New Horizons is an interplanetary space probe launched as a part of NASA's New Frontiers program. Launched in 2006, it became the first spacecraft to perform a flyby study and capture high-resolution photographs of Pluto and its moons during its encounter.
It was engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research Institute (SwRI), with a team led by Alan Stern. New Horizons is the fifth space probe to achieve the escape velocity needed to leave the Solar System.
On January 19, 2006, New Horizons was launched from Cape Canaveral Space Force Station by an Atlas V rocket directly into an Earth-and-solar escape trajectory with a speed of about 16.26 km/s (10.10 mi/s; 58,500 km/h; 36,400 mph) relative to the sun. It was the fastest (average speed with respect to Earth) human-made object ever launched from Earth. It is not the fastest speed recorded for a spacecraft, which, as of 2026, is that of the Parker Solar Probe. After a brief encounter with asteroid 132524 APL, New Horizons proceeded to Jupiter, making its closest approach on February 28, 2007, at a distance of 2.3 million kilometers (1.4 million miles). The Jupiter flyby provided a gravity assist that increased New Horizons' speed; the flyby also enabled a general test of New Horizons' scientific capabilities, returning data about the planet's atmosphere, moons, and magnetosphere.
Most of the post-Jupiter voyage was spent in hibernation mode to preserve onboard systems, except for brief annual checkouts. On December 6, 2014, New Horizons was brought back online for the Pluto encounter, and instrument check-out began. On January 15, 2015, the spacecraft began its approach phase to Pluto.
On July 14, 2015, at 11:49 UTC, it flew 12,500 km (7,800 mi) above the surface of Pluto, which at the time was 34 AU from the Sun, making it the first spacecraft to explore the dwarf planet. In August 2016, New Horizons was reported to have traveled at speeds of more than 84,000 km/h (52,000 mph). On October 25, 2016, at 21:48 UTC, the last recorded data from the Pluto flyby was received from New Horizons. Having completed its flyby of Pluto, New Horizons then maneuvered for a flyby of Kuiper belt object 486958 Arrokoth (then nicknamed Ultima Thule), which occurred on January 1, 2019, when it was 43.4 AU (6.49 billion km; 4.03 billion mi) from the Sun. In August 2018, NASA cited results by Alice on New Horizons to confirm the existence of a "hydrogen wall" at the outer edges of the Solar System. This "wall" was first detected in 1992 by the two Voyager spacecraft.
New Horizons is traveling through the Kuiper belt; it is 64.21 AU (9.61 billion km; 5.97 billion mi) from Earth and 64.45 AU (9.64 billion km; 5.99 billion mi) from the Sun as of April 2026. NASA has announced its plan to extend operations for New Horizons until the spacecraft exits the Kuiper belt, which is expected to occur in either 2028 or 2029. The White House's proposed budget for FY2026 would have cut funding for New Horizons, but the continued funding of the mission was heavily debated in the United States Congress and a final budget of 24.44 billion USD guaranteed the mission continuation.
History
In August 1992, JPL scientist Robert Staehle called Pluto discoverer Clyde Tombaugh, requesting permission to visit his planet. "I told him he was welcome to it," Tombaugh later remembered, "though he's got to go one long, cold trip." The call eventually led to a series of proposed Pluto missions leading up to New Horizons.
Stamatios "Tom" Krimigis, head of the Applied Physics Laboratory's space division, one of many entrants in the New Frontiers Program competition, formed the New Horizons team with Alan Stern in December 2000. Appointed as the project's principal investigator, Stern was described by Krimigis as "the personification of the Pluto mission". New Horizons was based largely on Stern's work since Pluto 350 and involved most of the team from Pluto Kuiper Express.
The New Horizons proposal was one of five that were officially submitted to NASA. It was later selected as one of two finalists to be subject to a three-month concept study in June 2001. The other finalist, POSSE (Pluto and Outer Solar System Explorer), was a separate but similar Pluto mission concept by the University of Colorado Boulder, led by principal investigator Larry W. Esposito, and supported by the JPL, Lockheed Martin and the University of California.
