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Observations From Hubble Reveal Pluto Has Two Additional Moons

By Kenneth Chang

And then there were three.

Astronomers using NASA’s Hubble Space Telescope announced Monday that they had spotted two small moons circling Pluto. That gives Pluto, the smallest of the nine planets, a total of three moons, or more than four of the other planets.

“I can’t really say we’re really surprised,” said Hal Weaver, a planetary scientist at Johns Hopkins University’s Applied Physics Laboratory in Laurel, Md., a co-leader of the observations. “We proposed this, to look for satellites around Pluto.”

Other astronomers had not been as optimistic. “Pretty much the community said: ‘Eh, why are you looking? You’re not going to find anything,”’ said another Hubble team member, Marc W. Buie, at the Lowell Observatory in Flagstaff, Ariz.

The proposal was turned down twice before being approved in September 2004, Weaver said.

Pluto’s largest moon, Charon, was discovered in 1978. Hubble spotted the second and third moons May 15 and May 18. For now, they are known only as S/2005 P1 and S/2005 P2. S/2005 P1, estimated to orbit 40,000 miles from Pluto, is up to 100 miles wide. P2 is likely 10 to 15 percent smaller and about 30,000 miles from Pluto.

The discovery does not directly play into the recent debate of whether Pluto should be considered a planet; a moon is not a prerequisite for planethood. (Mercury and Venus have none.)

The discovery, however, will aid planning for NASA’s New Horizons mission, which will be the first spacecraft to visit Pluto. It is scheduled for launching next year.

“We’ll have to divide our attention four ways instead of two,” said Dr. S. Alan Stern, director of the Southwest Research Institute’s Department of Space Studies in Boulder, Colo., the principal investigator for New Horizons. He was also a co-leader of the discovery team.