guided by intricate flight software – is the spacecraft's "brain." The processor distributes operating commands to each subsystem, collects and processes instrument data, and sequences information sent back to Earth. It also runs the advanced "autonomy" algorithms that allow the spacecraft to check the status of each system and, if necessary, correct any problems, switch to backup systems or contact operators on Earth for help.
Figure 4: Mongoose V MIPS 3000 controller (image credit: Synova)
For data storage, New Horizons carries two low-power solid-state recorders (one backup) that can hold up to 8 GB each. The main processor collects, compresses, reformats, sorts and stores science and housekeeping data on the recorder – similar to a flash memory card for a digital camera – for transmission to Earth through the telecommunications subsystem.
The Command and Data Handling processor, data recorder, power converters, Guidance and Control processor, radio science and tracking electronics, and interfaces between the processors and science instruments are housed in the Integrated Electronics Module (IEM), a space- and weight-saving device that combines the spacecraft's core avionics in a single box. New Horizons carries a redundant IEM as a backup.
Thermal Control Subsystem: New Horizons is designed to retain heat like a thermos bottle. The spacecraft is covered in lightweight, gold-colored, multilayered thermal insulation blankets, which hold in heat from operating electronics to keep the spacecraft warm. Heat from the electronics will keep the spacecraft operating at between 10-30ºCelsius throughout the journey.
New Horizons' sophisticated, automated heating system monitors power levels inside the craft to make sure the electronics are running at enough wattage to maintain safe temperatures. Any drop below that operating level (about 150 W) and it will activate small heaters around the craft to make up the difference. When the spacecraft is closer to Earth and the Sun, louvers (that act as heat vents) on the craft will open when internal temperatures are too high.
The thermal blanketing – 18 layers of Dacron mesh cloth sandwiched between aluminized Mylar and Kapton film – also helps to protect the craft from micrometeorites.
Propulsion: The propulsion system on New Horizons is used for course corrections and for pointing the spacecraft. It is not needed to speed the spacecraft to Pluto; that is done entirely by the launch vehicle.
The New Horizons propulsion system includes 16 small hydrazine-propellant thrusters mounted across the spacecraft in eight locations, a fuel tank, and associated distribution plumbing. Four thrusters that each provide 4.4 N of thrust will be used mostly for course corrections. The spacecraft will use 12 smaller thrusters – providing 0.8 N of thrust each – to point, spin up and spin down the spacecraft. Eight of the 16 thrusters aboard New Horizons are considered the primary set; the other eight comprise the backup (redundant) set.
At launch, the spacecraft will carry 77 kg of hydrazine, stored in a lightweight titanium tank. Helium gas pushes fuel through the system to the thrusters. Using a Jupiter gravity assist, along with the fact that New Horizons does not need to slow down enough to enter orbit around Pluto, reduces the amount of propellant needed for the mission.
Guidance and Control: New Horizons must be oriented in a particular direction to collect data with its scientific instruments, communicate with Earth, or maneuver through space. Attitude determination – knowing which direction New Horizons is facing – is performed using star-tracking cameras, Inertial Measurement Units (containing sophisticated gyroscopes and accelerometers that measure rotation and horizontal/vertical motion), and digital solar sensors. Attitude control for the spacecraft – whether in a steady, three-axis pointing mode or in a spin-stabilized mode – is accomplished using thrusters.
The IMUs and star trackers provide constant positional information to the spacecraft's Guidance and Control processor, which like the command and data handling processor is a 12 MHz Mongoose V. New Horizons carries two copies at each of these units for redundancy. The star-tracking cameras store a map of about 3,000 stars; 10 times per second one of the cameras snaps a wide-angle picture of space, compares the locations of the stars to its onboard map, and calculates the spacecraft's orientation. The IMU feeds motion information 100 times a second. If data shows New Horizons is outside a predetermined position, small hydrazine thrusters will fire to re-orient the spacecraft. The Sun sensors back up the star trackers; they would find and point New Horizons toward the Sun (with Earth nearby) if the other sensors couldn't find home in an emergency.
Operators use thrusters to maneuver the spacecraft, which has no internal reaction wheels. Its smaller thrusters will be used for fine pointing; thrusters that are approximately five times more powerful will be used during the trajectory course maneuvers that guide New Horizons toward its targets. New Horizons will spin – typically at 5 rpm (revolutions per minute)– during trajectory-correction maneuvers, long radio contacts with Earth, and while it "hibernates" during long cruise periods. Operators will steady and point the spacecraft during science observations and instrument-system checkouts.
Communications: New Horizons' X-band communications system is the spacecraft's link to Earth, returning science data, exchanging commands and status information, and allowing for precise radiometric tracking through NASA's Deep Space Network of antenna stations.
The system includes two broad-beam, low-gain antennas on opposite sides of the spacecraft for near-Earth communications: a 30 cm diameter medium-gain dish antenna and a large, 2.1 m diameter high-gain dish antenna. The antenna assembly on the spacecraft's top deck consists of the high, medium, and forward low-gain antennas; this stacked design provides a clear field of view for the low-gain antenna and structural support for the high and medium-gain dishes. Operators aim the antennas by turning the spacecraft toward Earth. The high-gain beam is only 0.3 degrees wide, so it must point directly at Earth. The medium-gain beam is wider (14 degrees), so it is used in conditions when the pointing might not be as accurate. All antennas have Right Hand Circular and Left Hand Circular polarization feeds.
Figure 5: RF telecommunications system block diagram (image credit: JHU/APL)
Data rates will depend on spacecraft distance, the power used to send the data and the size of the antenna on the ground. For most of the mission, New Horizons will use its high-gain antenna to exchange data with the Deep Space Network's largest antennas, 70 meters across. Even then, because New Horizons will be more than 5 billion km from Earth and radio signals will take more than four hours to reach the spacecraft, it can send information at about 700 bit/s. It will take nine months to send the full set of Pluto encounter science data back to Earth.
New Horizons will fly the most advanced digital receiver ever used for deep space communications. Advances include regenerative ranging and low power – the receiver consumes 66% less power than current deep space receivers. The Radio Science Experiment (REX) to examine Pluto's atmosphere is also integrated into the communications subsystem. — The entire telecom system on New Horizons is redundant, with two of everything except the high gain antenna structure itself.
Power: New Horizons' electrical power comes from a single radioisotope thermoelectric generator (RTG), which provides power through the natural radioactive decay of plutonium dioxide fuel. The New Horizons RTG, provided by the U.S. Department of Energy, carries approximately 11 kg of plutonium dioxide. Onboard systems manage the spacecraft's power consumption so it doesn't exceed the steady output from the RTG, which will decrease by about 3.5 W/year.
Typical of RTG-based systems, as on past outer-planet missions, New Horizons does not have a battery for storing power. At the start of the mission, the RTG will supply approximately 240 W (at 30 volts of direct current) – the spacecraft's shunt regulator unit maintains a steady input from the RTG and dissipates power the spacecraft cannot use at a given time. By July 2015 (the earliest Pluto encounter date) that supply decreases to 200 W at the same voltage, so New Horizons will ease the strain on its limited power source by cycling science instruments during planetary encounters.
The spacecraft's fully redundant Power Distribution Unit (PDU) – with 96 connectors and more than 3,200 wires – efficiently moves power through the spacecraft's vital systems and science instruments.
The PDU communicates with the spacecraft control system via two 1553 interfaces using redundant universal asynchronous receiver/transmitter (UART) serial links that pass critical commands and telemetry. PDU is in charge of delivering power to all loads of the spacecraft that are grouped in critical and non-essential loads. Critical loads are the Integrated Equipment Modules, the command receiver, Ultra-Stable Oscillators and Power Distribution Unit 1553 board – having their primary and redundant units powered at all times. To power off any of the redundant units, software and hardware-enabling would be needed, either through ground command or onboard fault-protection.
Figure 6: Image of the RTG block (image credit: NASA)
Figure 7: Alternate view of the New Horizons spacecraft (image credit: JHU/APL, NASA, SwRI)
Mission Overview: New Horizons will help us understand worlds at the edge of our solar system by making the first reconnaissance of Pluto and Charon – a "double planet" and the last planet in our solar system to be visited by spacecraft – Pluto's moons, and the Kuiper Belt objects beyond.
Packed with robust electronics and a full suite of science instruments, the compact New Horizons probe is fortified for a long voyage of discovery. Launched on a powerful Atlas V rocket, New Horizons will be the fastest spacecraft ever dispatched to the outer solar system, passing lunar orbit distance nine hours after launch and reaching Jupiter for a gravity assist and scientific studies just 13 months later. As early as 2015 it will conduct a five-month-long flyby study of the Pluto system. Then, as part of a potential extended mission, it will head deeper into the Kuiper Belt to study one or more of the icy mini-worlds in that vast region at least 1.6 billion kilometers beyond Neptune's orbit.
Sending New Horizons on this long journey will help us answer basic questions about the surface properties, geology, interior makeup and atmospheres on these mysterious relics of solar system formation – and tell us much about the origins and evolution of the worlds around us.
Launch: The New Horizons interplanetary space probe was launched on 19 January 2006 (19:00 UTC) on an Atlas V-551 vehicle of Lockheed Martin from Launch Complex 41 at Cape Canaveral Air Force Station, FL. The Atlas V-551 is NASA's most powerful launch vehicle. It features a Common Core Booster first stage, bolstered by five strap-on solid rocket boosters. Its second stage uses the Centaur booster. New Horizons also has a custom Boeing solid-propellant STAR 48B third-stage motor, which gives it a final push toward Jupiter and on to Pluto. 4)
Figure 8: New Horizons / Atlas V expanded view (image credit: NASA)
Planned mission overview
New Horizons will cross the Moon's orbit in just nine hours – something that took the Apollo astronauts more than three days to accomplish. Just 13 months later New Horizons will fly past Jupiter for a gravitational assist toward Pluto; the two most recent NASA missions sent to Jupiter, Galileo and Cassini, took six and four years, respectively, to reach the giant planet. And yet, Jupiter, almost half a billion miles away, is only a fraction of the distance to Pluto.
New Horizons takes advantage of a Jupiter gravity assist that shaves three to five years off the trip time to Pluto-Charon and the Kuiper Belt. New Horizons will pass through the Jupiter system at 21 km/s on a path that could get it to Pluto as early as 2015. The flyby increases New Horizons' speed away from the Sun by nearly 4 km/s.
The Jupiter gravity assist is a mission priority because, by reducing the flight time to Pluto, it reduces the risk of mission failure. But the Jupiter flyby also presents New Horizons a unique opportunity to flight-test its instruments and pointing capabilities on an exciting scientific target. New Horizons will venture at least three times closer to Jupiter than the Cassini spacecraft did in late 2000, when it used Jupiter for a gravity assist on the way to Saturn.
New Horizons will fly just outside of the orbit of Jupiter's large moon Callisto – about 2.27 million km from the giant planet. From this closer-range, New Horizons will perform a number of Jupiter system studies not possible from Cassini's greater flyby distance – science opportunities include Jovian meteorology, Jovian auroral studies, Jovian magnetospheric sampling, and dust sampling and ultraviolet mapping of the torus around Jupiter's volcanic moon, Io. Surface mapping, compositional mapping and atmospheric studies of Jupiter's moons are planned as well.
Hibernation: New Horizons will "sleep" for most of the cruise between Jupiter and Pluto in spin-stabilized hibernation mode, designed to reduce spacecraft operation costs and free up Deep Space Network tracking resources for other missions. Hibernation, during which much of the spacecraft is unpowered, also reduces wear and tear on spacecraft electronics – an important consideration for the long journey to Pluto.
Operators will put New Horizons into hibernation by turning off most of its electronics and setting it on a steady course, spinning at 5 rpm. The antenna dish will point toward Earth while the onboard flight computer monitors system health and, on command from home, broadcasts a weekly beacon tone through the medium-gain antenna. New Horizons will transmit a "green" coded tone if all is well, or send back one of seven coded "red" tones if it detects a problem and requires help from the operations team.
Approaching Pluto: After traveling some 5 billion kilometers, New Horizons must thread a celestial needle and fly through a circle only 300 kilometers in diameter to accomplish its science objectives. Fortunately, the team has a chance to guide New Horizons along the way.
As New Horizons gets closer to Pluto, it will take detailed pictures of the Pluto system, to help the team determine if the spacecraft is moving in the right direction (this is called "optical navigation."). New Horizons uses its smaller thrusters to spin "down" into a stable pointing mode and change direction. The large thrusters only have 4.4 newtons of force – not much for a spacecraft with a mass of 490 kg – but they only need to make small corrections.
The cameras and spectrometers on New Horizons will start taking data on the Pluto system five months before the spacecraft arrives. Pluto and Charon will first appear as small, bright dots, but the planet and its moons will appear larger as the encounter date approaches. About three months from the closest approach – when Pluto and Charon are about 100 million kilometers away – the cameras on the spacecraft can make the first maps. For those three months, the mission team would take pictures and spectra measurements.
Pluto and Charon each rotate once every 6.4 Earth days. For the last four Pluto days before encounter (26 Earth days), the team will compile maps and gather spectra measurements of Pluto and Charon every half-day. The team can then compare these maps to check changes over a Pluto day, at scales as good as about 48 km, that might indicate new snows or other weather.
The Encounter: The busiest part of the Pluto-Charon flyby will last a full Earth day, from about 12 hours before closest approach to about 12 hours after. On the way in, the spacecraft will study ultraviolet emissions from Pluto's atmosphere and make its best global maps of Pluto and Charon in green, blue, red and a special wavelength that is sensitive to methane frost on the surface. It will also take spectral maps in the near infrared, telling the science team about Pluto's and Charon's surface compositions at all locations, as well as the variation in temperature across the surface. New Horizons will also sample material coming from Pluto's atmosphere, and it will image all of Pluto's moons during this period.
During the half-hour when the spacecraft is closest to Pluto and Charon, it will take close-up pictures in both visible and near-infrared wavelengths. The best pictures of Pluto will depict surface features as small as 25 meters across.
Even after the spacecraft passes Pluto, Charon and their two smaller companion moons, its work is far from done. Looking back at the mostly dark side of Pluto or Charon is the best way to spot haze in the atmosphere, to look for rings, and to determine whether their upper surfaces are smooth or rough. Also, the spacecraft will fly through the shadows cast by Pluto and Charon and observe both the Earth and Sun setting, and then rising, through Pluto's atmosphere. It will look back at the Sun and Earth, and watch the light from the Sun and pick up radio waves from transmitters on Earth. These measurements will reveal the composition, structure, and thermal profile of Pluto's atmosphere in exquisite detail.
Figure 9: Pluto encounter timeline for 2015 arrival (image credit: JHU/APL, NASA)
Many of these types of measurements were made by spacecraft like the Voyagers and the Mariners on previous first flybys of planets. However, New Horizons also brings some revolutionary new capabilities to bear. These include temperature and composition mapping capabilities and a dust detector to pick up tiny debris particles near Pluto. The technology for these latter kinds of instruments was not available when the Mariner and Voyager spacecraft were flown.
New Horizons will approach Pluto from the planet's southern hemisphere – for a July 2015 encounter, the southern hemisphere will be sunlit and the northern cap dark. The spacecraft flies toward Pluto at a solar phase angle of 15 degrees – excellent lighting conditions for remote sensing.
Science sensor complement: (Alice, Ralph, REX, LORRI, SWAP, PEPSSI, SDC)
The New Horizons science payload consists of seven instruments – three optical instruments, two plasma instruments, a dust sensor and a radio science receiver/radiometer. This payload was designed to investigate the global geology, surface composition and temperature, and the atmospheric pressure, temperature and escape rate of Pluto and its largest moons. They will also be used to study the Jupiter system if the spacecraft is launched on a Jupiter-Pluto trajectory, as the team prefers. If an extended mission is approved, the instruments will probe additional Kuiper Belt Objects that the spacecraft can reach.
The payload is incredibly power efficient – with the instruments collectively drawing less than 28 watts – and represent a degree of miniaturization that is unprecedented in planetary exploration. The instruments were designed specifically to handle the cold conditions and low light levels at Pluto and in the Kuiper Belt beyond.
Alice is a sensitive ultraviolet imaging spectrometer designed to probe the composition and structure of Pluto's dynamic atmosphere. A spectrometer separates light into its constituent wavelengths (like a prism). An "imaging spectrometer" both separates the different wavelengths of light and produces an image of the target at each wavelength. The objective of Alice is to staudy atmospheric composition and structure.
The instrument has a mass of 4.5 kg and an average power demand of 4.4 W, developed at SwRI. The PI is Alan Stern of SwRI. Alice's spectroscopic range extends across both extreme and far-ultraviolet wavelengths from approximately 500 to 1,800 Å. The instrument will detect a variety of important atomic and molecular species in Pluto's atmosphere, and determine their relative abundances, giving scientists the first complete picture of Pluto's atmospheric composition. Alice will search for an ionosphere around Pluto and an atmosphere around Pluto's moon Charon. It will also probe the density of Pluto's atmosphere, and the atmospheric temperature of Pluto, both as a function of altitude.
Figure 10: Illustration of the Alice instrument (image credit: SwRI)
Alice consists of a compact telescope, a spectrograph, and a sensitive electronic detector with 1,024 spectral channels at each of 32 separate spatial locations in its long, rectangular field of view. Alice has two modes of operation: an "airglow" mode that measures ultraviolet emissions from atmospheric constituents, and an "occultation" mode, where it views the Sun or a bright star through an atmosphere and detects atmospheric constituents by the amount of sunlight they absorb. Absorption of sunlight by Pluto's atmosphere will show up as characteristic "dips" and "edges" in the ultraviolet part of the spectrum of light that Alice measures. This technique is a powerful method for measuring even traces of atmospheric gas.
A first-generation version of New Horizons' Alice (smaller and a bit less sophisticated) is flying successfully aboard the European Space Agency's Rosetta spacecraft, which will examine the surface of Comet 67P/Churyumov-Gerasimenko and study its escaping atmosphere and complex surface.
Ralph is the main "eyes" of New Horizons and is charged with making the maps that show what Pluto, its moons, and other Kuiper Belt Objects look like. (The instrument is so named because it's coupled with an ultraviolet spectrometer called Alice in the New Horizons remote-sensing package – a reference familiar to fans of "The Honeymooners" TV show.) Ralph consists of three panchromatic (black-and-white) and four color imagers inside its MVIC (Multispectral Visible Imaging Camera), as well as an infrared compositional mapping spectrometer called the LEISA (Linear Etalon Imaging Spectral Array). LEISA is an advanced, miniaturized short-wavelength infrared (1.25-2.50 µm) spectrometer provided by scientists from NASA's Goddard Space Flight Center. MVIC operates over the bandpass from 0.4 to 0.95 µm.
