Charon is the only known satellite of Pluto. It was discovered by astronomer James Christy on June 22, 1978 by carefully examining highly magnified images of Pluto on photographic plates taken a couple of months before and noticing that a slight bulge appeared periodically. Later, the bulge was confirmed on plates dating back to April 29, 1965. It received the temporary designation 1978 P 1, according to the then-recently instituted convention. It is not to be confused with the similarly named Chiron, another object in the outer solar system with an orbit between those of Saturn and Uranus.
Christy named it after the Greek mythological figure Charon but pronounced it differently. The "ch" at the beginning of the moon's name is soft so it sounds like "Sharon," after the astronomer's wife Charlene, nicknamed Char, which both have soft ch sounds. The mythological figure's name is pronounced with a hard "ch" sound like the modern letter "k", like "ch" in Christy's name. The name "Charon" was officially accepted by the IAU in 1985.
The discovery of Charon allowed astronomers to more accurately calculate Pluto's mass and size. Charon revolves around Pluto in 6.387 days, the same period as Pluto's rotation. The two objects are gravitationally locked (tidal locking) so they each keep the same face towards the other.
Charon's diameter is 1,172 km (728 miles), just under half the size of Pluto. It has 1/7th the mass of Pluto, and a surface area of 4,400,000 km2. Unlike Pluto, which is covered in nitrogen ice, Charon appears to be coated in water ice.
Due to the unusually small difference in size between it and Pluto, Pluto and Charon are sometimes considered to be a double planet. They are also sometimes thought of as not a planet and a satellite, but as the first two Trans-Neptunian objects.
Simulation work published in 2005 by Robin Canup suggested that Charon could have formed by a giant impact around 4.5 billion years ago, with a Kuiper belt object between 1600 and 2000 kilometres in diameter striking Pluto at a speed of 1 kilometre per second and Charon coalescing from the ring of debris kicked up by the colision. There is less evidence for this theory than there is for the similar theory regarding Earth's Moon, however, most notably a lack of information regarding the composition of the two bodies.
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