WASHINGTON (AFP) - Astronomers using NASAs powerful Hubble Space Telescope have spotted a tiny fourth moon around Pluto, the smallest ever glimpsed around the icy dwarf planet, the US space agency said Wednesday. The small moon, named P4 for now until a better name is decided upon, is only about eight to 21 miles (13-24 kilometers) in diameter. Plutos largest moon, Charon, is 648 miles (1,043 km) across. The other two, Nix and Hydra, are between 20 and 70 miles in diameter (32 to 113 km), NASA said. Hubble discovered Nix and Hydra in 2005. Astronomers at the US Naval Observatory glimpsed Charon in 1978.