New Horizons

New Horizons Mission

Spacecraft   Launch    Sensor Complement    Mission Status    References 

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

• June 22, 2022: Southwest Research Institute (SwRI) scientists combined data from NASA’s New Horizons mission with cutting-edge laboratory experiments and exospheric modeling to reveal the likely composition of the red cap on Pluto’s moon Charon and how it may have formed. This first-ever description of Charon’s dynamic methane atmosphere using new experimental data provides a fascinating glimpse into the origins of this moon’s red spot as described in two recent papers. 5)

- Charon is the largest of Pluto’s moons. At half the size of Pluto, it is the largest known satellite relative to its parent body. Charon orbits Pluto every 6.4 Earth days. James Christy and Robert Harrington discovered Charon in 1978 at the U.S. Naval Observatory in Flagstaff, Arizona.

- “Prior to New Horizons, the best Hubble images of Pluto revealed only a fuzzy blob of reflected light,” said SwRI’s Randy Gladstone, a member of the New Horizons science team. “In addition to all the fascinating features discovered on Pluto’s surface, the flyby revealed an unusual feature on Charon, a surprising red cap centered on its north pole.”

- Soon after the 2015 encounter, New Horizons scientists proposed that a reddish “tholin-like” material at Charon’s pole could be synthesized by ultraviolet light breaking down methane molecules. These are captured after escaping from Pluto and then frozen onto the moon’s polar regions during their long winter nights. Tholins are sticky organic residues formed by chemical reactions powered by light, in this case the Lyman-alpha ultraviolet glow scattered by interplanetary hydrogen atoms.

- “Our findings indicate that drastic seasonal surges in Charon’s thin atmosphere, as well as light breaking down the condensing methane frost, are key to understanding the origins of Charon’s red polar zone,” said SwRI’s Dr. Ujjwal Raut, lead author of a paper titled “Charon’s Refractory Factory” in the journal Science Advances. “This is one of the most illustrative and stark examples of surface-atmospheric interactions so far observed at a planetary body.” 6)


Figure 16: Southwest Research Institute scientists combined data from NASA’s New Horizons mission with novel laboratory experiments and exospheric modeling to reveal the likely composition of the red cap on Pluto’s moon Charon and how it may have formed. New findings suggest drastic seasonal surges in Charon’s thin atmosphere combined with light breaking down the condensing methane frost may be key to understanding the origins of Charon’s red polar zones (image credit: Courtesy NASA / Johns Hopkins APL / SwRI)

- The team realistically replicated Charon surface conditions at SwRI's new Center for Laboratory Astrophysics and Space Science Experiments (CLASSE) to measure the composition and color of hydrocarbons produced on Charon's winter hemisphere as methane freezes beneath the Lyman-alpha glow. The team fed the measurements into a new atmospheric model of Charon to show methane breaking down into residue on Charon's north polar spot.

- "Our team's novel 'dynamic photolysis' experiments provided new limits on the contribution of interplanetary Lyman-alpha to the synthesis of Charon's red material," Raut said. "Our experiment condensed methane in an ultra-high vacuum chamber under exposure to Lyman-alpha photons to replicate with high fidelity the conditions at Charon's poles."

- SwRI scientists also developed a new computer simulation to model Charon's thin methane atmosphere.

- "The model points to 'explosive' seasonal pulsations in Charon's atmosphere due to extreme shifts in conditions over Pluto's long journey around the Sun," said Dr. Ben Teolis, lead author of a related paper titled "Extreme Exospheric Dynamics at Charon: Implications for the Red Spot" in Geophysical Research Letters. 7)

- The team input the results from SwRI's ultra-realistic experiments into the atmospheric model to estimate the distribution of complex hydrocarbons emerging from methane decomposition under the influence of ultraviolet light. The model has polar zones primarily generating ethane, a colorless material that does not contribute to a reddish color.

- ”We think ionizing radiation from the solar wind decomposes the Lyman-alpha-cooked polar frost to synthesize increasingly complex, redder materials responsible for the unique albedo on this enigmatic moon," Raut said. "Ethane is less volatile than methane and stays frozen to Charon's surface long after spring sunrise. Exposure to the solar wind may convert ethane into persistent reddish surface deposits contributing to Charon's red cap."

- The team is set to investigate the role of solar wind in the formation of the red pole," said SwRI's Dr. Josh Kammer, who secured continued support from NASA's New Frontier Data Analysis Program.

• December 17, 2021: New Horizons remains healthy and continues to send valuable data from deep in the Kuiper Belt – more than 5 billion miles away — even as it speeds farther and farther from the Earth and Sun. — As 2021 winds down, I want to recount what the New Horizons project has accomplished this year, and also look ahead to tell you about our plans for 2022. 8)

- During a busy and productive 2021, our science team published or submitted for publication no less than 49 research papers detailing discoveries about our flyby targets in the Pluto system and at the Kuiper Belt object (KBO) Arrokoth, other KBOs and dwarf planets, the outer heliosphere of the Sun, and even cosmology! Meanwhile, our mission operations and engineering teams have planned and executed literally dozens of new scientific observations, tested and uploaded new main-computer software to enhance spacecraft data-collection capabilities, and tested and uploaded software that enables new capabilities for our Solar Wind Around Pluto (SWAP) and Alice spectrometers. We’ve also sent another year’s worth of data and six separate “metaproduct” datasets to NASA’s Planetary Data System for use by anyone in the world, researcher or private citizen, and we’ve continued outreach and communications activities that inform the public about discoveries and other New Horizons news.

- In addition to all of that, we’ve continued ground based searches for new KBOs to study or fly by, and we’ve been hard at work on our proposal to NASA, due next month, to continue the New Horizons mission from 2023 through 2025.

- As we plan for 2022, here are a few of the activities we will be concentrating on in the new year:

a) Completing the mission extension proposal due in January.

b) Uploading the last set of planned instrument software upgrades, this time for our Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) charged-particle spectrometer.

c) Testing and then executing our first spacecraft hibernation period since 2018, which will save both fuel and budget.

d) Testing a new power-saving capability that will allow the spacecraft to achieve maximum data transmission rates by using both data transmitters for at least five more years, even as spacecraft power levels decline due to the half-life of the plutonium in our nuclear battery.

e) Searching for KBOs to study or fly by using new machine learning tools and a deep-search camera filter that together will more than quadruple the number of objects we expect to detect.

f) Sending back most of the Arrokoth flyby data still on the spacecraft’s digital recorders; some of that data remains on board due to higher-priority data transmission needs and some downtime activities at NASA’s Deep Space Network, which we use to communicate with New Horizons.

g) Continuing to analyze data to produce and publish scientific discoveries from the mission.

