Concept

Hydra (moon)

Summary
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. Members of the New Horizons team suspected that Pluto and Charon might be accompanied by other small, distant moons, weakly bound to the Pluto system. They used the Hubble Space Telescope to test this hypothesis. This lead to the discovery of Nix and Hydra – both surprisingly close to Pluto/Charon – and that no significant moons existed in Pluto's extended sphere of influence. 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. 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.
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