New Horizons Reaches Pluto
In July 2015, NASA's New Horizons became the first spacecraft to visit Pluto, passing the dwarf planet and its moons after a journey of more than nine years. The flyby transformed Pluto from a blurry smudge into a world with mountains, plains, and an atmosphere, and it completed the first reconnaissance of the planets known at the time of the mission's launch.
Pluto was discovered in 1930 by Clyde Tombaugh at Lowell Observatory in Arizona and was counted as the ninth planet for decades. Even the best images from the Hubble Space Telescope showed only vague bright and dark patches. In the 1990s, astronomers began finding many icy bodies in the Kuiper Belt, the region beyond Neptune, and Pluto came to look like one of the largest of them rather than a solitary oddity. In 2006 the International Astronomical Union reclassified it as a dwarf planet, a decision that remains debated by some scientists and members of the public. Proposals for a Pluto mission had struggled for years before New Horizons, led by principal investigator Alan Stern, won approval and launched in January 2006.
The spacecraft, about the size of a grand piano and powered by a radioisotope generator, flew past Jupiter in 2007 for a gravity boost, then spent years mostly in hibernation. Its closest approach to Pluto came on 14 July 2015, at a distance of roughly 12,000 kilometers, and it was moving far too fast to enter orbit. Instead it gathered data in a brief, tightly planned window, then turned to send it home. Because of the vast distance and a very slow transmission rate, the full dataset took about sixteen months to reach Earth.
The results surprised scientists. Pluto proved to be geologically active or recently active, with few impact craters across some regions. Mountains of water ice rise several kilometers high, and a broad, bright plain informally named Sputnik Planitia appears to be covered in slowly churning nitrogen ice. Pluto also has a thin nitrogen atmosphere with layered hazes, and reddish material on its surface that is thought to arise from chemistry involving methane. Its large moon Charon showed canyons and a dark reddish polar cap, and the smaller moons were found to be irregular and tumbling.
New Horizons continued outward and in January 2019 flew past Arrokoth, a small Kuiper Belt object shaped like two joined lobes. The mission showed that small, distant worlds can retain surprising internal energy and complexity, and it gave planetary scientists a new baseline for understanding the outer solar system.