Charon, discovered in 1978, is, depending on the definition employed, either the largest moon of Pluto or one member of a double dwarf planet with Pluto being the other member.
With the discovery in 2005 of two other moons of Pluto (Nix and Hydra), Charon is now also referred to as Pluto I.
The New Horizons mission is scheduled to visit Charon and Pluto in July 2015.
Charon was discovered by astronomer James Christy on June 22, 1978 when he was examining highly magnified images of Pluto on photographic plates taken a couple of months before.
Christy noticed that a slight bulge appeared periodically.
Later, the bulge was confirmed on plates dating back to April 29, 1965.