Hydra (moon) the Hydra, the nine-headed serpent that battled Heracles in Greek mythology. The nine heads of Hydra are a reference to Pluto's tenure as the ninth planet; its initial, "H", refers to the Hubble Telescope, which discovered Hydra and, together with Nix, to the "New Horizons" mission whose safe passage was the motivation for taking the Hubble images. Hydra orbits the barycenter of the system in the same plane as Charon and Nix, at a distance of about 65,000 km. Its eccentricity of 0.0059 is small, but significantly non-zero, and the largest of those of Pluto's small moons (slightly larger than
HMS Hydra (A144) the first commission of HMS "Hydra". She was decommissioned and sold to the Indonesian Navy in 1986 and renamed KRI "Dewa Kembar" (Pennant Number 932); she was still in service in 2006. There have been eight ships of the name in the Royal Navy, named for the Hydra of Greek Mythology, a serpent with many heads (though nine is generally accepted as standard), the centre one of which was immortal. The monster was overcome and slain by Hercules. The ship's badge of HMS "Hydra" depicts the monster with seven heads. The ship's motto was "Ut Herculis Perseverantia" ("Like Hercules Persevere").