Hydra is a natural satellite of Pluto, with a diameter of approximately 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. Hydra has a highly reflective surface caused by the presence of water ice, similar to other Plutonian moons. Hydra's reflectivity is intermediate, in between those of Pluto and Charon. The New Horizons spacecraft imaged Pluto and its moons in July 2015 and returned multiple images of Hydra. Discovery Hydra was discovered by researchers of the Pluto Companion Search Team, consisting of Hal A. Weaver and many others involved in the New Horizons mission to Pluto, including Alan Stern and Marc W. Buie. The New Horizons team had suspected that Pluto and Charon might be accompanied by other smaller moons previously undiscovered, hence they used the Hubble Space Telescope to observe faint moons around Pluto. Since Hydra's brightness is about 5,000 times fainter than Pluto, long exposure images of Pluto were taken in order to find Hydra. The discovery images were taken on 15 May 2005 and 18 May 2005. Hydra and Nix were independently discovered by Max J. Mutchler on 15 June 2005 and by Andrew J. Steffl on 15 August 2005. The discoveries were announced on 31 October 2005, after confirmation by precovering archival Hubble images of Pluto from 2002. The two newly discovered moons were subsequently provisionally designated S/2005 P 1 for Hydra and S/2005 P 2 for Nix. The moons were informally referred to as "P1" and "P2" respectively, by the discovery team. Naming The name Hydra was approved on 21 June 2006 by the International Astronomical Union (IAU) and was announced along with the naming of Nix in the IAU Circular 8723. Hydra was named after the Lernaean Hydra, a nine-headed serpent that battled Heracles in Greek mythology. Particularly, the nine heads of Hydra subtly references Pluto's former ninth planetary status. The two newly named moons were intentionally named that the order of their initials N and H honors the New Horizons mission to Pluto, similarly to how the first two letters of Pluto's name honors Percival Lowell. Hydra's name was