Protecting earth: gravitational tractor could lure asteroids off course.

When a wayward asteroid is about to smash into Earth, scriptwriters for the movie Armageddon called in Bruce Willis to drill into the rock and nuke it. He succeeds but sacrifices his own life.

Now, two NASA scientists, both also astronauts, suggest a simpler, safer, and much more plausible way of diverting an offending asteroid. Their method relies on the gravitational tug of a massive, unmanned spacecraft to pull the rock away from a damaging rendezvous with Earth.

The gravitational tractor, as the researchers call their proposed craft, would require the sustained power of a nuclear-propulsion system to reach the asteroid and perform the maneuvers that would be required to deflect it. For general space exploration, NASA has already proposed a fleet of suitable vehicles, although their funding is currently uncertain.

As envisioned by Ed Lu and Stan Love of NASA'S Johnson Space Center in Houston, the gravitational tractor would hover some tens of meters from a spinning asteroid. Only the force of gravity would connect the two.

Careful control of the tractor's thrusters would keep the craft close to the asteroid as it slowly pulled the rock off its collision course. Given enough lead time, it would take just a year for a 20-ton spacecraft to drag a 200-meter-wide asteroid weighing about 60 million tons away from Earth's path, Lu and Love calculate in the Nov. 10 Nature. Towing would have to begin at least 20 years before the projected collision.

A 200-m-wide asteroid could cause significant Earth damage, and many asteroids are much larger. A bigger asteroid would require a heavier tractor to draw it off course.

A tractor would circumvent many of the problems posed by other asteroid-eradication methods. Bruce Willis to the contrary, no scientist has recently suggested nuking an asteroid, notes Love. That's because many asteroids are now known to be porous, loosely bound agglomerations of rubble.

Attempting to blow up a loose agglomeration of rocky material is a lot like firing a bullet into a pile of sandbags, notes Love. The shock is easily damped. "It's not like blowing up a glass bottle with a BB gun," says Love. "Life is not like the movies."

A more realistic protection plan--directly attaching a craft to an asteroid and using thrusters to push away the body--has its own complications. An asteroid's weak internal structure and low surface gravity would make it difficult for a craft to link up.

But the biggest problem is that asteroids rotate. Unless the attached craft stopped the asteroid from spinning or fired its engine only at specific times, each thrust would push the asteroid in a different direction rather than along a chosen course.

Because the proposed gravitational tractor hovers rather than binds, it provides a unidirectional tug, regardless of the asteroid's spin or composition, notes Love.

"It's very clever," comments planetary scientist William Bottke of the Southwest Research Institute in Boulder, Colo. Because the method requires several years of advance warning, it could also inspire more-detailed surveys of near-Earth asteroids, he adds.