However, the APL, in addition to being supported by Pluto Kuiper Express developers at the Goddard Space Flight Center and Stanford University were at an advantage; they had recently developed NEAR Shoemaker for NASA, which had successfully entered orbit around 433 Eros earlier that year, and would later land on the asteroid to scientific and engineering fanfare.
In November 2001, New Horizons was officially selected for funding as part of the New Frontiers program. However, the new NASA Administrator appointed by the Bush administration, Sean O'Keefe, was not supportive of New Horizons and effectively canceled it by not including it in NASA's budget for 2003. NASA's Associate Administrator for the Science Mission Directorate, Ed Weiler, prompted Stern to lobby for the funding of New Horizons in hopes of the mission appearing in the Planetary Science Decadal Survey, a prioritized "wish list", compiled by the United States National Research Council, that reflects the opinions of the scientific community.
After an intense campaign to gain support for New Horizons, the Planetary Science Decadal Survey of 2003–2013 was published in the summer of 2002. New Horizons topped the list of projects considered the highest priority among the scientific community in the medium-size category; ahead of missions to the Moon, and even Jupiter. Weiler stated that it was a result that "[his] administration was not going to fight". Funding for the mission was finally secured following the publication of the report. Stern's team was finally able to start building the spacecraft and its instruments, with a planned launch in January 2006 and arrival at Pluto in 2015. Alice Bowman became Mission Operations Manager (MOM).
Mission profile
New Horizons is the first mission in NASA's New Frontiers mission category, larger and more expensive than the Discovery missions but smaller than the missions of the Flagship Program. The cost of the mission, including spacecraft and instrument development, launch vehicle, mission operations, data analysis, and education/public outreach, is approximately $700 million over 15 years (2001–2016). The spacecraft was built primarily by Southwest Research Institute (SwRI) and the Johns Hopkins Applied Physics Laboratory. The mission's principal investigator is Alan Stern of the Southwest Research Institute (formerly NASA Associate Administrator).
After separation from the launch vehicle, overall control was taken by Mission Operations Center (MOC) at the Applied Physics Laboratory in Howard County, Maryland. The science instruments are operated at Clyde Tombaugh Science Operations Center (T-SOC) in Boulder, Colorado. Navigation is performed at various contractor facilities, whereas the navigational positional data and related celestial reference frames are provided by the Naval Observatory Flagstaff Station through Headquarters NASA and JPL.
KinetX is the lead on the New Horizons navigation team and is responsible for planning trajectory adjustments as the spacecraft speeds toward the outer Solar System. Coincidentally the Naval Observatory Flagstaff Station was where the photographic plates were taken for the discovery of Pluto's moon Charon. The Naval Observatory itself is not far from the Lowell Observatory where Pluto was discovered.
New Horizons was originally planned as a voyage to the only unexplored planet in the Solar System. When the spacecraft was launched, Pluto was still classified as a planet, later to be reclassified as a dwarf planet by the International Astronomical Union (IAU). Some members of the New Horizons team, including Alan Stern, disagree with the IAU definition and still describe Pluto as the ninth planet. Pluto's satellites Nix and Hydra also have a connection with the spacecraft: the first letters of their names (N and H) are the initials of New Horizons. The moons' discoverers chose these names for this reason, plus Nix and Hydra's relationship to the mythological Pluto.
Mementos
In addition to the science equipment, there are nine cultural artifacts traveling with the spacecraft. These include a collection of 434,738 names stored on a compact disc, a collection of images of New Horizons project personnel on another CD, a piece of Scaled Composites's SpaceShipOne, a "Not Yet Explored" USPS stamp, and two copies of the Flag of the United States.
About 30 grams (1 oz) of Clyde Tombaugh's ashes are aboard the spacecraft, to commemorate his discovery of Pluto in 1930. A Florida state quarter coin, whose design commemorates human exploration, is included, officially as a trim weight, as is a Maryland state quarter to honor the probe's builders. One of the science packages (a dust counter) is named after Venetia Burney, who, as a child, suggested the name "Pluto" after its discovery.
