also intentionally chosen that its initial H honors the Hubble Space Telescope used by the Pluto Companion Search Team to discover Hydra and Nix. 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. Origin 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. It was thought that Hydra had initially formed at a closer proximity to Pluto, and its orbit had undergone changes through tidal interactions. 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. Through 'tidal damping' by mutual tidal interactions with Charon, Hydra's orbit around the Pluto-Charon barycenter gradually became more circular over time. Hydra is believed to have formed from two smaller objects merging into one single object. Physical characteristics Hydra is irregular in shape, measuring along its longest axis and its shortest axis measuring across. This gives Hydra the measured dimensions of . The surface of Hydra is highly reflective due to the presence of water ice on its surface. The surface of Hydra displays a neutral spectrum similarly to Pluto's small moons, although the spectrum of Hydra appears slightly bluer. The water ice on Hydra's surface is relatively pure and shows no significant darkening compared to Charon. One explanation suggests that Hydra's surface is continually refreshed by micrometeorite impacts ejecting darker material from the surface of Hydra. The surface spectrum of Hydra is slightly bluish compared to that of Nix. 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. Derived from crater counting data from New Horizons, the surface of Hydra is estimated to be about four billion years old. Large craters and indentations on Hydra suggest that it may have lost some of its original mass from impact events since