Montage of our solar system

A team of radar astronomers who have been observing asteroids from the giant Arecibo radar/radio telescope have identified a near-Earth metal asteroid for the first time, the researchers reported today in Science magazine.

Orbiting from just beyond Earth to just inside Jupiter's orbit and back every 4.7 years, the 2-kilometer- (1.6-mile-) wide object called 1986 DA shows a radar brightness "far greater than any of the five dozen asteroids we have observed before or since," according to Dr. Steven Ostro of the NASA's Jet Propulsion Laboratory (JPL), leader of the team. "This supports the hypothesis that it is a large lump of iron, nickel, and other metals."

Although they observed the asteroid in 1986 soon after its discovery, the scientists needed almost 5 years to analyze the data and "build up a statistical population of observed asteroids," Dr. Ostro said. Only then was it clear that the asteroid's albedo, or reflected brightness, is extremely high, leading to the conclusion about its composition.

Earth-approaching asteroids, like this one, and Earth-crossing asteroids, whose orbits pass within Earth's orbit, are believed to have come originally from the main Asteroid Belt between Mars and Jupiter. So are meteorites, the stony or metallic fragments that survive after entering Earth's atmosphere. Using color or spectral differences obtained with optical astronomy, scientists have tentatively classified asteroids according to the compositions observed first-hand in meteorites. The asteroid 1986 DA is the first, however, for which clear and emphatic radar confirmation of a metallic composition has occurred.

This asteroid is also remarkable in its shape. Analysis of the radar echoes reveals an extremely lumpy body, possibly even multi-lobed like 1989 PB, observed and imaged by Ostro and colleagues almost two years ago.

These results may shed light on the early history of the solar system. 1986 DA may be a relic of a larger body which came together, melted, separated into layers of heavy metal and light rock, cooled, and then was smashed into fragments in a catastrophic collision with another large asteroid.

The new radar results also offer the prospect of a valuable mineral resource for eventual space colonists. Although meteoritic metal is mostly iron with about 8 percent nickel, it also contains 10 parts-per-million (one thousandth of one percent) of platinum-group metals and gold; that would be about 100,000 tons in a body the size of 1986 DA.

The scientific team includes Dr. Donald Campbell and Alice Hine of the National Astronomy and Ionosphere Center, Cornell University, which operates the Arecibo Observatory for the National Science Foundation; Dr. John Chandler and Dr. Irwin Shapiro of the Harvard-Smithsonian Center for Astrophysics; Dr. Scott Hudson of Washington University and Keith Rosema of the Jet Propulsion Laboratory.

This work is part of the Planetary Astronomy Program of NASA's Office of Space Science and Applications, with additional support by the National Science Foundation.

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