Charon (moon)

Charon is the only known satellite of Pluto.

Charon was discovered by astronomer James Christy[?] in 1978 using photographic plates which showed a bulge moving around Pluto. 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" (or more properly like the German "ch" in "Bach"), like in Christy's name.

The discovery of Charon allowed astonomers 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 kilometers (728 miles), just under half the size of Pluto. It has 1/7th the mass of Pluto. It has 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.