New Horizons : Spacecraft

New Horizons : Spacecraft



Spacecraft Systems and Components   Payload   Ground Stations   Data Collection

Systems and Components

A 3D model of NASA’s New Horizons, a mission to Pluto and the Kuiper Belt.
Click and drag to rotate
Source: NASA Visualization Technology Applications and Development (VTAD)

Designed and integrated at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland — with contributions from companies and institutions in the United States and abroad — the New Horizons spacecraft is a robust, lightweight observatory that has withstood 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 the Southwest Research Institute (SwRI), 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, piano-sized probe weighed 478 kilograms (1,054 pounds) at launch. Designed to operate on a limited power source — a single radioisotope thermoelectric generator (RTG) — New Horizons needs less power than a pair of 100-watt light bulbs to perform its mission.

On average, each of the seven science instruments uses between 2 and 10 watts — about the power of a night light — when turned on. The instruments send data to the two onboard solid-state memory banks, where data are recorded before later playback to Earth. During normal operations, the spacecraft communicates with Earth through its 2.1-meter (83-inch) wide high-gain antenna. Smaller antennas provide backup communications. When the spacecraft was in hibernation through long stretches of its voyage, its computer was programmed to monitor its systems and report its status back to Earth with a specially coded, low-energy beacon signal.

New Horizons’ “thermos bottle” design retains heat and keeps the spacecraft operating at room temperature without large heaters. Aside from protective covers on five instruments that were opened shortly after launch, and one small protective cover opened after the Jupiter encounter, New Horizons uses no deployable mechanisms or scanning platforms. It does have backup devices for all major electronics, its star-tracking navigation cameras and data recorders.

New Horizons has operated mostly in a spin-stabilized mode while cruising between planets, and also in a three-axis “pointing” mode that allows for pointing or scanning instruments during calibrations and planetary encounters (like the Jupiter flyby and, of course, at Pluto). 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 3 billion miles from Earth and light itself takes more than four and a half hours to go from Earth to the spacecraft.


New Horizons’ primary structure includes an aluminum central cylinder that supports the spacecraft body panels, supports the interface between the spacecraft and its RTG power source, and houses the propellant tank. It also served as the payload adapter fitting that connected the spacecraft to the launch vehicle.

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 was 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-megahertz 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.

For data storage, New Horizons carries two low-power solid-state recorders (one backup) that can hold up to 8 gigabytes each. The main processor collects, compresses, reformats, sorts and stores science and housekeeping (telemetry) 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 system is housed in an Integrated Electronics Module that also contains a vital guidance computer, the communication system and part of the REX instrument.

Thermal Control

New Horizons is designed to retain heat like a thermos bottle. The spacecraft is covered in lightweight, gold-colored, multilayered thermal insulation – like a survival camping blanket – which holds in heat from operating electronics to keep the spacecraft warm. Heat from the electronics has kept the spacecraft operating at between 10-30 degrees Celsius (about 50-85 degrees Fahrenheit) 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 watts) and it will activate small heaters around the craft to make up the difference. When the spacecraft was closer to Earth and the Sun, louvers (essentially heat vents) on the craft opened when internal temperatures were 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.


The propulsion system on New Horizons is used for course corrections and for pointing the spacecraft. It was not needed to speed the spacecraft along its trajectory to Arrokoth and beyond; that was done by the launch vehicle, with a boost from Jupiter’s gravity. But it does make small corrections to the flight path and tiny changes to the speed to ensure that New Horizons arrives when and where it can make the best observations. For example, after the Pluto encounter, it changed the trajectory very slightly to go to Arrokoth.

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 of these thrusters, each of which provides 4.4 Newtons (1 pound) of force are used for course corrections. Operators also employ 12 smaller thrusters – each providing 0.8 Newtons (about 3 ounces) 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 carried 77 kilograms (170 pounds) of hydrazine, stored in a lightweight titanium tank. Helium gas pushes fuel through the system to the thrusters. The Jupiter gravity assist, and the mission design calling for flybys at Pluto and Arrokoth (instead of entries into orbit), reduced the amount of propellant needed for the mission.

