Hydra is a natural satellite of Pluto, with a diameter of approximately 51 km (32 mi) across its longest dimension.[6] 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,[1] and was named after the Hydra, the nine-headed underworld serpent in Greek mythology.[14] By distance, Hydra is the fifth and outermost moon of Pluto, orbiting beyond Pluto's fourth moon Kerberos.[11]

Hydra has a highly reflective surface caused by the presence of water ice, similar to other Plutonian moons.[15] Hydra's reflectivity is intermediate, in between those of Pluto and Charon.[16] The New Horizons spacecraft imaged Pluto and its moons in July 2015 and returned multiple images of Hydra.[17]