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
• 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. 5)
- "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 16: 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 17: 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. 6)
- "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 18: 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. 7)
• 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. 8)
- 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 19: 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) 9)
Figure 20: 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 21: 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. 10)
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 22: 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 23: 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. 11)
• 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. 12)
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 24: 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. 13)
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 25: 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 26: 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. 14)
"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 27: 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 28: 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. 17).
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 29: 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 (image credit: NASA)
• New Horizons Captures Two of Pluto's Smaller Moons (21 July 2015)
Figure 30: 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) 15)
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. 16)
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 31). 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 31: 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 32: 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. 17) 18)
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 33: 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 34: Jupiter flyby science of New Horizons (image credit: NASA)
Figure 35: And surprises in Jupiter's magnetotail (image credit: NASA)
Figure 36: Other observations of New Horizons at Jupiter (image credit: NASA)
Figure 37: 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 37: 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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14) Bill Keeler, "One Year Later: New Horizons' Top 10 Discoveries at Pluto," NASA, 14 July 2016, updated 7 August 2017, URL: https://www.nasa.gov/feature/one-year-later-new-horizons-top-10-discoveries-at-pluto
15) Tricia Talbert, "New Horizons Captures Two of Pluto's Smaller Moons," NASA, 21 July 2015, updated 7 August 2017, URL: https://www.nasa.gov/image-feature/new-horizons-captures-two-of-plutos-smaller-moons
16) Tricia Talbert,"New Horizons Spacecraft Displays Pluto's Big Heart," NASA, 14 July 2015, updated 7 August 2017, URL: https://www.nasa.gov/feature/new-horizons-spacecraft-displays-pluto-s-big-heart-0
17) "Jupiter Encounter — Pluto-Bound New Horizons Sees Changes in Jupiter System," JHU/APL, 2007, URL: http://pluto.jhuapl.edu/Mission/The-Path-to-Pluto-and-Beyond.php#Jupiter-Encounter
18) "Pluto-Bound New Horizons Sees Changes in Jupiter System," NASA, 9 October 2007, URL: https://www.nasa.gov/mission_pages/newhorizons/news/nh_jupiter_oct09.html
19) Joanne Baker, "Grand Tour," URL: http://science.sciencemag.org/content/318/5848/215
20) Press Briefing: New Horizons at Jupiter, American Astronomical Society/Division for Planetary Sciences Meeting, Orlando, Florida, 9 October 2007, URL: http://pluto.jhuapl.edu/News-Center/News-Article.php?page=100907_pressGraphics
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).