Hubble Space Telescope discovery of Styx, Pluto's fifth moon.[a] (also informally known as P5) is a small natural satellite of Pluto whose discovery was announced on 11 July 2012. It is the fifth confirmed satellite of Pluto, and was found approximately one year after S/2011 (134340) 1 (or "P4"), Pluto's fourth discovered satellite. The moon is estimated to have a diameter of between 10 and 25 kilometers (6 and 16 mi), and orbital period of 20.2 ± 0.1 days.