Goal
The goal of the mission is to understand the formation of the Plutonian system, the Kuiper belt, and the transformation of the early Solar System. The spacecraft collected data on the atmospheres, surfaces, interiors, and environments of Pluto and its moons. It will also study other objects in the Kuiper belt. "By way of comparison, New Horizons gathered 5,000 times as much data at Pluto as Mariner did at the Red Planet."
Some of the questions the mission attempts to answer are: What is Pluto's atmosphere made of and how does it behave? What does its surface look like? Are there large geological structures? How do solar wind particles interact with Pluto's atmosphere?
Specifically, the mission's science objectives are to:
Map the surface compositions of Pluto and Charon
Characterize the geologies and morphologies of Pluto and Charon
Characterize the neutral atmosphere of Pluto and its escape rate
Search for an atmosphere around Charon
Map surface temperatures on Pluto and Charon
Search for rings and additional satellites around Pluto
Conduct similar investigations of one or more Kuiper belt objects
Design and construction
Spacecraft subsystems
The spacecraft is comparable in size and general shape to a grand piano and has been compared to a piano glued to a cocktail bar-sized satellite dish. As a point of departure, the team took inspiration from the Ulysses spacecraft, which also carried a radioisotope thermoelectric generator (RTG) and dish on a box-in-box structure through the outer Solar System. Many subsystems and components have flight heritage from APL's CONTOUR spacecraft, which in turn had heritage from APL's TIMED spacecraft.
New Horizons' body forms a triangle, almost 0.76 m (2.5 ft) thick. (The Pioneers have hexagonal bodies, whereas the Voyagers have decagonal bodies, Galileo an octagonal prism body, and Cassini–Huygens a dodecagonal prism body.) A 7075 aluminium alloy tube forms the main structural column, between the launch vehicle adapter ring at the "rear", and the 2.1 m (6 ft 11 in) radio dish antenna affixed to the "front" flat side. The titanium fuel tank is in this tube. The RTG attaches with a 4-sided titanium mount resembling a gray pyramid or stepstool.
Titanium provides strength and thermal isolation. The rest of the triangle is primarily sandwich panels of thin aluminum face sheet (less than 1⁄64 in or 0.40 mm) bonded to aluminum honeycomb core. The structure is larger than strictly necessary, with empty space inside. The structure is designed to act as shielding, reducing electronics errors caused by radiation from the RTG. Also, the mass distribution required for a spinning spacecraft demands a wider triangle.
The interior structure is painted black to equalize temperature by radiative heat transfer. Overall, the spacecraft is thoroughly blanketed to retain heat. Unlike the Pioneers and Voyagers, the radio dish is also enclosed in blankets that extend to the body. The heat from the RTG adds warmth to the spacecraft while it is in the outer Solar System. While in the inner Solar System, the spacecraft must prevent overheating, hence electronic activity is limited, power is diverted to shunts with attached radiators, and louvers are opened to radiate excess heat. While the spacecraft is cruising inactively in the cold outer Solar System, the louvers are closed, and the shunt regulator reroutes power to electric heaters.
Propulsion and attitude control
New Horizons has both spin-stabilized (cruise) and three-axis stabilized (science) modes controlled entirely with hydrazine monopropellant. Additional post launch delta-v of over 290 m/s (1,000 km/h; 650 mph) is provided by a 77 kg (170 lb) internal tank. Helium is used as a pressurant, with an elastomeric diaphragm assisting expulsion. The spacecraft's on-orbit mass including fuel is over 470 kg (1,040 lb) on the Jupiter flyby trajectory, but would have been only 445 kg (981 lb) for the backup direct flight option to Pluto. Significantly, had the backup option been taken, this would have meant less fuel for later Kuiper belt operations.
