Hubble discovered a new moon for Pluto, fourth in number

The moon was discovered as part of the Pluto system observations project, which aims to help plan the journey of the New Horizons spacecraft during its transit through the Pluto system in 2015

Astronomers using the Hubble Space Telescope have discovered a fourth moon orbiting the dwarf planet Pluto. The tiny moon - temporarily known as P4 - appeared in Hubble's survey looking for rings around the dwarf planet.

The new moon is the smallest yet discovered around Pluto. It is difficult to know exactly its diameter but it is estimated between 13 and 34 kilometers. By comparison, the diameter of Charon, Pluto's largest moon, is 1,043 kilometers and the other moons Nix and Hydra are between 32 and 113 kilometers in diameter.

"I find it impressive that the Hubble cameras were able to allow us to see such a small object clearly from over 5 billion kilometers away," said Mark Showalter of the ST Institute in Mountain View, California, who led the observing program for the Hubble.

The findings are the result of ongoing work in support of NASA's New Horizons mission, which is scheduled to fly by Pluto in 2015. The mission was designed to provide new insights into the worlds at the edge of the solar system. The mapping of Pluto's surface by Hubble and the discovery of its moons are very important for the planning of the New Horizons approach mission to Pluto.
"This is a great discovery," says New Horizons project principal investigator Alan Stern of the Southwest Research Institute in Boulder, Colorado. "Now that we know there is another moon in the Pluto system, we can focus our observations on it during the flyby."

The orbit of the new moon lies between the orbits of Nix and Hydra, the moons that Hubble discovered in 2005. Charon was discovered in 1978 by the US Naval Observatory and was first clearly imaged in 1990 by Hubble as a separate body from Pluto.

The dwarf planet Pluto's moon system is hypothesized to have formed in a collision between Pluto and a similar-sized body in the early history of the solar system. The collision caused a flight of materials that joined the family of satellites observed orbiting Pluto.

Moon rocks brought to Earth by the Apollo missions have led to the theory that our Moon was formed as a result of a collision between Earth and a Mars-sized body 4.4 billion years ago. The scientists believe that the moons are bombarded by the impact of micro meteorites originating from the ring surrounding Pluto, but in the images taken by the Hubble no such ring has been discovered so far.
"This surprising observation is a strong reminder of Hubble's ability as a multipurpose astronomical observatory to make exciting discoveries," said John Morse, director of the Astrophysics Division at NASA's Washington Headquarters.

P4 was first seen in an image taken by Hubble's Wide Field Camera 3 on June 28. It was confirmed by repeated observations on July 3 and 18. The moon was not visible in previous photos because the exposure times in these photos were shorter. There is a chance that it appeared as a smear in the 2006 images that was mostly obscured by other bodies.