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Since its discovery in 1930, Pluto was thought to be a solitary object. So it came as quite a surprise in 1978 when astronomer James Christy discovered that Pluto actually has a Moon.
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Christy was examining highly magnified photographic plates take of Pluto and noticed that it had a strange bulge that appeared periodically. Further observations teased out that Pluto was indeed being orbited by a smaller Moon.
Charon’s diameter is about half the size of Pluto, measuring 1,207 km (750 miles) across. Unlike Pluto, which seems to be dominated by nitrogen and methane ice, Charon surface is covered by water ice.
Astronomers recently detected patches of ammonia hydrates and water crystals on the surface of Charon, suggesting that the tiny moon has active cryo-volcanism, like several of Saturn’s moons.
Detecting Charon was a boon for astronomers, since it allowed them to then measure the mass of Pluto. Determining the mass of an object in the Universe is difficult, unless you have another object orbiting it. Then it’s a straightforward calculation.
Pluto and Charon are tidally locked, and always show the same face to each other. They take 6.387 days to revolve around each other. Another interesting fact is that the center of gravity for Pluto and Charon is outside Pluto itself. This means that the two objects could actually be called a binary planet. But the IAU’s 2006 redefinition of planethood kept Charon out of being named a dwarf planet.
The name Charon comes from Greek legends. He was the ferryman of Hades, who carried the newly dead across the river Styx. Since Pluto is the Roman God of the Underworld, it’s an appropriate name for a companion moon.