There are 16 thrusters on New Horizons: four 4.4 N (1.0 lbf) and twelve 0.9 N (0.2 lbf) plumbed into redundant branches. The larger thrusters are used primarily for trajectory corrections, and the small ones (previously used on Cassini and the Voyager spacecraft) are used primarily for attitude control and spinup/spindown maneuvers. Two star cameras are used to measure the spacecraft attitude. They are mounted on the face of the spacecraft and provide attitude information while in spin-stabilized or 3-axis mode. In between the time of star camera readings, spacecraft orientation is provided by dual redundant miniature inertial measurement units. Each unit contains three solid-state gyroscopes and three accelerometers. Two Adcole Sun sensors provide attitude determination. One detects the angle to the Sun, whereas the other measures spin rate and clocking.
Power
A cylindrical radioisotope thermoelectric generator (RTG) protrudes in the plane of the triangle from one vertex of the triangle. The RTG provided 245.7 W of power at launch, and was predicted to drop approximately 3.5 W every year, decaying to 202 W by the time of its encounter with the Plutonian system in 2015 and will decay too far to power the transmitters in the 2030s. There are no onboard batteries since RTG output is predictable, and load transients are handled by a capacitor bank and fast circuit breakers. As of January 2019, the power output of the RTG is about 190 W.
The RTG, model "GPHS-RTG", was originally a spare from the Cassini mission. The RTG contains 9.75 kg (21.5 lb) of plutonium-238 oxide pellets. Each pellet is clad in iridium, then encased in a graphite shell. It was developed by the U.S. Department of Energy at the Materials and Fuels Complex, a part of the Idaho National Laboratory.
The original RTG design called for 10.9 kg (24 lb) of plutonium, but a unit less powerful than the original design goal was produced because of delays at the United States Department of Energy, including security activities, that delayed plutonium production. The mission parameters and observation sequence had to be modified for the reduced wattage; still, not all instruments can operate simultaneously. The Department of Energy transferred the space battery program from Ohio to Argonne in 2002 because of security concerns.
The amount of radioactive plutonium in the RTG is about one-third the amount on board the Cassini–Huygens probe when it launched in 1997. The Cassini launch had been protested by multiple organizations, due to the risk of such a large amount of plutonium being released into the atmosphere in case of an accident. The United States Department of Energy estimated the chances of a launch accident that would release radiation into the atmosphere at 1 in 350, and monitored the launch because of the inclusion of an RTG on board. It was estimated that a worst-case scenario of total dispersal of on-board plutonium would spread the equivalent radiation of 80% the average annual dosage in North America from background radiation over an area with a radius of 105 km (65 mi).
Flight computer
The spacecraft carries two computer systems: the Command and Data Handling system and the Guidance and Control processor. Each of the two systems is duplicated for redundancy, for a total of four computers. The processor used for its flight computers is the Mongoose-V, a 12 MHz radiation-hardened version of the MIPS R3000 CPU. Multiple redundant clocks and timing routines are implemented in hardware and software to help prevent faults and downtime. To conserve heat and mass, spacecraft and instrument electronics are housed together in IEMs (integrated electronics modules). There are two redundant IEMs. Including other functions such as instrument and radio electronics, each IEM contains 9 boards. The software of the probe runs on Nucleus RTOS operating system.
There have been two "safing" events, that sent the spacecraft into safe mode:
On March 19, 2007, the Command and Data Handling computer experienced an uncorrectable memory error and rebooted itself, causing the spacecraft to go into safe mode. The craft fully recovered within two days, with some data loss on Jupiter's magnetotail. No impact on the subsequent mission was expected.
On July 4, 2015, there was a CPU safing event triggered by an over-assignment of commanded science operations on the craft's approach to Pluto. Fortunately, the craft was able to recover within two days without major impacts on its mission. NASA scientists therefore reduced the number of scientific operations on the craft to prevent future events, which could happen during the approach with Pluto.
Telecommunications and data handling
Communication with the spacecraft is via X band. The craft had a communication rate of 38 kbit/s at Jupiter; at Pluto's distance, a rate of approximately 1 kbit/s per transmitter was expected. Besides the low data rate, there is also a latency of about 4.5 hours one-way at Pluto's distance. The 70 m (230 ft) NASA Deep Space Network (DSN) dishes are used to relay commands once the spacecraft is beyond Jupiter. The spacecraft uses dual modular redundancy transmitters and receivers, and either right- or left-hand circular polarization.
