Charon is the largest satellite of the dwarf planet Pluto. It was discovered in 1978 at the United States Naval Observatory Flagstaff Station. Following the 2005 discovery of two other natural satellites of Pluto (Nix and Hydra), Charon may also be referred to as (134340) Pluto I. The New Horizons mission is scheduled to visit Charon and Pluto in July 2015.
Charon should not be confused with the similarly named Chiron, a smaller outer solar system centaur.
Charon was discovered by astronomer James Christy on June 22, 1978, when he was examining highly magnified images of Pluto on photographic plates taken a couple of months earlier. Christy noticed that a slight elongation appeared periodically. The discovery was announced on July 7, 1978. Later, the bulge was confirmed on plates dating back to April 29, 1965.
Subsequent observations of Pluto determined that the bulge was due to a smaller accompanying body. The periodicity of the bulge corresponded to Pluto's rotation period, which was previously known from Pluto's light curve. This indicated a synchronous orbit, which strongly suggested that the bulge effect was real and not spurious.
All doubts were erased when Pluto and Charon entered a five-year period of mutual eclipses and transits between 1985 and 1990. This occurs when the Pluto-Charon orbital plane is edge-on as seen from Earth, which only happens at two intervals in Pluto's 248-year orbital period. It was fortuitous that one of these intervals happened to occur so soon after Charon's discovery.
Images showing Pluto and Charon resolved into separate disks were taken for the first time by the Hubble Space Telescope in the 1990s. Later, the development of adaptive optics made it possible to resolve Pluto and Charon into separate disks using ground-based telescopes.
Charon's diameter is about 1,207 kilometres (750 mi), just over half that of Pluto, with a surface area of 4,580,000 square kilometres (1,770,000 sq mi). Unlike Pluto, which is covered with nitrogen and methane ices, the Charonian surface appears to be dominated by less volatile water ice, and also appears to have no atmosphere. In 2007, observations by the Gemini Observatory of patches of ammonia hydrates and water crystals on the surface of Charon suggested the presence of active cryo-geysers. (See also Cryovolcano.) Mutual eclipses of Pluto and Charon in the 1980s allowed astronomers to take spectra of Pluto and then the combined spectrum of the pair. By subtracting Pluto's spectrum from the total, astronomers were able to spectroscopically determine the surface composition of Charon.
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