The largest satellite of Pluto; also known as Pluto I. It orbits 19 591 km from Pluto’s centre in 6.387 days. Its orbital period and its axial rotation are the same as the rotation period of Pluto itself, so Charon not only keeps one face permanently turned towards Pluto but also hangs stationary over one point on its surface. Charon is 1212 km in diameter, over half the size of Pluto. Its mass is one-tenth that of Pluto, and the barycentre of the system lies above Pluto’s surface. Charon’s density is about 1.7 g/cm3, suggesting a composition of rock and ice. Charon’s surface consists mostly of water ice with some ammonia compounds. The New Horizons spacecraft that flew past Pluto and Charon in 2015 revealed a belt of fractures and canyons extending around Charon’s equator. A dark-reddish area near its north pole is composed of organic material produced from gases such as methane and nitrogen escaping from Pluto’s atmosphere. Charon was discovered in 1978 by the American astronomer James Walter Christy (1938– ).
http://photojournal.jpl.nasa.gov/catalog/PIA19968 Charon seen by NASA’s New Horizons space probe in 2015.