Ralph has a mass of 10.3 kg and an average power of 6.3 W. The instrument was developed at BATC (Ball Aerospace and Technologies Corporation), at NASA/GSFC and at SwRI. PI: Alan Stern. The objective is to study surface geology and morphology; obtain surface composition and surface temperature maps.
Ralph's suite of eight detectors – seven CCDs similar to those found in a digital camera, and a single infrared array detector – are fed by a single, sensitive magnifying telescope with a resolution more than 10 times better than the human eye can see. The entire package operates on less than half the wattage of a night light.
Ralph will take images twice daily as New Horizons approaches, flies past and then looks back at the Pluto system. Ultimately, MVIC will map landforms in black-and-white and color with a best resolution of about 250 m/pixel, take stereo images to determine surface topography, and help scientists refine the radii and orbits of Pluto and its moons. It will aid the search for clouds and hazes in Pluto's atmosphere, and for rings and additional satellites around Pluto and other Kuiper Belt Objects. It will also obtain images of Pluto's night side, illuminated by "Charon-light."
At the same time, LEISA will map the amounts of nitrogen, methane, carbon monoxide, and frozen water and other materials, including organic compounds, across the sunlit surfaces of Pluto and its moons (and later Kuiper Belt Objects). It will also let scientists map surface temperatures across Pluto and Charon by sensing the spectral features of frozen nitrogen, water and carbon monoxide. And Pluto is so far from the Sun that Ralph must work with light levels 1,000 times fainter than daylight at Earth – or 400 times fainter than conditions Mars probes face – so it is incredibly sensitive.
Figure 11: Illustration of the Ralph instrument (image credit: BATC, SwRI)
REX (Radio Science Experiment)
REX consists only of a small printed circuit board containing sophisticated signal-processing electronics integrated into the New Horizons telecommunications system. Because the telecom system is redundant within New Horizons, the spacecraft carries two copies of REX. Both can be used simultaneously to improve the data return from the radio science experiment.
REX has a mass of 100 grams and an average power demand of 2.1 W. REX was developed at JHU/APL and at Stanford University. PI: Len Tyler, Stanford University. The objective of REX is to measure the atmospheric temperature and pressure (down to the surface); measure density of the ionosphere; search for atmospheres around Charon and other KBOs.
REX will use an occultation technique to probe Pluto's atmosphere and to search for an atmosphere around Charon. After New Horizons flies by Pluto, its 2.1 m dish antenna will point back at Earth. On Earth, powerful transmitters in NASA's largest Deep Space Network antennas will beam radio signals to the spacecraft as it passes behind Pluto. The radio waves will bend according to the average molecular weight of gas in the atmosphere and the atmospheric temperature. The same phenomenon could happen at Charon if the large moon has a substantial atmosphere, but Earth-based studies indicate this is unlikely.
Space missions typically conduct this type of experiment by sending a signal from the spacecraft through a planet's atmosphere and back to Earth (this is called a "downlink" radio experiment). New Horizons will be the first to use a signal from Earth – the spacecraft will be so far from home and moving so quickly past Pluto-Charon that only a large, ground-based antenna can provide a strong enough signal. This new technique, called an "uplink" radio experiment, is an important advance beyond previous outer planet missions.
REX will also measure the weak radio emissions from Pluto and other bodies the spacecraft flies by, such as Jupiter and Charon. Scientists will use the data to derive accurate globally averaged day-side and night-side temperature measurements. Also, by using REX to track slight changes in the spacecraft's path, scientists will measure the masses of Pluto and Charon and possibly the masses of additional Kuiper Belt Objects. By timing the length of the radio occultations of Pluto and Charon, REX will also yield improved radii measurements for Pluto and Charon.
LORRI (Long Range Reconnaissance Imager)
LORRI, the "eagle eyes" of New Horizons, is a panchromatic high-magnification imager, consisting of a telescope with an 20.8 cm aperture that focuses visible light onto a CCD (Charge-Coupled Device). It's essentially a digital camera with a large telephoto telescope – only fortified to operate in the cold, hostile environs near Pluto.
LORRI has a mass of 8.8 kg and an average power demand of 5.8 W. Theinstrument was developed at JHU/APL and the PI is Andy Cheng of APL. The objective is to staudy geology; provide high-resolution approach and highest-resolution encounter images.
LORRI images will be New Horizons' first of the Pluto system, starting about 200 days before closest approach. At the time, Pluto and its moons will resemble little more than bright dots, but these system-wide views will help navigators keep the spacecraft on course and help scientists refine their orbit calculations of Pluto and its moons. At 90 days before closest approach – with the system more than 100 million kilometers away – LORRI images will surpass Hubble-quality resolution, providing never-before-seen details each day. At closest approach, LORRI will image select sections of Pluto's sunlit surface at football-field-size resolution, resolving features at least 50 meters across.
Figure 12: Illustration of the LORRI instrument (image credit: JHU/APL)
This range of images will give scientists an unprecedented look at the geology on Pluto, Charon, and additional Kuiper Belt Objects – including the number and size of craters on each surface, revealing the history of impacting objects in that distant region. LORRI will also yield important information on the history of Pluto's surface, search for activity such as geysers on that surface, and look for hazes in Pluto's atmosphere. LORRI will also provide the highest resolution images of any Kuiper Belt Objects New Horizons would fly by in an extended mission.
LORRI has no color filters or moving parts – operators will take images by pointing the LORRI side of the spacecraft directly at their target. The instrument's innovative silicon carbide construction will keep its mirrors focused through the extreme temperature dips New Horizons will experience on the way to and past Pluto-Charon.
SWAP (Solar Wind at Pluto)
The SWAP instrument will measure interactions of Pluto with the solar wind – the high-speed stream of charged particles flowing from the Sun. The incredible distance of Pluto from the Sun required the SWAP team to build the largest-aperture instrument ever used to measure the solar wind.
SWAP has a mass of 3.3 kg and an average power demand of 2.3 W. The instrument was developed at SwRI. The PI is David McComas at SwRI. The objective is to study the solar wind interactions and atmospheric escape.
Pluto's small gravitational acceleration (approximately 1/16 of Earth's gravity) leads scientists to think that about 75 kg of material escape its atmosphere every second. If so, then the planet behaves like a comet, though Pluto is more than 1,000 times larger than a typical comet nucleus. The atmospheric gases that escape Pluto's weak gravity leave the planet as neutral atoms and molecules. These atoms and molecules are ionized by ultraviolet sunlight (similar to the Earth's upper atmosphere and ionosphere). Once they become electrically charged, the ions and electrons become "picked up" and are carried away by the solar wind. In the process, these pick-up ions gain substantial energy (thousands of electron-volts). This energy comes from the solar wind, which is correspondingly slowed down and diverted around Pluto. SWAP measures low-energy interactions, such as those caused by the solar wind. By measuring how the solar wind is perturbed by the interaction with Pluto's escaping atmosphere, SWAP will determine the escape rate of atmospheric material from Pluto.
Figure 13: Illustration of the SWAP instrument (image credit: SwRI)
At the top of its energy range SWAP can detect some pickup ions (up to 6.5 keV). SWAP combines a RPA (Retarding Potential Analyzer) with anESA (Electrostatic Analyzer) to enable extremely fine, accurate energy measurements of the solar wind, allowing New Horizons to measure minute changes in solar wind speed.
The amount of Pluto's atmosphere that escapes into space provides critical insights into the structure and destiny of the atmosphere itself.
PEPSSI (Pluto Energetic Particle Spectrometer Science Investigation)
PEPSSI, the most compact, lowest-power directional energetic particle spectrometer flown on a space mission, will search for neutral atoms that escape Pluto's atmosphere and become charged by their interaction with the solar wind. It will detect the material that escapes from Pluto's atmosphere (such as molecular nitrogen, carbon monoxide and methane), which break up into ions and electrons after absorbing the Sun's ultraviolet light, and stream away from Pluto as "pick up" ions carried by the solar wind.
PEPSSI has a mass of 1.5 kg and a power demand of 2.5 W. The instrument was developed at JHU/APL. The PI is Ralph McNutt Jr. of APL.
The instrument will likely get its first taste of Pluto's atmosphere when the planet is still millions of kilometers away. By using PEPSSI to count particles, and knowing how far New Horizons is from Pluto at a given time, scientists will be able to tell how quickly the planet's atmosphere is escaping and gain new information about what the atmosphere is made of.
Figure 14: Illustration of the PEPSSI instrument (image credit: JHU/APL)
PEPSSI is a classic "time-of-flight" particle instrument: particles enter the detector and knock other particles (electrons) from a thin foil; they zip toward another foil before hitting a solid-state detector. The instrument clocks the time between the foil collisions to tell the particle's speed (measuring its mass) and figures its total energy when it collides with the solid-state detector. From this, scientists can determine the composition of each particle. PEPSSI can measure energetic particles up to 1,000 keV, many times more energetic than SWAP can. Together the two instruments make a powerful combination for studying the Pluto system.
SDC (Student Dust Counter)
Designed and built by students at the University of Colorado at Boulder, the SDC will detect microscopic dust grains produced by collisions among asteroids, comets, and Kuiper Belt Objects during New Horizons' long journey. Officially a New Horizons Education and Public Outreach project, SDC is the first science instrument on a NASA planetary mission to be designed, built and "flown" by students.
The SDC has a mass of 1.9 kg and an average power demand of 5 W. The instrument was developed at LASP (Laboratory for Atmospheric and Space Physics) of the University of Colorado at Boulder. The PI is Mihaly Horanyi, University of Colorado at Boulder. The objective is to measure the concentration of dust particles in outer solar system.
The SDC will count and measure the sizes of dust particles along New Horizons' entire trajectory and produce information on the collision rates of such bodies in the deep outer solar system. SDC will also be used to search for dust in the Pluto system; such dust might be generated by collisions of tiny impactors on Pluto's small moons.The instrument includes two major pieces: an 45 x 30 cm detector assembly, which is mounted on the outside of the spacecraft and exposed to the dust particles; and an electronics box inside the spacecraft that, when a hit occurs on the detector, deciphers the data and determines the mass and speed of the particle. Because no dust detector has ever flown beyond 18 astronomical units from the Sun (nearly 2.7 billion km, about the distance from Uranus to the Sun), SDC data will give scientists an unprecedented look at the sources and transport of dust in the solar system.
Figure 15: Illustration of the SDC instrument (image credit: LASP)
With faculty support, the University of Colorado students will also distribute and archive data from the instrument, and lead a comprehensive education and outreach effort to bring their results and experiences to classrooms of all grades over the next two decades.
Status of the New Horizons mission
- I'm going to focus this PI's Perspective on a major upcoming mission mile marker — namely, New Horizons being 50 astronomical units (AU) from the Sun next month. But first, some mission news.
- Our biggest news is that most of our latest flight software upgrades, which will provide new scientific capabilities on the spacecraft, are in final test and on track to be uplinked in July. In fact, one of those updates, for our solar wind instrument called SWAP, is already aboard the spacecraft — and being used to produce new science! That software, transmitted to New Horizons in mid-February and tested for a week at the end of February, allows us to see much finer structures in the solar wind as we plow toward the heliopause, the outer edge of the heliosphere that surrounds the solar system.
- We're also preparing another search for Kuiper Belt objects (KBOs) to study as we pass by them; those same summertime searches will also look for a new flyby target KBO, just as we did in our 2020 searches. Keep in mind, the search for Arrokoth (2014 MU69) took four years—and this search will go on for years, too, because it's a needle in a haystack challenge to find flyby KBOs! But this time, we're applying a new tool—artificial intelligence. Using machine-learning software, mission co-investigator JJ Kavelaars and collaborating scientist Wes Patrick have sped up and made those searches far more productive. In fact, when they reran the 2020 search data through their new software tools, it not only worked 100 times faster, but it turned up dozens of new KBOs that human searchers had not found in the search images! We'll be taking advantage of this important new tool again later this year, and next year and after that as well.
- And one last news item: We're wrapping up development on a flight plan and command load to study three KBOs in May, determining their surface properties, shapes, and more. These kinds of studies we're doing from within the Kuiper Belt can't be done from Earth or even orbiting telescopes, and New Horizons has now studied almost 30 KBOs this way. For some of those KBOs, we we're close enough to search for and find satellites around them at resolutions even the Hubble Space Telescope cannot match, providing an important new window into how KBOs formed. Additional KBO observations are planned in September and December.
Reaching Rare Space
- I mentioned earlier our big upcoming milestone: New Horizons will cross the amazing 50 AU distance marker on April 17 or 18, depending on your time zone here on Earth.). At 50 AU, we'll be 50 times as far from the Sun as Earth is! That's a milestone that only four operating spacecraft — Pioneers 10 and 11, and Voyagers 1 and 2 — have reached before us. That's so far away, in fact — almost 5 billion miles (7.5 billion kilometers) — that the Sun itself is smaller in the sky there than Jupiter is from Earth!
- Of course, the Pioneers (now out of power and derelict) and the Voyagers (both still operating) are much farther out than New Horizons. In fact, they are so much farther out that none of them are the nearest spacecraft to us. That spacecraft is Juno, orbiting Jupiter 10 times closer to the Sun than New Horizons is now! And note that in mid-April, we'll have a news release, with some very special images we've taken from our perch so far away in the Kuiper Belt, so keep an eye out for that!
- Looking back at the flight of New Horizons from Earth to 50 AU almost seems like a dream. Most of us on the flight team have been a part of it all the way, and during that time our kids have grown up, our parents (and we ourselves!) have grown older, and the first exploration of Pluto and the first KBO has been accomplished!
- Looking ahead, just like other NASA planetary missions in extended (post-prime mission) operations, every three years we have to propose a new mission and science plan to NASA. If we are approved, we are funded for the next three years; if not, the mission will be terminated. Our next proposal will be due in early 2022. If New Horizons continues to be funded, it'll fly on, exploring the outer Kuiper Belt and the Sun's outer heliosphere. That's something no other spacecraft can do: we're the only one in this region!
- We hope to continue proposing and performing science for many years. By the late 2030s, though, New Horizons may be too low on power to operate. That's because of the half-life of our plutonium power supply, which produces 3.3 watts less every year, and 33 watts less every decade. By the time it can't produce enough power to run the main spacecraft systems, New Horizons will be at or near 100 AU from the Sun—twice as far out as we are now. But even once the spacecraft is derelict—either because it runs out of power or fuel, or for any other reason—it will continue to coast outward, into the galaxy at a speed of nearly 3 AU (about 300 million miles) per year. In fact, even when the day comes in a few billion years that the Sun goes red giant and engulfs Earth, New Horizons, like the Pioneers and Voyagers, will still be out there, outliving even its home planet!
- While you ponder that sobering thought, I'll conclude this report. In the meantime, I hope you'll keep on exploring — just as we do!, Alan Stern.
• January 19, 2021: New Horizons is healthy and continues to send data back from the Kuiper Belt, even as it speeds farther and farther from the Earth and the Sun. The PI's perspective (Alan Stern) on the 15th anniversary of the launch. 6)
- But the mission's jam-packed plans for new Kuiper Belt exploration this year are not the subject of this PI Perspective. Instead, I want to concentrate on a very special anniversary, taking place today — our 15th anniversary of launch!
- That's right, New Horizons lifted into skies above the Florida coast —from the same Cape Canaveral pad used to launch NASA's storied Voyagers—at 2 pm Eastern Time on Thursday, Jan. 19, 2006. I witnessed the launch from the launch control center and gave my final "GO" as the mission principal investigator to proceed less than a minute before our Atlas V rocket ignited.
- In the 5,840 days since launch, New Horizons has blazed through 4.5 billion miles of space to reach its present perch in the Kuiper Belt, the third and most distantly explored region of our planetary system.
- But to me, it's not really about the miles. Nor is it about our speed—even if our spacecraft is still moving at well over 30,000 miles per hour, about 100 times faster than an average jetliner. It's not even about our gravity assist and flight-test flyby at Jupiter in 2007, or the first exploration of the Pluto system in 2015, or even the first exploration of a Kuiper Belt object in 2019 – though those historic achievements were our mission's natal objectives, each accomplished spectacularly and revolutionized humankind's knowledge of our solar system.
- To me, the biggest memes of the past 15 years have been a doubleheader of inspiring public engagement and intensive teamwork. On the public engagement front, New Horizons broke so many NASA records that I've lost count, but I look back proudly knowing that we touched lives and even awed a sometimes cynical world convinced that great things can't happen in our own time. On teamwork, the crew of New Horizons—the flight controllers, engineers, scientists, and others totaling about 200 belly buttons who guided the project and our spacecraft—showed that by working together we could accomplish something both larger than life and greater than any one of us could ever have hoped to do alone, no matter how talented or hard working an individual might be.
- The launch of New Horizons was itself the culmination of 17 years of struggle in the U.S. planetary science community to see NASA commit to and fund the exploration of Pluto and the Kuiper Belt—unfinished business left to my generation by the pioneers who explored all eight of the other classical planets known at the birth of the Space Age.
Figure 16: The flight of New Horizons [image credit: NASA/Johns Hopkins APL/Southwest Research Institute)]
- It took six separate and failed mission concepts before New Horizons was born. It took a Decadal Survey's endorsement. It took a David and Goliath proposal battle that the David in this story, the New Horizons team, won against tall odds. It took years of Washington politics, followed by the blur of a nearly record-setting spacecraft development and nuclear launch approval process during 2002-2005 to reach the launch pad in time for the single, three-week launch window in 2006 that was needed to reach Pluto by 2015. And, all told, it took the dedication and talents of nearly 2,500 men and women.
- And when we reached that launch day—when everything depended on perfection in the rocket and the spacecraft—it all worked! It worked so well, in fact, that it seemed almost too easy. It worked so well that just a few hours later, we burned the launch malfunction procedures in an evening beach bonfire to celebrate that we would never need them. It worked so well that we needed just half of the predicted fuel to refine New Horizons' course toward the Jupiter flyby and gravity assist, leaving a present to ourselves—bonus fuel for the exploration of the Kuiper Belt we are now carrying out!
- Now, so many miles and so many smiles later, I just want to say one thing to all the people who contributed to making New Horizons successful; to our funders, our mission team, our NASA colleagues, our launch team, the Deep Space Network that provides tracking and communications, our navigation team, our media and education teams, and all those, including all of you, who supported us from the outside: Thank you for making a dream come true, for making a nation and world proud of what humans can accomplish,. And thank you for your commitment to excellence and teamwork that still shines brightly today as New Horizons flies ever onward, exploring, setting records, seeking knowledge, and blazing a trail across our solar system that will ultimately lead it to the stars.
