Close Encounters

A recent mission to an asteroid reveals secrets about these drifting space rocks.

Asteroids have been getting a bad rap of late, which is better than the rap they used to get, which was none at all. But several developments in the past few months--including fresh data from a spacecraft's visit to one asteroid and now the nightly appearances of another--have fostered a new interest in asteroids among professional and amateur astronomers alike.

Ever since January 1, 1801, when Italian monk and astronomer Giuseppe Piazzi discovered a relatively small celestial object he later christened Ceres, asteroids have been the orphans of the solar system--planets without portfolio. Maybe they were pieces of a planet that never quite coalesced. Maybe they were pieces of a planet that did coalesce but at some later point suffered a catastrophic collision. Maybe they were pieces of neither, but simply an odd assortment of space rocks left over from the solar system's birth and swept over time into the gap between the orbits of Mars and Jupiter by the give-and-take of gravity.

Maybe "maybe" doesn't belong in the scientific vocabulary--though in the case of asteroids, astronomers were willing to make an exception. There seemed to be no end to the number of asteroids, and no compelling reason to seek one. Occasionally an observer would stumble across a new asteroid, name it (often after a spouse, friend, or pet), and then pretty much forget about it. In 1887 a Scottish astronomer thought he'd found a new star, only to realize he'd rediscovered an asteroid that had first been sighted eighty years earlier. In 1916 U.S. astronomers Seth Nicholson and Harlow Shapley discovered a new asteroid (which they named after Shapley's wife, Mildred), only to lose it for much of the rest of the century (it resurfaced in 1991).

Today the state of asteroid knowledge' can still be as spotty as the sky during a meteor shower. Most meteorites, in fact, are fragments of asteroids; astronomers can now say at least that much with certainty. But ask how many asteroids there might be out there, and the estimates range from hundreds of thousands to billions. Ask whether any of the asteroids that are not part of the main belt between Mars and Jupiter might be on an eventual collision course with Earth, and the answer is even less precise. Nobody knows.

Thanks to Hollywood's special-effects artists, as well as to an astronomer's mistaken prediction a couple of years ago that an asteroid could potentially be heading for an impact with our planet some thirty years hence, it's the apocalyptic possibilities of asteroids that have most captured the popular imagination. For scientists, however, these minor planets have begun to ignite a different kind of speculation: What if at least some asteroids are remnants of the primordial solar system?

Last February the spacecraft NEAR Shoemaker entered a close orbit with the asteroid Eros, specifically to investigate this possibility. Still, nobody at NASA had dared hope for what happened on May 4. For half an hour, while instruments on the NEAR tracked the 21-mile-long asteroid from a distance of only 31 miles, a flare from the Sun washed over Eros, causing its elements to radiate X rays--and, in the process, to reveal the asteroid's chemical composition. Preliminary results show a collection of magnesium, silicon, and aluminum that has never undergone the kind of intense heating and melting that would characterize the development of a more mature terrestrial body. Therefore, Eros is indeed very likely a relic from the first stage of the solar system's formation.

That's not true of all asteroids, however, and one prominent exception has been visible in the night sky in recent months. Spectroscopic studies of Vesta during the 1990s revealed that lava once flowed on its surface, which means that this rather massive asteroid is a kind of cousin to Earth. With its diameter of approximately 335 miles, Vesta is the third-largest asteroid in the main belt. It is also the only main-belt asteroid occasionally seen with the naked eye by observers on Earth.

As September begins, Vesta is wrapping up a four-month period of such visibility. For the first few days of the month, in the hours just after nightfall, it will be shining in the southern sky at magnitude 5.8--just about the limit of our unaided viewing capabilities. From about September 7-19 it will wash out in the glare from the Moon, but when Vesta returns, it will be appearing near a reasonably conspicuous marker in the sky: the 5.5-magnitude star SAO 188192. Although by that time you'll need binoculars to see Vesta, careful monitoring should reveal the asteroid moving against the background stars from one night to the next.

Vesta won't be visible to the naked eye again until February 2003. Until then it may be out of sight but, like asteroids themselves these days, no longer out of mind.

Richard Panek is the author of Seeing and Believing: How the Telescope Opened Our Eyes and Minds to the Heavens (Penguin, 1999).