Hydra is a natural satellite of Pluto, with a diameter of approximately 51 km (32 mi) across its longest dimension. It is the second-largest moon of Pluto, being slightly larger than Nix. Hydra was discovered along with Nix by astronomers using the Hubble Space Telescope on 15 May 2005, and was named after the Hydra, the nine-headed underworld serpent in Greek mythology. By distance, Hydra is the fifth and outermost moon of Pluto, orbiting beyond Pluto's fourth moon Kerberos.
Discovery images of Nix and Hydra
Hubble image of Pluto's moons (annotated)
Image: Pure Ice Hydra Charon Spectra composite
Image: Pure Ice Hydra Charon Spectra composite
The dwarf planet Pluto has five natural satellites. In order of distance from Pluto, they are Charon, Styx, Nix, Kerberos, and Hydra. Charon, the largest, is mutually tidally locked with Pluto, and is massive enough that Pluto and Charon are sometimes considered a binary dwarf planet.
Charon and Pluto, to scale. Photo taken by New Horizons on approach.
The Hubble discovery image of Nix and Hydra
Discovery image of Styx, overlaid with orbits of the satellite system
Formation of Pluto's moons. 1: a Kuiper belt object approaches Pluto; 2: it collides with Pluto; 3: a dust ring forms around Pluto; 4: the debris aggregates to form Charon; 5: Pluto and Charon relax into spherical bodies.