- Regarding our proposal to NASA to extend New Horizons funding and operations across 2023-2025, we have ambitious plans. If a new flyby target is found, we will concentrate on that flyby. But if no target is found, we will convert New Horizons into a highly-productive observatory conducting planetary science, astrophysics and heliospheric observations that no other spacecraft can — simply because New Horizons is the only spacecraft in the Kuiper Belt and the Sun’s outer heliosphere, and far enough away to perform some unique kinds of astrophysics. Those studies would range from unique new astronomical observations of Uranus, Neptune and dwarf planets, to searches for free floating black holes and the local interstellar medium, along with new observations of the faint optical and ultraviolet light of extragalactic space. All of this, of course, depends on NASA’s peer review evaluation of our proposal.

- And those are only a sampling of the things New Horizons will be doing. If our proposal is approved, I’ll give a more thorough accounting of these and other 2023-2025 plans later next year.

- For this update, I also want to mention to you that a number of Pluto system and Arrokoth surfaces features have received official names that our project team proposed. These include Pluto features honoring pioneering early 20th century aviatrix Bessie Coleman and pioneering late 20th century astronaut Sally Ride ( They also include the first surface feature named on Pluto’s moon Nix, and an official name, “Sky,” for Arrokoth’s largest crater.

- Finally, I’d like to give you a small sampling of the research that appeared in our 49 new scientific publications this year:

A) Results on the ages of Pluto’s surface feature ages from careful crater count chronologies, by Kelsi Singer and colleagues.

B) The first comprehensive, close-up look at the far side geology of Pluto’s largest moon, Charon, by Ross Beyer and colleagues.

C) A detailed compilation of everything known about the bright ring at the junction (or “neck”) between Arrokoth’s two lobes, by myself (Alan Stern) and a host of colleagues.

D) Results on the outer heliosphere’s all-important “pick up ions” that dominate the thermal pressure of this faraway region of space, by Dave McComas and colleagues.

• November 22, 2021: A new study led by Southwest Research Institute determined the brightness of the galactic Lyman-alpha background using a SwRI-developed instrument aboard NASA’s Kuiper Belt space probe, New Horizons. 9)

- The space Lyman-alpha ultraviolet background was first detected in the 1960s, and its existence was later confirmed in 1971. This ultraviolet glow permeates space and can be used to characterize the tenuous wind of hydrogen atoms which blows through our solar system. It has also been used by SwRI instruments on NASA spacecraft to image permanently dark craters near the north and south poles of the Moon.

- In most of our solar system, the background is dominated by Lyman-alpha photons emitted by the sun and scattered by interstellar hydrogen atoms that are passing through. In the outer solar system, however, where the New Horizons spacecraft travels, the scattered sunlight component of the Lyman-alpha signal is far less bright and the fainter components from the nearby regions of the Milky Way become easier to distinguish.

- “The galactic Lyman-alpha background comes from hot regions around massive stars which ionize all the matter near them, which is primarily hydrogen, as that is the most abundant element in the universe,” said Dr. Randy Gladstone, the study’s lead author. “When the electrons and protons eventually get back together, or recombine, they nearly always emit Lyman-alpha photons.” 10)

- Hydrogen atoms between the stars scatter these photons into a roughly uniform glow throughout space. They are detectable, Gladstone said, but only at the Lyman-alpha wavelength, which is at a wavelength about four times shorter than can be seen by human eyes.

- “The Lyman-alpha background has been studied a lot near the Earth’s orbit, and is bright enough that if we could see it, the night sky would never get darker than twilight,” Gladstone explained. “It’s so bright from solar Lyman-alpha that we weren’t certain how much the Milky Way galaxy contributed to its overall brightness. It’s like standing near a streetlamp on a foggy night. The fog scatters the lamp’s light, making it hard to see anything else.”


Figure 17: This false-color map shows several scans of the Lyman-alpha background over the sky, obtained by the Alice ultraviolet spectrograph on the New Horizons spacecraft when it was 45 AU from the Sun. The data agrees well with an underlying model of the solar component of the Lyman-alpha background to which a constant brightness from the Milky Way has been added. The background is brighter at both directions near our Sun, which is marked here by an orange dot (image credit: SwRI)

- With the SwRI-led Alice UV imaging spectrograph aboard New Horizons, Gladstone was able to accurately measure the brightness of the galactic component of the Lyman-alpha background for the first time.

- “New Horizons has been flying away from the Sun for more than 15 years now,” Gladstone explained. “The farther we moved away from the Sun, the less we were blinded by the solar component of the Lyman-alpha background.”

- With New Horizons now far beyond Pluto, Gladstone was able to measure the brightness of the Lyman-alpha background from the Milky Way for the first time: about 20 times less bright than the Lyman-alpha background is near Earth.

- “This has been something that’s been guessed at by astronomers for decades,” Gladstone said. “Now we have a much more precise number.”

- Gladstone hopes that this discovery will help astronomers better understand the nearby regions of the Milky Way galaxy.

- “The unique position of New Horizons in the far away Kuiper Belt allows it to make discoveries like this that no other spacecraft can,” said New Horizons principal investigator and SwRI space division associate vice president Dr. Alan Stern. “What a great resource New Horizons is, not just for the exploration of the Kuiper Belt, but also to understand more about our galaxy and even the universe beyond our galaxy through this and other observations by our scientific instrument payload.”

- The Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, designed, built and operates the New Horizons spacecraft, and manages the mission for NASA's Science Mission Directorate. The MSFC Planetary Management Office provides the NASA oversight for the New Horizons. Southwest Research Institute, based in San Antonio, directs the mission via Principal Investigator Stern, and leads the science team, payload operations and encounter science planning. New Horizons is part of the New Frontiers Program managed by NASA's Marshall Space Flight Center in Huntsville, Alabama.

