Since its discovery in 2005, Pluto's moon Hydra has been known only as a fuzzy dot of uncertain shape, size, and reflectivity. Imaging during New Horizons' historic transit of the Pluto-Charon system definitively resolved these fundamental properties of Pluto's outermost moon. Long Range Reconnaissance Imager (LORRI) observations from a distance of approximately 640,000 kilometers revealed an irregularly shaped body characterized by significant brightness variations over the surface. With a resolution of 3 kilometers per pixel, the LORRI image shows the tiny potato-shaped moon measures 43 kilometers by 33 kilometers.
Like that of Pluto's largest moon Charon, Hydra's surface is probably covered with water ice. Observed within Hydra's bright regions is a darker circular structure approximately 10 kilometers in diameter. Hydra's reflectivity is intermediate between that of Pluto and Charon.