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 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 was ordered in 1885 in response to a crisis in the Balkans and Ottoman naval expansion. The ship was launched in 1889 and delivered to Greece by 1892. She was armed with a main battery of three guns and five guns, and had a top speed of . "Hydra" and her sisters saw