• April 15, 2021: Now 50 times as far from the Sun as Earth, History-Making Pluto Explorer Photographs Voyager 1’s Location from the Kuiper Belt. 11)

- As New Horizons crossed the solar system, and its distance from Earth jumped from millions to billions of miles, that time between contacts grew from a few minutes to several hours. And on April 18 at 12:42 UTC (or April 17 at 8:42 p.m. EDT), New Horizons will reach a rare deep-space milepost — 50 astronomical units from the Sun, or 50 times farther from the Sun than Earth is.

- New Horizons is just the fifth spacecraft to reach this great distance, following the legendary Voyagers 1 and 2 and their predecessors, Pioneers 10 and 11. It’s almost 5 billion miles (7.5 billion km) away; a remote region where one of those radioed commands, even traveling at the speed of light, needs seven hours to reach the far flung spacecraft. Then add seven more hours before its control team on Earth finds out if the message was received.

- “It’s hard to imagine something so far away,” said Alice Bowman, the New Horizons mission operations manager at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland. “One thing that makes this distance tangible is how long it takes for us on Earth to confirm that the spacecraft received our instructions. This went from almost instantaneous to now being on the order of 14 hours. It makes the extreme distance real.”

- To mark the occasion, New Horizons recently photographed the star field where one of its long-distance cousins, Voyager 1, appears from New Horizons’ unique perch in the Kuiper Belt. Never before has a spacecraft in the Kuiper Belt photographed the location of an even more distant spacecraft, now in interstellar space. Although Voyager 1 is far too faint to be seen directly in the image, its location is known precisely due to NASA’s radio tracking.


Figure 18: Hello, Voyager! From the distant Kuiper Belt at the solar system’s frontier, on Christmas Day, Dec. 25, 2020, NASA’s New Horizons spacecraft pointed its Long Range Reconnaissance Imager in the direction of the Voyager 1 spacecraft, whose location is marked with the yellow circle. Voyager 1, the farthest human-made object and first spacecraft to actually leave the solar system, is more than 152 astronomical units (AU) from the Sun—about 14.1 billion miles or 22.9 billion km—and was 11.2 billion miles (18 billion km) from New Horizons when this image was taken. Voyager 1 itself is about 1 trillion times too faint to be visible in this image. Most of the objects in the image are stars, but several of them, with a fuzzy appearance, are distant galaxies. New Horizons reaches the 50 AU mark on April 18, 2021, and will join Voyagers 1 and 2 in interstellar space in the 2040s (image credits: NASA/Johns Hopkins APL/Southwest Research Institute)

- “That’s a hauntingly beautiful image to me,” said Alan Stern, New Horizons principal investigator from the Southwest Research Institute in Boulder, Colorado.

- “Looking back at the flight of New Horizons from Earth to 50 AU almost seems in some way like a dream,” he continued. “Flying a spacecraft across our entire solar system to explore Pluto and the Kuiper Belt had never been done before New Horizons. Most of us on the team have been a part of this mission since it was just an idea, and during that time our kids have grown up, and our parents, and we ourselves, have grown older. But most importantly, we made many scientific discoveries, inspired countless STEM careers, and even made a little history.”


Figure 19: Currently exploring the Kuiper Belt beyond Pluto, New Horizons is just one of five spacecraft to reach 50 astronomical units – 50 times the distance between the Sun and Earth – on its way out of the solar system and, eventually, into interstellar space (image credits: NASA/Johns Hopkins APL/Southwest Research Institute)

- New Horizons was practically designed to make history. Dispatched at 36,400 miles per hour (58,500 km/h) on Jan. 19, 2006, New Horizons was and is still the fastest human-made object ever launched from Earth. Its gravity-assist flyby of Jupiter in February 2007 not only shaved about three years from its voyage to Pluto, but allowed it to make the best views ever of Jupiter’s faint ring, and capture the first movie of a volcano erupting anywhere in the solar system except Earth.

- New Horizons successfully pulled off the first exploration of the Pluto system in July 2015, followed by the farthest flyby in history – and first close-up look at a Kuiper Belt object (KBO) — with its flight past Arrokoth on New Year’s day 2019. From its unique perch in the Kuiper Belt, New Horizons is making observations that can’t be made from anywhere else; even the stars look different from the spacecraft’s point of view.

- New Horizons team members use giant telescopes like the Japanese Subaru observatory to scan the skies for another potential (and long-shot) KBO flyby target, New Horizons itself remains healthy, collecting data on the solar wind and space environment in the Kuiper Belt, other Kuiper Belt objects, and distant planets like Uranus and Neptune. This summer, the mission team will transmit a software upgrade to boost New Horizons’ scientific capabilities. For future exploration, the spacecraft’s nuclear battery should provide enough power to keep New Horizons operating until the late-2030s.

• March 23, 2021: New Horizons remains healthy and continues to send valuable data from the Kuiper Belt, even as it speeds farther and farther from Earth and the Sun (Figure 20). 12)

- I'm going to focus this PI's Perspective on a major upcoming mission mile marker — namely, New Horizons being 50 astronomical units (AU) from the Sun next month. But first, some mission news.

- Our biggest news is that most of our latest flight software upgrades, which will provide new scientific capabilities on the spacecraft, are in final test and on track to be uplinked in July. In fact, one of those updates, for our solar wind instrument called SWAP, is already aboard the spacecraft — and being used to produce new science! That software, transmitted to New Horizons in mid-February and tested for a week at the end of February, allows us to see much finer structures in the solar wind as we plow toward the heliopause, the outer edge of the heliosphere that surrounds the solar system.

- We're also preparing another search for Kuiper Belt objects (KBOs) to study as we pass by them; those same summertime searches will also look for a new flyby target KBO, just as we did in our 2020 searches. Keep in mind, the search for Arrokoth (2014 MU69) took four years—and this search will go on for years, too, because it's a needle in a haystack challenge to find flyby KBOs! But this time, we're applying a new tool—artificial intelligence. Using machine-learning software, mission co-investigator JJ Kavelaars and collaborating scientist Wes Patrick have sped up and made those searches far more productive. In fact, when they reran the 2020 search data through their new software tools, it not only worked 100 times faster, but it turned up dozens of new KBOs that human searchers had not found in the search images! We'll be taking advantage of this important new tool again later this year, and next year and after that as well.

