When is a comet not a comet?--When it is an asteroid. And when is an asteroid not an asteroid?--When it is a comet. Asteroids are not supposed to have tails, yet comets are. Comets are not supposed to have completely stable orbits in the inner solar system, yet asteroids are.
Although the distinction between "comet" and "asteroid" has been becoming less and less clear for some decades now, few astronomers were prepared for something that happened this month. In the space a less than a week, two completely opposite extremes were reported. Something that looks precisely like an asteroid was discovered moving in an entirely cometary orbit with a revolution period about the sun of several millennia, and something moving in an orbit completely characteristic of the main belt of asteroids has a tail!
"Asteroid" 1996 PW was identified by Gareth Williams at the Minor Planet Center at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, in data assembled at the Jet Propulsion Laboratory in Pasadena, California, by the Near-Earth Asteroid Tracking team, led by Eleanor Helin, that examines electronic images obtained with a telescope operated by the U.S. Air Force in Haleakala, Hawaii. His latest computation of the orbit has the object currently at its closest to the sun and located in the main asteroid belt between Mars and Jupiter, although at the outer extreme of its 7000-year-period orbit 1996 PW moves out to more than 20 times the distance of Neptune.
"Comet" 1996 N2 was identified by Eric Elst at the Belgian Royal Observatory in Brussels on photographs taken by Guido Pizarro at the European Southern Observatory in Chile. The object's tail, present on these photographs, has been fully confirmed on electronic images by several observers, who are not, however, able to detect the fuzzy head or coma that comets usually show. Computations place 1996 N2 in a typically asteroidal orbit, nearly circular and close to the earth's orbital plane, at some three times the earth's distance from the sun and traveling around it in rather less than six years.
One speculation is that 1996 PW is an inert comet, with its ices either completely dispersed or--more likely--completely smothered by the dust and dirt comets typically contain mixed with their ices. On earlier passages near the sun, particularly the one that initially brought it in from the the vast reservoir of the "Oort Cloud" surrounding the solar system--it perhaps had the more characteristic appearance of a comet. Alternatively, it might just be an asteroid deflected from the main belt as the result of a collision with another small body. Until now, the known object most like 1996 PW is Damocles, an apparent asteroid with an eccentric, inclined orbit stretching from the orbit of Mars to just beyond the orbit of Uranus.
As for 1996 N2, it seems likely that the tail is of rather recent origin, conceivably dust raised from the surface, again perhaps as the result of a recent collision. One suspects the object to be an ordinary asteroid, and indeed, two probable observations of it have now been recognized on photographs taken in 1979. Until now, the known object most like 1996 N2 is an object named Wilson-Harrington, noticed to have a tail on just a single night in 1949, although its orbit is that of a typical short-period comet, not that of a main-belt asteroid.
Brian G. Marsden, 1996 Aug. 26
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