Distant vista. View of Pluto and Charon from the surface of one of the new moons, as seen in this artist's conception.

Three's Company for Pluto

Astronomers have discovered two new moons around Pluto, raising the planet's known number of satellites to three. Measuring some 50 to 150 kilometers across, the new moons are much smaller than Charon, the 1192-km diameter satellite of Pluto discovered in 1978. Astronomers believe the small moons, temporarily designated S/2005 P1 and S/2005 P2, may be rubble from the same primordial collision that gave birth to Charon and could shed new light on the early history of the outer solar system.

The new moons were first spotted on 15 June by Max Mutchler, an instrument analyst at the Space Telescope Science Institute in Baltimore, Maryland. He was studying photos of Pluto taken on 15 and 18 May by the Hubble Space Telescope as part of a dedicated satellite search. Last week, astronomers Marc Buie of Lowell Observatory in Flagstaff, Arizona, and Eliot Young of the Southwest Research Institute (SwRI) in Boulder, Colorado, also found the two tiny moons on Hubble photos made on 14 June 2002. The discovery was subsequently announced on Halloween in an electronic circular by the International Astronomical Union. "I thought it was a Trick-or-Treat joke for a moment," says Jim Christy, the discoverer of Charon.

Alan Stern of SwRI, a member of the discovery team headed by Hal Weaver of Johns Hopkins University in Baltimore, says he is surprised by how compact Pluto's satellite system is. Charon orbits Pluto at a distance of some 22,000 kilometers, while the new moons are about 50,000 and 65,000 kilometers away. Christy says he thought additional satellites this close to the planet would be unlikely, given the disruptive gravitational tug of Pluto and Charon.

Attempts in September to see the new satellites with large ground-based telescopes were unsuccessful, and final confirmation of their existence will have to await new Hubble observations in mid-February 2006. By then, NASA will have launched its New Horizons spacecraft, set to flyby Pluto in the summer of 2015 (ScienceNOW, 11 August 2004:). "We [now] have four objects to divide our attention among at Pluto, rather than two," says Stern, who is New Horizon's principal investigator. "I wish we had a second spacecraft, given the richness of the system."

Related sites
Background information on the discovery
New Horizons mission to Pluto
Pluto and Charon