Magazine article Science News

Solar System Masquerade

Magazine article Science News

Solar System Masquerade

Article excerpt

Call them the great pretenders.

Two solar system wayfarers that mask their true identity-one looks like an asteroid but orbits like a comet, while the other looks like a comet but orbits like an asteroid-were discovered last month.

The first body, dubbed 1996 PW, has the unadorned appearance of an ordinary asteroid and currently lies in the main asteroid belt between Mars and Jupiter. However, Gareth V. Williams of the Smithsonian Astrophysical Observatory (SAO) in Cambridge, Mass., finds that the object has a highly elongated orbit resembling that of a comet.

Williams analyzed images from the Near-Earth Asteroid Tracking team, led by Eleanor F. Helin at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif.

The pictures were taken Aug. 9 by an Air Force telescope atop Mount Haleakala, Hawaii.

He calculates that during its 7,000-year orbit, 1996 PW ventures 20 times farther from the sun than Neptune does, journeying through a region of the outer solar system where only comets had been thought to roam. Indeed, a proposed reservoir for comets, the Oort cloud, is believed to inhabit this remote area. If 1996 PW is truly an asteroid, it would be the first one known to traverse comet territory.

Comets display tails when they visit the inner solar system, where the sun's heat turns the ice on their surface into jets of steam that carry light-reflecting dust. It's possible, notes Brian G. Marsden of SAO, that 1996 PW is a dormant comet whose surface ice has been depleted or smothered by dust and dirt. …

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