Guidance and Control

New Horizons must be oriented precisely 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 (IMUs) (containing sophisticated gyroscopes and accelerometers that measure rotation and horizontal/vertical motion), and digital Sun 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 of 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 measures spacecraft angular rates 100 times a second. If data shows New Horizons is outside a predetermined position, small hydrazine thrusters will fire to reorient 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 are used for fine pointing; thrusters that are approximately five times more powerful are used during the trajectory course maneuvers that guide New Horizons toward its targets. New Horizons spins – typically at 5 revolutions per minute (RPM) – during trajectory-correction maneuvers and long radio contacts with Earth, and while it “hibernates” during long cruise periods. Operators steady and point the spacecraft during science observations and instrument-system checkouts.


Accurate Ranging:

New Horizons is the first mission to use onboard regenerative ranging to track the distance between the spacecraft and Earth. When a spacecraft is far from home, the ranging tone sent from the ground to measure distance is weak (or “noisy”) by the time it reaches the spacecraft’s communications system. In normal ranging, the spacecraft simply amplifies and sends the noisy tone back to Earth, which adds errors to the range measurement. In regenerative ranging, the spacecraft’s advanced electronics track and “regenerate” the tone without the noise. The ground station on Earth receives a much clearer signal – giving navigators and operators a more accurate lock on the spacecraft’s distance, and improving their ability to guide New Horizons through the solar system.

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, which were used early in the mission for near-Earth communications; as well as a 30-centimeter (12-inch) diameter medium-gain dish antenna and a large, 2.1-meter (83-inch) 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 wider medium-gain beam (4 degrees) is used in conditions when the pointing might not be as accurate. All antennas have Right Hand Circular and Left Hand Circular polarization feeds.

Data rates 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 has used its high-gain antenna to exchange data with the Deep Space Network’s largest antennas, 70 meters across. Even at Arrokoth, because New Horizons was be more than 4 billion miles from Earth and radio signals took more than six hours to reach the spacecraft, it was able to send information at about 1,000 bits per second.

New Horizons is flying 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 earlier deep-space receivers. The Radio Science Experiment (REX) that examined 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.


New Horizons’ electrical power comes from a single radioisotope thermoelectric generator (RTG). The RTG provides power through the natural radioactive decay of plutonium dioxide fuel, which creates a huge amount of heat. Unlike a normal reactor, the Plutonium-238 used in the RTG cannot undergo a chain reaction. But, its normal decay rate is high enough (that is, its half-life is short enough) that it always releases heat at a high rate. That heat is converted directly to electrical power by thermocouples.

The New Horizons RTG, provided by the U.S. Department of Energy, carries approximately 11 kilograms (24 pounds) of plutonium dioxide. Onboard systems manage the spacecraft’s power consumption so it doesn’t exceed the steady output from the RTG, which has decreased by about 3.5 watts per year since launch.

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 supplied approximately 245 watts (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 January 2019 (when New Horizons flew past Arrokoth) that supply decreased to about 190 watts at the same voltage, so New Horizons eased the strain on its limited power source by cycling science instruments during the encounter.

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.

A description of the RTG power system on New Horizons mission and the final EIS can be found below:

Nuclear Safety Issues and Answers »

Read about the radioisotope power systems that enabled some of NASA’s greatest voyages »

Flying with an RTG

How hot is the RTG? Sensors attached to the outside of the RTG case before launch pegged the case temperature at about 245 C (nearly 475 F). When New Horizons reaches Pluto, engineers estimate the temperature will have dropped to around 208 C (406 F) – thanks to a combination of distance from the Sun and fuel decay.

The RTG is not hot enough to produce visible light, but it does emit infrared (or thermal) radiation.

At launch the fuel produced almost 4,000 watts of thermal power; of that, New Horizons used about 25 watts of the waste heat to warm the spacecraft. Electrical power output of the RTG was about 245 watts. Some of that electrical power (about 120 watts) is also reused after powering components to help heat the spacecraft. The rest of the RTG heat and any extra electrical power are radiated into space.

Many scientific instruments work better when they are cold; that’s one reason why they are located on the opposite side of the spacecraft from the RTG. New Horizons also has a heat shield around the base of the RTG to avoid a direct line of sight from the instruments to the RTG. These design features help avoid any interference from the RTG with scientific measurements.

Nuclear Safety Issues and Answers


The Final Environmental Impact Statement for the mission was released to the public on Aug. 5, 2005. NASA issued its National Environmental Policy Act (NEPA) record of decision for New Horizons on Sept. 7, 2005 – deciding to complete preparations for launch in January-February 2006, and to operate the mission. The White House Office of Science Technology Policy gave approval for the launch to proceed.

