The New Horizons spacecraft
FROM MISSION PRESS KIT
Posted: January 8, 2006
Designed and integrated at the Johns Hopkins University Applied Physics Laboratory (APL) 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 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 weighs 478 kilograms (1,054 pounds). Designed to operate on a limited power source - a single radioisotope thermoelectric generator - New Horizons needs less power than a pair of 100-watt light bulbs to complete its mission at Pluto.
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 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-meter (83-inch) wide high-gain antenna. Smaller antennas provide backup and nearEarth 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.
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 3 billion miles from Earth and round-trip transmission time is nine hours.
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.
Alice's spectroscopic range extends across both extreme and far-ultraviolet wavelengths from approximately 500 to 1,800 Angstroms. 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.
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/ChuryumovGerasimenko 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 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) 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 meters (820 feet) per 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.
Radio Science Experiment (REX)
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 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-meter (83-inch) 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.
Long Range Reconnaissance Imager (LORRI)
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.
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 (60 million miles) away - LORRI images will surpass Hubble-quality resolution, providing never-beforeseen details each day. At closest approach, LORRI will image select sections of Pluto's sunlit surface at football-fieldsize resolution, resolving features at least 50 meters across.
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.
Solar Wind at Pluto (SWAP)
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.
Pluto's small gravitational acceleration (approximately 1/16 of Earth's gravity) leads scientists to think that about 75 kilograms (165 pounds) 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.
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)
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.
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.
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 SWAP can. Together the two instruments make a powerful combination for studying the Pluto system.
Student Dust Counter (SDC)
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 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 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 flown beyond 18 astronomical units from the Sun (nearly 1.7 billion miles, 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.
With faculty support, 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.
Spacecraft Systems and Components
Command and Data Handling
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 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.
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 newtons (1 pound) of thrust will be used mostly for course corrections. The spacecraft will use 12 smaller thrusters - 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 will carry 77 kilograms (170 pounds) 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
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 revolutions per minute (RPM)- 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.
The system includes two broad-beam, low-gain antennas on opposite sides of the spacecraft for near-Earth communications: 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 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.
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 3 billion miles from Earth and radio signals will take more than four hours to reach the spacecraft, it can send information at about 700 bits per second. 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.
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 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 July 2015 (the earliest Pluto encounter date) that supply decreases to 200 watts 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.
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