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 1897–1900, "Hydra" and her sister-ships were partially rearmed; "Hydra" was modified at the La Seyne shipyard. Their small-caliber guns were replaced with one gun forward, eight guns, four 3-pounders, and ten 1-pounder revolver cannon. One of the 14-inch torpedo tubes was replaced with a weapon. In 1908–1910, the old 5.9 in guns were replaced with new, longer L/45 models. The Balkan League, of which Greece was a member, declared war on the Ottoman Empire in October 1912. Two months later, the Ottoman fleet attacked the Greek navy, in an attempt to disrupt the naval blockade surrounding the Dardanelles. The Ottoman