- And one last news item: We're wrapping up development on a flight plan and command load to study three KBOs in May, determining their surface properties, shapes, and more. These kinds of studies we're doing from within the Kuiper Belt can't be done from Earth or even orbiting telescopes, and New Horizons has now studied almost 30 KBOs this way. For some of those KBOs, we we're close enough to search for and find satellites around them at resolutions even the Hubble Space Telescope cannot match, providing an important new window into how KBOs formed. Additional KBO observations are planned in September and December.

Reaching Rare Space

- I mentioned earlier our big upcoming milestone: New Horizons will cross the amazing 50 AU distance marker on April 17 or 18, depending on your time zone here on Earth.). At 50 AU, we'll be 50 times as far from the Sun as Earth is! That's a milestone that only four operating spacecraft — Pioneers 10 and 11, and Voyagers 1 and 2 — have reached before us. That's so far away, in fact — almost 5 billion miles (7.5 billion kilometers) — that the Sun itself is smaller in the sky there than Jupiter is from Earth!

- Of course, the Pioneers (now out of power and derelict) and the Voyagers (both still operating) are much farther out than New Horizons. In fact, they are so much farther out that none of them are the nearest spacecraft to us. That spacecraft is Juno, orbiting Jupiter 10 times closer to the Sun than New Horizons is now! And note that in mid-April, we'll have a news release, with some very special images we've taken from our perch so far away in the Kuiper Belt, so keep an eye out for that!

- Looking back at the flight of New Horizons from Earth to 50 AU almost seems like a dream. Most of us on the flight team have been a part of it all the way, and during that time our kids have grown up, our parents (and we ourselves!) have grown older, and the first exploration of Pluto and the first KBO has been accomplished!

- Looking ahead, just like other NASA planetary missions in extended (post-prime mission) operations, every three years we have to propose a new mission and science plan to NASA. If we are approved, we are funded for the next three years; if not, the mission will be terminated. Our next proposal will be due in early 2022. If New Horizons continues to be funded, it'll fly on, exploring the outer Kuiper Belt and the Sun's outer heliosphere. That's something no other spacecraft can do: we're the only one in this region!

- We hope to continue proposing and performing science for many years. By the late 2030s, though, New Horizons may be too low on power to operate. That's because of the half-life of our plutonium power supply, which produces 3.3 watts less every year, and 33 watts less every decade. By the time it can't produce enough power to run the main spacecraft systems, New Horizons will be at or near 100 AU from the Sun—twice as far out as we are now. But even once the spacecraft is derelict—either because it runs out of power or fuel, or for any other reason—it will continue to coast outward, into the galaxy at a speed of nearly 3 AU (about 300 million miles) per year. In fact, even when the day comes in a few billion years that the Sun goes red giant and engulfs Earth, New Horizons, like the Pioneers and Voyagers, will still be out there, outliving even its home planet!

- While you ponder that sobering thought, I'll conclude this report. In the meantime, I hope you'll keep on exploring — just as we do!, Alan Stern.

• January 19, 2021: New Horizons is healthy and continues to send data back from the Kuiper Belt, even as it speeds farther and farther from the Earth and the Sun. The PI's perspective (Alan Stern) on the 15th anniversary of the launch. 13)

- But the mission's jam-packed plans for new Kuiper Belt exploration this year are not the subject of this PI Perspective. Instead, I want to concentrate on a very special anniversary, taking place today — our 15th anniversary of launch!

- That's right, New Horizons lifted into skies above the Florida coast —from the same Cape Canaveral pad used to launch NASA's storied Voyagers—at 2 pm Eastern Time on Thursday, Jan. 19, 2006. I witnessed the launch from the launch control center and gave my final "GO" as the mission principal investigator to proceed less than a minute before our Atlas V rocket ignited.

- In the 5,840 days since launch, New Horizons has blazed through 4.5 billion miles of space to reach its present perch in the Kuiper Belt, the third and most distantly explored region of our planetary system.

- But to me, it's not really about the miles. Nor is it about our speed—even if our spacecraft is still moving at well over 30,000 miles per hour, about 100 times faster than an average jetliner. It's not even about our gravity assist and flight-test flyby at Jupiter in 2007, or the first exploration of the Pluto system in 2015, or even the first exploration of a Kuiper Belt object in 2019 – though those historic achievements were our mission's natal objectives, each accomplished spectacularly and revolutionized humankind's knowledge of our solar system.

- To me, the biggest memes of the past 15 years have been a doubleheader of inspiring public engagement and intensive teamwork. On the public engagement front, New Horizons broke so many NASA records that I've lost count, but I look back proudly knowing that we touched lives and even awed a sometimes cynical world convinced that great things can't happen in our own time. On teamwork, the crew of New Horizons—the flight controllers, engineers, scientists, and others totaling about 200 belly buttons who guided the project and our spacecraft—showed that by working together we could accomplish something both larger than life and greater than any one of us could ever have hoped to do alone, no matter how talented or hard working an individual might be.

- The launch of New Horizons was itself the culmination of 17 years of struggle in the U.S. planetary science community to see NASA commit to and fund the exploration of Pluto and the Kuiper Belt—unfinished business left to my generation by the pioneers who explored all eight of the other classical planets known at the birth of the Space Age.


Figure 20: The flight of New Horizons [image credit: NASA/Johns Hopkins APL/Southwest Research Institute)]

- It took six separate and failed mission concepts before New Horizons was born. It took a Decadal Survey's endorsement. It took a David and Goliath proposal battle that the David in this story, the New Horizons team, won against tall odds. It took years of Washington politics, followed by the blur of a nearly record-setting spacecraft development and nuclear launch approval process during 2002-2005 to reach the launch pad in time for the single, three-week launch window in 2006 that was needed to reach Pluto by 2015. And, all told, it took the dedication and talents of nearly 2,500 men and women.

- And when we reached that launch day—when everything depended on perfection in the rocket and the spacecraft—it all worked! It worked so well, in fact, that it seemed almost too easy. It worked so well that just a few hours later, we burned the launch malfunction procedures in an evening beach bonfire to celebrate that we would never need them. It worked so well that we needed just half of the predicted fuel to refine New Horizons' course toward the Jupiter flyby and gravity assist, leaving a present to ourselves—bonus fuel for the exploration of the Kuiper Belt we are now carrying out!

