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
Greek ironclad Hydra the engagement. By 1914, "Hydra" had been withdrawn from active duty to serve as a gunnery training ship. At the outbreak of World War I in at the end of July 1914, Greece's pro-German monarch, Constantine I, decided to remain neutral. The Entente powers landed troops in Salonika in 1915, which was a source of tension between France and Greece. In October 1916, "Hydra" and a pair of torpedo boats defected to the Venizelist faction in the National Schism. Ultimately, the French seized the Greek Navy on 19 October; the heavy units of the Greek fleet were disarmed and placed