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 in reserve for the remainder of the war. "Hydra" was decommissioned in 1918 and stricken from the naval register the next year. She was to be sold for scrapping in 1921, but instead returned to her role as a naval artillery school from 1922 until 1929 when she was broken up for scrap. Greek ironclad Hydra Hydra () was an ironclad warship of the Greek Navy, named for Hydra, one of the Saronic Gulf islands which played a key role in the war at sea during the Greek War of Independence. The lead ship of her class of ironclads, she