An appendix in the Final EIS includes public comments on the Draft Environmental Impact Statement, gathered during a 45-day period from February-April 2005, and during two community meetings in Cocoa, Florida, in March 2005.

Final Environmental Impact Statement

Disclaimer: While the National Aeronautics and Space Administration (NASA) has taken reasonable and prudent measures to ensure the accuracy, integrity and security of the electronic version of the Final Environmental Impact Statement for the New Horizons mission, NASA cannot guarantee that the electronic version is identical to the printed version, nor that it cannot be tampered with by a third party.

Download the document (PDF) in two sections:

The Draft Environmental Impact Statement is also available here.

Public Involvement


On Feb. 25, 2005, the U.S. Environmental Protection Agency’s notice of availability for the New Horizons Draft Environmental Impact Statement was published in the Federal Register. Interested parties had 45 days to submit written or electronic comments on environmental concerns on the Draft Environmental Impact Statement; these comments had to be postmarked on or before April 11, 2005.

The Final Environmental Impact Statement was released to the public in August 2005; an appendix in the Final EIS includes public comments on the Draft EIS and NASA’s responses. NASA issued its National Environmental Policy Act (NEPA) record of decision for New Horizons on Sept. 7, 2005 — deciding to complete preparations for launch in January-February 2006, and to operate the mission. The White House Office of Science and Technology Policy approved the launch in January 2006.

Public Meetings

On March 29-30, 2005, NASA hosted meetings during which the public was invited to participate in an open exchange of information and to submit comments on the New Horizons Draft Environmental Impact Statement. Each public meeting began with an opportunity for informal discussions with project personnel, followed by a brief NASA presentation on the New Horizons mission, and concluded with an invitation to attendees to ask questions or submit formal comments.

The meetings were held at the Florida Solar Energy Center in Cocoa, Florida. Click on a presenter’s name to see an individual presentation, or click here for the full presentation.

  • Kurt Lindstrom, program executive, NASA Headquarters: mission overview
  • Hal Weaver, project scientist, Johns Hopkins Applied Physics Laboratory: mission science
  • Glen Fountain, project manager, Johns Hopkins Applied Physics Laboratory: mission design
  • Kurt Lindstrom, program executive: Draft EIS overview
  • Kenneth Kumor, NASA Headquarters: National Environmental Policy Act (NEPA) process

NASA included comments received during the meeting – and during the public comment period – in the Final Environmental Impact Statement.


Other Resources


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 moons.

The same payload was used to explore Arrokoth, the most distant object ever targeted for a flyby.

The payload is incredibly power efficient – with the instruments collectively drawing less than 28 watts – and represents a degree of miniaturization that is unprecedented in planetary exploration. The instruments were designed specifically to handle the cold conditions and low light levels in the Kuiper Belt.


Mass: 4.5 kilograms (9.9 pounds)
Average Power: 4.4 watts
Development: Southwest Research Institute
Principal Investigator: Alan Stern, Southwest Research Institute
Purpose: Study atmospheric composition and structure

Alice is a sensitive ultraviolet imaging spectrometer designed to probe the composition and structure of Pluto’s dynamic atmosphere. Where 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. Alice’s spectroscopic range extends across both extreme and far-ultraviolet wavelengths from approximately 500 to 1,800 Angstroms. The instrument detected a variety of important chemicals in Pluto’s atmosphere, and determined their relative abundances, giving scientists the first complete picture of Pluto’s atmospheric composition. Alice set tight upper limits on the maximum density of any ionosphere around Pluto and on that of any atmosphere around Pluto’s largest moon, Charon. It also searched for (but didn’t find) an atmosphere or exosphere around Arrokoth.

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 the atmosphere at Pluto showed up as characteristic “dips” and “edges” in the ultraviolet part of the spectrum of light that Alice measured. This technique is a powerful method for measured even traces of atmospheric gas.

A first-generation version of New Horizons’ Alice (smaller and a bit less sophisticated) flew aboard the European Space Agency’s Rosetta spacecraft, used to explore the escaping atmosphere and complex surface of a comet. Later versions are flying or scheduled to fly aboard LRO, JUNO, JUICE and Europa Clipper.


