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New Horizons is the first mission of NASA’s New Frontiers programme of medium-class planetary exploration missions launched every five years. The mission was approved in 2000 at a time when Pluto was still considered by scientists to be a planet, before it was reclassified as a dwarf planet in 2006.
Most of the probe’s instruments are from the United States, but several European scientists are on the New Horizons science team.
New Horizons is one of the fastest spacecraft ever built. After a gravity boost from Jupiter in 2007 and a journey of some five billion kilometres, the one-tonne probe, smaller than a grand piano, flew by the Pluto system in July 2015. The flyby was completely automatic.
The first flyby of the dwarf planet Pluto, the largest Kuiper Belt object (KBO), spanning roughly 2,380 km, its main satellite Charon and four small moons (Styx, Nix, Kerberos and Hydra) collected a wealth of data. While the flyby only lasted a few days, it took more than a year to beam the data back to Earth.
New Horizons observed clearly visible layers of nitrogen haze in Pluto’s atmosphere, a great diversity of relief including some cryovolcanoes, the notable absence of craters in certain regions (particularly Sputnik Planitia, the largest nitrogen ice glacier in the solar system), mountains of water ice and moving glaciers. The dwarf planet appears to be young.
The Pluto-Charon system can be likened to the Earth-Moon system, except Charon is half the size of Pluto. New Horizons also discovered a phenomenon not seen in the solar system before: Pluto’s atmosphere escapes towards Charon in ‘summer’, depositing heavy hydrocarbons on Charon’s surface that are visible at its north pole, giving it its dark hue. These hydrocarbons are tholins, complex organic substances containing varying levels of nitrogen that are abundant in the solar system but not found on Earth.
Pluto’s small moons show traces of impacts and could be fragments from a collision between Pluton and a large KBO.
New Horizons’ mission has been extended and it is scheduled to fly within 3,500 km of a small KBO spanning roughly 40 km called 2014 MU69 on 1 January 2019. The objective of this flyby is to study a common object in the belt like the thousands of others already catalogued to understand how such small bodies have evolved since they were formed.
No further close flybys are planned after this encounter, but the probe’s camera should allow other KBOs to be observed for the next 20 to 40 years. All being well, the probe will continue to send back images until its radioisotope thermal electric generator (RTG) runs out of fuel. We will thus be able to learn more about the outer reaches of our solar system and possibly beyond. With its missions accomplished and propelled by its momentum, New Horizons will then become only the fifth man-made spacecraft to leave the solar system.