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Pluto and Charon
Hubble Space Telescope image of Pluto and Charon taken in 1994 when the two bodies were at maximum elongation of 0.9 arc seconds
The largest of the three known moons of Pluto. Charon was discovered in 1978 by U.S. Naval Observatory astronomer James Christy (1938–). With just over half the diameter of Pluto and one-seventh the mass, Charon is easily the largest satellite relative to its planet in the Solar System.

During the 1980s, Earth crossed the orbital plane of Charon so that, from our vantage point, Charon and Pluto alternately passed in front of each other. These eclipses enabled the size of the two objects and other valuable data on the Pluto-Charon system to be collected. Albedo measurements suggest that Charon is covered mainly with water-ice, while Pluto has a coating of frozen nitrogen. Also Charon's density (about 1.2 g/cm3) is significantly lower than that of Pluto (about 2.0 g/cm3) suggesting that the moon has very little rock and adding to the controversy of how Charon formed. Set against the favored view that Charon stemmed from the collision between Pluto and another large object in the early days of the Solar System, is the suggestion that Pluto and Charon formed independently.

The likely presence of water-ice, together with the possibility that Charon is subject to significant tidal heating as a result of Pluto's gravity, has led to the hypothesis that there may be a sub-ice liquid ocean of water on the moon which could harbor microbial life. Charon is due to be observed at close range by the New Horizons probe in 2015.

discovery 1978, James Christy at U.S. Naval Observatory
mean distance from Pluto 19,600 km (12,180 miles)
diameter 1,186 km (737 miles)
mean density 1.2 g/cm3
escape velocity 0.61 km/s (2,196 km/h, 1,365 mph)
orbital period 6.387 days (6 days 9 hours)
orbital eccentricity 0.00
orbital inclination 98.80°
axial period 6.387 days
albedo 0.5
surface temperature ~-240°C (-400°F)


In Greek mythology, Charon was the son of Erebus and Nyx. It was his duty to ferry over the Rivers Styx and Acheron the souls of the dead who had received the rites of burial. His payment for this was a coin, which was placed in the mouth of the corpse. If this rite was neglected, Charon refused to convey the soul across, and it was doomed to wander restlessly along the shores of Acheron.

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