Mass: 10.3 kilograms (22.7 pounds)
Average Power: 6.3 watts
Development: Ball Aerospace Corporation, NASA Goddard Space Flight Center, Southwest Research Institute
Principal Investigator: Cathy Olkin, Southwest Research Institute
Purpose: Study surface geology and morphology; obtain surface composition and surface temperature maps

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 the Alice ultraviolet spectrometer 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 Multispectral Visible Imaging Camera (MVIC), as well as an infrared compositional mapping spectrometer called the Linear Etalon Imaging Spectral Array (LEISA). LEISA is an advanced, miniaturized short-wavelength infrared (1.25-2.50 micron) spectrometer provided by scientists from NASA’s Goddard Space Flight Center. MVIC operates over the bandpass from 0.4 to 0.95 microns. Ralph’s suite of eight detectors – seven charge-coupled devices (CCDs) like 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 an appliance light bulb.

Ralph took images at increasing frequency as New Horizons approached, flew past, and then looked back at the Pluto system. The MVIC images helped scientists to map landforms in black-and-white and color with a best resolution of about 250 meters (820 feet) per pixel, create stereo images to determine surface topography, and refine the radii and orbits of Pluto and its moons. With the LORRI instrument, it found clouds and hazes in Pluto’s atmosphere, and searched for rings and additional satellites around Pluto. It also obtained images of Pluto’s night side, illuminated by “Charon-light.” At the same time, LEISA mapped 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.

It also let scientists map surface temperatures across Pluto and Charon by sensing the spectral features of frozen nitrogen, water and carbon monoxide.

Radio Science Experiment (REX)

Mass: 100 grams (3.5 ounces)
Average Power: 2.1 watts
Development: Johns Hopkins University Applied Physics Laboratory, Stanford University
Principal Investigators: Len Tyler and Ivan Linscott, Stanford University
Purpose: Measure atmospheric temperature and pressure (down to the surface); measure density of the ionosphere; search for atmospheres around Charon and other KBOs

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 used an occultation technique to probe Pluto’s atmosphere and to search for an atmosphere around Charon. After New Horizons flew by Pluto, its 2.1-meter (83-inch) dish antenna pointed back at Earth. On Earth, powerful transmitters in NASA’s largest Deep Space Network antennas beamed radio signals to the spacecraft as it passed behind Pluto. The radio waves bent according to the average molecular weight of gas in the atmosphere and the atmospheric temperature.

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 was the first to use a signal from Earth – the spacecraft was so far from home and moving so quickly past Pluto and Charon that only a large, ground-based antenna could provide a strong enough signal. This new technique, called an “uplink” radio experiment, is an important advance beyond previous outer planet missions.

Scientists use REX 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 measured the masses of Pluto and Charon. By timing the length of the radio occultations of Pluto and Charon, REX also yielded improved radii measurements for each body.

Long Range Reconnaissance Imager (LORRI)

Mass: 8.8 kilograms (19.4 pounds)
Average Power: 5.8 watts
Development: Johns Hopkins University Applied Physics Laboratory
Principal Investigator: Hal Weaver, Applied Physics Laboratory
Purpose: Study geology; provide high-resolution approach and highest-resolution encounter images

LORRI, the “eagle eyes” of New Horizons, is a panchromatic high-magnification imager, consisting of a telescope with an 8.2-inch (20.8-centimeter) aperture that focuses visible light onto a charge-coupled device (CCD). It’s essentially a digital camera with a large telephoto telescope – only fortified to operate in the cold, hostile environs near Pluto and beyond.

During the encounter, LORRI images were New Horizons’ first of the Pluto system, starting about 180 days before closest approach. Pluto and its moons still resembled little more than bright dots, but these system-wide views helped navigators keep the spacecraft on course and helped scientists refine their orbit calculations of Pluto and its moons. Approximately 60 days before closest approach – around mid-May 2015 – LORRI images began to surpass Hubble-quality resolution, providing never-before-seen details each day. At closest approach, LORRI imaged select sections of Pluto’s sunlit surface at football-field-size resolution, resolving features at about 50 meters across.

This range of images gave scientists an unprecedented look at the geology on Pluto and its moons– including the number and size of craters on each surface, revealing the history of impacting objects in that distant region. LORRI also yielded important information on the history of Pluto’s surface, searched for activity such as geysers on that surface, and looked for hazes in Pluto’s atmosphere. LORRI is also providing the highest resolution and sensitivity images of many Kuiper Belt Objects as New Horizons passes them in the extended mission.

