Hubble discovered Pluto’s moons Nix and Hydra in 2005. Astronomers found the two faint satellites in Hubble images taken on May 15 and 18, 2005, while searching the Pluto system for additional moons that might pose a hazard to the New Horizons spacecraft.
The discoveries were announced on October 31, 2005, after researchers confirmed them using earlier archival Hubble images from 2002. The moons initially received the provisional designations S/2005 P 1 and S/2005 P 2; Hydra was P 1 and Nix was P 2.
Nix and Hydra are small, irregular moons orbiting Pluto and Charon. New Horizons later photographed both during its July 2015 flyby. Hydra is the outermost of Pluto’s five known moons, while Nix orbits closer to Pluto than Hydra.
A common mix-up is confusing the discovery year with the naming year. The International Astronomical Union approved the names Nix and Hydra in 2006, but the Hubble discoveries themselves occurred in 2005.