- Now, so many miles and so many smiles later, I just want to say one thing to all the people who contributed to making New Horizons successful; to our funders, our mission team, our NASA colleagues, our launch team, the Deep Space Network that provides tracking and communications, our navigation team, our media and education teams, and all those, including all of you, who supported us from the outside: Thank you for making a dream come true, for making a nation and world proud of what humans can accomplish,. And thank you for your commitment to excellence and teamwork that still shines brightly today as New Horizons flies ever onward, exploring, setting records, seeking knowledge, and blazing a trail across our solar system that will ultimately lead it to the stars.


Figure 21: Widow of Pluto discoverer Clyde Tombaugh, Patsy Tombaugh (1912-2012), then 93, points toward Pluto on the afternoon of our launch, Jan. 19, 2006 (photo credit: Michael Soluri)

• January 12, 2021: How dark is the sky, and what does that tell us about the number of galaxies in the visible universe? Astronomers can estimate the total number of galaxies by counting everything visible in a Hubble deep field and then multiplying them by the total area of the sky. But other galaxies are too faint and distant to directly detect. Yet while we can’t count them, their light suffuses space with a feeble glow. 14)

- To measure that glow, astronomical satellites have to escape the inner solar system and its light pollution, caused by sunlight reflecting off dust. A team of scientists has used observations by NASA’s New Horizons mission to Pluto and the Kuiper Belt to determine the brightness of this cosmic optical background. Their result sets an upper limit to the abundance of faint, unresolved galaxies, showing that they only number in the hundreds of billions, not 2 trillion galaxies as previously believed.

- How dark does space get? If you get away from city lights and look up, the sky between the stars appears very dark indeed. Above the Earth’s atmosphere outer space dims even further, fading to an inky pitch-black. And yet even there, space isn’t absolutely black. The universe has a suffused feeble glimmer from innumerable distant stars and galaxies.

- New measurements of that weak background glow show that the unseen galaxies are less plentiful than some theoretical studies suggested, numbering only in the hundreds of billions rather than the previously reported two trillion galaxies.

- “It’s an important number to know – how many galaxies are there?” said Marc Postman of the Space Telescope Science Institute in Baltimore, Maryland, a lead author on the study. “We simply don’t see the light from two trillion galaxies.”

- The earlier estimate was extrapolated from very deep sky observations by NASA’s Hubble Space Telescope. It relied on mathematical models to estimate how many galaxies were too small and faint for Hubble to see. That team concluded that 90% of the galaxies in the universe were beyond Hubble’s ability to detect in visible light. The new findings, which relied on measurements from NASA’s distant New Horizons mission, suggest a much more modest number.


Figure 22: This artist’s illustration shows NASA’s New Horizons spacecraft in the outer solar system. In the background lies the Sun and a glowing band representing zodiacal light, caused by sunlight reflecting off of dust. By traveling beyond the inner solar system and its accompanying light pollution, New Horizons was able to answer the question: How dark is space? At lower right are background stars of the Milky Way (image credit: STScI, Joe Olmsted)

- “Take all the galaxies Hubble can see, double that number, and that’s what we see – but nothing more,” said Tod Lauer of NSF’s NOIRLab (National Optical-Infrared Astronomy Research Laboratory), a lead author on the study.

- These results will be presented on Wednesday, Jan. 13th at a meeting of the American Astronomical Society, which is open to registered participants.

- The cosmic optical background that the team sought to measure is the visible-light equivalent of the more well-known cosmic microwave background – the weak afterglow of the big bang itself, before stars ever existed.

- “While the cosmic microwave background tells us about the first 450,000 years after the big bang, the cosmic optical background tells us something about the sum total of all the stars that have ever formed since then,” explained Postman. “It puts a constraint on the total number of galaxies that have been created, and where they might be in time.”

- As powerful as Hubble is, the team couldn’t use it to make these observations. Although located in space, Hubble orbits Earth and still suffers from light pollution. The inner solar system is filled with tiny dust particles from disintegrated asteroids and comets. Sunlight reflects off those particles, creating a glow called the zodiacal light that can be observed even by skywatchers on the ground.

- To escape the zodiacal light, the team had to use an observatory that has escaped the inner solar system. Fortunately the New Horizons spacecraft, which has delivered the closest ever images of Pluto and the Kuiper Belt object Arrokoth, is far enough to make these measurements. At its distance (more than 4 billion miles away when these observations were taken), New Horizons experiences an ambient sky 10 times darker than the darkest sky accessible to Hubble.

- “These kinds of measurements are exceedingly difficult. A lot of people have tried to do this for a long time,” said Lauer. “New Horizons provided us with a vantage point to measure the cosmic optical background better than anyone has been able to do it.”

- The team analyzed existing images from the New Horizons archives. To tease out the feeble background glow, they had to correct for a number of other factors. For example, they subtracted the light from the galaxies expected to exist that are too faint to be identifiable. The most challenging correction was removing light from Milky Way stars that was reflected off interstellar dust and into the camera.

- The remaining signal, though extremely faint, was still measurable. Postman compared it to living in a remote area far from city lights, lying in your bedroom at night with the curtains open. If a neighbor a mile down the road opened their refrigerator looking for a midnight snack, and the light from their refrigerator reflected off the bedroom walls, it would be as bright as the background New Horizons detected.

- So, what could be the source of this leftover glow? It’s possible that an abundance of dwarf galaxies in the relatively nearby universe lie just beyond detectability. Or the diffuse halos of stars that surround galaxies might be brighter than expected. There might be a population of rogue, intergalactic stars spread throughout the cosmos. Perhaps most intriguing, there may be many more faint, distant galaxies than theories suggest. This would mean that the smooth distribution of galaxy sizes measured to date rises steeply just beyond the faintest systems we can see – just as there are many more pebbles on a beach than rocks.

- NASA’s upcoming James Webb Space Telescope may be able to help solve the mystery. If faint, individual galaxies are the cause, then Webb ultra-deep field observations should be able to detect them. — This study is accepted for publication in the Astrophysical Journal. 15)

• October 25, 2020: Like Earth, the dwarf planet of Pluto has mountains. Like their earthly cousins, some of those peaks are covered in blankets of white. But the origins of these ice-like deposits are very different. 16)

- According to new research published on October 13, 2020, in Nature Communications, some of the mountains discovered on Pluto during the flyby of the New Horizons spacecraft in 2015 are covered by a blanket of methane ice. An international team of scientists analyzed data from Pluto’s atmosphere and surface and used numerical simulations of its climate to reveal that these ice caps are created through different processes than they are on Earth.

