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Charon, discovered in 1978, is either the largest moon of Pluto or the smaller member of a double dwarf planet with Pluto, depending on the definition employed. With the discovery in 2005 of two other moons of Pluto (Nix and Hydra), Charon is now also referred to as Pluto I. The New Horizons mission is scheduled to visit Charon and Pluto in July 2015.
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 before. Christy noticed that a slight bulge appeared periodically. The discovery was announced on 1978 July 7. 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.
Any final doubts were erased when Pluto and Charon entered a five-year period of mutual eclipses 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 also resolve Pluto and Charon into separate disks using ground-based telescopes.
Charon's diameter is about 1207 km, just over half that of Pluto, with a surface area of 4 580 000 km². 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.
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.
Charon's volume and mass allow calculation of its density from which it can be told that Charon is largely an icy body and contains less rock by proportion than its partner Pluto. This supports the idea Charon was created by a giant impact into Pluto's icy mantle (see Formation below.) There are two conflicting theories about Charon's internal structure: some scientists believe it to be a differentiated body like Pluto with a rocky core and an icy mantle while others believe Charon to be of uniform composition throughout. Evidence in support of the former position was found in 2007, when 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. The fact that the ice was still in crystalline form suggested it had been recently deposited, as solar radiation would have degraded older ice to an amorphous state after 30 000 years or so.
Charon and Pluto revolve about each other every 6.387 days. The two objects are gravitationally locked, so each keeps the same face towards the other. The average distance between Charon and Pluto is 19 570 km. The discovery of Charon allowed astronomers to accurately calculate the mass of the Plutonian system, and mutual occultations revealed their sizes. However, neither indicated the two bodies' individual masses, which could only be estimated, until the discovery of Pluto's outer moons in late 2005. Details in the orbits of the outer moons reveal that Charon has approximately 11.65% of the mass of Pluto. This shows it to have a density of 1.65 ± 0.06 g/cm³, suggesting a composition of 55 ± 5% "rock" to 45% ice, whereas Pluto is somewhat denser and about 70% "rock".
