Pluto’s outermost known moon, an irregular body with a bright, water-ice-rich surface.
Hydra was discovered in 2005 together with Nix in observations made with the Hubble Space Telescope. Of Pluto’s five known moons, Hydra follows the outermost orbit. It is irregular rather than spherical and does not keep one face pointed toward Pluto.
Like Nix, Hydra tumbles rather than rotating in simple synchronous lockstep. Hubble observations and dynamical models show that its spin is chaotic in the changing gravitational field of Pluto and Charon.
New Horizons found Hydra to be highly reflective, and infrared spectroscopy showed that its surface is dominated by nearly pristine crystalline water ice. That composition offers clues to the debris from which Pluto’s small moons may have formed.
NASA’s Eyes on the Solar System places Pluto in its wider solar system setting and lets you examine the Pluto–Charon system in three dimensions. Rotate the view, zoom in or out, and use the time controls to follow the modeled motion of the system.
NASA Eyes is an interactive visualization provided by NASA/JPL and loads from eyes.nasa.gov. If the embedded view is unavailable, use the full-screen link above.
Pluto has five known moons. Charon is by far the largest and forms a closely coupled pair with Pluto. Farther out, the four much smaller moons—Styx, Nix, Kerberos and Hydra—follow nearly circular orbits around the Pluto–Charon system.
Diagram is schematic; sizes and orbital distances are not to scale.
NASA Science provides mission background, observations and image collections; Wikipedia offers a broader reference overview with citations.