- The study authors noted that “within the dark equatorial region of Cthulhu, bright frost containing methane is observed coating crater rims and walls as well as mountain tops, providing spectacular resemblance to terrestrial snow-capped mountain chains.”


Figure 23: The image on the left was acquired on July 14, 2015, when the New Horizons spacecraft approached Pluto. The Long-Range Reconnaissance Imager (LORRI) detected the presence of patchy bright deposits atop the Pigafetta Montes and Elcano Montes mountain ranges. The spacecraft’s Multispectral Visible Imaging Camera (data not shown) revealed signatures of methane. — The right image is a natural-color view of a section of the Alps range in Europe and was acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite on March 19, 2020 (image credit: NASA Earth Observatory image by Lauren Dauphin, using MODIS data from NASA EOSDIS/LANCE and GIBS/Worldview. Pluto imagery courtesy of NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute. Story by Frank Tavares, NASA Ames, with Mike Carlowicz)


Figure 24: Researchers find that ice caps on the mountains of Pluto are made of methane, but develop through an opposite process from that on Earth (image credit: NASA Earth Observatory)


Figure 25: On Earth, atmospheric temperatures decrease with altitude, and that cool air chills land surfaces at high elevations. When a moist wind moves toward and over a mountain on Earth, its water vapor cools and condenses, forming clouds and snow, as seen on mountaintops like the Alps. — But on Pluto, the opposite occurs. The dwarf planet’s atmosphere actually gets warmer with altitude because methane gas absorbs solar radiation. However, the atmosphere is too thin to affect surface temperatures, which remain constant with altitude. And unlike the way winds tend to ride up over mountains on Earth, the winds on Pluto mostly travel downslope (image credit: NASA Earth Observatory)

- “It is particularly remarkable to see that two very similar landscapes on Earth and Pluto can be created by two very dissimilar processes,” said Tanguy Bertrand, a postdoctoral researcher at NASA’s Ames Research Center and lead author on the paper. “Though theoretically objects like Neptune’s moon Triton could have a similar process, no other place in our solar system has ice-capped mountains like this besides Earth.”

- To understand how similar landscapes develop from different conditions and chemistries, the researchers at the Laboratoire de Météorologie Dynamique (France) developed a three-dimensional model simulating the atmosphere and surface of Pluto. They found that the dwarf planet’s atmosphere has more gaseous methane at its warmer, higher altitudes. That gas can saturate, condense, and then freeze directly on mountain peaks without any clouds forming. At lower altitudes on Pluto, there is no methane frost because there is too little methane for condensation to occur.

- This condensation process not only creates methane ice caps on Pluto’s mountains, but also similar features on its crater rims as well. The mysterious bladed terrain found in the Tartarus Dorsa region around Pluto's equator can also be explained by this cycle.

- “Pluto really is one of the best natural laboratories we have to explore the physical and dynamic processes involved when compounds that regularly transition between solid and gas states interact with a planetary surface,” said Bertrand. “The New Horizons flyby revealed astonishing glacial landscapes we continue to learn from.”

• July 15, 2020: Collaborative observations with NASA's New Horizons mission have been ongoing at the Subaru Telescope since May 2020. Hyper Suprime-Cam (HSC), the wide field camera mounted on the prime focus of the Subaru Telescope, is used for the observations to search for target candidates for New Horizons' next observations. Astronomers from Japan are participating in the observation team together with ones from the New Horizons mission. - Note: The Subaru Telescope of NAOJ (National Astronomical Observatory of Japan) is a very large optical infrared telescope (aperture of 8.2 m) installed near the summit of Maunakea, Hawai‘i, USA. 17)

- New Horizons is going to continue its exploration in the outer Solar System. The Subaru Telescope was selected as one of the facilities to search for target candidates for New Horizons' next observations. Dr. Alan Stern of Southwest Research Institute, the Principal Investigator of the New Horizons mission, emphasizes the importance of the observations using the Subaru Telescope, saying "We are using the Subaru Telescope because it is the best in the world for our search purposes. This is due to its unique combination of telescope size—one of the very largest anywhere, and Hyper Suprime-Cam's wide field of view—which can discover many Kuiper Belt objects at once."

- The Subaru Telescope is observing an area equivalent in size to 18 full moons (which can be covered in 2 pointings by HSC) in the constellation of Sagittarius, where New Horizons is now cruising. From the observations with the Subaru Telescope, the team expects to discover 100s of new Kuiper Belt objects, of which about 50 should be observable at a distance with the New Horizons spacecraft. Observing with both the Subaru Telescope and New Horizons is important to discern the nature of mysterious objects in the outer Solar System.

- When a distant object in the outer Solar System is viewed from the Earth, it always appears as a "full moon" illuminated almost head-on by the sunlight. On the other hand, when New Horizons looks at an object orbiting near it, the object may appear as a "half moon" with the sunlight hitting it from the side. Observing an object at different phase angles will allow us to know the detailed characteristics of the surface.

- In addition, the team will investigate whether or not the newly-discovered Kuiper Belt objects include one that is suitable for a future flyby study by New Horizons, like Arrokoth was.

- The observation runs at the Subaru Telescope happened in May and June 2020, and another will follow in August. The team will search for new distant objects by comparing images taken at different times, and then determine their orbits precisely.

- Dr. Tsuyoshi Terai, a core member of the observation team and a Support Astronomer for the Subaru Telescope in charge of HSC, comments, "The search area is within the Milky Way, and thus there are many nearby stars including bright ones, which make the observations even more difficult. The observation team is doing its best to take high quality data by utilizing the unique capabilities of the Subaru Telescope, and to investigate the origin of the Solar System together with New Horizons."

