Nov. 9, 2005— The gravity of a spacecraft could one day be used to alter the course of an Earth-threatening asteroid, according to astronauts at NASA's Johnson Space Center.

The so-called gravitational tractor design not only capitalizes on advanced rocket technology already planned for some NASA programs, but also provides a cheaper and more fastidious solution than other methods.

"If you're trying to deflect an asteroid from hitting the Earth, you better be very controlled before you do it. You don't want to do the silly Hollywood movie thing, which is to blow it up," said astronaut Edward Lu, who with astronaut Stanley Love published the proposal in the Nov. 10 issue of Nature.

To date, astronomers have discovered more than 800 asteroids about two-thirds of a mile wide circling the sun in orbits similar to Earth's.

A much smaller version of one these asteroids, named 99942 Apophis, is expected to pass nearer to Earth in 2036 than current orbiting communication satellites.

The 400-meter rock has about a 1 in 5,000 chance of hitting our planet, where it could destroy a city, ignite earthquakes, or unleash tsunamis.

Blowing up an asteroid on a potential collision course with Earth is not under serious consideration, but docking a spacecraft onto the rock and using thrusters to redirect it is one option being pondered.

However, flying a spacecraft to a distant asteroid is difficult because conventional rockets transport limited quantities of fuel and are designed to burn it up quickly to provide thrust.

Even if the ship could get to the asteroid, it would have a hard time landing because asteroids typically have uneven and gravely surfaces.

And because asteroids often tumble in unpredictable ways through space, a great deal of fuel and propellant could be wasted trying to accurately redirect it.

Lu and Love suggest a way that doesn't involve landing on an asteroid at all. In a hypothetical scenario, they describe a 20-ton ship, with a mass similar to a city bus, approaching an asteroid 200 meters in diameter.

The relatively weak gravitational force between both objects — about the force of grasping one apple — would bind them.

To keep from colliding with the asteroid, the rocket ship would turn on its thrusters, which would be angled to the sides of the asteroids instead of directly at the surface.

This would reduce the amount of dust stirred up and the actual energy needed to eventually change the asteroid's speed.

With the thrusters on, the spacecraft would essentially hover above the asteroid and slow it down. Even a tiny change in velocity years before impact would result in a big miss down the road.

"All you do is maintain a parked distance and pull the asteroid along," said Dan Durda, senior research scientist at Southwest Research Institute in Boulder, Colo., and an expert in asteroids.

"This idea can make use of the same nuclear-electric propulsion technology that you're using to go and do other exploration in space."

In fact, Lu and Love sketched out their plan based on spacecraft that were under development by NASA in the Prometheus program, which was focused on building ships with nuclear-electric propulsion systems to accomplish long-distance missions such as landing on Mars or on some of Jupiter's moons.

The program is currently on hold, but according to Lu, NASA could use a more modest spacecraft to redirect Apophis using the gravitational tractor design, should future space surveys currently underway determine that an impact is inevitable.