As of summer 2012, astronomers had discovered five moons orbiting Pluto. The largest, Charon, discovered in 1978, is roughly half the diameter of Pluto itself. Of the other moons, two have been named by the International Astronomical Union -- Nix and Hydra. The other two, discovered in 2011 and 2012, have received the provisional designations P4 and P5, pending approval of formal names. Nix, Hydra, P4, and P5 are all quite small, with none larger than about 100 miles (160 km) in diameter, with the smallest only about 10 percent of that size.