BOSTON — Earthlings are about to get their first up-close peek at one of the most primitive objects in the solar system - a Rhode Island-sized chunk of rock known as 253 Mathilde.
The asteroid's photo session, which takes place tomorrow morning, represents the first scientific payback from the Near-Earth Asteroid Rendezvous (NEAR) spacecraft. NEAR is spearheading a new class of low-cost, high-tech US space probes designed to help unravel the solar system's unsolved mysteries as it hurtles toward a 1999 meeting with another asteroid, 433 Eros.
Despite asteroids' starring roles in apocalyptic action films, they remain largely a mystery. Current scientific theory holds that asteroids are construction debris left over from the solar system's formation more than 4.5 billion years ago. Ranging from pebble-size to hundreds of miles across, they have attracted widespread attention for the enormous destructive impact they can have on civilization. In 1908, an asteroid estimated at 330 feet across flattened a half-million acres of forest in Siberia. On a positive note, though, they have also been touted as a potential source of minerals for future Earth- and space-based industries.
Much of what astronomers do know about asteroids comes from ground-based telescopes, from meteorites, or from two previous but distant flybys as another probe, Galileo, headed to Jupiter. NEAR will give scientists a wealth of new data. Its primary goal is to orbit Eros - at times within 10 miles of the surface - and spend a year feeding detailed information on the asteroid back to earthbound astronomers. Mathilde became involved when mission planners plotted NEAR's route to Eros and discovered that the probe would pass through Mathilde's neighborhood.
"This is bonus science," says Robert Farquhar, a scientist at the Johns Hopkins University's Applied Physics Laboratory and director of the NEAR mission. "It's a real target of opportunity."
ROUGHLY spherical and about 38 miles across, Mathilde is the largest asteroid ever visited. Until the NEAR mission was announced, however, astronomers hadn't lavished much attention on it, says Andrew Cheng, the mission's project scientist. Interest grew as astronomers discovered that Mathilde is a type of asteroid never visited before.
Based on the minuscule amount of sunlight it reflects, scientists concluded that it belonged to a family of asteroids made up largely of carbon, instead of metals or silicates. Carbonaceous asteroids are thought to be about 75 percent of all asteroids.
But photographing Mathilde will not be a waltz. Noting that NEAR's camera setup was designed with a leisurely close-in orbit in mind, and not a 22,000-mile-an-hour flyby shooting of an object 750 miles away, "this is not the way I'd design a flyby mission," Dr. Farquhar wryly observes.
Mathilde's position isn't known with sufficient precision to aim NEAR's camera in one direction, nor can the camera pan as NEAR passes the asteroid. So the whole craft will have to be constantly slued in hopes that at least one of the 534 exposures made during the 25-minute encounter will provide the much-sought Kodak moment.
Still, the potential payback is enormous. The images, Farquhar says, should be sharp enough to pick out craters as small as the Rose Bowl. Combined with other measurements, including those to be made later this year by the Aricebo radiotelescope in Puerto Rico, NEAR's data should help pin down vital statistics ranging from Mathilde's mass to its density.
The latter is of particular interest, Dr. Cheng says. If the density is high, perhaps three or four times that of water, it would suggest a solidly packed hunk of rock. If the density is close to that of water, however, it would indicate that Mathilde is little more than a lazily spinning pile of rubble that the sun's gravity never allows to stray more than 300 million miles from the solar system's center.