Pluto, formerly the ninth planet in the Solar System, now merely the second dwarf planet, is known to possess three moons. By far the largest is Charon, with a diameter of 1,207 km (750 miles), just over half that of Pluto. Charon is so comparably large it is sometimes considered to be part of a dual dwarf planet system with Pluto rather than as a satellite. This is because the barycenter (gravitational center of mass) of the system is located outside rather than within Pluto. Pluto's other two moons, Nix and Hydra, have diameters of 46−137 km and 61−167 km respectively.
The diameters of these satellites cannot be determined for sure because they are too small and distant for contemporary telescopes to resolve. Pluto, being named after the Roman god of the underworld, has its satellites named after minor figures found in the underworld of Roman mythology. Charon, for example, is named after a ferryman who took the souls of the dead across the river Styx to their final resting place, Hades.
In contrast to Pluto, whose appearance is slightly reddish with large black splotches, Charon is mostly black with white splotches at the poles. Pluto and Charon are the largest known Solar System bodies to be tidally locked to one another — that is, the same side of each faces the other. If you were to be on the side of Pluto facing Charon, it would always stay at the same position in the sky, never changing — if you were to stand on the opposite side, Charon would never be visible at all.
Charon was discovered by astronomer James Cristy in 1978, 48 years after the discovery of Pluto itself. It appeared as a slight bulge on photographic plates with images of Pluto. In 1985 and 1990, Charon and Pluto entered into a series of mutual eclipses from our perspective on Earth, a rare event that only happens twice in Pluto's 248 year orbital period. Observations of these mutual eclipses confirmed the existence of Charon beyond a shadow of a doubt. In the 1990's, observations by the Hubble Space Telescope resolved Pluto and Charon into separate discs for the first time.
Nix and Hydra, the other moons, are quite small, and little is known about them. These moons were discovered in June 2005 by the Hubble Space Telescope Pluto Companion Search Team.