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Image of Charon compiled from bightness variations
Image of Charon compiled from bightness variations
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
relative sizes of the Moon, Pluto and Charon
Relative sizes of the Moon, Pluto and Charon
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.

Charon orbits a mere 18,000km (11,200 miles) above surface of Pluto, which is more than 20 times closer than the Moon is from the Earth. Owing to their closeness and the fact that Charon is so large, the common center of gravity of Pluto and Charon, known as the barycentre, lies in the space between the two worlds.

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. 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 center of Pluto 19,600 km (12,180 miles)
diameter 1,206 km (749 miles)
mean density 1.65 g/cm3
escape velocity 0.58 km/s (2,088 km/h, 1,297 mph)
orbital period 6.387 days (6 days 9 hours)
orbital eccentricity 0.00
orbital inclination 0.001° (to Pluto's equator); 119.6° (to Pluto's orbit)
axial period 6.387 days (synchronous)
albedo 0.37
surface temperature ~-220°C (-364°F, 53K)


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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