Figure 17: Widow of Pluto discoverer Clyde Tombaugh, Patsy Tombaugh (1912-2012), then 93, points toward Pluto on the afternoon of our launch, Jan. 19, 2006 (photo credit: Michael Soluri)
• January 12, 2021: How dark is the sky, and what does that tell us about the number of galaxies in the visible universe? Astronomers can estimate the total number of galaxies by counting everything visible in a Hubble deep field and then multiplying them by the total area of the sky. But other galaxies are too faint and distant to directly detect. Yet while we can't count them, their light suffuses space with a feeble glow. 7)
- To measure that glow, astronomical satellites have to escape the inner solar system and its light pollution, caused by sunlight reflecting off dust. A team of scientists has used observations by NASA's New Horizons mission to Pluto and the Kuiper Belt to determine the brightness of this cosmic optical background. Their result sets an upper limit to the abundance of faint, unresolved galaxies, showing that they only number in the hundreds of billions, not 2 trillion galaxies as previously believed.
- How dark does space get? If you get away from city lights and look up, the sky between the stars appears very dark indeed. Above the Earth's atmosphere outer space dims even further, fading to an inky pitch-black. And yet even there, space isn't absolutely black. The universe has a suffused feeble glimmer from innumerable distant stars and galaxies.
- New measurements of that weak background glow show that the unseen galaxies are less plentiful than some theoretical studies suggested, numbering only in the hundreds of billions rather than the previously reported two trillion galaxies.
- "It's an important number to know – how many galaxies are there?" said Marc Postman of the Space Telescope Science Institute in Baltimore, Maryland, a lead author on the study. "We simply don't see the light from two trillion galaxies."
- The earlier estimate was extrapolated from very deep sky observations by NASA's Hubble Space Telescope. It relied on mathematical models to estimate how many galaxies were too small and faint for Hubble to see. That team concluded that 90% of the galaxies in the universe were beyond Hubble's ability to detect in visible light. The new findings, which relied on measurements from NASA's distant New Horizons mission, suggest a much more modest number.
Figure 18: This artist's illustration shows NASA's New Horizons spacecraft in the outer solar system. In the background lies the Sun and a glowing band representing zodiacal light, caused by sunlight reflecting off of dust. By traveling beyond the inner solar system and its accompanying light pollution, New Horizons was able to answer the question: How dark is space? At lower right are background stars of the Milky Way (image credit: STScI, Joe Olmsted)
- "Take all the galaxies Hubble can see, double that number, and that's what we see – but nothing more," said Tod Lauer of NSF's NOIRLab (National Optical-Infrared Astronomy Research Laboratory), a lead author on the study.
- These results will be presented on Wednesday, Jan. 13th at a meeting of the American Astronomical Society, which is open to registered participants.
- The cosmic optical background that the team sought to measure is the visible-light equivalent of the more well-known cosmic microwave background – the weak afterglow of the big bang itself, before stars ever existed.
- "While the cosmic microwave background tells us about the first 450,000 years after the big bang, the cosmic optical background tells us something about the sum total of all the stars that have ever formed since then," explained Postman. "It puts a constraint on the total number of galaxies that have been created, and where they might be in time."
- As powerful as Hubble is, the team couldn't use it to make these observations. Although located in space, Hubble orbits Earth and still suffers from light pollution. The inner solar system is filled with tiny dust particles from disintegrated asteroids and comets. Sunlight reflects off those particles, creating a glow called the zodiacal light that can be observed even by skywatchers on the ground.
- To escape the zodiacal light, the team had to use an observatory that has escaped the inner solar system. Fortunately the New Horizons spacecraft, which has delivered the closest ever images of Pluto and the Kuiper Belt object Arrokoth, is far enough to make these measurements. At its distance (more than 4 billion miles away when these observations were taken), New Horizons experiences an ambient sky 10 times darker than the darkest sky accessible to Hubble.
- "These kinds of measurements are exceedingly difficult. A lot of people have tried to do this for a long time," said Lauer. "New Horizons provided us with a vantage point to measure the cosmic optical background better than anyone has been able to do it."
- The team analyzed existing images from the New Horizons archives. To tease out the feeble background glow, they had to correct for a number of other factors. For example, they subtracted the light from the galaxies expected to exist that are too faint to be identifiable. The most challenging correction was removing light from Milky Way stars that was reflected off interstellar dust and into the camera.
- The remaining signal, though extremely faint, was still measurable. Postman compared it to living in a remote area far from city lights, lying in your bedroom at night with the curtains open. If a neighbor a mile down the road opened their refrigerator looking for a midnight snack, and the light from their refrigerator reflected off the bedroom walls, it would be as bright as the background New Horizons detected.
- So, what could be the source of this leftover glow? It's possible that an abundance of dwarf galaxies in the relatively nearby universe lie just beyond detectability. Or the diffuse halos of stars that surround galaxies might be brighter than expected. There might be a population of rogue, intergalactic stars spread throughout the cosmos. Perhaps most intriguing, there may be many more faint, distant galaxies than theories suggest. This would mean that the smooth distribution of galaxy sizes measured to date rises steeply just beyond the faintest systems we can see – just as there are many more pebbles on a beach than rocks.
- NASA's upcoming James Webb Space Telescope may be able to help solve the mystery. If faint, individual galaxies are the cause, then Webb ultra-deep field observations should be able to detect them. — This study is accepted for publication in the Astrophysical Journal. 8)
• October 25, 2020: Like Earth, the dwarf planet of Pluto has mountains. Like their earthly cousins, some of those peaks are covered in blankets of white. But the origins of these ice-like deposits are very different. 9)
- According to new research published on October 13, 2020, in Nature Communications, some of the mountains discovered on Pluto during the flyby of the New Horizons spacecraft in 2015 are covered by a blanket of methane ice. An international team of scientists analyzed data from Pluto's atmosphere and surface and used numerical simulations of its climate to reveal that these ice caps are created through different processes than they are on Earth.
- The study authors noted that "within the dark equatorial region of Cthulhu, bright frost containing methane is observed coating crater rims and walls as well as mountain tops, providing spectacular resemblance to terrestrial snow-capped mountain chains."
Figure 19: The image on the left was acquired on July 14, 2015, when the New Horizons spacecraft approached Pluto. The Long-Range Reconnaissance Imager (LORRI) detected the presence of patchy bright deposits atop the Pigafetta Montes and Elcano Montes mountain ranges. The spacecraft's Multispectral Visible Imaging Camera (data not shown) revealed signatures of methane. — The right image is a natural-color view of a section of the Alps range in Europe and was acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite on March 19, 2020 (image credit: NASA Earth Observatory image by Lauren Dauphin, using MODIS data from NASA EOSDIS/LANCE and GIBS/Worldview. Pluto imagery courtesy of NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute. Story by Frank Tavares, NASA Ames, with Mike Carlowicz)
Figure 20: Researchers find that ice caps on the mountains of Pluto are made of methane, but develop through an opposite process from that on Earth (image credit: NASA Earth Observatory)
Figure 21: On Earth, atmospheric temperatures decrease with altitude, and that cool air chills land surfaces at high elevations. When a moist wind moves toward and over a mountain on Earth, its water vapor cools and condenses, forming clouds and snow, as seen on mountaintops like the Alps. — But on Pluto, the opposite occurs. The dwarf planet's atmosphere actually gets warmer with altitude because methane gas absorbs solar radiation. However, the atmosphere is too thin to affect surface temperatures, which remain constant with altitude. And unlike the way winds tend to ride up over mountains on Earth, the winds on Pluto mostly travel downslope (image credit: NASA Earth Observatory)
- "It is particularly remarkable to see that two very similar landscapes on Earth and Pluto can be created by two very dissimilar processes," said Tanguy Bertrand, a postdoctoral researcher at NASA's Ames Research Center and lead author on the paper. "Though theoretically objects like Neptune's moon Triton could have a similar process, no other place in our solar system has ice-capped mountains like this besides Earth."
- To understand how similar landscapes develop from different conditions and chemistries, the researchers at the Laboratoire de Météorologie Dynamique (France) developed a three-dimensional model simulating the atmosphere and surface of Pluto. They found that the dwarf planet's atmosphere has more gaseous methane at its warmer, higher altitudes. That gas can saturate, condense, and then freeze directly on mountain peaks without any clouds forming. At lower altitudes on Pluto, there is no methane frost because there is too little methane for condensation to occur.
- This condensation process not only creates methane ice caps on Pluto's mountains, but also similar features on its crater rims as well. The mysterious bladed terrain found in the Tartarus Dorsa region around Pluto's equator can also be explained by this cycle.
- "Pluto really is one of the best natural laboratories we have to explore the physical and dynamic processes involved when compounds that regularly transition between solid and gas states interact with a planetary surface," said Bertrand. "The New Horizons flyby revealed astonishing glacial landscapes we continue to learn from."
• July 15, 2020: Collaborative observations with NASA's New Horizons mission have been ongoing at the Subaru Telescope since May 2020. Hyper Suprime-Cam (HSC), the wide field camera mounted on the prime focus of the Subaru Telescope, is used for the observations to search for target candidates for New Horizons' next observations. Astronomers from Japan are participating in the observation team together with ones from the New Horizons mission. - Note: The Subaru Telescope of NAOJ (National Astronomical Observatory of Japan) is a very large optical infrared telescope (aperture of 8.2 m) installed near the summit of Maunakea, Hawai‘i, USA. 10)
- New Horizons is going to continue its exploration in the outer Solar System. The Subaru Telescope was selected as one of the facilities to search for target candidates for New Horizons' next observations. Dr. Alan Stern of Southwest Research Institute, the Principal Investigator of the New Horizons mission, emphasizes the importance of the observations using the Subaru Telescope, saying "We are using the Subaru Telescope because it is the best in the world for our search purposes. This is due to its unique combination of telescope size—one of the very largest anywhere, and Hyper Suprime-Cam's wide field of view—which can discover many Kuiper Belt objects at once."
- The Subaru Telescope is observing an area equivalent in size to 18 full moons (which can be covered in 2 pointings by HSC) in the constellation of Sagittarius, where New Horizons is now cruising. From the observations with the Subaru Telescope, the team expects to discover 100s of new Kuiper Belt objects, of which about 50 should be observable at a distance with the New Horizons spacecraft. Observing with both the Subaru Telescope and New Horizons is important to discern the nature of mysterious objects in the outer Solar System.
- When a distant object in the outer Solar System is viewed from the Earth, it always appears as a "full moon" illuminated almost head-on by the sunlight. On the other hand, when New Horizons looks at an object orbiting near it, the object may appear as a "half moon" with the sunlight hitting it from the side. Observing an object at different phase angles will allow us to know the detailed characteristics of the surface.
- In addition, the team will investigate whether or not the newly-discovered Kuiper Belt objects include one that is suitable for a future flyby study by New Horizons, like Arrokoth was.
- The observation runs at the Subaru Telescope happened in May and June 2020, and another will follow in August. The team will search for new distant objects by comparing images taken at different times, and then determine their orbits precisely.
- Dr. Tsuyoshi Terai, a core member of the observation team and a Support Astronomer for the Subaru Telescope in charge of HSC, comments, "The search area is within the Milky Way, and thus there are many nearby stars including bright ones, which make the observations even more difficult. The observation team is doing its best to take high quality data by utilizing the unique capabilities of the Subaru Telescope, and to investigate the origin of the Solar System together with New Horizons."
• June 22, 2020: The accretion of new material during Pluto's formation may have generated enough heat to create a liquid ocean that has persisted beneath an icy crust to the present day, despite the dwarf planet's orbit far from the sun in the cold outer reaches of the solar system. 11)
- This "hot start" scenario, presented in a paper published June 22 in Nature Geoscience, contrasts with the traditional view of Pluto's origins as a ball of frozen ice and rock in which radioactive decay could have eventually generated enough heat to melt the ice and form a subsurface ocean. 12)
- "For a long time people have thought about the thermal evolution of Pluto and the ability of an ocean to survive to the present day," said coauthor Francis Nimmo, professor of Earth and planetary sciences at UC Santa Cruz. "Now that we have images of Pluto's surface from NASA's New Horizons mission, we can compare what we see with the predictions of different thermal evolution models."
- Because water expands when it freezes and contracts when it melts, the hot-start and cold-start scenarios have different implications for the tectonics and resulting surface features of Pluto, explained first author and UCSC graduate student Carver Bierson.
- "If it started cold and the ice melted internally, Pluto would have contracted and we should see compression features on its surface, whereas if it started hot it should have expanded as the ocean froze and we should see extension features on the surface," Bierson said. "We see lots of evidence of expansion, but we don't see any evidence of compression, so the observations are more consistent with Pluto starting with a liquid ocean."
- The thermal and tectonic evolution of a cold-start Pluto is actually a bit complicated, because after an initial period of gradual melting the subsurface ocean would begin to refreeze. So compression of the surface would occur early on, followed by more recent extension. With a hot start, extension would occur throughout Pluto's history.
- "The oldest surface features on Pluto are harder to figure out, but it looks like there was both ancient and modern extension of the surface," Nimmo said.
Figure 22: Extensional faults (arrows) on the surface of Pluto indicate expansion of the dwarf planet's icy crust, attributed to freezing of a subsurface ocean (image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker)
- The next question was whether enough energy was available to give Pluto a hot start. The two main energy sources would be heat released by the decay of radioactive elements in the rock and gravitational energy released as new material bombarded the surface of the growing protoplanet.
- Bierson's calculations showed that if all of the gravitational energy was retained as heat, it would inevitably create an initial liquid ocean. In practice, however, much of that energy would radiate away from the surface, especially if the accretion of new material occurred slowly.
- "How Pluto was put together in the first place matters a lot for its thermal evolution," Nimmo said. "If it builds up too slowly, the hot material at the surface radiates energy into space, but if it builds up fast enough the heat gets trapped inside."
- The researchers calculated that if Pluto formed over a period of less that 30,000 years, then it would have started out hot. If, instead, accretion took place over a few million years, a hot start would only be possible if large impactors buried their energy deep beneath the surface.
- The new findings imply that other large Kuiper belt objects probably also started out hot and could have had early oceans. These oceans could persist to the present day in the largest objects, such as the dwarf planets Eris and Makemake.
- "Even in this cold environment so far from the sun, all these worlds might have formed fast and hot, with liquid oceans," Bierson said.
- In addition to Bierson and Nimmo, the paper was coauthored by Alan Stern at the Southwest Research Institute, the principal investigator of the New Horizons mission.
• June 10, 2020: For the first time, a spacecraft has sent back pictures of the sky from so far away that some stars appear to be in different positions than we'd see from Earth. 13)
- More than four billion miles from home and speeding toward interstellar space, NASA's New Horizons has traveled so far that it now has a unique view of the nearest stars. "It's fair to say that New Horizons is looking at an alien sky, unlike what we see from Earth," said Alan Stern, New Horizons principal investigator from Southwest Research Institute (SwRI) in Boulder, Colorado. "And that has allowed us to do something that had never been accomplished before — to see the nearest stars visibly displaced on the sky from the positions we see them on Earth."
- On April 22-23, the spacecraft turned its long-range telescopic camera to a pair of the "closest" stars, Proxima Centauri and Wolf 359, showing just how they appear in different places than we see from Earth. Scientists have long used this "parallax effect" – how a star appears to shift against its background when seen from different locations — to measure distances to stars.
Figure 23: This two-frame animation of Proxima Centauri blinks back and forth between New Horizons and Earth images of each star, clearly illustrating the different view of the sky New Horizons has from its deep-space perch. The image was obtained on April 22 at 12:51 UT (8:51 a.m. ET) by a remotely operated 0.4-meter telescope at the Siding Spring node of the Las Cumbres Observatory in Australia. The timing accounts for New Horizons being nearly three light hours closer to Proxima Centauri than Earth when the images were taken (image credit: NASA, JHU/APL, SwRI)
- An easy way to see parallax is to place one finger at arm's length and watch it jump back and forth when you view it successively with each eye. Similarly, as Earth makes it way around the Sun, the stars shift their positions. But because even the nearest stars are hundreds of thousands of times farther away than the diameter of Earth's orbit, the parallax shifts are tiny, and can only be measured with precise instrumentation.
- "No human eye can detect these shifts," Stern said.
- But when New Horizons images are paired with pictures of the same stars taken on the same dates by telescopes on Earth, the parallax shift is instantly visible. The combination yields a 3D view of the stars "floating" in front of their background star fields.
Figure 24: This two-frame animation of Wolf 359 blinks back and forth between New Horizons and Earth images of each star, clearly illustrating the different view of the sky New Horizons has from its deep-space perch. The image was obtained on April 23 at 04:37 UT (12:37 a.m. ET) with the University of Louisville 0.6-meter telescope located at Mt. Lemmon Observatory, near Tucson, Arizona, operated remotely by John F. Kielkopf (University of Louisville) and Karen A. Collins (Harvard and Smithsonian Center for Astrophysics). This is 37 minutes later than the New Horizons image, relative to Wolf 359 time. The timing accounts for New Horizons being nearly four light hours farther from Wolf 359 than Earth when the images were taken (image credit: NASA, JHU/APL, SwRI)
- "The New Horizons experiment provides the largest parallax baseline ever made — over 4 billion miles — and is the first demonstration of an easily observable stellar parallax," said Tod Lauer, New Horizons science team member from the National Science Foundation's National Optical-Infrared Astronomy Research Laboratory who coordinated the parallax demonstration.
- "The New Horizons spacecraft is truly a mission of firsts, and this demonstration of stellar parallax is no different" said Kenneth Hansen, New Horizons program scientist at NASA Headquarters in Washington. "The New Horizons spacecraft continues to speed away from Earth toward interstellar space and is continuing to return exciting new data for planetary science."
Working in Stereo
- Lauer, New Horizons Deputy Project Scientist John Spencer, of SwRI, and science team collaborator, astrophysicist, Queen guitarist and stereo imaging enthusiast Brian May created the images that clearly show the effect of the vast distance between Earth and the two nearby stars.
