New Horizons

• 1. Introduction

  ›

  New Horizons left Earth on the 19th of January 2006, traveling faster than any human-made object ever launched from our planet. It cleared the Moon's orbit in just nine hours. At 16.26 km/s, it was already racing toward a destination that would take nearly a decade to reach.

  The target was Pluto. Not as a curiosity, but as the last unexplored world in what was then still called the Solar System's planetary family. When New Horizons lifted off from Pad 41 at Cape Canaveral, Pluto was still classified as a full planet. By the time the probe arrived, the rules had changed, but the mission had not.

  What engineers at Johns Hopkins University and the Southwest Research Institute built was a spacecraft roughly the size and shape of a grand piano, powered by a nuclear battery originally meant for a different mission. It carried the ashes of the man who discovered Pluto, a USPS stamp that read "Not Yet Explored," and the names of 434,738 people on a compact disc.

  The probe would take high-resolution photographs of Pluto for the first time in history. It would fly past a distant, double-lobed object called Arrokoth on New Year's Day 2019. And as of April 2026, it was still out there, 64.21 AU from Earth, drifting through the Kuiper belt. What did it find, and what does it mean for everything beyond our Sun?

• 2. A Long, Cold Trip

  ›

  In August 1992, a JPL scientist named Robert Staehle called Clyde Tombaugh, the man who discovered Pluto in 1930, and asked permission to visit his planet. Tombaugh's reply was generous: "I told him he was welcome to it, though he's got to go one long, cold trip."

  That call seeded years of mission proposals. By December 2000, Stamatios "Tom" Krimigis, head of the Applied Physics Laboratory's space division, had formed the New Horizons team alongside Alan Stern. Krimigis described Stern as "the personification of the Pluto mission" and named him principal investigator. The proposal drew heavily on Stern's earlier work with Pluto 350 and incorporated most of the team behind the Pluto Kuiper Express concept.

  Five proposals competed for the New Frontiers slot. New Horizons was selected as one of two finalists for a concept study in June 2001. The other finalist was POSSE, the Pluto and Outer Solar System Explorer, led by Larry W. Esposito of the University of Colorado Boulder and backed by JPL and Lockheed Martin. The APL team had a recent advantage: their NEAR Shoemaker spacecraft had successfully orbited and then landed on asteroid 433 Eros earlier that year.

  New Horizons was officially selected for funding in November 2001. Then the new NASA Administrator, Sean O'Keefe, effectively killed it by leaving it out of the 2003 budget. Associate Administrator Ed Weiler pushed Stern to lobby for the mission's inclusion in the Planetary Science Decadal Survey, the scientific community's ranked wish list compiled by the National Research Council.

  The Decadal Survey results, published in the summer of 2002, placed New Horizons at the top of the medium-size mission category, ahead of missions to the Moon and Jupiter. Weiler stated that his administration was "not going to fight" that result. Funding was secured, and Alice Bowman became Mission Operations Manager. The planned launch window opened in January 2006, with arrival at Pluto set for 2015.

• 3. What Is Aboard

  ›

  Nine cultural artifacts ride with New Horizons alongside its science instruments. Among them: two copies of the American flag, a Florida state quarter chosen because its design commemorates exploration, and a Maryland state quarter honoring the probe's builders at the Applied Physics Laboratory in Howard County. Officially, the coins serve as trim weights.

  About 1 oz of Clyde Tombaugh's ashes are aboard, a tribute to his 1930 discovery of Pluto. The dust sensor is named the Venetia Burney Student Dust Counter, after the girl who, at age 11, suggested the name "Pluto" right after the planet was found. A short film about that instrument won an Emmy Award for student achievement in 2006.

  The science payload itself consists of seven instruments. LORRI, the Long-Range Reconnaissance Imager, is a 208.3 mm aperture telescope with a 1024 by 1024 pixel monochromatic CCD built from silicon carbide mirrors to prevent warping in the cold. The Ralph telescope, 75 mm in aperture, handles color and near-infrared imaging; it was deliberately named after Alice's husband on The Honeymooners, since the Alice ultraviolet spectrometer was designed first. On the 23rd of June 2017, NASA renamed Ralph's infrared channel the Lisa Hardaway Infrared Mapping Spectrometer, honoring Ralph's program manager at Ball Aerospace, who died in January 2017 at age 50.

