Charon is the largest of the five known moons of the dwarf planet Pluto. It's about the size of Alaska (1,200 km in diameter) and compared to other moons in the solar system is very large relative to its parent body.
This image of Charon, taken from a distance of 470,000 km, displays a surprisingly complex geological history, including bright and dark crater rays, and the conspicuous dark north polar region at the top of the image. Surface features are amazingly similar to Pluto.
Astronomer James W. Christy of the US Naval Observatory noticed that images of Pluto appeared elongated while other plates showed the planet as a tiny round dot. After eliminating the possibility that the elongations are produced by plate defects and background stars, the only plausible explanation was that they are caused by a moon orbiting very close to Pluto.
The discovery was formally announced to the world on July 7th, 1978. Christy proposed the name "Charon", after the mythological ferryman who carried souls across the river Acheron, one of the five mythical rivers that surrounded Pluto's underworld.