Figure 25: Stereo for 3D Glasses: These anaglyph images can be viewed with red-blue stereo glasses to reveal the stars' distance from their backgrounds. On the left is Proxima Centauri and on the right is Wolf 359 (image credit: NASA, JHU/APL, SwRI)
- "It could be argued that in astro-stereoscopy — 3D images of astronomical objects – NASA's New Horizons team already leads the field, having delivered astounding stereoscopic images of both Pluto and the remote Kuiper Belt object Arrokoth," May said. "But the latest New Horizons stereoscopic experiment breaks all records. These photographs of Proxima Centauri and Wolf 359 – stars that are well-known to amateur astronomers and science fiction aficionados alike — employ the largest distance between viewpoints ever achieved in 180 years of stereoscopy!"
- The companion images of Proxima Centauri and Wolf 359 were provided by the Las Cumbres Observatory, operating a remote telescope at Siding Spring Observatory in Australia, and astronomers John Kielkopf, University of Louisville, and Karen Collins, Harvard and Smithsonian Center for Astrophysics, operating a remote telescope at Mt. Lemmon Observatory in Arizona.
- "The professional and amateur astronomy communities had been waiting to try this, and were very excited to make a little space exploration history," said Lauer. "The images collected on Earth when New Horizons was observing Proxima Centauri and Wolf 359 really exceeded my expectations."
Figure 26: Parallel Stereo of Proxima Centauri: Use a stereo viewer for these images; if you don't have a viewer, change your focus from the image by looking "through" it (and the screen) and into the distance. This creates the effect of a third image in the middle, and try setting your focus on that third image. The New Horizons image is on the left (image credit: NASA, JHU/APL, SwRI)
An Interstellar Navigation First
- Throughout history, navigators have used measurements of the stars to establish their position on Earth. Interstellar navigators can do the same to establish their position in the galaxy, using a technique that New Horizons has demonstrated for the first time. While radio tracking by NASA's Deep Space Network is far more accurate, its first use is a significant milestone in what may someday become human exploration of the galaxy.
- At the time of the observations, New Horizons was more than 4.3 billion miles (about 7 billion kilometers) from Earth, where a radio signal, traveling at the speed of light, needed just under 6 hours and 30 minutes to reach home.
- Launched in 2006, New Horizons is the first mission to Pluto and the Kuiper Belt. It explored Pluto and its moons in July 2015 — completing the space-age reconnaissance of the planets that started 50 years earlier — and continued on its unparalleled voyage of exploration with the close flyby of Kuiper Belt object Arrokoth in January 2019. New Horizons will eventually leave the solar system, joining the Voyagers and Pioneers on their paths to the stars.
- The Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, designed, built and operates the New Horizons spacecraft, and manages the mission for NASA's Science Mission Directorate. The MSFC Planetary Management Office provides the NASA oversight for the New Horizons. Southwest Research Institute, based in San Antonio, directs the mission via Principal Investigator Stern, and leads the science team, payload operations and encounter science planning. New Horizons is part of the New Frontiers Program managed by NASA's Marshall Space Flight Center in Huntsville, Alabama.
• February 13, 2020: Data from NASA's New Horizons mission are providing new insights into how planets and planetesimals – the building blocks of the planets – were formed. 14)
- Using detailed data on the object's shape, geology, color and composition – gathered during a record-setting flyby that occurred more than four billion miles from Earth – researchers have apparently answered a longstanding question about planetesimal origins, and therefore made a major advance in understanding how the planets themselves formed.
- The team reports those findings in a set of three papers in the journal Science, and at a media briefing Feb. 13 at the annual American Association for the Advancement of Science meeting in Seattle.
- "Arrokoth is the most distant, most primitive and most pristine object ever explored by spacecraft, so we knew it would have a unique story to tell," said New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute in Boulder, Colorado. "It's teaching us how planetesimals formed, and we believe the result marks a significant advance in understanding overall planetesimal and planet formation."
- The first post-flyby images transmitted from New Horizons last year showed that Arrokoth had two connected lobes, a smooth surface and a uniform composition, indicating it was likely pristine and would provide decisive information on how bodies like it formed. These first results were published in Science last May.
- "This is truly an exciting find for what is already a very successful and history-making mission" said Lori Glaze, director of NASA's Planetary Science Division. "The continued discoveries of NASA's New Horizons spacecraft astound as it reshapes our knowledge and understanding of how planetary bodies form in solar systems across the universe."
- Over the following months, working with more and higher-resolution data as well as sophisticated computer simulations, the mission team assembled a picture of how Arrokoth must have formed. Their analysis indicates that the lobes of this "contact binary" object were once separate bodies that formed close together and at low velocity, orbited each other, and then gently merged to create the 22-mile long object New Horizons observed.
- This indicates Arrokoth formed during the gravity-driven collapse of a cloud of solid particles in the primordial solar nebula, rather than by the competing theory of planetesimal formation called hierarchical accretion. Unlike the high-speed collisions between planetesimals in hierarchical accretion, in particle-cloud collapse, particles merge gently, slowly growing larger.
- "Just as fossils tell us how species evolved on Earth, planetesimals tell us how planets formed in space," said William McKinnon, a New Horizons co-investigator from Washington University in St. Louis, and lead author of an Arrokoth formation paper in Science this week. "Arrokoth looks the way it does not because it formed through violent collisions, but in more of an intricate dance, in which its component objects slowly orbited each other before coming together."
- Two other important pieces of evidence support this conclusion. The uniform color and composition of Arrokoth's surface shows the KBO formed from nearby material, as local cloud collapse models predict, rather than a mishmash of matter from more separated parts of the nebula, as hierarchical models might predict.
- The flattened shapes of each of Arrokoth's lobes, as well as the remarkably close alignment of their poles and equators, also point to a more orderly merger from a collapse cloud. Further still, Arrokoth's smooth, lightly cratered surface indicates its face has remained well preserved since the end of the planet formation era.
- "Arrokoth has the physical features of a body that came together slowly, with ‘local' materials in the solar nebula," said Will Grundy, New Horizons composition theme team lead from Lowell Observatory in Flagstaff, Arizona, and the lead author of a second Science paper. "An object like Arrokoth wouldn't have formed, or look the way it does, in a more chaotic accretion environment."
- The latest Arrokoth reports significantly expand on the May 2019 Science paper, led by Stern. The three new papers are based on 10 times as much data as the first report, and together provide a far more complete picture of Arrokoth's origin.
- "All of the evidence we've found points to particle-cloud collapse models, and all but rule out hierarchical accretion for the formation mode of Arrokoth, and by inference, other planetesimals," Stern said.
- New Horizons continues to carry out new observations of additional Kuiper Belt objects it passes in the distance. New Horizons also continues to map the charged-particle radiation and dust environment in the Kuiper Belt. The new KBOs being observed now are too far away to reveal discoveries like those on Arrokoth, but the team can measure aspects such as each object's surface properties and shape. This summer the mission team will begin using large ground-based telescopes to search for new KBOs to study in this way, and even for another flyby target if fuel allows.
- The New Horizons spacecraft is now 4.4 billion miles (7.1 billion kilometers) from Earth, operating normally and speeding deeper into the Kuiper Belt at nearly 31,300 miles (50,400 kilometers) per hour.
• February 13, 2020: New observations on the farthest, most primitive object in the Solar System ever to be visited by a spacecraft – a tiny, frozen, bi-lobed world known as Arrokoth – offer a unique glimpse into the early formation of our Solar System and perhaps the planet Earth. 17)
- The new findings expand upon the first published observations of the Kuiper Belt object, announced in a May 2019 issue of Science, which were based on a small amount of data sent from the New Horizons spacecraft shortly after its New Year's Day 2019 encounter.
- Using more than ten times as much data – including the highest-resolution images from the flyby – the New Horizons team describes Arrokoth in unprecedented detail across three reports in the February 14 issue of Science.
- Also known as 2014 MU69 and previously nicknamed Ultima Thule, the Kuiper Belt planetesimal was recently officially named Arrokoth, a Native American term meaning "sky" in the Powhattan/Algonquin language.
- These studies provide a far more complete picture of the composition and origin of Arrokoth and help to resolve a longstanding scientific controversy about how planetesimals – the primordial building blocks of our Solar System's planets – were formed.
- After passing Pluto in 2015, NASA's New Horizons spacecraft continued its mission deeper into the Kuiper Belt – a vast ring of icy bodies spread beyond the orbit of Neptune – to investigate the objects observed slowly orbiting in the outer reaches of the Solar System. Its target: Arrokoth, a peanut-shaped Kuiper Belt object discovered by the Hubble Telescope the previous year. New Horizons flew past the distant bi-lobed world after a journey spanning more than three years and one billion miles.
- Like the asteroids of the well-known asteroid belt between Mars and Jupiter, many of the objects drifting in the Kuiper Belt are remnants left over from the formation of the Solar System. One of the class of bodies called Cold Classical Kuiper Belt Objects, Arrokoth is an ancient relic preserved from the time when the planets first began to form nearly 4.5 billion years ago. Scientists still debate the process by which the dust and gases surrounding the new Sun coalesced to become the eight planets we recognize today.
Figure 27: Peanut-shaped Arrokoth is the farthest and most primitive object in the Solar System ever to be visited by a spacecraft (image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Roman Tkachenko)
- "To understand planet formation, we have to understand planetesimal formation, since this is a key first step," said Lowell Observatory astronomer William Grundy, lead author of one of the studies. "Arrokoth is a planetesimal – an essential stepping-stone on the way to building planets, starting from dust in a protoplanetary nebula." 18)
- While there are planetesimals closer to Earth in the asteroid belt, according to Grundy, those visited via space probes have all been badly battered by colliding with each other or cooked by the sun, looking far different than when they first formed. However, in the cold, vast and remote Kuiper Belt, primordial planetesimals like Arrokoth remain largely untouched – even by the heat of the Sun – thus avoiding many of the processes that have obscured or erased the earliest histories of other worlds. As a result, Kuiper Belt planetesimals are the best-preserved time capsules of the early stages of planet formation known to exist.
- "For decades, we've been thinking about how planetesimals are made and debating the pros and cons of the various hypothesized mechanisms," said Grundy. "So, it's thrilling to finally be able to see one still pretty much just as it was after its formation."
- New Horizon's visit to Arrokoth was brief – the spacecraft rocketed past the roughly 22-mile-long object at almost 32,000 miles per hour before careening even deeper into the Kuiper Belt. However, the encounter was close and long enough to characterize, map and image the distant object's geological and geophysical features, including its most striking aspect – its unique shape.
- Images from New Horizons show that Arrokoth is what's known as a contact binary. Its shape – something akin to a slightly flattened snowman – is the product of two separate, mutually orbiting planetesimals, which gently merged with one another. According to John Spencer, a planetary scientist at the Southwest Research Institute and lead author of another study in the issue, this is something that could only happen during the early stages of Solar System formation.
- "The gentle merger provides strong support for the idea that planetesimals formed from local clouds of material that collapsed under their own gravity," rather than the longstanding theory of hierarchical accretion, said Spencer.
- The hierarchical accretion hypothesis – whereby colliding dust grains become larger, and so on up through pebbles, cobbles and boulders, colliding ever more forcefully into one another as they gradually grow to become the size of planets – is the currently accepted method of planetesimal formation.
- However, the formation and merging of Arrokoth's lobes bear no signs of such violent processes. Instead, the new evidence from Arrokoth suggest a rapid but gentle process of planetesimal formation – a formally underdog hypothesis known as local cloud collapse – where local concentrations of protoplanetary dust and debris come together under the influence of their own collective gravity.
- "It's hard to communicate about something to the public when we don't even have a good idea of what it looks like," said Grundy. "Just knowing that it plays an important role as a building block of planets isn't enough."
- "Now we finally have that picture and a much clearer idea of how it forms," he said.
- New Horizons is speeding onwards in the outer fringes of the Solar System, already 300 million miles beyond Arrokoth, and its exploration of the Kuiper Belt is ongoing. According to the researchers, distant observations of other Kuiper Belt Objects will help to place Arrokoth's observations in context. And, however unlikely, the team hopes to find another potential target for a close flyby.
- "I entered this field because I was inspired by Carl Sagan's point that only a couple of generations of scientists in all of human history would be the first to send craft to explore the planets of our Solar System," said NASA's Alan Stern, New Horizons' principal investigator. "I have been fortunate to be a part of that scientific revolution."
- "We successfully accomplished the most distant flyby in the history of planetary exploration of a world we didn't know existed five years ago," said Spencer. "Seeing a new world for the first time – there's nothing else like that experience."
• December 2, 2019: Measurements taken by the SWAP (Solar Wind Around Pluto) instrument aboard NASA's New Horizons spacecraft are providing important new insights from some of the farthest reaches of space ever explored. In a paper recently published in the Astrophysical Journal, a team led by Southwest Research Institute shows how the solar wind — the supersonic stream of charged particles blown out by the Sun — evolves at increasing distances from the Sun. 19)
- "Previously, only the Pioneer 10 and 11 and Voyager 1 and 2 missions have explored the outer solar system and outer heliosphere, but now New Horizons is doing that with more modern scientific instruments," said Dr. Heather Elliott, a staff scientist at SwRI, Deputy Principal Investigator of the SWAP instrument and lead author of the paper. "Our Sun's influence on the space environment extends well beyond the outer planets, and SWAP is showing us new aspects of how that environment changes with distance." 20)
- The solar wind fills a bubble-like region of space encompassing our solar system, called the heliosphere. From aboard New Horizons, SWAP collects detailed, daily measurements of the solar wind as well as other key components called "interstellar pickup ions" in the outer heliosphere. These interstellar pickup ions are created when neutral material from interstellar space enters the solar system and becomes ionized by light from the Sun or by charge exchange interactions with solar wind ions.
- As the solar wind moves farther from the Sun, it encounters an increasing amount of material from interstellar space. When interstellar material is ionized, the solar wind picks up the material and, researchers theorized, slows and heats in response. SWAP has now detected and confirmed this predicted effect.
Figure 28: Solar wind speed diagram. The SWAP instrument aboard NASA's New Horizons spacecraft has confirmed that the solar wind slows as it travels farther from the Sun. This schematic of the heliosphere shows the solar wind begins slowing at approximately 4 AU radial distance from the Sun and continues to slow as it moves toward the outer solar system and picks up interstellar material. Current extrapolations reveal the termination shock may currently be closer than found by the Voyager spacecraft. However, increasing solar activity will soon expand the heliosphere and push the termination shock farther out, possibly to the 84-94 AU range encountered by the Voyager spacecraft (image credit: SwRI, background artist rendering by NASA and Adler Planetarium)
- The SWAP team compared the New Horizons solar wind speed measurements from 21 to 42 astronomical units to the speeds at 1 AU from both the Advanced Composition Explorer (ACE) and Solar TErrestrial RElations Observatory (STEREO) spacecraft. (One AU is equal to the distance between the Sun and Earth.) By 21 AU, it appeared that SWAP could be detecting the slowing of the solar wind in response to picking up interstellar material. However, when New Horizons traveled beyond Pluto, between 33 and 42 AU, the solar wind measured 6-7% slower than at the 1 AU distance, confirming the effect.
- In addition to confirming the slowing of the solar wind at great distances, the change in the solar wind temperature and density could also provide a means to estimate when New Horizons will join the Voyager spacecraft on the other side of the termination shock, the boundary marking where the solar wind slows to less than the sound speed as it approaches the interstellar medium. Voyager 1 crossed the termination shock in 2004 at 94 AU, followed by Voyager 2 in 2007 at 84 AU. Based on current lower levels of solar activity and lower solar wind pressures, the termination shock is expected to have moved closer to the Sun since the Voyager crossings. Extrapolating current trends in the New Horizons measurements also indicates that the termination shock might now be closer than when it was intersected by Voyager. At the earliest, New Horizons will reach the termination shock in the mid-2020s. As the solar cycle activity increases, the increase in pressure will likely expand the heliosphere. This could push the termination shock to the 84-94 AU range found by the Voyager spacecraft before New Horizons has time to reach the termination shock.
- New Horizons' journey through the outer heliosphere contrasts Voyager's in that the current solar cycle is mild compared to the very active solar cycle Voyager experienced in the outer heliosphere. In addition to measuring the solar wind, New Horizons' SWAP is extremely sensitive and simultaneously measures the low fluxes of interstellar pickup ions with unprecedented time resolution and extensive spatial coverage. New Horizons is also the only spacecraft in the solar wind beyond Mars (1.5 AU) and, consequently, the only spacecraft measuring interactions between the solar wind and the interstellar material in the outer heliosphere during the current mild solar cycle. New Horizons is on course to be the first spacecraft to measure both the solar wind and interstellar pickup ions at the termination shock.
- "New Horizons has significantly advanced our knowledge of distant planetary objects, and it's only fitting that it is now also revealing new knowledge about our own Sun and its heliosphere," said New Horizons Principal Investigator Dr. Alan Stern of the SwRI.
• November 12, 2019: In a fitting tribute to the farthest flyby ever conducted by spacecraft, the Kuiper Belt object 2014 MU69 has been officially named Arrokoth, a Native American term meaning "sky" in the Powhatan/Algonquian language. 21)
- With consent from Powhatan Tribal elders and representatives, NASA's New Horizons team – whose spacecraft performed the record-breaking reconnaissance of Arrokoth four billion miles from Earth – proposed the name to the International Astronomical Union and Minor Planets Center, the international authority for naming Kuiper Belt objects. The name was announced at a ceremony today at NASA Headquarters in Washington, DC.
- "The name ‘Arrokoth' reflects the inspiration of looking to the skies and wondering about the stars and worlds beyond our own," said Alan Stern, New Horizons principal investigator from Southwest Research Institute, Boulder, Colorado. "That desire to learn is at the heart of the New Horizons mission, and we're honored to join with the Powhatan community and people of Maryland in this celebration of discovery."
- New Horizons launched in January 2006; it then zipped past Jupiter for a gravity boost and scientific studies in February 2007 and conducted an historic first flight through the Pluto system on July 14, 2015. The spacecraft continued its unparalleled voyage on New Year's 2019 with the exploration of Arrokoth – which the team had nicknamed "Ultima Thule" — a billion miles beyond Pluto, and the farthest flyby ever conducted.
- Arrokoth is one of the thousands of known small icy worlds in the Kuiper Belt, the vast "third zone" of the solar system beyond the inner terrestrial planets and the outer gas giant planets. It was discovered in 2014 by a New Horizons team – which included Marc Buie, of the Southwest Research Institute – using the powerful Hubble Space Telescope.
- "Data from the newly-named Arrokoth, has given us clues about the formation of planets and our cosmic origins," said Buie. "We believe this ancient body, composed of two distinct lobes that merged into one entity, may harbor answers that contribute to our understanding of the origin of life on Earth."
