To steer an asteroid away from Earth, try a space 'tractor'

If an asteroid ever threatens to collide with Earth, scientists have a toolkit of ideas worthy of a Hollywood blockbuster. They might blow it up or divert it by smacking it with a projectile or planting a rocket motor on its surface.

Now, two NASA astronauts are proposing a far more subtle approach: a space "tractor" that uses gravity to tow those hurtling space rocks onto a nonthreatening orbit.

The issue: Astronomers have their eye on an asteroid called 99942 Apophis, discovered last year. If it hits a gravitational "sweet spot" during a close approach to Earth in 2029, astronomers say it would hit the planet when it returns in 2035 or 2036. The likelihood that Apophis will thread the eye of this gravitational needle is probably vanishingly small, they add, but they haven't been able to calculate the asteroid's orbit with enough precision yet to know for sure.

If diversion of Apophis, or any other asteroid, becomes necessary, the typical toolkit of approaches falls short, says astronaut Edward Lu. He and fellow astronaut Stanley Love describe the tractor concept in a paper appearing in the current issue of the journal Nature.

"You want a system with predictable results," he says. Unfortunately, approaches discussed so far don't guarantee astronomers would get the desired effect. Some have dubbed them "blast and hope" methods, he says.

Dr. Lu and Mr. Love figured there had to be a better way. Their high-tech John Deere is a pendulum-like spacecraft with most of its mass at one end and thrusters at the other. The craft would hover above the asteroid's surface with the heavy end closest to the space rock. Mutual gravitational attraction between the tractor and the asteroid connects the two objects. Using nozzles carefully aimed to avoid the exhaust hitting the asteroid, and relatively gentle "puffs" of thrust, the tug could haul an asteroid into a new orbit in a predictable way. If the asteroid has its own tiny moons - as an increasing number of asteroids appear to have - they get pulled along as well.

"It's a beautiful and entirely new idea," notes Clark Chapman, a scientist at the Southwest Research Institute in Boulder, Colo., who studies asteroids, comets, and other small bodies in the solar system.

A significant challenge to blast-and-hope approaches is that their effect depends a great deal on whether the asteroid is a rubble pile or a chunk of metal. Indeed, some researchers have argued that if an asteroid threatens, humans would need to mount a robotic reconnaissance mission to find out how the object is put together before they could figure out how to deal with it effectively.

With a gravitational tractor, it doesn't matter if the asteroid "has the consistency of a mountain of metal or a mountain of cotton candy. It can be moved without having to interact with it," Dr. Chapman explains.

Lu adds that asteroids can have odd shapes, and they tumble as they move along their orbits. A rocket motor place on the asteroid's surface would face serious steering problems.

The key to their idea, he and Love hold, is the right propulsion system - nuclear-electric motors. These are the only type of motors that can develop the velocity needed to close in on a potentially hazardous asteroid, then provide the gentle thrust over the decade or more needed to adjust the asteroid's orbit. Researchers have sent craft to asteroids using chemical propulsion, he acknowledges. But mission planners have had the luxury of picking tortoise-paced targets relative to Earth's motion. Most asteroids that make up the population of near-Earth objects move much faster.

As elegant as Lu and Love's approach appears, it may not lift off the pages of Nature very soon. NASA has shelved a project to develop nuclear propulsion - a casualty of the agency's effort to focus technology development on a replacement for the space shuttles, due for retirement in five years.

The good news? The duo calculates that Apophis is one of the exceptions that could be handled by a 1-ton "gravitational tractor" using chemical propulsion.