LORRI has no color filters or moving parts – operators take images by pointing the LORRI side of the spacecraft directly at their target. The instrument’s innovative silicon carbide construction keeps its mirrors focused through the extreme temperature dips New Horizons has experienced on the way to and through the Kuiper Belt.

Solar Wind Around Pluto (SWAP)

Mass: 3.3 kilograms (7.3 pounds)
Average Power: 2.3 watts
Development: Southwest Research Institute
Principal Investigator: David McComas, Princeton University
Purpose: Study solar wind interactions and atmospheric escape

The SWAP instrument will measure interactions of Pluto with the solar wind – the stream of fast 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.

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 Earth’s upper atmosphere and ionosphere). Once they become electrically charged, the ions and electrons are “picked up” and carried away by the solar wind. In the process, these pickup 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 helped to determine the escape rate of atmospheric material from Pluto.

At the top of its energy range SWAP can detect some pickup ions (up to 6.5 kiloelectron volts, or keV). SWAP combines a retarding potential analyzer (RPA) with an electrostatic analyzer (ESA) 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.

Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI)

Mass: 1.5 kilograms (3.3 pounds)
Average Power: 2.5 watts
Development: Johns Hopkins University Applied Physics Laboratory
Principal Investigator: Ralph McNutt Jr., Applied Physics Laboratory
Purpose: Study the density, composition, and nature of energetic particles and plasmas resulting from the escape of Pluto’s atmosphere

PEPSSI, the most compact, lowest-power directional energetic particle spectrometer flown on a space mission, searched for neutral atoms that escape Pluto’s atmosphere and become charged by their interaction with the solar wind. It detected the material that escapes from Pluto’s atmosphere (such as molecular nitrogen, carbon monoxide and methane), which breaks up into ions and electrons after absorbing the Sun’s ultraviolet light, and streams away from Pluto as “pickup” ions carried by the solar wind.

By using PEPSSI data to count particles, and knowing how far New Horizons was 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.

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 kiloelectron volts (keV), many times more energetic than what SWAP can measure. Together the two instruments made a powerful combination for studying the Pluto system.

Venetia Burney Student Dust Counter (SDC)

Mass: 1.9 kilograms (4.2 pounds)
Average Power: 5 watts
Development: Laboratory for Atmospheric and Space Physics, University of Colorado at Boulder
Principal Investigator: Mihaly Horanyi, University of Colorado at Boulder
Purpose: Measure concentration of dust particles in outer solar system

Designed and built by students at the University of Colorado at Boulder, the SDC detects microscopic dust grains produced by collisions among asteroids, comets, and even 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 counts and measures the sizes of dust particles, producing information on the collision rates of such bodies in the outer solar system. At Pluto, SDC was also used to search for dust that might be generated by collisions of tiny “impactors” on Pluto’s small moons.

The instrument includes two major pieces: an 18-by-12-inch 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 operated beyond 18 astronomical units from the Sun (nearly 1.7 billion miles, about the distance from Uranus to the Sun), SDC data is giving scientists an unprecedented look at the sources and transport of dust in the solar system.

With faculty support, University of Colorado students have been distributing and archiving data from the instrument, and lead a comprehensive education and outreach effort to bring their results and experiences to classrooms of all grades.

In June 2006 the instrument was named for Venetia Burney, who at age 11 offered the name “Pluto” for the newly discovered ninth planet in 1930.  

Ground Stations

You need pretty large antennas to send data over billions of miles – and fortunately, NASA has them.

Image of Satellite Dish

The 70-meter diameter antenna at the Deep Space Network’s Goldstone Complex in the Mojave Desert, California.

The New Horizons mission operations team communicates with the spacecraft through NASA’s Deep Space Network (DSN) of antenna stations. The DSN consists of facilities in California’s Mojave Desert; near Madrid, Spain; and near Canberra, Australia. These stations are separated in longitude by about 120 degrees, assuring that any spacecraft can be observed without interruption as Earth rotates.

Visit the DSN website for more information.

Sending Commands to the Spacecraft

All commands sent to New Horizons must pass a rigorous development and review process to ensure the safety of the spacecraft. The mission operations team works closely with the instrument, science and spacecraft teams to develop the commands that perform New Horizons’ activities. After the command sequences are tested on a New Horizons simulator, the New Horizons Mission Operations Center at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland, sends them to the DSN, which is operated and managed by NASA’s Jet Propulsion Laboratory in Pasadena, California.

