Hitching a ride on a cosmic 'rubble pile'

This weekend, a tiny spacecraft begins the most critical phase of a 35-month journey. It will end with the probe orbiting - or even landing on - an asteroid that may provide a window on the evolution of the solar system.

On Sunday, the Near Earth Asteroid Rendezvous orbiter will take final aim toward a Jan. 10 encounter with a potato-shaped chunk of space debris twice the size of Manhattan. Once it arrives, the craft will circle the asteroid for at least a year, giving scientists their first detailed look at one of the most primitive objects in the solar system.

"It's extremely important to know the composition and structure of these objects," says Donald Yeomans, a planetary scientist at Cal Tech's Jet Propulsion Laboratory in Pasadena, Calif., and one of the lead members of NEAR's research team.

Until now, planetary scientists have had to rely on meteorite fragments, telescopes, and three blink-and-you-miss-it flybys to study these celestial leftovers. But these orbits eventually will take NEAR to within 100 meters of the surface of the asteroid, called 433 Eros, and perhaps to the surface itself. From there, the craft's six instruments will help scientists determine what the asteroid is made of and how it's put together.

Some asteroids are thought to be building blocks of planets that never formed; others are held to be the remains of fledgling planets that formed, only to disintegrate under heavy bombardment from other asteroids or protoplanets. Each of these possibilities would leave a unique signature that NEAR might be able to detect, helping researchers determine the asteroid's origins.

The information NEAR will collect also has potential future applications. Data on composition and structure would be critical for determining which near-Earth asteroids hold the highest potential as sources for raw materials such as titanium, iron, or water.

In addition, such intelligence would be indispensable to any plans for diverting an asteroid if it appears to be on a collision course with Earth.

Thus, while NEAR's mission is one of scientific discovery, it also represents "a prototype of the kind of instrument package that you would have to carry" to do research and prospecting on asteroids, says Joseph Veverka, an astronomy professor at Cornell University in Ithaca, N.Y., and another of the lead members of NEAR's science team.

Part of a new program

Launched on February 1996, the $211.5 million mission was the first to be sent into space under the National Aeronautics and Space Administration's "Discovery" program. The program is aimed at speeding the pace of unmanned planetary exploration by using smaller, less comprehensive spacecraft that could be launched more frequently and at lower cost than previous planetary probes.

On Sunday, when the more powerful of NEAR's two rocket motors fires to begin its rendezvous approach, the mission will enter its trickiest phase, according to Robert Farquhar, mission manager at The Johns Hopkins University's Applied Physics Laboratory.

"This is the critical part right now," he says. "If these burns don't go off as planned, we could end up flying by this thing and then trying to get back. The rendezvous could be postponed by several months," he explains, adding that the craft also could miss its rendezvous with the asteroid altogether.

Prep work

The science begins even before NEAR takes up its orbit around Eros. As the craft approaches the asteroid, its camera will be looking for satellite pieces that may be orbiting Eros.

In addition, scientists will use radio signals from the craft to determine the strength of the asteroid's gravity and how it varies with the object's irregular shape.

This information will help mission planners refine the spacecraft's orbit to allow for ever-closer passes. The gravity data also will be combined with maps of the object, derived from a laser rangefinder and NEAR's camera, to calculate the asteroid's density.

Using the same technique during NEAR's June 1997 flyby of the main-belt asteroid 253 Mathilde, which lies between the orbits of Mars and Jupiter, NEAR scientists discovered that the world of asteroids may hold more variety than they previously had thought.

Up to that point, Dr. Yeomans says, "we tended to think of asteroids as big slabs of rock."

Mathilde helped disabuse planetary scientists of that notion.

"We were really surprised at the numbers, because the density was very low," Dr. Farquhar says. Had the asteroid been much less dense, "this thing would have floated," he says. "This suggested that the asteroid was very porous, a loose rubble pile."

Researchers, who hope to build a movie of Eros's motion from images NEAR beams back shortly after it arrives, say they expect this asteroid to provide similar surprises.