Rocky Relics: Getting the Lowdown on Near-Earth Asteroids

By Cowen, Ron | Science News, February 5, 1994 | Go to article overview

Rocky Relics: Getting the Lowdown on Near-Earth Asteroids


Cowen, Ron, Science News


Earth shares the inner solar system with a swarm of objects--some of which pass closer to us than the moon. Astronomers know relatively little about these roughly 200 odd-shaped bodies, called near-Earth asteroids (NEAs), which range from 40 kilometers to 10 meters in length. A few thousand more of these NEAs--some with the potential to crash into our planet--may await discovery. But scientists have only recently begun efforts to obtain close-up images of these rocky bodies. Indeed, even their origin remains a matter of debate.

"I'm interested in them just out of natural curiosity; they're nearby and we really don't know much about them," says Lucy A. McFadden, an astronomer at the University of Maryland at College Park. "Every time we study NEAs, we're constantly surprised. The more we look, the more we find and the more we see things like [possible evidence of] new types of asteroid populations."

Indeed, the proximity of NEAs affords a unique opportunity to study the general properties of asteroids. Astronomers believe these chunks of rocky material are leftover rubble from the formation of the solar system--planetary wanna-bes that lacked the gravitational glue to stick together to form another Mercury, Venus, Earth, or Mars.

The composition, shape, and surface appearance of asteroids may harbor important clues to conditions in the early solar system, says planetary scientist Richard P. Binzel of the Massachusetts Institute of Technology. And new evidence provides further hints that asteroids--NEAs in particular--may be closely related to two other relics from the solar system's infancy--comets and meteorites.

"These bodies [NEAs] may provide the missing links between meteorites, comets, and main-belt asteroids," Binzel says.

NEAs represent only a tiny fraction of the million or so asteroids 1 km or larger in diameter thought to orbit the inner solar system. Most asteroids reside in a belt-shaped region that lies between the orbits of Mars and Jupiter. However, astronomers have discovered some 200 asteroids that periodically roam much closer to our planet. Some of these have orbits that intersect that of Earth (see sidebar).

The NEAs so far detected may represent just the tip of the iceberg. Astronomers estimate that there exist 5,000 to 10,000 near-Earth objects with diameters of 0.5 kilometers or larger. These objects come in three classes: Amors, Apollos, and Atens. Amors cross the orbit of Mars and approach that of Earth, and 10 percent of them cross Earth's path over the course of a few hundred to a few thousand years. Apollos cross Earth's orbit, and a few even come closer than the moon. Atens, for most of their orbit, lie closer to the sun than Earth does, but may intersect Earth's path at their farthest point from the sun.

Astronomers have found about half the known NEAs in the past five years and discover two or three new ones each month, thanks in part to improved search techniques, more sensitive photographic films, and the use of sensitive electronic light detectors, known as charged-coupled devices. In addition, astronomers are now using several telescopes--mostly small instruments--for the sole purpose of searching for these small bodies.

One of these telescopes, the 0.9-meter Spacewatch, sits atop Arizona's Kitt Peak and automatically scans the sky each night for near-Earth objects. In 1991, Tom Gehrels, David L. Rabinowitz, James V. Scotti, and their colleagues at the University of Arizona in Tucson used the 72-year-old telescope to detect the closest known approach of an asteroid (SN: 11/30/91, p.358). Dubbed 1991 BA, this NEA came within 171,000 km of Earth--less than half the distance to the moon. Moreover, at 10 m in diameter, the asteroid ranks as the smallest near-Earth object ever detected.

Last February, after analyzing the properties of several NEAs found by Spacewatch, Rabinowitz announced that his team may have found a new class of asteroid (SN: 2/20/93, p. …

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