Solar system masquerade.

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.

Alternatively, says Alan W. Harris of JPL, 1996 PW may be just what it appears to be-an asteroid traveling as a comet would. He notes that both comets and asteroids are remnants of material that assembled into planets. Many comets migrated to the fringes of the solar system; a few asteroids may have followed suit, Harris suggests. Scientists haven't observed such an oddball asteroid before simply because asteroids are much rarer than comets in the solar system, he adds.

"There's no dearth of explanations to account for this object," Harris observes wryly.

In contrast, he is at a loss to explain another discovery-a body that sports a tail yet orbits as if it were a main-belt asteroid. Eric Elst of the Belgian Royal Observatory in Brussels identified such an object, 1996 N2, by analyzing pictures taken by Guido Pizarro at the European Southern Observatory in La Silla, Chile. The tail points along the body's travel path rather than away from the sun, as comet tails typically do. Helin has reviewed images taken in 1979 that appear to show 1996 N2 without a tail. One scenario for 1996 N2's acquisition of a tail assumes that the body is an asteroid. The tail might represent dust blasted from 1996 N2's rocky surface during a recent collision with another asteroid. Harris notes, however, that collisions in the asteroid belt are rare. Observations over the next few months should determine whether the tail changes or even vanishes.