The downlink signal is amplified by dual redundant 12-watt traveling-wave tube amplifiers (TWTAs) mounted on the body under the dish. The receivers are low-power designs. The system can be controlled to power both TWTAs at the same time, and transmit a dual-polarized downlink signal to the DSN that nearly doubles the downlink rate. DSN tests early in the mission with this dual polarization combining technique were successful, and the capability was declared to be operational (when the spacecraft power budget permits both TWTAs to be powered).
In addition to the high-gain antenna, there are two backup low-gain antennas and a medium-gain dish. The high-gain dish has a Cassegrain reflector layout, composite construction, of 2.1-meter (7 ft) diameter providing over 42 dBi of gain and a half-power beam width of about a degree. The prime-focus medium-gain antenna, with a 0.3-meter (1 ft) aperture and 10° half-power beam width, is mounted to the forward-facing side of the high-gain antenna's secondary reflector. The forward low-gain antenna is stacked atop the feed of the medium-gain antenna. The aft low-gain antenna is mounted within the launch adapter at the rear of the spacecraft. This antenna was used only for early mission phases near Earth, just after launch and for emergencies if the spacecraft had lost altitude control.
New Horizons recorded scientific instrument data to its solid-state memory buffer at each encounter, then transmitted the data to Earth. Data storage is done on two low-power solid-state recorders (one primary, one backup) holding up to 8 gigabytes each. Because of the extreme distance from Pluto and the Kuiper belt, only one buffer load at those encounters can be saved. This is because New Horizons would require approximately 16 months after leaving the vicinity of Pluto to transmit the buffer load back to Earth. At Pluto's distance, radio signals from the space probe back to Earth took four hours and 25 minutes to traverse 4.7 billion km of space.
Part of the reason for the delay between the gathering of and transmission of data is that all of the New Horizons instrumentation is body-mounted. In order for the cameras to record data, the entire probe must turn, and the one-degree-wide beam of the high-gain antenna was not pointing toward Earth. Previous spacecraft, such as the Voyager program probes, had a rotatable instrumentation platform (a "scan platform") that could take measurements from virtually any angle without losing radio contact with Earth. New Horizons was mechanically simplified to save weight, shorten the schedule, and improve reliability during its 15-year designed lifetime.
Instruments
New Horizons carries seven instruments: three optical instruments, two plasma instruments, a dust sensor and a radio science receiver/radiometer. The instruments are to be used to investigate the global geology, surface composition, surface temperature, atmospheric pressure, atmospheric temperature and escape rate of Pluto and its moons. The rated power is 21 watts, though not all instruments operate simultaneously. In addition, New Horizons has an Ultrastable Oscillator subsystem, which may be used to study and test the Pioneer anomaly towards the end of the spacecraft's life.
Long-Range Reconnaissance Imager (LORRI)
The Long-Range Reconnaissance Imager (LORRI) is a long-focal-length imager designed for high resolution and responsivity at visible wavelengths. The instrument is equipped with a 1024×1024 pixel by 12-bits-per-pixel monochromatic CCD imager giving a resolution of 5 μrad (~1 arcsec). The CCD is chilled far below freezing by a passive radiator on the antisolar face of the spacecraft. This temperature differential requires insulation and isolation from the rest of the structure. The 208.3 mm (8.20 in) aperture Ritchey–Chretien mirrors and metering structure are made of silicon carbide to boost stiffness, reduce weight and prevent warping at low temperatures. The optical elements sit in a composite light shield and mount with titanium and fiberglass for thermal isolation. Overall mass is 8.6 kg (19 lb), with the optical tube assembly (OTA) weighing about 5.6 kg (12 lb), for one of the largest silicon-carbide telescopes flown at the time (now surpassed by Herschel). For viewing on public web sites the 12-bit per pixel LORRI images are converted to 8-bit per pixel JPEG images. These public images do not contain the full dynamic range of brightness information available from the raw LORRI images files.