The Latest: The largest of Pluto's five moons, Charon, was discovered 40 years ago in June 1978 by James Christy and Robert Harrington at the U.S. Naval Observatory in Flagstaff, Arizona―only about six miles from where Pluto itself was discovered at Lowell Observatory. They weren't even looking for moons. They were trying to refine Pluto's orbit around the Sun.
Charon was discovered in 1978 when sharp-eyed astronomer James Christy noticed images of Pluto were strangely elongated. The blob seemed to move around Pluto. The direction of elongation cycled back and forth over 6.39 days―Pluto's rotation period. Searching through their archives of Pluto images taken years before, Christy found more cases where Pluto appeared elongated. Additional images confirmed he had discovered the first known moon of Pluto.
At half the size of Pluto, Charon is the largest of Pluto's moons and the largest known satellite relative to its parent body. Pluto-Charon is our solar system's only known double planetary system. The same surfaces of Charon and Pluto always face each other, a phenomenon called mutual tidal locking. Charon orbits Pluto every 6.4 Earth days.
Charon is almost half the size of Pluto. The little moon is so big that Pluto and Charon are sometimes referred to as a double dwarf planet system. The distance between them is 19,640 km (12,200 miles).
The Hubble Space Telescope photographed Pluto and Charon in 1994 when Pluto was about 30 AU from Earth. These photos showed that Charon is more neutral grey than Pluto (which has a red tinge), indicating that they have different surface compositions and structure.
Charon's orbit around Pluto takes 6.4 Earth days, and one Pluto rotation (a Pluto day) takes 6.4 Earth days. Charon neither rises nor sets, but hovers over the same spot on Pluto's surface, and the same side of Charon always faces Pluto―this is called tidal locking. Compared with most of the planets and moons, the Pluto-Charon system is tipped on its side, like Uranus. Pluto's rotation is retrograde: it rotates backwards, from east to west (Uranus and Venus also have retrograde rotations).
How Charon got its Name
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. Apart from the mythological connection for this name, Christy chose it because the first four letters also matched the name of his wife, Charlene.