• June 22, 2020: The accretion of new material during Pluto’s formation may have generated enough heat to create a liquid ocean that has persisted beneath an icy crust to the present day, despite the dwarf planet’s orbit far from the sun in the cold outer reaches of the solar system. 18)

- This “hot start” scenario, presented in a paper published June 22 in Nature Geoscience, contrasts with the traditional view of Pluto’s origins as a ball of frozen ice and rock in which radioactive decay could have eventually generated enough heat to melt the ice and form a subsurface ocean. 19)

- “For a long time people have thought about the thermal evolution of Pluto and the ability of an ocean to survive to the present day,” said coauthor Francis Nimmo, professor of Earth and planetary sciences at UC Santa Cruz. “Now that we have images of Pluto’s surface from NASA’s New Horizons mission, we can compare what we see with the predictions of different thermal evolution models.”

- Because water expands when it freezes and contracts when it melts, the hot-start and cold-start scenarios have different implications for the tectonics and resulting surface features of Pluto, explained first author and UCSC graduate student Carver Bierson.

- “If it started cold and the ice melted internally, Pluto would have contracted and we should see compression features on its surface, whereas if it started hot it should have expanded as the ocean froze and we should see extension features on the surface,” Bierson said. “We see lots of evidence of expansion, but we don’t see any evidence of compression, so the observations are more consistent with Pluto starting with a liquid ocean.”

- The thermal and tectonic evolution of a cold-start Pluto is actually a bit complicated, because after an initial period of gradual melting the subsurface ocean would begin to refreeze. So compression of the surface would occur early on, followed by more recent extension. With a hot start, extension would occur throughout Pluto’s history.

- “The oldest surface features on Pluto are harder to figure out, but it looks like there was both ancient and modern extension of the surface,” Nimmo said.


Figure 26: Extensional faults (arrows) on the surface of Pluto indicate expansion of the dwarf planet’s icy crust, attributed to freezing of a subsurface ocean (image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker)

- The next question was whether enough energy was available to give Pluto a hot start. The two main energy sources would be heat released by the decay of radioactive elements in the rock and gravitational energy released as new material bombarded the surface of the growing protoplanet.

- Bierson’s calculations showed that if all of the gravitational energy was retained as heat, it would inevitably create an initial liquid ocean. In practice, however, much of that energy would radiate away from the surface, especially if the accretion of new material occurred slowly.

- “How Pluto was put together in the first place matters a lot for its thermal evolution,” Nimmo said. “If it builds up too slowly, the hot material at the surface radiates energy into space, but if it builds up fast enough the heat gets trapped inside.”

- The researchers calculated that if Pluto formed over a period of less that 30,000 years, then it would have started out hot. If, instead, accretion took place over a few million years, a hot start would only be possible if large impactors buried their energy deep beneath the surface.

- The new findings imply that other large Kuiper belt objects probably also started out hot and could have had early oceans. These oceans could persist to the present day in the largest objects, such as the dwarf planets Eris and Makemake.

- “Even in this cold environment so far from the sun, all these worlds might have formed fast and hot, with liquid oceans,” Bierson said.

- In addition to Bierson and Nimmo, the paper was coauthored by Alan Stern at the Southwest Research Institute, the principal investigator of the New Horizons mission.

• June 10, 2020: For the first time, a spacecraft has sent back pictures of the sky from so far away that some stars appear to be in different positions than we'd see from Earth. 20)

- More than four billion miles from home and speeding toward interstellar space, NASA's New Horizons has traveled so far that it now has a unique view of the nearest stars. “It’s fair to say that New Horizons is looking at an alien sky, unlike what we see from Earth,” said Alan Stern, New Horizons principal investigator from Southwest Research Institute (SwRI) in Boulder, Colorado. “And that has allowed us to do something that had never been accomplished before — to see the nearest stars visibly displaced on the sky from the positions we see them on Earth.”

- On April 22-23, the spacecraft turned its long-range telescopic camera to a pair of the “closest” stars, Proxima Centauri and Wolf 359, showing just how they appear in different places than we see from Earth. Scientists have long used this “parallax effect” – how a star appears to shift against its background when seen from different locations — to measure distances to stars.

Figure 27: This two-frame animation of Proxima Centauri blinks back and forth between New Horizons and Earth images of each star, clearly illustrating the different view of the sky New Horizons has from its deep-space perch. The image was obtained on April 22 at 12:51 UT (8:51 a.m. ET) by a remotely operated 0.4-meter telescope at the Siding Spring node of the Las Cumbres Observatory in Australia. The timing accounts for New Horizons being nearly three light hours closer to Proxima Centauri than Earth when the images were taken (image credit: NASA, JHU/APL, SwRI)

- An easy way to see parallax is to place one finger at arm’s length and watch it jump back and forth when you view it successively with each eye. Similarly, as Earth makes it way around the Sun, the stars shift their positions. But because even the nearest stars are hundreds of thousands of times farther away than the diameter of Earth’s orbit, the parallax shifts are tiny, and can only be measured with precise instrumentation.

- “No human eye can detect these shifts,” Stern said.

- But when New Horizons images are paired with pictures of the same stars taken on the same dates by telescopes on Earth, the parallax shift is instantly visible. The combination yields a 3D view of the stars “floating” in front of their background star fields.

Figure 28: This two-frame animation of Wolf 359 blinks back and forth between New Horizons and Earth images of each star, clearly illustrating the different view of the sky New Horizons has from its deep-space perch. The image was obtained on April 23 at 04:37 UT (12:37 a.m. ET) with the University of Louisville 0.6-meter telescope located at Mt. Lemmon Observatory, near Tucson, Arizona, operated remotely by John F. Kielkopf (University of Louisville) and Karen A. Collins (Harvard and Smithsonian Center for Astrophysics). This is 37 minutes later than the New Horizons image, relative to Wolf 359 time. The timing accounts for New Horizons being nearly four light hours farther from Wolf 359 than Earth when the images were taken (image credit: NASA, JHU/APL, SwRI)

- “The New Horizons experiment provides the largest parallax baseline ever made — over 4 billion miles — and is the first demonstration of an easily observable stellar parallax,” said Tod Lauer, New Horizons science team member from the National Science Foundation's National Optical-Infrared Astronomy Research Laboratory who coordinated the parallax demonstration.

- "The New Horizons spacecraft is truly a mission of firsts, and this demonstration of stellar parallax is no different" said Kenneth Hansen, New Horizons program scientist at NASA Headquarters in Washington. "The New Horizons spacecraft continues to speed away from Earth toward interstellar space and is continuing to return exciting new data for planetary science."

