Charon is the largest of Pluto’s five moons and was discovered by astronomers at the United States Naval Observatory Flagstaff Station in 1978.

Diameter: 1,206km
Mass: 1.52 x 1021kg
Composition: Rock and ice
Orbit: 6.38 Earth days

Astronomer James Christy saw Charon as a smudge when he was examining photographic plates with images of Pluto. Charon appeared as a periodic smudge on plate images from observations as far back as 1965. The moon was originally designated s/1978 P1, but acquired its name as a scientific form of ‘Char’, the nickname of Christy’s wife.

Subsequent observations showed that Charon and Pluto are in synchronous orbit, each body showing the same hemisphere to the other. Between 1985 and 1990, astronomers observed a series of mutual transits and eclipses of the Pluto-Charon orbital system.

Charon has 11.6% of the mass of Pluto. The centre of gravity of the Charon-Pluto orbit lies outside both bodies. This has led astronomers to suggest that the system should be considered a binary planet. But the International Astronomical Unions maintains that Charon is a satellite of Pluto.

Charon’s diameter is slightly more than half that of Pluto. Unlike the dwarf planet, which has an atmosphere of nitrogen and methane ice, Charon seems to have no atmosphere. Astronomers believe that that the moon was formed from the collision of a celestial body into Pluto’s mantle. But there is controversy about the internal structure of Charon. Some astronomers believe that the moon has a differentiated structure with a core of rock and an icy mantle, while others believe that its density is uniform throughout.

In 2007, observation at the Gemini Observatory telescopes in Hawaii and Chile revealed ammonia hydrate and water ice crystals on Charon’s surface. There are also indications of active cryo-geysers, or volcano-like eruptions of ice from the surface. The ice deposits are believed to be recent as solar radiation would have resulted in their degradation within 30,000 years.

Pluto was discovered in 1930 and declared to be a planet. However, it was relegated to dwarf planet status in 2006 by the International Astronomical Union. This is the term used to describe icy bodies in the Kuiper Belt. Astronomers were surprised by the discovery of Pluto’s moons, as they thought that dwarf planets could not have such satellites.

NASA’s New Horizons mission will reach Pluto in July 2015. However, the fact that it has moons in orbit around it means that the mission may be more hazardous than astronomers had anticipated.

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