  The Alice instrument, derived from a twin aboard ESA's Rosetta spacecraft, weighs 4.4 kg and draws 4.4 watts. Its primary job is to read the composition of Pluto's atmosphere. In August 2018, Alice's results confirmed the existence of a hydrogen wall at the outer edges of the Solar System, a boundary first detected in 1992 by the two Voyager spacecraft.

  Power comes from an RTG, model GPHS-RTG, originally a spare from the Cassini mission. It was loaded with 9.75 kg of plutonium-238 oxide pellets, each clad in iridium and encased in graphite. The unit provided 245.7 watts at launch, decaying by roughly 3.5 watts per year; it was projected to fall too low to power the transmitters sometime in the 2030s.

• 4. The Jupiter Slingshot

  ›

  On the 28th of February 2007, New Horizons made its closest approach to Jupiter at 05:43:40 UTC, passing 2.3 million kilometers from the planet. The flyby accelerated the probe by 4 km/s, pushing it to 23 km/s relative to the Sun and cutting three years off the journey to Pluto.

  The encounter ran for four months, from January through June, and served as a full dress rehearsal for the Pluto flyby. Because Jupiter is far closer to Earth than Pluto, the communications link could handle multiple buffer downloads; the mission ultimately returned more data from the Jovian system than it was ever expected to transmit from Pluto.

  The cameras caught the Little Red Spot, spanning up to 70 percent of Earth's diameter, from close range for the first time. Heat-induced lightning in Jupiter's polar regions and wave patterns indicating violent storm activity were measured. New Horizons also found debris inside Jupiter's faint ring system, left over from recent collisions or unexplained phenomena, though a search for undiscovered moons came up empty.

  Of Jupiter's moons, Io drew the most attention. New Horizons observed eleven eruptions, three of them never seen before. The volcano Tvashtar threw material to an altitude of up to 330 km, giving scientists an unprecedented view of a rising plume and its collapse. Infrared signatures of a further 36 volcanoes were also detected.

  On the 19th of March 2007, weeks after the flyby, the Command and Data Handling computer suffered an uncorrectable memory error and rebooted itself, sending the spacecraft into safe mode. The craft recovered within two days, though some data on Jupiter's magnetotail was lost. After that recovery, most of the voyage to Pluto was spent in hibernation to preserve onboard systems.

• 5. Twelve Thousand Kilometers from Pluto

  ›

  On the 6th of December 2014, mission controllers sent the wake-up command to New Horizons. Its reply arrived on Earth at 02:30 UTC on December 7th. The spacecraft had been dormant through most of its voyage, activated for roughly two months each year for instrument checks and calibration.

  Distant-encounter operations began on the 4th of January 2015. At that point, images of Pluto from the LORRI camera were only a few pixels wide. Engineers used those images alongside starfield photographs to refine the spacecraft's approach trajectory down to the precision needed for a successful flyby.

  On the 4th of July 2015, a software timing flaw sent New Horizons into safe mode again, just ten days before closest approach. Two tasks had been commanded simultaneously: compressing previously collected data and copying the approach command sequence. Together, they overloaded the primary computer. The spacecraft switched to its backup system, entered safe mode, and sent a distress signal that reached Earth that afternoon. Engineers resolved the problem by July 7th; the science observations lost were judged to have no impact on the mission's main objectives.

  At 11:49 UTC on the 14th of July 2015, New Horizons flew 12,500 km above Pluto's surface, with Pluto then 34 AU from the Sun. During the flyby, LORRI captured images at up to 50 m resolution; MVIC produced four-color global maps at 1.6 km resolution; LEISA obtained near-infrared maps ranging from 7 km per pixel globally down to 0.6 km per pixel for selected areas. The spacecraft then went silent for 22 hours, pointing its instruments toward the Pluto system rather than Earth.