Working in Stereo

- Lauer, New Horizons Deputy Project Scientist John Spencer, of SwRI, and science team collaborator, astrophysicist, Queen guitarist and stereo imaging enthusiast Brian May created the images that clearly show the effect of the vast distance between Earth and the two nearby stars.


Figure 29: Stereo for 3D Glasses: These anaglyph images can be viewed with red-blue stereo glasses to reveal the stars' distance from their backgrounds. On the left is Proxima Centauri and on the right is Wolf 359 (image credit: NASA, JHU/APL, SwRI)

- “It could be argued that in astro-stereoscopy — 3D images of astronomical objects – NASA’s New Horizons team already leads the field, having delivered astounding stereoscopic images of both Pluto and the remote Kuiper Belt object Arrokoth,” May said. “But the latest New Horizons stereoscopic experiment breaks all records. These photographs of Proxima Centauri and Wolf 359 – stars that are well-known to amateur astronomers and science fiction aficionados alike — employ the largest distance between viewpoints ever achieved in 180 years of stereoscopy!”

- The companion images of Proxima Centauri and Wolf 359 were provided by the Las Cumbres Observatory, operating a remote telescope at Siding Spring Observatory in Australia, and astronomers John Kielkopf, University of Louisville, and Karen Collins, Harvard and Smithsonian Center for Astrophysics, operating a remote telescope at Mt. Lemmon Observatory in Arizona.

- “The professional and amateur astronomy communities had been waiting to try this, and were very excited to make a little space exploration history,” said Lauer. “The images collected on Earth when New Horizons was observing Proxima Centauri and Wolf 359 really exceeded my expectations.”


Figure 30: Parallel Stereo of Proxima Centauri: Use a stereo viewer for these images; if you don’t have a viewer, change your focus from the image by looking "through" it (and the screen) and into the distance. This creates the effect of a third image in the middle, and try setting your focus on that third image. The New Horizons image is on the left (image credit: NASA, JHU/APL, SwRI)

An Interstellar Navigation First

- Throughout history, navigators have used measurements of the stars to establish their position on Earth. Interstellar navigators can do the same to establish their position in the galaxy, using a technique that New Horizons has demonstrated for the first time. While radio tracking by NASA’s Deep Space Network is far more accurate, its first use is a significant milestone in what may someday become human exploration of the galaxy.

- At the time of the observations, New Horizons was more than 4.3 billion miles (about 7 billion kilometers) from Earth, where a radio signal, traveling at the speed of light, needed just under 6 hours and 30 minutes to reach home.

- Launched in 2006, New Horizons is the first mission to Pluto and the Kuiper Belt. It explored Pluto and its moons in July 2015 — completing the space-age reconnaissance of the planets that started 50 years earlier — and continued on its unparalleled voyage of exploration with the close flyby of Kuiper Belt object Arrokoth in January 2019. New Horizons will eventually leave the solar system, joining the Voyagers and Pioneers on their paths to the stars.

- The Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, designed, built and operates the New Horizons spacecraft, and manages the mission for NASA's Science Mission Directorate. The MSFC Planetary Management Office provides the NASA oversight for the New Horizons. Southwest Research Institute, based in San Antonio, directs the mission via Principal Investigator Stern, and leads the science team, payload operations and encounter science planning. New Horizons is part of the New Frontiers Program managed by NASA's Marshall Space Flight Center in Huntsville, Alabama.

• February 13, 2020: Data from NASA’s New Horizons mission are providing new insights into how planets and planetesimals – the building blocks of the planets – were formed. 21)

- Using detailed data on the object’s shape, geology, color and composition – gathered during a record-setting flyby that occurred more than four billion miles from Earth – researchers have apparently answered a longstanding question about planetesimal origins, and therefore made a major advance in understanding how the planets themselves formed.

- The team reports those findings in a set of three papers in the journal Science, and at a media briefing Feb. 13 at the annual American Association for the Advancement of Science meeting in Seattle.

- “Arrokoth is the most distant, most primitive and most pristine object ever explored by spacecraft, so we knew it would have a unique story to tell,” said New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute in Boulder, Colorado. “It’s teaching us how planetesimals formed, and we believe the result marks a significant advance in understanding overall planetesimal and planet formation.”

- The first post-flyby images transmitted from New Horizons last year showed that Arrokoth had two connected lobes, a smooth surface and a uniform composition, indicating it was likely pristine and would provide decisive information on how bodies like it formed. These first results were published in Science last May.

- “This is truly an exciting find for what is already a very successful and history-making mission” said Lori Glaze, director of NASA's Planetary Science Division. “The continued discoveries of NASA’s New Horizons spacecraft astound as it reshapes our knowledge and understanding of how planetary bodies form in solar systems across the universe.”

- Over the following months, working with more and higher-resolution data as well as sophisticated computer simulations, the mission team assembled a picture of how Arrokoth must have formed. Their analysis indicates that the lobes of this “contact binary” object were once separate bodies that formed close together and at low velocity, orbited each other, and then gently merged to create the 22-mile long object New Horizons observed.

- This indicates Arrokoth formed during the gravity-driven collapse of a cloud of solid particles in the primordial solar nebula, rather than by the competing theory of planetesimal formation called hierarchical accretion. Unlike the high-speed collisions between planetesimals in hierarchical accretion, in particle-cloud collapse, particles merge gently, slowly growing larger.

- “Just as fossils tell us how species evolved on Earth, planetesimals tell us how planets formed in space,” said William McKinnon, a New Horizons co-investigator from Washington University in St. Louis, and lead author of an Arrokoth formation paper in Science this week. “Arrokoth looks the way it does not because it formed through violent collisions, but in more of an intricate dance, in which its component objects slowly orbited each other before coming together.”

- Two other important pieces of evidence support this conclusion. The uniform color and composition of Arrokoth’s surface shows the KBO formed from nearby material, as local cloud collapse models predict, rather than a mishmash of matter from more separated parts of the nebula, as hierarchical models might predict.

- The flattened shapes of each of Arrokoth’s lobes, as well as the remarkably close alignment of their poles and equators, also point to a more orderly merger from a collapse cloud. Further still, Arrokoth’s smooth, lightly cratered surface indicates its face has remained well preserved since the end of the planet formation era.