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Near-Earth asteroids: class consciousness.

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.C.). Details will appear in an April issue of ASTROPHYSICAL JOURNAL.

Spacewatch researchers, who include James V. Scotti, say they aren't sure how the Arjuna class formed. These bodies might represent material gouged out of the lunar surface when other, larger asteroids slammed into it, Gehrels says. Or the Arjuna asteroids might be fragments of comets that passed close to Earth. But he adds that such fragments, like their parent comets, would likely have parabolic rather than circular- orbits. Gehrels says he now favors another model: that the Arjuna asteroids are the secondary fragments, or grandchildren, of members of the main asteroid belt, which lies between Mars and Jupiter.

Asteroid chunks leave the main belt under the influence of Jupiter's gravity, and some become kilometer-size, nearEarth objects. According to Gehrels, some of these chunks may collide with each other, and these small "fragments of fragments" might constitute the Arjuna class.

Marsden says that although the small size of these near-Earth asteroids is intriguing, scientists currently lack compelling evidence that all of the bodies were created by the same physical process. "It's a grave mistake to name a class that may not have any significance;' he says.

Spectra of these fleeting objects may settle the controversy, Rabinowitz says. "Five years ago, [astronomers] had only observed about 100 near-Earth asteroids he notes. "Now, in the past five years, scientists have found about 200." Researchers, he says, are in the midst of searching for telltale patterns among these bodies.
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Title Annotation:new asteroids identified
Author:Cowen, Ron
Publication:Science News
Date:Feb 20, 1993
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