Charon, also called (134340) Pluto I, is the largest of the five known moons of the dwarf planetPluto. It was discovered in 1978 at the United States Naval Observatory in Washington, D.C., using photographic plates taken at the United States Naval Observatory Flagstaff Station (NOFS). With half the diameter and one eighth the mass of Pluto, it is a very large moon in comparison to its parent body. Its gravitational influence is such that the barycenter of the Pluto–Charon system lies outside Pluto.
The New Horizons spacecraft, to date the only probe to have visited the Pluto system, approached within 27,000 km (17,000 mi) of Charon.