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Moon of Pluto

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, By distance, Hydra is the fifth and outermost moon of Pluto, orbiting beyond Pluto's fourth moon Kerberos. 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 led 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. Hydra was named after the Lernaean Hydra, a nine headed serpent that battled Heracles in Greek mythology. The names of features on the bodies in the Pluto system are related to mythology and the literature and history of exploration. In particular, the names of features on Hydra must be related to legendary serpents and dragons from literature, mythology, and history.

Pluto's smaller moons, including Hydra, were thought to have formed from debris ejected from a massive collision between Pluto and another Kuiper belt object, similarly to how the Moon is believed to have formed from debris ejected by a large collision of Earth. The ejecta from the collision would then coalesce into the moons of Pluto. In this case, Hydra along with the smaller moons of Pluto would have migrated outwards with Charon into their current orbits around the Pluto Charon barycenter. Explanations for Hydra's bluish color suggest that the surface of Hydra has a higher amount of water ice compared to Nix, which could also explain Hydra's very high geometric albedo, or its reflectivity, of 83 percent. Hydra's chaotic tumbling is largely caused by the varying gravitational influences of Pluto and Charon as they orbit around their barycenter. Hydra has an orbital period of approximately 38.2 days and is resonant with other moons of Pluto. Hydra is in a 2:3 orbital resonance with Nix, and a 6:11 resonance with Styx (the ratios represent numbers of orbits completed per unit time; the period ratios are the inverses). As a result of this "Laplace like" 3 body resonance, it has conjunctions with Styx and Nix in a 5:3 ratio. with a timing discrepancy of 0.3%. A hypothesis explaining the near resonance suggests that the resonance originated before the outward migration of Charon after the formation of all five known moons, and is maintained by the periodic local fluctuation of 5% in the Pluto–Charon gravitational field strength.

The New Horizons spacecraft visited the Pluto system and imaged Pluto and its moons during its flyby on 14 July 2015. At the time of the New Horizons flyby, Hydra was behind Pluto and was further away from New Horizons at closest approach. Hydra's surface composition, reflectivity, and other basic physical properties were later measured by New Horizons during the flyby.