Comet, Asteroid Distinction Blurred

Comets and asteroids, small chunks of drifting solar-system material that sometimes collide with planets and produce fireworks, have long been considered separate types of celestial bodies. But the distinction between them has blurred, and last month a comet that is behaving like an asteroid turned up, and a supposed asteroid has sprouted a cometlike tail.

In general, said Brian Marsden of the Smithsonian Astrophysical Observatory in Cambridge, Mass., comets are expected to have very elongated orbits that take them out to the distant reaches of the solar system, while asteroids tend to gather in relatively circular orbits that remain relatively close to the sun, many of them in the main asteroid belt between Jupiter and Mars.

But Asteroid 1996 PW, identified by Gareth Williams at the Smithsonian observatory from data supplied by the Jet Propulsion Laboratory in Pasadena, Calif., has violated the rule. This little object is currently cruising, asteroidlike, in the main asteroid belt, but its orbit resembles that of a comet.