Look Who’s “Talking” to New Horizons

NASA’s Deep Space Network is our link to New Horizons, providing the capability to communicate with our explorer as it crosses the void of space nearly 4 billion miles from home. The Eyes on the Solar System Deep Space Network page shows which DSN stations are in contact with New Horizons or any other spacecraft, 24 hours a day. (The code for New Horizons is NHPC.)

Data Collection

New Horizons is on a one-way journey to the Kuiper Belt and beyond. Unlike missions that return to Earth, New Horizons sends back all of its data using a radio transmitter and its 83-inch (2.1-meter) diameter radio antenna. It receives commands over this link, and returns both science data and information on the spacecraft’s temperature and power.

Sending Commands to the Spacecraft

All commands sent to the New Horizons spacecraft must first pass a rigorous development and review process to ensure the safety of the spacecraft. The science team will work closely with the instrument mission operations and spacecraft teams to develop the commands that trigger New Horizons’ scientific activity. After the command sequences are tested on the ground, they will be sent by the New Horizons Mission Operations Center at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland, to NASA’s Deep Space Network (DSN), which is operated and managed by the Jet Propulsion Laboratory in Pasadena, California.

Spacecraft Pointing

Usually, New Horizons must be oriented in a particular direction to take data with its scientific instruments. For example, its various telescopes must be accurately pointed at a specific target (such as a location on the surface of Pluto). New Horizons has an advanced Guidance and Control (G&C) system for determining its orientation. An Inertial Measurement Unit (IMU), which is a sophisticated gyroscope, provides relatively coarse positional information and keeps the spacecraft stable. Star-tracker cameras employing charge-coupled devices (CCDs) image the sky, and the positions of the detected stars are used to accurately determine the orientation of the spacecraft. The star tracker feeds star-position information to the G&C computer, which compares the observed position to the commanded position. If the difference is outside some predetermined tolerance, small hydrazine thrusters will fire to re-orient the spacecraft to the desired position.

The thrusters provide the only mechanism for maneuvering the New Horizons spacecraft, and the amount of hydrazine thruster fuel is carefully watched to ensure that the mission’s scientific objectives are fulfilled. Besides the small thrusters that are used to fine-point the spacecraft, thrusters that are approximately five times more powerful are used during trajectory correction maneuvers (TCMs) that keep New Horizons on the proper path to its targets.

Science Instruments

New Horizons carries seven scientific instruments, which collect several types of data. (The instrument names and main functions are described in the science payload section) As an instrument makes an observation, data is transferred to a solid-state recorder (similar to a flash memory card for a digital camera), where they are compressed (if necessary), reformatted and transmitted to Earth through the spacecraft’s radio telecommunications system.

The Data Rate Challenge

A major challenge for the New Horizons mission is the relatively low “downlink” rate at which data can be transmitted to Earth, especially when you compare it to rates now common for high-speed Internet surfers.

During the Jupiter flyby in February 2007, New Horizons sent data home at about 38 kilobits per second (kbps), which is slightly slower than the transmission speed was for acoustic computer modems which operated over telephone lines. The average downlink rate after New Horizons passed Pluto (and sent the bulk of its encounter data back to Earth) was approximately 2,000 bits per second, a rate the spacecraft achieved by downlinking with both of its transmitters through NASA’s largest antennas. Even then, it took until late 2016 to bring down all the encounter data stored on the spacecraft’s recorders.

Since NASA’s Deep Space Network has to track other missions besides New Horizons, the team produced a lossy compressed browse data set that could be sent down more quickly. The browse dataset was downlinked before the end of 2015; the complete dataset came down after the browse dataset.

New Horizons Mission Operations Center

Data received on Earth through the Deep Space Network is sent to the New Horizons Mission Operations Center at APL, where data is “unpacked” and stored. The mission operations and instrument teams scour the engineering data for performance trend information, while science data is copied to the Science Operations Center at the Southwest Research Institute in Boulder, Colorado. At the Science Ops Center, data passes through “pipeline” software that converts the data from instrumental units to scientific units, based on calibration data obtained for each instrument. Both the raw and calibrated data files are formatted for New Horizons science team members to analyze. Both the raw and calibrated data, along with various ancillary files (such as documents describing the pipeline process or the science instruments) are archived at the Small Bodies Node of NASA’s Planetary Data System.

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