- In accordance with IAU (International Astronomical Union) naming conventions, the discovery team earned the privilege of selecting a permanent name for the celestial body. The team used this convention to associate the culture of the native peoples who lived in the region where the object was discovered; in this case, both the Hubble Space Telescope (at the Space Telescope Science Institute) and the New Horizons mission (at the Johns Hopkins Applied Physics Laboratory) are operated out of Maryland — a tie to the significance of the Chesapeake Bay region to the Powhatan people.
- "We graciously accept this gift from the Powhatan people," said Lori Glaze, director of NASA's Planetary Science Division. "Bestowing the name Arrokoth signifies the strength and endurance of the indigenous Algonquian people of the Chesapeake region. Their heritage continues to be a guiding light for all who search for meaning and understanding of the origins of the universe and the celestial connection of humanity."
- The Pamunkey Reservation in King William County, Virginia, is the oldest American Indian reservation in the U.S. — formed by a treaty with England in the 1600s and finally receiving federal recognition in July 2015. The Pamunkey tribe and its village were significant in the original Powhatan Confederacy; today, Pamunkey tribal members work collaboratively with other Powhatan tribes in Virginia and also have descendants who are members of the Powhatan-Renape Nation in New Jersey. Many direct descendants still live on the Pamunkey reservation, while others have moved to Northern Virginia, Maryland, D.C., New York and New Jersey.
- The Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, designed, built and operates the New Horizons spacecraft, and manages the mission for NASA's Science Mission Directorate. NASA's Marshall Space Flight Center (MSFC) Planetary Management Office, in Huntsville, Alabama, provides the NASA oversight for the New Horizons. The Southwest Research Institute, based in San Antonio, directs the mission via Principal Investigator Stern, and leads the science team, payload operations and encounter science planning. New Horizons is part of the New Frontiers Program managed by NASA's MSFC.
Figure 29: New Horizons Path of Exploration through the KBOs (Kuiper Belt Objects), image credit: NASA
• July 8, 2019: Scientists have unlocked clues about the earliest forms of life on Earth by studying fossils found across our planet. Similarly, we're now learning more about the earliest formation of our solar system from a different kind of fossil – a Kuiper Belt Object (KBO) known as 2014 MU69. 22)
Figure 30: Scientists are unlocking clues about the earliest formation of our solar system from a Kuiper Belt Object known as 2014 MU69 (video credit: Science At NASA, Published on 8 July 2019)
- Travel back in time to the solar system's very beginning. There, two objects were formed that eventually came together in a body resembling a flattened bowling pin. When looking at the object from the vicinity of the Earth, which is 4 billion miles (6.4 billion km) away, MU69 looked like a point of light, even when using the powerful Hubble Space Telescope. But thanks to the New Horizons spacecraft, this is what it looked like on New Year's Day, 2019, from approximately 4,100 miles (6,600 km) away, about 7 minutes before the spacecraft's closest approach.
- New Horizons is a grand piano-sized spacecraft that was launched back in 2006, with the undertaking of exploring the Kuiper Belt – that donut-shaped disc of space that begins just beyond the orbit of Neptune. Dwarf planet Pluto and its largest moon Charon reside in the Kuiper Belt, as do over 100,000 KBOs. MU69 was discovered by the New Horizons team, and was found to be located in the neighborhood of New Horizons' trajectory when the spacecraft reached the Kuiper Belt in 2015. Its location led to its selection as a flyby target.
- New Horizons Project Scientist Hal Weaver, of the Johns Hopkins Applied Physics Lab, notes three wondrous impressions about this 22 mile (35 km) long primeval object.
- "First, this is the most primitive object ever encountered by a spacecraft. By that I mean the least changed since the time of the solar system's formation."
- "Second, the shape of MU69's body is giving us new insights into how planets formed. Scientific hypotheses change as New Horizons delivers new data. Until we saw it up close, we didn't know for sure if MU69 was a single object or two distinct pieces. Now we know it's actually composed of two distinct pieces – a large, flat lobe and a smaller, rounder lobe, that have merged into one entity. This fusion gives us clues regarding the initial steps that were taken to form a planet."
- "Third, the flyby showed MU69's red color – redder even than Pluto – and we believe that this may come from organic material – the same material that may have contributed to the origin of life on Earth."
- Fossils of once-living things on Earth convey vital clues about the past. In space, KBOs can also convey fossil-like clues – about 4.5 billion years' worth of solar system history. Scientists are poring over the information they've received so far, and the data transmissions from the MU69 flyby will continue through the late summer of 2020. In the meantime, New Horizons is traveling farther into the Kuiper Belt at about 31,500 miles (51,000 km) per hour. The spacecraft is now observing additional KBOs and exploring the region's concentration of charged particle radiation and dust within the Kuiper Belt.
• May 16, 2019: NASA's New Horizons mission team has published the first profile of the farthest world ever explored, a planetary building block and Kuiper Belt object called 2014 MU69. 23)
- Analyzing just the first sets of data gathered during the New Horizons spacecraft's New Year's 2019 flyby of MU69 (nicknamed Ultima Thule) the mission team quickly discovered an object far more complex than expected. The team publishes the first peer-reviewed scientific results and interpretations – just four months after the flyby – in the May 17 issue of the journal Science. 24)
- In addition to being the farthest exploration of an object in history – four billion miles from Earth – the flyby of Ultima Thule was also the first investigation by any space mission of a well-preserved planetesimal, an ancient relic from the era of planet formation.
- The initial data summarized in Science reveal much about the object's development, geology and composition. It's a contact binary, with two distinctly differently shaped lobes. At about 22 miles (36 kilometers) long, Ultima Thule consists of a large, strangely flat lobe (nicknamed "Ultima") connected to a smaller, somewhat rounder lobe (nicknamed "Thule"), at a juncture nicknamed "the neck." How the two lobes got their unusual shape is an unanticipated mystery that likely relates to how they formed billions of years ago.
- The lobes likely once orbited each other, like many so-called binary worlds in the Kuiper Belt, until some process brought them together in what scientists have shown to be a "gentle" merger. For that to happen, much of the binary's orbital momentum must have dissipated for the objects to come together, but scientists don't yet know whether that was due to aerodynamic forces from gas in the ancient solar nebula, or if Ultima and Thule ejected other lobes that formed with them to dissipate energy and shrink their orbit. The alignment of the axes of Ultima and Thule indicates that before the merger the two lobes must have become tidally locked, meaning that the same sides always faced each other as they orbited around the same point.
Figure 31: This composite image of the primordial contact binary Kuiper Belt Object 2014 MU69 (nicknamed Ultima Thule) – featured on the cover of the May 17 issue of the journal Science – was compiled from data obtained by NASA's New Horizons spacecraft as it flew by the object on Jan. 1, 2019. The image combines enhanced color data (close to what the human eye would see) with detailed high-resolution panchromatic pictures (image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Roman Tkachenko)
- "We're looking into the well-preserved remnants of the ancient past," said New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute, Boulder, Colorado. "There is no doubt that the discoveries made about Ultima Thule are going to advance theories of solar system formation."
- As the Science paper reports, New Horizons researchers are also investigating a range of surface features on Ultima Thule, such as bright spots and patches, hills and troughs, and craters and pits on Ultima Thule. The largest depression is a 5-mile-wide (8 km wide) feature the team has nicknamed Maryland crater – which likely formed from an impact. Some smaller pits on the Kuiper Belt object, however, may have been created by material falling into underground spaces, or due to exotic ices going from a solid to a gas (called sublimation) and leaving pits in its place.
- In color and composition, Ultima Thule resembles many other objects found in its area of the Kuiper Belt. It's very red – redder even than much larger, 1,500-mile (2,400 km) wide Pluto, which New Horizons explored at the inner edge of the Kuiper Belt in 2015 – and is in fact the reddest outer solar system object ever visited by spacecraft; its reddish hue is believed to be caused by modification of the organic materials on its surface New Horizons scientists found evidence for methanol, water ice, and organic molecules on Ultima Thule's surface – a mixture very different from most icy objects explored previously by spacecraft.
- Data transmission from the flyby continues, and will go on until the late summer 2020. In the meantime, New Horizons continues to carry out new observations of additional Kuiper Belt objects it passes in the distance. These additional KBOs are too distant to reveal discoveries like those on MU69, but the team can measure aspects such as the object's brightness. New Horizons also continues to map the charged-particle radiation and dust environment in the Kuiper Belt.
- The New Horizons spacecraft is now 4.1 billion miles (6.6 billion kilometers) from Earth, operating normally and speeding deeper into the Kuiper Belt at nearly 33,000 miles (53,000 kilometers) per hour.
- The Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, designed, built and operates the New Horizons spacecraft, and manages the mission for NASA's Science Mission Directorate. The MSFC Planetary Management Office provides the NASA oversight for the New Horizons. Southwest Research Institute, based in San Antonio, directs the mission via Principal Investigator Stern, and leads the science team, payload operations and encounter science planning. New Horizons is part of the New Frontiers Program managed by NASA's Marshall Space Flight Center in Huntsville, Alabama.
• March 18, 2019: The farthest object ever explored is slowly revealing its secrets, as scientists piece together the puzzles of Ultima Thule – the Kuiper Belt object NASA's New Horizons spacecraft flew past on New Year's Day, four billion miles from Earth. 25) 26)
- Analyzing the data New Horizons has been sending home since the flyby of Ultima Thule (officially named 2014 MU69), mission scientists are learning more about the development, geology and composition of this ancient relic of solar system formation. The team discussed those findings today at the 50th Lunar and Planetary Science Conference in The Woodlands, Texas.
- Ultima Thule is the first unquestionably primordial contact binary ever explored. Approach pictures of Ultima Thule hinted at a strange, snowman-like shape for the binary, but further analysis of images, taken near closest approach – New Horizons came to within just 2,200 miles (3,500 km) – have uncovered just how unusual the KBO's (Kuiper Belt Object) shape really is. At 22 miles (35 km) long, Ultima Thule consists of a large, flat lobe (nicknamed "Ultima") connected to a smaller, rounder lobe (nicknamed "Thule").
- This strange shape is the biggest surprise, so far, of the flyby. "We've never seen anything like this anywhere in the solar system," said New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute, Boulder, Colorado. "It is sending the planetary science community back to the drawing board to understand how planetesimals – the building blocks of the planets – form."
- Because it is so well preserved, Ultima Thule is offering our clearest look back to the era of planetesimal accretion and the earliest stages of planetary formation. Apparently Ultima Thule's two lobes once orbited each other, like many so-called binary worlds in the Kuiper Belt, until something brought them together in a "gentle" merger.
- "This fits with general ideas of the beginning of our solar system," said William McKinnon, a New Horizons co-investigator from Washington University in St. Louis. "Much of the orbital momentum of the Ultima Thule binary must have been drained away for them to come together like this. But we don't know yet what processes were most important in making that happen."
- That merger may have left its mark on the surface. The "neck" connecting Ultima and Thule is reworked, and could indicate shearing as the lobes combined, said Kirby Runyon, a New Horizons science team member from the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland.
Figure 32: The most distant world we've ever explored, Ultima Thule, keeps getting weirder — but NASA scientists are beginning to unravel its mysteries (image credit: NASA; JHU/APL, SwRI, ESA)
- Runyon and fellow team geologists are describing and trying to understand Ultima Thule's many surface features, from bright spots and patches, to hills and troughs, to craters and pits. The craters, while at first glance look like impact craters, could have other origins. Some may be pit craters, where material drains into underground cracks, or a result of sublimation, where ice went directly from solid to gas and left pits in its place. The largest depression is a 5-mile-wide (8-kilometer-wide) feature the team has nicknamed Maryland crater. It could be an impact crater, or it could have formed in one of the other above-mentioned ways.
- "We have our work cut out to understand Ultima Thule's geology, that is for sure," Runyon said.
- In color and composition, New Horizons data revealed that Ultima Thule resembles many other objects found in its region of the Kuiper Belt. Consistent with pre-flyby observations from the Hubble Telescope, Ultima Thule is very red – redder even than Pluto, which New Horizons flew past on the inner edge of the Kuiper Belt in 2015 – and about the same color as many other so-called "cold classical" KBOs. ("Cold" referring not to temperature but to the circular, uninclined orbits of these objects; "classical" in that their orbits have changed little since forming, and represent a sample of the primordial Kuiper Belt.)
- "This is the first time one of these 'ultra red' objects has been explored, and our observations open all kinds of new questions," said Carly Howett, a New Horizons science team member from SwRI. "The color imaging even reveals subtle differences in coloration across the surface, and we really want to know why."
- New Horizons scientists have also seen evidence for methanol, water ice and organic molecules on the surface. "The spectrum of Ultima Thule is similar to some of the most extreme objects we've seen in the outer solar system," said Silvia Protopapa, a New Horizons co-investigator from SwRI. "So New Horizons is giving us an incredible opportunity to study one of these bodies up close."
- The Ultima Thule data transmission continues, though all of the data from the flyby won't be on the ground until late summer 2020. In the meantime, New Horizons continues to carry out distant observations of additional Kuiper Belt objects and mapping the charged-particle radiation and dust environment in the Kuiper Belt.
- The New Horizons spacecraft is 4.1 billion miles (6.6 billion km) from Earth, operating normally and speeding deeper into the Kuiper Belt at nearly 33,000 miles (53,000 km) per hour.
• February 28, 2019: Using New Horizons data from the Pluto-Charon flyby in 2015, a Southwest Research Institute-led team of scientists have indirectly discovered a distinct and surprising lack of very small objects in the Kuiper Belt. The evidence for the paucity of small Kuiper Belt objects (KBOs) comes from New Horizons imaging that revealed a dearth of small craters on Pluto's largest satellite, Charon, indicating that impactors from 100 meters to 1.6 km in diameter must also be rare. 27)
- The Kuiper Belt is a donut-shaped region of icy bodies beyond the orbit of Neptune. Because small Kuiper Belt objects were some of the "feedstock" from which planets formed, this research provides new insights into how the solar system originated. This research was published in the March 1 issue of the prestigious journal Science.
- "These smaller Kuiper Belt objects are much too small to really see with any telescopes at such a great distance," said SwRI's Dr. Kelsi Singer, the paper's lead author and a co-investigator of NASA's New Horizons mission. "New Horizons flying directly through the Kuiper Belt and collecting data there was key to learning about both large and small bodies of the Belt."
- "This breakthrough discovery by New Horizons has deep implications," added the mission's principal investigator, Dr. Alan Stern, also of SwRI. "Just as New Horizons revealed Pluto, its moons, and more recently, the KBO nicknamed Ultima Thule in exquisite detail, Dr. Singer's team revealed key details about the population of KBOs at scales we cannot come close to directly seeing from Earth."
- Craters on solar system objects record the impacts of smaller bodies, providing hints about the history of the object and its place in the solar system. Because Pluto is so far from Earth, little was known about the dwarf planet's surface until the epic 2015 flyby. Observations of the surfaces of Pluto and Charon revealed a variety of features, including mountains that reach as high as 4 km and vast glaciers of nitrogen ice. Geologic processes on Pluto have erased or altered some of the evidence of its impact history, but Charon's relative geologic stasis has provided a more stable record of impacts.
- "A major part of the mission of New Horizons is to better understand the Kuiper Belt," said Singer, whose research background studying the geology of the icy moons of Saturn and Jupiter positions her to understand the surface processes seen on KBOs. "With the successful flyby of Ultima Thule early this year, we now have three distinct planetary surfaces to study. This paper uses the data from the Pluto-Charon flyby, which indicate fewer small impact craters than expected. And preliminary results from Ultima Thule support this finding."
- Typical planetary models show that 4.6 billion years ago, the solar system formed from the gravitational collapse of a giant molecular cloud. The Sun, the planets and other objects formed as materials within the collapsing cloud clumped together in a process known as accretion. Different models result in different populations and locations of objects in the solar system.
- "This surprising lack of small KBOs changes our view of the Kuiper Belt and shows that either its formation or evolution, or both, were somewhat different than those of the asteroid belt between Mars and Jupiter," said Singer. "Perhaps the asteroid belt has more small bodies than the Kuiper Belt because its population experiences more collisions that break up larger objects into smaller ones."
- The paper published in Science is titled "Impact craters on Pluto and Charon indicate a deficit of small Kuiper Belt objects." The Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, designed, built and operates the New Horizons spacecraft, and manages the mission for NASA's Science Mission Directorate. The MSFC Planetary Management Office provides the NASA oversight for the New Horizons mission. Southwest Research Institute, based in San Antonio, directs the mission via Principal Investigator Stern, and leads the science team, payload operations and encounter science planning. New Horizons is part of the New Frontiers Program managed by NASA's Marshall Space Flight Center in Huntsville, Alabama. 28)
• February 22, 2019: The mission team called it a "stretch goal" – just before closest approach, precisely point the cameras on NASA's New Horizons spacecraft to snap the sharpest possible pics of the Kuiper Belt object nicknamed Ultima Thule, its New Year's flyby target and the farthest object ever explored. 29)
- Now that New Horizons has sent those stored flyby images back to Earth, the team can enthusiastically confirm that its ambitious goal was met.
- These new images of Ultima Thule – obtained by the telephoto Long-Range Reconnaissance Imager (LORRI) just 6½ minutes before New Horizons' closest approach to the object (officially named 2014 MU69) at 12:33 a.m. EST on Jan. 1 – offer a resolution of about 110 feet (33 meters) per pixel. Their combination of high spatial resolution and a favorable viewing angle gives the team an unprecedented opportunity to investigate the surface, as well as the origin and evolution, of Ultima Thule – thought to be the most primitive object ever encountered by a spacecraft.
- "Bullseye!" said New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute (SwRI). "Getting these images required us to know precisely where both tiny Ultima and New Horizons were — moment by moment – as they passed one another at over 32,000 miles per hour in the dim light of the Kuiper Belt, a billion miles beyond Pluto. This was a much tougher observation than anything we had attempted in our 2015 Pluto flyby.
- "These 'stretch goal' observations were risky, because there was a real chance we'd only get part or even none of Ultima in the camera's narrow field of view," he continued. "But the science, operations and navigation teams nailed it, and the result is a field day for our science team! Some of the details we now see on Ultima Thule's surface are unlike any object ever explored before."
- The higher resolution brings out a many surface features that weren't readily apparent in earlier images. Among them are several bright, enigmatic, roughly circular patches of terrain. In addition, many small, dark pits near the terminator (the boundary between the sunlit and dark sides of the body) are better resolved. "Whether these features are craters produced by impactors, sublimation pits, collapse pits, or something entirely different, is being debated in our science team," said John Spencer, deputy project scientist from SwRI.
