Hydra is the outermost known moon of Pluto. It was discovered along with Nix in June 2005, and was visited along with Pluto by New Horizons in July 2015. Hydra's surface is probably covered with water ice. Observed within Hydra's bright regions is a darker circular structure with a diameter of approximately 10 kilometers (5 miles). Hydra's reflectivity (the percentage of incident light reflected from the surface) is intermediate between those of Pluto and Charon.