Charon

Charon is Pluto's largest moon, discovered in 1978 by the American astronomer James Christy. Charon orbits Pluto closely, at a distance of 12,200 miles (19,600 km), with a period of 6.39 days. Both objects are rotationally locked to each other, always showing the same face to one another. Furthermore, the barycenter of their mutual orbit does not lie within either body. This configuration is unique within the Solar System.

Between 1985 and 1990, Pluto and Charon were situated in such a way that many mutual eclipses and transits occurred. Observation of these events led to a wealth of improved data on the Pluto-Charon system, including much-improved values for both objects' size and mass. Charon is about half the size of Pluto, with a diameter of 745 miles (1200 km). It is even less dense than Pluto, leading to speculation that Pluto and Charon formed at different locations.

A rough surface albedo map of Charon generated from occultation observations shows light and dark areas, but little other detail. Charon's surface is believed to be similar to Pluto's, covered with frozen nitrogen and methane, and has an average temperature is -370°F (-220°C). Much more is expected to be revealed by the New Horizons spacecraft, which will fly by Pluto and Charon in 2015.