- Project Scientist Hal Weaver, of the JHU/APL, noted that the latest images have the highest spatial resolution of any New Horizons has taken – or may ever take – during its entire mission. Swooping within just 3,500 km, New Horizons flew approximately three times closer to Ultima than it zipped past its primary mission target, Pluto, in July 2015.
- Ultima Thule is smaller than Pluto, but the Ultima flyby was done with the highest navigation precision ever achieved by any spacecraft. This unprecedented precision was achieved thanks to the ground-based occultation campaigns from 2017 and 2018 conducted in Argentina, Senegal, South Africa and Colombia, as well as the European Space Agency's Gaia mission, which provided the locations of the stars that were used during the occultation campaigns.
- Look for these and other LORRI images on the New Horizons LORRI website this week. Raw images from the camera are posted to the site each Friday.
- Mission operations manager Alice Bowman, of APL, reports that the spacecraft continues to operate flawlessly. New Horizons is nearly 4.13 billion miles (6.64 billion kilometers) from Earth; at that distance, radio signals, traveling at light speed, reach the large antennas of NASA's Deep Space Network six hours and nine minutes after New Horizons sends them. Follow New Horizons on its trek through the Kuiper Belt.
Figure 33: The most detailed images of Ultima Thule — obtained just minutes before the spacecraft's closest approach at 12:33 a.m. EST on Jan. 1 — have a resolution of about 33 m/pixel. Their combination of higher spatial resolution and a favorable viewing geometry offer an unprecedented opportunity to investigate the surface of Ultima Thule, believed to be the most primitive object ever encountered by a spacecraft. — This processed, composite picture combines nine individual images taken with the Long Range Reconnaissance Imager (LORRI), each with an exposure time of 0.025 seconds, just 6 ½ minutes before the spacecraft's closest approach to Ultima Thule (officially named 2014 MU69). The image was taken at 5:26 UT (12:26 a.m. EST) on Jan. 1, 2019, when the spacecraft was 6,628 km from Ultima Thule and 6.6 billion km from Earth. The angle between the spacecraft, Ultima Thule and the Sun – known as the "phase angle" – was 33 degrees (image credit: NASA, JHU/APL, SwRI, National Optical Astronomy Observatory)
• February 15, 2019: Scientists who study the solar system tend to ask big questions: How was our solar system formed? Where did the building blocks of life come from? What hazards from above threaten life on our planet? To find answers, they're looking more and more at small worlds. 30)
Figure 34: Small Worlds hold keys to questions about our solar system and the origin of life on Earth (video credit: Science@NASA, published on 15 February 2019)
- What are small worlds? Asteroids for sure. Comets too. Also the many small satellites or moons that orbit large planets as well as the icy worlds at the distance of Pluto and beyond. Some have combined, only to be broken apart later by collisions and tidal forces. Others have gone largely untouched since the dawn of the solar system. Some carry water and organic compounds, others are almost entirely composed of metal. And all hold keys to questions about our solar system and the origin of life on Earth.
- Dr. Adriana Ocampo, Program Executive for NASA's New Horizons mission, says "Water is key to life as we know it. Learning where water is found in our solar system provides pieces to the puzzle of understanding the origins of life. New Horizons recently surprised us by discovering a large abundance of water ice at Pluto." More surprises are in store, as New Horizons transmits the data from its January 1, 2019 flyby of the Kuiper Belt object 2014 MU69 back to Earth!
- Small worlds can be found in a wide range of locations across the solar system, from the inner solar system all the way out to the Kuiper Belt. When they are studied together, these remnants of the early solar system can help tell the story of solar system formation.
- Dawn recently completed a mission to the Main Asteroid Belt, visiting the dwarf planet Ceres and the Belt's largest asteroid, Vesta. OSIRIS-REx has arrived at Bennu, a near-Earth asteroid about 500 m across, and will return to Earth in 2023 with a sample so scientists can begin to understand Bennu's origin and history. The Lucy mission will be traveling to six Trojan asteroids, trapped in the orbit of Jupiter. These objects are the only remaining unexplored population of small worlds in the solar system. The Psyche mission will be visiting a metal object in the Main Asteroid Belt that could be the remnant core of a protoplanet similar in size to Vesta!
- While those missions travel to their individual targets, NEOWISE, a repurposed space telescope in low-Earth orbit, has made infrared measurements of hundreds of near-Earth objects and tens of thousands of other small worlds in the solar system. These diverse worlds offer insights into how our solar system formed and evolved.
- Dr. Tom Statler, Planetary Science Program Scientist at NASA Headquarters notes, "This is not your grandparent's solar system, and things are not as orderly as we once believed."
- "The data we've gleaned from these objects so far have changed the way we think about the origin of the planets. For example, the small worlds in the Kuiper Belt are leading us to think that Uranus and Neptune formed much closer to the Sun than where they reside now, then gradually moved to their current orbits."
- The biggest misperception about small worlds? Their distance to each other. Statler explains, "In the movies, they always show an asteroid belt with millions of rocks almost touching each other, whereas in reality there is much more empty space. You have to travel hundreds of thousands of miles to get from one asteroid to another."
- Yet scientists are also looking closer to home. Determining the orbits and physical characteristics of objects that might impact Earth is critical to understanding the consequences of any such impact; and responding to an actual impact threat, if one is ever discovered. NASA knows of no asteroid or comet currently on a collision course with Earth. But, to prepare for that scenario NASA is developing the DART (Double Asteroid Redirection Test) mission as the first demonstration of the kinetic impact technique that could be used to change the motion of a hazardous asteroid away from Earth.
• February 8, 2019: An evocative new image sequence from NASA's New Horizons spacecraft offers a departing view of the Kuiper Belt object (KBO) nicknamed Ultima Thule—the target of its New Year's 2019 flyby and the most distant world ever explored. 31)
- These aren't the last Ultima Thule images New Horizons will send back to Earth – in fact, many more are to come — but they are the final views New Horizons captured of the KBO (officially named 2014 MU69) as it raced away at over 31,000 miles/hour (50,000 km/hour) on January 1. The images were taken nearly 10 minutes after New Horizons crossed its closest approach point.
- "This really is an incredible image sequence, taken by a spacecraft exploring a small world four billion miles away from Earth," said mission Principal Investigator Alan Stern, of Southwest Research Institute. "Nothing quite like this has ever been captured in imagery."
- The newly released images also contain important scientific information about the shape of Ultima Thule, which is turning out to be one of the major discoveries from the flyby.
- The first close-up images of Ultima Thule – with its two distinct and, apparently, spherical segments – had observers calling it a "snowman." However, more analysis of approach images and these new departure images have changed that view, in part by revealing an outline of the portion of the KBO that was not illuminated by the Sun, but could be "traced out" as it blocked the view to background stars.
- Stringing 14 of these images into a short departure movie, New Horizons scientists can confirm that the two sections (or "lobes") of Ultima Thule are not spherical. The larger lobe, nicknamed "Ultima," more closely resembles a giant pancake and the smaller lobe, nicknamed "Thule," is shaped like a dented walnut.
- "We had an impression of Ultima Thule based on the limited number of images returned in the days around the flyby, but seeing more data has significantly changed our view," Stern said. "It would be closer to reality to say Ultima Thule's shape is flatter, like a pancake. But more importantly, the new images are creating scientific puzzles about how such an object could even be formed. We've never seen something like this orbiting the Sun."
Figure 35: The Crescent View: New Horizons took this image of the Kuiper Belt object 2014 MU69 (nicknamed Ultima Thule) on Jan. 1, 2019, when the NASA spacecraft was 5,494 miles (8,862 km) beyond it. The image to the left is an "average" of ten images taken by the Long Range Reconnaissance Imager (LORRI); the crescent is blurred in the raw frames because a relatively long exposure time was used during this rapid scan to boost the camera's signal level. Mission scientists have been able to process the image, removing the motion blur to produce a sharper, brighter view of Ultima Thule's thin crescent (image credit: NASA/Johns Hopkins Applied Physics Laboratory/Southwest Research Institute/National Optical Astronomy Observatory)
- The departure images were taken from a different angle than the approach photos and reveal complementary information on Ultima Thule's shape. The central frame of the sequence was taken on Jan. 1 at 05:42:42 UT (12:42 a.m. EST), when New Horizons was 5,494 miles (8,862 km) beyond Ultima Thule, and 4.1 billion miles (6.6 billion km) from Earth. The object's illuminated crescent is blurred in the individual frames because a relatively long exposure time was used during this rapid scan to boost the camera's signal level – but the science team combined and processed the images to remove the blurring and sharpen the thin crescent.
- Many background stars are also seen in the individual images; watching which stars "blinked out" as the object passed in front them allowed scientists to outline the shape of both lobes, which could then be compared to a model assembled from analyzing pre-flyby images and ground-based telescope observations. "The shape model we have derived from all of the existing Ultima Thule imagery is remarkably consistent with what we have learned from the new crescent images," says Simon Porter, a New Horizons co-investigator from the Southwest Research Institute, who leads the shape-modeling effort.
- "While the very nature of a fast flyby in some ways limits how well we can determine the true shape of Ultima Thule, the new results clearly show that Ultima and Thule are much flatter than originally believed, and much flatter than expected," added Hal Weaver, New Horizons project scientist from the Johns Hopkins Applied Physics Laboratory. "This will undoubtedly motivate new theories of planetesimal formation in the early solar system."
- The images in this sequence will be available on the New Horizons LORRI website this week. Raw images from the camera are posted to the site each Friday.
Figure 36: Scientists' understanding of Ultima Thule has changed as they review additional data. The "old view" in this illustration is based on images taken within a day of New Horizons' closest approach to the Kuiper Belt object on Jan. 1, 2019, suggesting that both of "Ultima" (the larger section, or lobe) and "Thule" (the smaller) were nearly perfect spheres just barely touching each other. - But as more data were analyzed, including several highly evocative crescent images taken nearly 10 minutes after closest approach, a "new view" of the object's shape emerged. Ultima more closely resembles a "pancake," and Thule a "dented walnut."The bottom view is the team's current best shape model for Ultima Thule, but still carries some uncertainty as an entire region was essentially hidden from view, and not illuminated by the Sun, during the New Horizons flyby. The dashed blue lines span the uncertainty in that hemisphere, which shows that Ultima Thule could be either flatter than, or not as flat as, depicted in this figure (image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)
Figure 37: This animation depicts a shape model of Ultima Thule created by the New Horizons science team based on its analysis of all the pre-flyby images sent to Earth so far. The first half of the movie mimics the view from the New Horizons spacecraft as it approached Ultima Thule and has the "snowman" shape that was so frequently mentioned in the days surrounding the New Year's 2019 flyby. - The movie then rotates to a side-view that illustrates what New Horizons might have seen had its cameras been pointing toward Ultima Thule only a few minutes after closest approach. While that wasn't the case, mission scientists have been able to piece together a model of this side-view, which has been at least partially confirmed by a set of crescent images of Ultima Thule (link). There is still considerable uncertainty in the sizes of "Ultima" (the larger section, or lobe) and "Thule" (the smaller) in the vertical dimension, but it's now clear that Ultima looks more like a pancake than a sphere, and that Thule is also very non-spherical. - The rotation in this animation is not the object's actual rotation, but is used purely to illustrate its shape (video credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)
• January 24, 2018: The wonders – and mysteries – of Kuiper Belt object 2014 MU69 continue to multiply as NASA's New Horizons spacecraft beams home new images of its New Year's Day 2019 flyby target. 32)
- Obtained with the wide-angle MVIC (Multicolor Visible Imaging Camera) component of New Horizons' Ralph instrument, this image was taken when the KBO was 4,200 miles (6,700 km) from the spacecraft, at 05:26 UT (12:26 a.m. EST) on Jan. 1 – just seven minutes before closest approach. With an original resolution of 135 m/pixel, the image was stored in the spacecraft's data memory and transmitted to Earth on Jan. 18-19. Scientists then sharpened the image to enhance fine detail. (This process – known as deconvolution – also amplifies the graininess of the image when viewed at high contrast.)
- The oblique lighting of this image reveals new topographic details along the day/night boundary, or terminator, near the top. These details include numerous small pits up to about 0.4 miles (0.7 km) in diameter. The large circular feature, about 4 miles (7 km) across, on the smaller of the two lobes, also appears to be a deep depression. Not clear is whether these pits are impact craters or features resulting from other processes, such as "collapse pits" or the ancient venting of volatile materials.
- Both lobes also show many intriguing light and dark patterns of unknown origin, which may reveal clues about how this body was assembled during the formation of the solar system 4.5 billion years ago. One of the most striking of these is the bright "collar" separating the two lobes.
- "This new image is starting to reveal differences in the geologic character of the two lobes of Ultima Thule, and is presenting us with new mysteries as well," said Principal Investigator Alan Stern, of the Southwest Research Institute in Boulder, Colorado. "Over the next month there will be better color and better resolution images that we hope will help unravel the many mysteries of Ultima Thule."
- New Horizons is approximately 4.13 billion miles (6.64 billion km) from Earth, operating normally and speeding away from the Sun (and Ultima Thule) at more than 31,500 miles/h (50,700 km/h). At that distance, a radio signal reaches Earth six hours and nine minutes after leaving the spacecraft.
Figure 38: This image, taken during the historic Jan. 1 flyby of what's informally known as Ultima Thule, is the clearest view yet of this remarkable, ancient object in the far reaches of the solar system – and the first small KBO (Kuiper Belt Object) ever explored by a spacecraft (image credit: NASA, JHU/APL, SwRI)
• January 2, 2019: Scientists from NASA's New Horizons mission released the first detailed images of the most distant object ever explored — the Kuiper Belt object nicknamed Ultima Thule. Its remarkable appearance, unlike anything we've seen before, illuminates the processes that built the planets four and a half billion years ago. 33)
- "This flyby is a historic achievement," said New Horizons Principal Investigator Alan Stern of the Southwest Research Institute in Boulder, Colorado. "Never before has any spacecraft team tracked down such a small body at such high speed so far away in the abyss of space. New Horizons has set a new bar for state-of-the-art spacecraft navigation."
Figure 39: This image, taken by LORRI (Long-Range Reconnaissance Imager), is the most detailed of Ultima Thule returned so far by the New Horizons spacecraft. It was taken at 5:01 UTC on January 1, 2019, just 30 minutes before closest approach from a range of 28,000 km, with an original scale of 140 m/pixel (image credit: NASA, JHU/APL, SwRI)
- The new images — taken from as close as 27,000 km on approach — revealed Ultima Thule as a "contact binary," consisting of two connected spheres. End to end, the world measures 31 km in length. The team has dubbed the larger sphere "Ultima" (19 km across) and the smaller sphere "Thule" (14 km across).
- The team says that the two spheres likely joined as early as 99 percent of the way back to the formation of the solar system, colliding no faster than two cars in a fender-bender.
- "New Horizons is like a time machine, taking us back to the birth of the solar system. We are seeing a physical representation of the beginning of planetary formation, frozen in time," said Jeff Moore, New Horizons Geology and Geophysics team lead. "Studying Ultima Thule is helping us understand how planets form — both those in our own solar system and those orbiting other stars in our galaxy."
- Data from the New Year's Day flyby will continue to arrive over the next weeks and months, with much higher resolution images yet to come.
- "In the coming months, New Horizons will transmit dozens of data sets to Earth, and we'll write new chapters in the story of Ultima Thule — and the solar system," said Helene Winters, New Horizons Project Manager.
Figure 40: The first color image of Ultima Thule, taken at a distance of 137,000 km at 4:08 UTC January 1, 2019 (and published on 2 January 2019), highlights its reddish surface. At left is an enhanced color image taken by the Multispectral Visible Imaging Camera (MVIC), produced by combining the near infrared, red and blue channels. The center image taken by the Long-Range Reconnaissance Imager (LORRI) has a higher spatial resolution than MVIC by approximately a factor of five. At right, the color has been overlaid onto the LORRI image to show the color uniformity of the Ultima and Thule lobes. Note the reduced red coloring at the neck of the object (image credit: NASA, JHU/APL, SwRI)
• January 1, 2019: NASA's New Horizons spacecraft flew past Ultima Thule in the early hours of New Year's Day, ushering in the era of exploration from the enigmatic Kuiper Belt, a region of primordial objects that holds keys to understanding the origins of the solar system. 34)
- "Congratulations to NASA's New Horizons team, Johns Hopkins Applied Physics Laboratory and the Southwest Research Institute for making history yet again. In addition to being the first to explore Pluto, today New Horizons flew by the most distant object ever visited by a spacecraft and became the first to directly explore an object that holds remnants from the birth of our solar system," said NASA Administrator Jim Bridenstine. "This is what leadership in space exploration is all about."
- Signals confirming the spacecraft is healthy and had filled its digital recorders with science data on Ultima Thule reached the mission operations center at the Johns Hopkins Applied Physics Laboratory (APL) today at 10:29 a.m. EST, almost exactly 10 hours after New Horizons' closest approach to the object.
- "New Horizons performed as planned today, conducting the farthest exploration of any world in history — 4 billion miles from the Sun," said Principal Investigator Alan Stern, of the Southwest Research Institute in Boulder, Colorado. "The data we have look fantastic and we're already learning about Ultima from up close. From here out the data will just get better and better!"
- Images taken during the spacecraft's approach — which brought New Horizons to within just 2,200 miles (3,500 km) of Ultima at 12:33 a.m. EST — revealed that the Kuiper Belt object may have a shape similar to a bowling pin, spinning end over end, with dimensions of approximately 32 by 16 km. Another possibility is Ultima could be two objects orbiting each other. Flyby data have already solved one of Ultima's mysteries, showing that the Kuiper Belt object is spinning like a propeller with the axis pointing approximately toward New Horizons. This explains why, in earlier images taken before Ultima was resolved, its brightness didn't appear to vary as it rotated. The team has still not determined the rotation period.
- As the science data began its initial return to Earth, mission team members and leadership reveled in the excitement of the first exploration of this distant region of space.
- "New Horizons holds a dear place in our hearts as an intrepid and persistent little explorer, as well as a great photographer," said Johns Hopkins Applied Physics Laboratory Director Ralph Semmel. "This flyby marks a first for all of us — APL, NASA, the nation and the world — and it is a great credit to the bold team of scientists and engineers who brought us to this point."
- "Reaching Ultima Thule from 4 billion miles away is an incredible achievement. This is exploration at its finest," said Adam L. Hamilton, president and CEO of the Southwest Research Institute in San Antonio. "Kudos to the science team and mission partners for starting the textbooks on Pluto and the Kuiper Belt. We're looking forward to seeing the next chapter."