  Telemetry confirming a healthy spacecraft arrived at Earth on the 15th of July 2015 at 00:52:37 UTC. The mission statement afterward was direct: "The New Horizons flyby of the Pluto system was fully successful, meeting and in many cases exceeding, the Pluto objectives set out for it by NASA and the National Academy of Sciences."

  Because all instruments are body-mounted, the probe had to rotate to capture data, meaning its antenna was not pointed at Earth during observations. The complete data set, transferred at 1 to 2 kilobits per second, took just over 15 months to download. The last piece of Pluto data, a segment from the Ralph/LEISA imager, arrived at the Johns Hopkins University Applied Physics Laboratory on the 25th of October 2016 at 21:48 UTC.

• 6. New Year's Day at Arrokoth

  ›

  Finding a target for the extended mission was harder than finding Pluto. After the flyby, New Horizons had only 33 kg of hydrazine propellant remaining. Any follow-on object had to fall within a cone less than a degree wide extending from Pluto's direction, and within 55 AU, beyond which the communications link would weaken too far and the RTG's decaying output would limit observations.

  Ground-based telescopes including the twin 6.5-meter Magellan Telescopes in Chile and the 8.2-meter Subaru Observatory in Hawaii found roughly 143 candidate Kuiper belt objects, but none sat close enough to the flight path. Only the Hubble Space Telescope was judged capable of finding a suitable target in time. Hubble was granted search time on the 16th of June 2014. On the 15th of October 2014, three candidates were announced, temporarily labeled PT1, PT2, and PT3. All had estimated diameters in the 30-55 km range. PT1, given the designation 486958 Arrokoth and nicknamed Ultima Thule, was selected as the flyby target on the 28th of August 2015.

  Four targeting maneuvers, carried out between the 22nd of October and the 4th of November 2015, adjusted the trajectory. The spacecraft was brought out of hibernation at approximately 00:33 UTC on the 5th of June 2018 for approach preparations. The official approach phase ran from the 16th of August 2018 through the 24th of December 2018.

  Closest approach occurred on the 1st of January 2019 at 05:33 UTC, with Arrokoth 43.4 AU from the Sun. The spacecraft flew within 3,500 km at 51,500 kph. At that distance, radio signals took six hours one-way; confirmation that the digital recorders had filled arrived on Earth ten hours after closest approach, at 15:29 UTC. Arrokoth is the first object ever targeted for a flyby that was discovered after the spacecraft carrying out that flyby had already launched.

  As of July 2022, roughly 10 percent of the Arrokoth data was still being received at Earth, trickling back at 1-2 kilobits per second.

• 7. Beyond the Edge

  ›

  In April 2020, New Horizons turned its LORRI camera toward Proxima Centauri and Wolf 359 simultaneously with telescopes on Earth, from a vantage point more than 6.4 billion km away. The comparison of the two sets of images produced what researchers described as the first easily observable stellar parallax demonstration.

  On the 5th of December 2017, when the spacecraft was 40.9 AU from Earth, a calibration image of the Wishing Well star cluster set a new record as the most distant photograph ever taken by a spacecraft, breaking the 27-year record held by Voyager 1's Pale Blue Dot. Two hours later, New Horizons broke its own record.

  New Horizons has since observed the dwarf planets Eris, Haumea, Makemake, and Quaoar, along with several large Kuiper belt objects including Ixion, Triton in 2019, and others. By December 2023, the spacecraft had discovered roughly 100 Kuiper belt objects and flown close enough to about 20 of them to measure shape, rotational period, possible moons, and surface composition.

  As of April 2026, New Horizons was 64.21 AU from Earth and 64.45 AU from the Sun, still traveling through the Kuiper belt. NASA plans to extend operations until the spacecraft exits the Kuiper belt, expected sometime in 2028 or 2029. The White House's proposed FY2026 budget would have ended the mission, but a final appropriation of 24.44 billion USD secured its continuation after debate in Congress.

  In the 2030s, the RTG will decay too far to power the transmitters, and New Horizons will go silent. It will become the fifth human-made object to achieve escape velocity from the Solar System, carrying Clyde Tombaugh's ashes outward past the boundary his planet once marked.