Pluto, which was once the ninth planet in the Solar System but is now only the second dwarf planet, is known to have three moons. Charon is by far the largest, with a diameter of 1,207 kilometers (750 miles), just over half of Pluto’s. Because Charon is so large in comparison to Pluto, it is sometimes considered a dual dwarf planet system with Pluto rather than a satellite. This is because the system’s barycenter (gravitational center of mass) is outside, rather than inside Pluto. Nix and Hydra, Pluto’s other two moons, have diameters of 46137 km and 61167 km, respectively.
The diameters of these satellites cannot be determined with certainty because they are too small and far away to be resolved by modern telescopes. Pluto, named after the Roman god of the underworld, has satellites named after minor figures from Roman mythology’s underworld. For example, Charon is named after a ferryman who transported the souls of the dead across the river Styx to Hades, their final resting place.
Charon is mostly black with white splotches at the poles, unlike Pluto, which is slightly reddish with large black splotches. Pluto and Charon are the largest known Solar System bodies that are tidally locked — that is, they face each other from the same side. If you stood on Pluto’s side facing Charon, it would always stay in the same place in the sky, never changing; if you stood on the opposite side, Charon would never be visible.
Charon was discovered in 1978 by astronomer James Cristy, 48 years after Pluto was discovered. On photographic plates of Pluto, it appeared as a slight bulge. From Earth’s perspective, Charon and Pluto experienced a series of mutual eclipses between 1985 and 1990, a rare occurrence that occurs only twice in Pluto’s 248-year orbital period. The existence of Charon was proven beyond a shadow of a doubt by observations of these mutual eclipses. For the first time, Hubble Space Telescope observations separated Pluto and Charon into separate discs in the 1990s.
The other moons, Nix and Hydra, are quite small, and little is known about them. The Pluto Companion Search Team of the Hubble Space Telescope discovered these moons in June 2005.