- The New Horizons spacecraft will continue downloading images and other data in the days and months ahead, completing the return of all science data over the next 20 months. When New Horizons launched in January 2006, George W. Bush was in the White House, Twitter had just been launched and Time Magazine's Person of the Year was "you — all the worldwide web users." Nine years into its journey, the spacecraft began its exploration of the Kuiper Belt with a flyby of Pluto and its moons. Almost 13 years after the launch, the spacecraft will continue its exploration of the Kuiper Belt until at least 2021. Team members plan to propose more Kuiper Belt exploration.
Figure 41: At left is a composite of two images taken by New Horizons' high-resolution Long-Range Reconnaissance Imager (LORRI), which provides the best indication of Ultima Thule's size and shape so far. Preliminary measurements of this Kuiper Belt object suggest it is approximately 20 miles long by 10 miles wide (32 kilometers by 16 kilometers). An artist's impression at right illustrates one possible appearance of Ultima Thule, based on the actual image at left. The direction of Ultima's spin axis is indicated by the arrows (image credit: NASA, JHU/APL, SwRI; sketch courtesy of James Tuttle Keane)
- New Horizons acquired gigabytes of photos and other observations during the pass. It will now send these home over the coming months. The radio message from the robotic craft was picked up by one of Nasa's big antennas, in Madrid, Spain. It had taken fully six hours and eight minutes to traverse the great expanse of space between Ultima and Earth.
• NASA's New Horizons spacecraft is on track to perform the farthest flyby in history, when it zips past a Kuiper Belt object nicknamed Ultima Thule – more than four billion miles (~6.5 billion km) from Earth – at 12:33 a.m. EST (05:33 UTC) on 1 January 2019. 35)
• December 30, 2018: The Hubble Space Telescope Paved the Way for the New Horizons Mission to Pluto and Ultima Thule. Years before a team of researchers proposed a mission called New Horizons to explore the dwarf planet Pluto, NASA's Hubble Space Telescope had already made initial observations of the world at the dim outer fringes of our celestial neighborhood. Over many years, Hubble's pioneering observations repeatedly accomplished what ground-based telescopes could not — imaging features on Pluto's surface, finding new Plutonian moons, and tracking down a destination to visit after Pluto — an even tinier, icy object in a vast region of small worlds beyond the orbit of Neptune called the Kuiper Belt. 36)
- Thus began a decades-long relationship between Hubble and NASA's New Horizons mission: A legendary space-based telescope and a pioneering space probe hurtling through space at about 32,000 miles/hour (51,500 km/hour).
- In 1990, Hubble produced the first image that illuminated Pluto and its large moon Charon. After Hubble's optical repair in 1993, scientists captured even sharper images. New Horizons Principal Investigator Alan Stern of the Southwest Research Institute in Boulder, Colorado, led the imaging projects while co-investigator Marc Buie, now at SwRI Boulder but then at Lowell Observatory where Pluto was discovered, led the data analysis.
- "We got eight pixels of Pluto in 1994. Each pixel represented more than 150 square miles of Pluto's surface. Fast forward to 2002 and we got even fewer pixels per image. We had to wring every bit of information from each pixel possible," Buie said. "It was a time- and computer-intensive process, but we were able to create the first maps of Pluto's surface, and they were truly spectacular for their time."
- Those crude but valuable maps provided the best evidence that Pluto was not simply a homogenous ball of ices, but has a complex, variegated surface — a promising aspect for close-up inspection by a visiting spacecraft. Hubble's cameras revealed nearly a dozen distinctive bright features, none of which had ever been seen before, including a "ragged" northern polar cap bisected by a dark strip, a puzzling high-contrast bright spot seen rotating with the planet, a cluster of dark spots, and a bright linear marking. That bright spot feature, unusually rich in carbon monoxide frost, became the prime target for New Horizons to examine up close after NASA funded the mission in 2002.
Figure 42: Full trajectory: side view: This image shows New Horizons' current position along its full planned trajectory. The green segment of the line shows where New Horizons has traveled since launch; the red indicates the spacecraft's future path. Positions of stars with magnitude 12 or brighter are shown from this perspective, which is slightly above the orbital plane of the planets (image credit: JHU/APL, NASA, SwRI) 37)
Figure 43: Full trajectory: overhead view: This image shows New Horizons' current position along its full planned trajectory. The green segment of the line shows where New Horizons has traveled since launch; the red indicates the spacecraft's future path. Positions of stars with magnitude 12 or brighter are shown from this perspective, which is above the Sun and "north" of Earth's orbit (image credit: JHU/APL, NASA, SwRI)
- Although Charon was discovered in 1978 using ground-based telescopes, Hubble detected all four of Pluto's other moons: Nix and Hydra in 2005, Kerberos in 2011, and Styx in 2012. These moons were spotted in the Hubble images by New Horizons team members, most notably Project Scientist Hal Weaver of the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, and co-investigator Mark Showalter of the SETI Institute in Mountain View, California. By the time the latter two moons were discovered, New Horizons was in the final years of its almost decade-long, 3-billion-mile sprint from Earth to Pluto.
- The discovery of these four small satellites was critical to overall Pluto flyby planning by identifying potential hazards, verifying the optimal spacecraft trajectory, and establishing the need for time to include observations of them as part of the flyby observing sequence. Without Hubble, New Horizons would have discovered these moons only a few months before the encounter — too late to effectively plan for their detailed study. To examine the possibility for an extended mission into the Kuiper Belt, the New Horizons team used Hubble in 2014 to conduct a needle-in-a-haystack search for a suitable Kuiper Belt Object that New Horizons could visit after passing Pluto. Hubble's sensitive telescope allowed it to look for fainter KBOs than ground-based telescopes can see. Hubble took deep exposures on 20 areas of the sky and found three suitable KBO targets about one billion miles beyond Pluto. Following NASA approval for a mission extension in 2016, Stern selected 2014 MU69, since nicknamed Ultima Thule, as the target for its January 2019 flyby. In the years since, Hubble has measured the target's red color and refined its orbit with dozens of additional observations.
- "Without Hubble there would be no flyby of Ultima Thule," said Stern. "And without Hubble, New Horizons would not have been as productive studying Pluto's small moons. In fact, without Hubble's early images revealing how interesting Pluto's surface markings are, there might have never been a mission to explore this fascinating dwarf planet."
Figure 44: NASA's Hubble Space Telescope discovered the next target for the New Horizons spacecraft — 2014 MU69, nicknamed Ultima Thule — in June 2014. Seen in these five overlaid images, the object resides more than one billion miles beyond Pluto in the frigid outer reaches of the Kuiper Belt. New Horizons will reach Ultima Thule on New Year's Day 2019 (image credit: NASA, STScI, JHU/APL, SwRI)
• New Horizons just over 100 days from Ultima Thule (19 September 2018)
NASA's New Horizons spacecraft is just over 100 days away from a high-risk, high-reward flyby of an ancient world on the outskirts of the solar system. 38)
On New Year's Day 2019, the spacecraft will come within 3,500 km of 2014 MU69, an estimated 37 km-wide object the mission team has nicknamed Ultima Thule. The encounter will take place 6.6 billion km from Earth, where it takes more than 6 hours for radio signals traveling at the speed of light to reach NASA's Deep Space Network.
There will only be one chance for New Horizons to perfectly aim its cameras and science instruments at Ultima Thule as it zips past at 14 km/s and there may be unforeseen hazards in the spacecraft's path. Nevertheless, Jim Green, the director of NASA's planetary science division, is looking forward to the drama. "Are there debris in the way? Will the spacecraft make it? I mean, you know, you can't get any better than that," he said Wednesday, during a "science chat" broadcast from the Johns Hopkins University Applied Physics Laboratory in Maryland. "And, we'll get spectacular images on top of that. What's not to like?"
Figure 45: Artist's impression of New Horizons encountering 2014 MU69 (Ultima Thule), image credit: NASA, JHU/APL, SwRI, Steve Gribben
Scientists think Ultima Thule is a building block of larger objects in the Kuiper Belt, a band of frigid worlds that orbit the Sun beyond Neptune. Objects in the belt serve as time capsules from the dawn of the solar system, when a cloud of dust and gas collapsed to form the Sun, planets, and other small bodies. The temperature on Ultima Thule is expected to be just 40 or 50 degrees above absolute zero, said Alan Stern, the principal investigator of the New Horizons mission. "It's a big deal because we're going 4 billion years into the past," he said. "Nothing that we've ever explored in the entire history of space exploration has been kept in this kind of deep freeze the way Ultima has."
New Horizons made worldwide headlines when it flew past Pluto in 2015, revealing a complex world with icy mountains and frozen plains. The big reveal of Ultima Thule probably won't garner as much publicity — but that hardly means it's going to be a cakewalk. "Everything about this flyby is tougher," said Stern.
For the Pluto encounter, the team knew exactly where to aim, thanks to decades of telescopic observations that refined our understanding of its orbit. That won't be the case with Ultima Thule, which was only discovered in 2014 by the Hubble Space Telescope. The New Horizons team must continually track the object using optical navigation cameras, and make the appropriate course corrections.
"The orbit is not very well characterized," said Alice Bowman, the mission operations manager for New Horizons. "That's why we're doing optical navigation measurements, continuously looking to make sure we know the point in space we want to target."
That's a tricky task because Ultima Thule is dark and reddish, and only reflects about as much light as potting soil, Stern said. The object is also set against a particularly crowded region of space that is flush with background stars.
Figure 46: New Horizons Ultima Thule encounter timeline (image credit: NASA)
The central objective of the New Horizons prime mission was to make the first exploration of Pluto and its system of moons. Following that, New Horizons has been approved for its first extended mission, which has the objectives of extensively studying the Kuiper Belt environment, observing numerous Kuiper Belt Objects (KBOs) and Centaurs in unique ways, and making the first close flyby of the KBO 486958 2014 MU69. This review summarizes the objectives and plans for this approved mission extension, and briefly looks forward to potential objectives for subsequent extended missions by New Horizons. 39)
• Pluto Features Given First Official Names (7 September 2017)
It's official: Pluto's "heart" now bears the name of pioneering American astronomer Clyde Tombaugh, who discovered Pluto in 1930. And a crater on Pluto is now officially named after Venetia Burney, the British schoolgirl who in 1930 suggested the name "Pluto," Roman god of the underworld, for Tombaugh's newly-discovered planet. 40)
Tombaugh Regio and Burney crater are among the first set of official Pluto feature names approved by the International Astronomical Union (IAU), the internationally recognized authority for naming celestial bodies and their surface features.
These and other names were proposed by NASA's New Horizons team following the first reconnaissance of Pluto and its moons by the New Horizons spacecraft in 2015. The New Horizons science team had been using these and other place names informally to describe the many regions, mountain ranges, plains, valleys and craters discovered during the first close-up look at the surfaces of Pluto and its largest moon, Charon.
A total of 14 Pluto place names have now been made official by the IAU; many more will soon be proposed to the IAU, both on Pluto and on its moons. "The approved designations honor many people and space missions who paved the way for the historic exploration of Pluto and the Kuiper Belt, the farthest worlds ever explored," said Alan Stern, New Horizons principal investigator from Southwest Research Institute, Boulder, Colorado.
"We're very excited to approve names recognizing people of significance to Pluto and the pursuit of exploration as well as the mythology of the underworld. These names highlight the importance of pushing to the frontiers of discovery," said Rita Schulz, chair of the IAU Working Group for Planetary System Nomenclature. "We appreciate the contribution of the general public in the form of their naming suggestions and the New Horizons team for proposing these names to us."
Figure 47: Pluto's first official surface-feature names are marked on this map, compiled from images and data gathered by NASA's New Horizons spacecraft during its flight through the Pluto system in 2015 (image credit: NASA, JHU/APL, SwRI, Ross Beyer)
Stern applauded the work of the New Horizons Nomenclature Working Group, which along with Stern included science team members Mark Showalter — the group's chairman and liaison to the IAU — Ross Beyer, Will Grundy, William McKinnon, Jeff Moore, Cathy Olkin, Paul Schenk and Amanda Zangari.
The team gathered many ideas during the "Our Pluto" online naming campaign in 2015. Following on Venetia Burney's original suggestion, several place names on Pluto come from underworld mythology. "I'm delighted that most of the approved names were originally recommended by members of the public," said Showalter, of the SETI Institute, Mountain View, California.
The approved Pluto surface feature names are listed in Table 1. The names pay homage to the underworld mythology, pioneering space missions, historic pioneers who crossed new horizons in exploration, and scientists and engineers associated with Pluto and the Kuiper Belt.
Table 1: Overview of approved Pluto surface names
The New Horizons spacecraft – built and operated at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, with a payload and science investigation led by SwRI — is speeding toward its next flyby, this one with the ancient Kuiper Belt object 2014 MU69, about 1.6 billion kilometers beyond Pluto, on 1 January 2019.
• NASA Video Soars over Pluto's Majestic Mountains and Icy Plains (14 July 2017)
In July 2015, NASA's New Horizons spacecraft sent home the first close-up pictures of Pluto and its moons – amazing imagery that inspired many to wonder what a flight over the distant worlds' icy terrain might be like. 41)
Wonder no more. Using actual New Horizons data and digital elevation models of Pluto and its largest moon Charon, mission scientists have created flyover movies that offer spectacular new perspectives of the many unusual features that were discovered and which have reshaped our views of the Pluto system – from a vantage point even closer than the spacecraft itself.
Figure 48: New Horizons flyover of Pluto. Using actual New Horizons data and digital elevation models of Pluto and its largest moon Charon, mission scientists have created flyover movies that offer spectacular new perspectives of the many unusual features that were discovered and which have reshaped (video credit: NASA, JHU/APL, SwRI, Paul Schenk and John Blackwell, Lunar and Planetary Institute)
This dramatic Pluto flyover begins over the highlands to the southwest of the great expanse of nitrogen ice plain informally named Sputnik Planitia. The viewer first passes over the western margin of Sputnik, where it borders the dark, cratered terrain of Cthulhu Macula, with the blocky mountain ranges located within the plains seen on the right. The tour moves north past the rugged and fractured highlands of Voyager Terra and then turns southward over Pioneer Terra — which exhibits deep and wide pits — before concluding over the bladed terrain of Tartarus Dorsa in the far east of the encounter hemisphere.
Figure 49: New Horizons Flyover of Charon (video credit: NASA, JHU/APL, SwRI, Paul Schenk and John Blackwell, Lunar and Planetary Institute)
The equally exciting flight over Charon begins high over the hemisphere New Horizons saw on its closest approach, then descends over the deep, wide canyon of Serenity Chasma. The view moves north, passing over Dorothy Gale crater and the dark polar hood of Mordor Macula. The flight then turns south, covering the northern terrain of Oz Terra before ending over the relatively flat equatorial plains of Vulcan Planum and the "moated mountains" of Clarke Montes.
The topographic relief is exaggerated by a factor of two to three times in these movies to emphasize topography; the surface colors of Pluto and Charon also have been enhanced to bring out detail.
• New Horizons' Top 10 Discoveries at Pluto (14 July 2016)
Five billion kilometers from Earth, NASA's New Horizons spacecraft, moving at speeds that would get it from New York to Los Angeles in about four minutes, was pointing cameras, spectrometers, and other sensors at Pluto and its moons – distant worlds that humankind had never seen up close – recording hundreds of pictures and other data that would forever change our view of the outer solar system. 42)
"New Horizons not only completed the era of first reconnaissance of the planets, the mission has intrigued and inspired. Who knew that Pluto would have a heart?" said NASA's Director of Planetary Science Jim Green. "Even today, New Horizons captures our imagination, rekindles our curiosity, and reminds us of what's possible."
To say that New Horizons shook the foundation of planetary science is an understatement—discoveries already culled from the pictures and compositional and space environment readings have not only introduced us to the Pluto system, but hint at what awaits as scientists examine other worlds in the Kuiper Belt. New Horizons Principal Investigator Alan Stern of the Southwest Research Institute, Boulder, Colorado, lists the mission's most surprising and amazing findings from Pluto (so far):
- The complexity of Pluto and its satellites is far beyond what we expected.
- The degree of current activity on Pluto's surface and the youth of some surfaces on Pluto are simply astounding.
- Pluto's atmospheric hazes and lower-than-predicted atmospheric escape rate upended all of the pre-flyby models.
- Charon's enormous equatorial extensional tectonic belt hints at the freezing of a former water ice ocean inside Charon in the distant past. Other evidence found by New Horizons indicates Pluto could well have an internal water-ice ocean today.
Figure 50: NASA's New Horizons spacecraft captured this high-resolution enhanced color view of Pluto's moon Charon just before closest approach on July 14, 2015. Charon's striking reddish north polar region is informally named Mordor Macula (image credits: NASA, JHU/APL, SwRI)
- All of Pluto's moons that can be age-dated by surface craters have the same, ancient age—adding weight to the theory that they were formed together in a single collision between Pluto and another planet in the Kuiper Belt long ago.
- Charon's dark, red polar cap is unprecedented in the solar system and may be the result of atmospheric gases that escaped Pluto and then accreted on Charon's surface.
- Pluto's vast 1,000-kilometer-wide heart-shaped nitrogen glacier (informally called Sputnik Planum) that New Horizons discovered is the largest known glacier in the solar system.
- Pluto shows evidence of vast changes in atmospheric pressure and, possibly, past presence of running or standing liquid volatiles on its surface – something only seen elsewhere on Earth, Mars and Saturn's moon Titan in our solar system.
- The lack of additional Pluto satellites beyond what was discovered before New Horizons was unexpected.
- Pluto's atmosphere is blue. Who knew?
"It's strange to think that only a year ago, we still had no real idea of what the Pluto system was like," said Hal Weaver, New Horizons project scientist from the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. "But it didn't take long for us to realize Pluto was something special, and like nothing we ever could have expected. We've been astounded by the beauty and complexity of Pluto and its moons and we're excited about the discoveries still to come."
New Horizons is now nearly 500 million km beyond Pluto, speeding to its next destination deeper into the Kuiper Belt, following NASA approval of an extended mission. About 80 percent of the data stored on the spacecraft's recorders has been sent to Earth; transmission of the remainder will be complete by October.
Figure 51: Illustration of Pluto and its next science target, 2014 MU69, with the trajectory of New Horizons in yellow (image credit: Alex Parker)
"Our entire team is proud to have accomplished the first exploration of Pluto and the Kuiper Belt—something many of us had worked to achieve since the 1990s," said Stern. "The data that New Horizons sent back about Pluto and its system of moons has revolutionized planetary science and inspired people of all ages across the world about space exploration. It's been a real privilege to be able to do that, for which I'll be forever indebted to our team and our nation."
