P4 is one of Pluto’s moons. It was discovered in 2011 by a large team of NASA and European Space Agency (ESA) astronomers led by Mark Showalter and using the Hubble Space Telescope.

Diameter: 13 to 34km
Mass: Unknown
Composition: Unknown
Orbit: 32.1 Earth days

P4 was the fourth of Pluto’s five moons to have been discovered by astronomers and is the third most distant moon from the planet. It is situated between the orbital paths of Hydra and Nix, which were discovered in 2005. Pluto’s largest moon, Charon, was discovered by the U.S. Naval Observatory in 1978. Hubble was used to refine the image and confirm that it is separate from Pluto. Hubble is an international space cooperation project between NASA and the European Space Agency (ESA).

Astronomers believe that, as with Pluto’s other moons, P4 may have been created during a collision between Pluto and another celestial body. Material that may have been blasted from Pluto’s moons could form a meteor belt or rings around the dwarf planet, but these have yet to be identified by observations. However, the unexpected discovery of P4 leads astronomers to believe that there will be similar future surprises.

Originally designated S/2011 (134340) 1, P4 is only about 10% as bright as Nix. It has the same kind of surface but with one third of the diameter and one tenth of the reflecting areas. It is located at a distance of 5 billion kilometres from Earth.

The first sighting of P4 was in a photograph in June 2011 taken with the Hubble Wide Field Camera. This was confirmed by later Hubble pictures in July. The moon had not been identified on previous Hubble images because exposure times were not long enough. The moon may have appeared on some 2006 photographs as a faint smudge.

Pluto was discovered in 1930. Its status as a planet was removed in 2006 by the International Astronomical Union and it was then called a dwarf planet. This is a category that describes icy bodies in the Kuiper Belt beyond the orbit of Neptune. Before Pluto’s moons were discovered, astronomers thought that these kind of bodies could not have a system of satellites orbiting them.

NASA’s New Horizons mission is due to fly past Pluto in July 2015. The discovery of P4 indicates that the spacecraft may enter a different kind of space environment, even a more hostile one, than was previously assumed.

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