a fourth moon orbiting the icy dwarf planet Pluto. The tiny, new
satellite, temporarily designated P4, was uncovered in a Hubble
survey searching for rings around the dwarf planet.
The new moon is the smallest discovered around Pluto. It has an
estimated diameter of 8 to 21 miles (13 to 34 km). By comparison,
Charon, Pluto's largest moon, is 648 miles (1,043 km) across, and the
other moons, Nix and Hydra, are in the range of 20 to 70 miles in
diameter (32 to 113 km).
"I find it remarkable that Hubble's cameras enabled us to see such a
tiny object so clearly from a distance of more than 3 billion miles
(5 billion km)," said Mark Showalter of the SETI Institute in
Mountain View, Calif., who led this observing program with Hubble.
The finding is a result of ongoing work to support NASA's New Horizons
mission, scheduled to fly through the Pluto system in 2015. The
mission is designed to provide new insights about worlds at the edge
of our solar system. Hubble's mapping of Pluto's surface and
discovery of its satellites have been invaluable to planning for New
Horizons' close encounter.
"This is a fantastic discovery," said New Horizons' principal
investigator Alan Stern of the Southwest Research Institute in
Boulder, Colo. "Now that we know there's another moon in the Pluto
system, we can plan close-up observations of it during our flyby."
The new moon is located between the orbits of Nix and Hydra, which
Hubble discovered in 2005. Charon was discovered in 1978 at the U.S.
Naval Observatory and first resolved using Hubble in 1990 as a
separate body from Pluto.
The dwarf planet's entire moon system is believed to have formed by a
collision between Pluto and another planet-sized body early in the
history of the solar system. The smashup flung material that
coalesced into the family of satellites observed around Pluto.
Lunar rocks returned to Earth from the Apollo missions led to the
theory that our moon was the result of a similar collision between
Earth and a Mars-sized body 4.4 billion years ago. Scientists believe
material blasted off Pluto's moons by micrometeoroid impacts may form
rings around the dwarf planet, but the Hubble photographs have not
detected any so far.
"This surprising observation is a powerful reminder of Hubble's
ability as a general purpose astronomical observatory to make
astounding, unintended discoveries," said Jon Morse, astrophysics
division director at NASA Headquarters in Washington.
P4 was first seen in a photo taken with Hubble's Wide Field Camera 3
on June 28. It was confirmed in subsequent Hubble pictures taken on
July 3 and July 18. The moon was not seen in earlier Hubble images
because the exposure times were shorter. There is a chance it
appeared as a very faint smudge in 2006 images, but was overlooked
because it was obscured.
Hubble is a project of international cooperation between NASA and the
European Space Agency. NASA's Goddard Space Flight Center in
Greenbelt, Md., manages the telescope. The Space Telescope Science
Institute (STScI) in Baltimore conducts Hubble science operations.
STScI is operated for NASA by the Association of Universities for
Research in Astronomy Inc. in Washington.