• Pluto Flyby in 2015
The 15 July 2015 flyby of Pluto was the culmination of a decades-long quest to see and understand the Solar System's most distant planet. But it turned out to be a new beginning, revealing for the first time the potential complexity of small (or dwarf) planet systems in the Kuiper belt. New Horizons' seven instruments took over 50 Gb of data at Pluto — on approach, passing through the system, and on departure. Pluto was revealed to be a geologically and meteorologically active world. Its size and density were accurately determined; the former turned out to be at the upper range of previous Earth-based estimates. Pluto remains, for now, the physically largest world in the Kuiper belt. Made largely of rock, about 2/3 by mass, but with substantial ice and carbonaceous (organic) material, Pluto appears to have differentiated, possessing a rock core and predominantly water ice mantle. Surface layers and deposits of volatile ices such as nitrogen (N2), methane (CH4), and carbon monoxide (CO), along with dark, reddish, organic matter often referred to as tholins, complete the picture (Ref. 45).
Pluto's volatile ices can sublimate and condense under the influence of even the feeble sunlight at Pluto's distance, and N2-ice in particular is soft enough to flow across Pluto's incredibly cold surface (about 40 K as measured by New Horizons). And flow it does, from high standing water-ice-rich terrains, through gaps in the mountains, and down to a vast, frozen nitrogen-ice sea called Sputnik Planitia (SP for short). This icy plain is actually a giant ice sheet, filling to a depth of several kilometers a great impact basin over 1000 km across, itself the product of the collision of a large (roughly 200-km wide) Kuiper belt object with Pluto over 4 billion years ago. Overall, Pluto's topography is rugged, with some water-ice mountain blocks reaching heights of 5 km. The density of impact craters on Pluto varies widely, from heavily cratered terrains in Cthulhu Regio that must date from the earliest days of the Solar System, to Sputnik Planitia itself, which betrays no impact craters at all in New Horizons' highest resolution images (about 70 m/pixel). The solid nitrogen ice in SP is organized into giant convection cells, slowly churning on a several 100,000 year time scale, transporting Pluto's modest interior heat outward. The position of SP on Pluto's tidal axis (the imaginary line that runs through the centers of Pluto and Charon) may not be coincidence; rather, it could be evidence of the slip, or polar wander, of Pluto's surface to a preferred orientation. Such a slip is circumstantial evidence that under Pluto's water ice mantle, perhaps at a depth of close to 150 km, lies an inner ocean of water.
Pluto's atmosphere was measured by New Horizons UV solar and radio occultations. At present it is quite thin, about 11.5 µbar at the surface, and dominated by N2 gas in vapor-pressure equilibrium with surface N2 ice. Methane is the next most abundant gas and several simple hydrocarbons were discovered as well. The temperature in the atmosphere rises over the first 25 km or so of altitude, peaking at around 115 K, and gradually falling with increasing altitude thereafter. The temperature at Pluto's exobase, 100s of km above the surface, was much lower than previously thought, which means that Pluto's atmosphere is not escaping as fast as previously believed. Even more interesting, the dominant gas escaping is the lighter component, CH4, not N2. As thin as Pluto's atmosphere is, it is capable of transporting sublimed ices from one part of Pluto's surface to another, and downslope wind speeds are great enough to form methane particle "sand dunes" at the edge of SP. But the best surprise was saved for last. Turning New Horizons' imaging telescopes to look back at Pluto on departure, multiple haze layers were discovered wrapping completely around the planet, extending over several 100 km in height above the surface.
Pluto's satellites did not disappoint as well. Charon, the large moon, has a surface dominated by water ice and no atmosphere. New Horizons' LEISA spectrometer confirmed the presence of some form of ammonia ice across much of the surface, along with concentrations associated with specific impact craters. Charon's poles are apparently stained red from CH4 escaping from Pluto but cold trapped and radiation processed there. Charon is not geologically active today, and has sufficient impact craters on its surface the New Horizons science team members estimate its overall surface age to approach 4 billion years. But that does not mean its geological past was anything but spectacular. Much of Charon's northern hemisphere visible to New Horizons at closest approach is broken into great crustal blocks and canyons many kilometers deep. In contrast, the southern hemisphere is slightly younger plains, informally named Vulcan Planitia (VP), and appears to have been resurfaced by a series of thick, icy — or cryovolcanic — flows. The totally of Charon's geological history suggests that it too is differentiated and possessed an internal ocean, but owing to the moon's smaller size relative to Pluto, it cooled and its ocean froze, causing it surface to expand and rupture, which in turn facilitated the cryovolcanic eruptions we see in VP.
Rounding out the tour of the Pluto system were studies of the 4 small moons, Styx, Nix, Kerberos, and Hydra. These moons, discovered after the New Horizons mission started, were imaged on a best effort basis. They are irregular, on the order of 10 to 50 km across, and highly reflective. Infrared spectra indicate surfaces of almost pure water ice. These moons orbit in the same plane as Pluto and Charon but at sufficiently great distances from the two that tides have not slowed any of the small moons to the synchronous spin state common among regular satellites elsewhere in the solar system. In fact, some of spin states are bizarre: all four highly elongated objects rotate much faster than synchronous, with their rotational poles highly inclined relative to those of Pluto and Charon (which are aligned). The relativities and surface compositions of Pluto's small moons are quite different from most Kuiper belt objects in their size class, which are much darker and spectrally featureless, but are consistent with being debris from the giant impact that created the Pluto-Charon binary at the beginning of solar system history.
Figure 52: Pluto's intriguing moons. We talk a lot about Charon, Pluto's largest moon that's about half the size of its host planet. But what about Pluto's other moons? They're strange, to say the least. Pluto's four smaller moons - Styx, Nix, Kerberos and Hydra - are differently fascinating than Charon (video credit: NASA)
• New Horizons Captures Two of Pluto's Smaller Moons (21 July 2015)
Figure 53: Pluto's moon Nix (left), shown here in enhanced color as imaged by the New Horizons Ralph instrument, has a reddish spot that has attracted the interest of mission scientists. The data were obtained on the morning of July 14, 2015, and received on the ground on July 18. At the time the observations were taken New Horizons was about 165,000 km from Nix. The image shows features as small as approximately 3 km across on Nix, which is estimated to be 42 km long and 36 km wide (image credit: NASA, JHU/APL, SWRI) 43)
Pluto's small, irregularly shaped moon Hydra (right) is revealed in this black and white image taken from New Horizons' LORRI instrument on July 14, 2015, from a distance of about 231,000 km. Features as small as1.2 km are visible on Hydra, which measures 55 km in length.
While Pluto's largest moon Charon has grabbed most of the lunar spotlight so far, these two smaller and lesser-known satellites are now getting some attention. Nix and Hydra – the second and third moons to be discovered – are approximately the same size, but their similarity ends there.
New Horizons' first color image of Pluto's moon Nix, in which colors have been enhanced, reveals an intriguing region on the jelly bean-shaped satellite, which is estimated to be 42 km long and 36 km wide.
Although the overall surface color of Nix is neutral grey in the image, the newfound region has a distinct red tint. Hints of a bull's-eye pattern lead scientists to speculate that the reddish region is a crater. "Additional compositional data has already been taken of Nix, but is not yet downlinked. It will tell us why this region is redder than its surroundings," said mission scientist Carly Howett, Southwest Research Institute, Boulder, Colorado. She added, "This observation is so tantalizing, I'm finding it hard to be patient for more Nix data to be downlinked."
Meanwhile, the sharpest image yet received from New Horizons of Pluto's satellite Hydra shows that its irregular shape resembles the state of Michigan. The new image was made by the LORRI (Long Range Reconnaissance Imager) on 14 July 2015 from a distance of 231,000 km, and shows features as small as 1.2 km across. There appear to be at least two large craters, one of which is mostly in shadow. The upper portion looks darker than the rest of Hydra, suggesting a possible difference in surface composition. From this image, mission scientists have estimated that Hydra is 55 km long and 40 km wide. Commented mission science collaborator Ted Stryk of Roane State Community College in Tennessee, "Before last week, Hydra was just a faint point of light, so it's a surreal experience to see it become an actual place, as we see its shape and spot recognizable features on its surface for the first time."
Images of Pluto's most recently discovered moons, Styx and Kerberos, are expected to be transmitted to Earth no later than mid-October.
Nix and Hydra were both discovered in 2005 using Hubble Space Telescope data by a research team led by New Horizons project scientist Hal Weaver, JHU/APL (Johns Hopkins University /Applied Physics Laboratory), Laurel, Maryland. New Horizons' findings on the surface characteristics and other properties of Nix and Hydra will help scientists understand the origins and subsequent history of Pluto and its moons.
• New Horizons Spacecraft Displays Pluto's Big Heart (14 July 2015)
Five billion kilometers away, Pluto has sent a "love note" back to Earth, via NASA's New Horizons spacecraft. 44)
At about 4 p.m. EDT on July 13 - about 16 hours before closest approach - New Horizons captured this stunning image of one of Pluto's most dominant features (Figure 54). The "heart," estimated to be 1,000 miles (1,600 km) across at its widest point rests just above the equator (the angle of view displays mostly the northern hemisphere). The heart's diameter is about the same distance as from Denver to Chicago, in America's heartland.
"Wow!" said New Horizons principal investigator Alan Stern, Southwest Research Institute, Boulder, Colorado, as the image was unveiled before the New Horizons science team at JHU/APL (Johns Hopkins University /Applied Physics Laboratory) in Laurel, Maryland. "My prediction was that we would find something wonderful, and we did. This is proof that good things really do come in small packages."
The newest image from the Long Range Reconnaissance Imager (LORRI) shows an almost perfectly shaped left half of a bright, heart-shaped feature centered just above Pluto's equator, while the right side of the heart appears to be less defined.
The image shows for the first time that some surfaces on Pluto are peppered with impact craters and are therefore relatively ancient, perhaps several billion years old. Other regions, such as the interior of the heart, show no obvious craters and thus are probably younger, indicating that Pluto has experienced a long and complex geological history. Some craters appear partially destroyed, perhaps by erosion. There are also hints that parts of Pluto's crust have been fractured, as indicated by the series of linear features to the left of the heart.
Below the heart are dark terrains along Pluto's equator, including, on the left, the large dark feature informally known as the "whale." Craters pockmark part of the whale's head; areas that appear smooth and featureless may be a result of image compression.
New Horizons traveled nearly a decade to receive its summer valentine, launching on January 19, 2006.
This is just the latest in a series of the New Horizons Pluto "picture show." On Wednesday July 15, more images of surface close-ups will make the more than four-hour journey to Earth at the speed of light to give Pluto fans details as small as New York's Central Park.
"Our data tomorrow (Wednesday, July 15) will have ten times the resolution of what we see today and it will knock your socks off," said Stern.
Curt Niebur, New Horizons program scientist with NASA Headquarters in Washington notes, "The science is amazing, but the team's excitement reminds me of why we really do this."
At 7:49 a.m. EDT on Tuesday, July 14 New Horizons sped past Pluto at 30,800 miles/hr (49,600 km/hr), with a suite of seven science instruments. As planned, New Horizons went incommunicado as it hurtled through the Pluto-Charon system busily gathering data. The New Horizons team will breathe a sigh of relief when New Horizons "phones home" at approximately 9:02 p.m. EDT on July 14. The mission to the icy dwarf planet completes the initial reconnaissance of the solar system.
Figure 54: Pluto nearly fills the frame in this image from the LORRI (Long Range Reconnaissance Imager) aboard NASA's New Horizons spacecraft, taken on July 13, 2015 when the spacecraft was 768,000 km from the surface. This is the last and most detailed image sent to Earth before the spacecraft's closest approach to Pluto on July 14. The color image has been combined with lower-resolution color information from the Ralph instrument that was acquired earlier on July 13. This view is dominated by the large, bright feature informally named the "heart," which measures approximately 1,600 km across. The heart borders darker equatorial terrains, and the mottled terrain to its east (right) are complex. However, even at this resolution, much of the heart's interior appears remarkably featureless—possibly a sign of ongoing geologic processes (image credit: NASA ,APL, SwRI)
Figure 55: New Horizons is the first mission to the Kuiper Belt, a gigantic zone of icy bodies and mysterious small objects orbiting beyond Neptune. This region also is known as the "third" zone of our solar system, beyond the inner rocky planets and outer gas giants (video credit: NASA, published in 12 June 2015)
• Jupiter Encounter in 2007
The voyage of NASA's Pluto-bound New Horizons spacecraft through the Jupiter system provided a bird's-eye view of a dynamic planet that had changed since previous close-up looks by NASA spacecraft. 45) 46)
New Horizons passed Jupiter on 28 February 2007, riding the planet's gravity to boost its speed and shave three years off its trip to Pluto. It was the eighth spacecraft to visit Jupiter – but a combination of trajectory, timing and technology allowed it to explore details no probe had seen before, such as lightning near the planet's poles, the life cycle of fresh ammonia clouds, boulder-size clumps speeding through the planet's faint rings, the structure inside volcanic eruptions on its moon Io, and the path of charged particles traversing the previously unexplored length of the planet's long magnetic tail.
"The Jupiter encounter was successful beyond our wildest dreams," says New Horizons Principal Investigator Alan Stern. "Not only did it prove out our spacecraft and put it on course to reach Pluto in 2015, it was a chance for us to take sophisticated instruments to places in the Jovian system where other spacecraft couldn't go, and to return important data that adds tremendously to our understanding of the solar system's largest planet and its moons, rings and atmosphere."
From January through June 2007, New Horizons' seven science instruments made more than 700 separate observations of the Jovian system – twice the activity planned at Pluto – with most of them coming in the eight days around closest approach to Jupiter. "We carefully selected observations that complemented previous missions, so that we could focus on outstanding scientific issues that needed further investigation," says New Horizons Jupiter Science Team Leader Jeff Moore, of NASA Ames Research Center, Moffett Field, Calif. "The Jupiter system is constantly changing and New Horizons was in the right place at the right time to see some exciting developments."
Figure 56: Jupiter encounter of New Horizons (image credit: NASA)
Jovian weather was high on the list, as New Horizons' visible light, infrared and ultraviolet remote-sensing instruments probed Jupiter's atmosphere for data on cloud structure and composition. They saw clouds form from ammonia welling up from the lower atmosphere and heat-induced lightning strikes in the polar regions – the first polar lightning ever observed beyond Earth, demonstrating that heat moves through water clouds at virtually all latitudes across Jupiter. They made the most detailed size and speed measurements yet of "waves" that run the width of planet and indicate violent storm activity below. Additionally, New Horizons snapped the first close-up images of the Little Red Spot, a nascent storm about half the size of Jupiter's larger Great Red Spot and about 70 percent of Earth's diameter, gathering new information on storm dynamics.
Under a range of lightning and viewing angles, New Horizons also captured the clearest images ever of the tenuous Jovian ring system. In them, scientists spotted clumps of debris that may indicate a recent impact inside the rings, or some more exotic phenomenon; movies made from New Horizons images also offer an unprecedented look at ring dynamics, with the tiny inner moons Metis and Adrastea shepherding the materials around the rings. A search for smaller moons inside the rings – and possible new sources of the dusty material – found no bodies wider than a kilometer.
The mission's investigations of Jupiter's four largest moons focused on Io, the closest to Jupiter and whose active volcanoes blast tons of material into the Jovian magnetosphere (and beyond). New Horizons spied 11 different volcanic plumes of varying size, three of which were seen for the first time and one – a spectacular 200-mile-high eruption rising above the volcano Tvashtar – that offered an unprecedented opportunity to trace the structure and motion of the plume as it condensed at high altitude and fell back to the moon's surface. In addition, New Horizons spotted the infrared glow from at least 36 Io volcanoes, and measured lava temperatures up to 1,900 degrees Fahrenheit (1040ºC), similar to many terrestrial volcanoes.
New Horizons' global map of Io's surface backs the moon's status as the solar system's most active body, showing more than 20 geological changes since the Galileo Jupiter orbiter provided the last close-up look in 2001. The remote imagers also kept watch on Io in the darkness of Jupiter's shadow, noting mysterious glowing gas clouds above dozens of volcanoes. Scientists suspect that this gas helps to resupply Io's atmosphere.
New Horizons' flight down Jupiter's magnetotail gave it an unprecedented look at the vast region dominated by the planet's strong magnetic field. Looking specifically at the fluxes of charged particles that flow hundreds of millions of miles beyond the giant planet, the New Horizons particle detectors saw evidence that tons of material from Io's volcanoes move down the tail in large, dense, slow-moving blobs. By analyzing the observed variations in particle fluxes over a wide range of energies and scales, New Horizons scientists are exploring how the volcanic gases from Io are ionized, trapped and energized by Jupiter's magnetic field, then ultimately ejected from the system.
Figure 57: Jupiter flyby science of New Horizons (image credit: NASA)
Figure 58: And surprises in Jupiter's magnetotail (image credit: NASA)
Figure 59: Other observations of New Horizons at Jupiter (image credit: NASA)
Figure 60: This is a montage of New Horizons images of Jupiter and its volcanic moon Io, taken during the spacecraft's Jupiter flyby in early 2007. The Jupiter image is an infrared color composite taken by the spacecraft's near-infrared imaging spectrometer, the Linear Etalon Imaging Spectral Array (LEISA) at 1:40 UTC on 28 February, 2007 (image credit: NASA, JHU/APL)
Legend to Figure 60: The infrared wavelengths used (red: 1.59 µm, green: 1.94 µm, blue: 1.85 µm) highlight variations in the altitude of the Jovian cloud tops, with blue denoting high-altitude clouds and hazes, and red indicating deeper clouds. The prominent bluish-white oval is the Great Red Spot. The observation was made at a solar phase angle of 75 degrees but has been projected onto a crescent to remove distortion caused by Jupiter's rotation during the scan. The Io image, taken at 00:25 UT on 1 March 2007, is an approximately true-color composite taken by the panchromatic Long-Range Reconnaissance Imager (LORRI), with color information provided by the 0.5 µm ("blue") and 0.9 µm ("methane") channels of the Multispectral Visible Imaging Camera (MVIC). The image shows a major eruption in progress on Io's night side, at the northern volcano Tvashtar. Incandescent lava glows red beneath a 330-kilometer high volcanic plume, whose uppermost portions are illuminated by sunlight. The plume appears blue due to scattering of light by small particles in the plume.
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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 (firstname.lastname@example.org).