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Observation of Pluto's Satellite Challenges Planetary Theories

By Jeffrey B. Chasnow

A recent citing of Charon, believed to be a moon of Pluto, challenges theories about the origins and characteristics of that planet, Harvard astronomers said yesterday.

Alistair R. Walker, an astronomer at the South African Astronomical observatory, observed Charon April 6 as it passed in front of a distant star and blocked the star's light for 50 seconds. After observing the event--known as an occultation--Walker calculated that Charon has a minimum diameter of 1200 kilometers.

Walker's finding indicates that Charon is almost half as large as Pluto, an unusual proportion for a satellite and a planet in the solar system, Brian G. Marsden, lecturer in Astronomy, said yesterday. The Earth's moon is one-fourth the size of the Earth, Marsden added.

"To find Pluto with a large satellite causes people to scratch their heads a little," Fred Franklin, associate of the Harvard College Observatory, said yesterday.

The occultation, a rare occurrence but an event Franklin said is very valuable for determining the size of distant planets and satellites, gives further evidence of the existence of Charon, which has not been officially recognized as a satellite of Pluto since astronomers at the U.S. Naval Observatory discovered it in July 1978.

James Elliot, director of the Wallace Astrophysical Observatory at the Massachusetts Institute of Technology, said yesterday Charon had previously appeared only as "a bump" on photos of Pluto.

Albert G.W. Cameron, chairman of the Astronomy Department, said yesterday the satellite was found "as a faint patch of light on a photograph of Pluto."

Franklin and Cameron said they are satisfied that Charon exists, but Marsden questioned Walker's single observation.

"I'm not convinced that this observation can be regarded as a confirmation of the existence [of Charon]," Marsden, who has done many celestial observations, said. The observed occultation could have been caused by nothing more than a cloud, he added.

Elliot said the occultation was anticipated, but Marsden said the accuracy of the predictions was "chancy." Because of the great distances of the occulted stars, predicting occultations is difficult, Marsden said, adding, "This is the first time there has been hint of success."

Cameron said he believes proof of the existence of Charon negates a hypothesis which postulates that Pluto originated as an escape satellite of Neptune. "It's difficult for Pluto to be an escape satellite and have a satellite of its own," he said.

Franklin said some evidence indicates that Pluto may be in a separate planetary system, adding that observing future occultations will help scientists learn more about the planet and its moons.

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