Near-Earth Asteroids: Class Consciousness

Article excerpt

After examining the properties of several surprisingly small near-Earth asteroids discovered during the past year, astronomers last week reported that they have identified a new asteroid class. Unofficially named Arjuna, in honor of the hero of an epic Hindu poem, this class contains asteroids measuring no more than 100 meters across and orbiting the sun in a nearly circular path.

Detected by the 0.9-meter Spacewatch Telescope on Arizona's Kitt Peak, these asteroids appear unusual on several counts, says David L. Rabinowitz, a member of the Spacewatch team at the University of Arizona in Tucson. Ten near-Earth asteroids this small far exceeds the number astronomers had predicted Spacewatch could detect, he notes. But most striking, adds Rabinowitz, is that about five of these rocky bodies- those smaller than 50 meters - move about the sun in nearly circular orbits. Current theories about asteroids can't adequately explain why all five of these objects should have circular orbits.

While Rabinowitz considers only these five asteroids to belong to the Arjuna group, Spacewatch director Tom Gehrels believes that all 10 of the small bodies including those with slightly more elliptical orbits - may belong to a special class. Gehrels notes that all 10 move no closer to the sun than the Earth does. However, Brian G. Marsden of the Smithsonian Astrophysical Observatory in Cambridge, Mass., says that a smaller, but significant percentage of larger nearEarth asteroids observed from the ground have similar orbits.

Rabinowitz described the work last week during a seminar at the Carnegie Institution of Washington (D. …