"Gravity Tractor" Could Deflect Earth-Bound Asteroids

Stefan Lovgren
for National Geographic News
November 9, 2005

A spacecraft could use a gravity "towline" to alter the course of an Earth- bound asteroid, a new study by two NASA astronauts suggests.

Previous schemes to deflect an incoming space rock range from landing a spacecraft on the asteroid and pushing it off course to blowing it up with nuclear weapons.

The new plan takes a gentler approach. A spacecraft would hover above the asteroid and gradually pull it off course using nothing more than the gravitational attraction between the two bodies.

"If an asteroid is found to be at an impact trajectory with Earth … you will have many decades of notice. And it turns out that you only need to change its velocity by a very small amount in order to prevent a collision," said Edward Lu, a NASA astronaut at the Johnson Space Center in Houston, Texas.

Lu devised the plan with fellow NASA astronaut Stanley Love. The pair report their findings in tomorrow's issue of the science journal Nature.

Nuclear Option

Formed during the creation of the solar system, most asteroids are made of rock, while about 3 percent are made of metals like iron. Most orbit the vast region of space between Mars and Jupiter.

The probability of a large asteroid hitting Earth is extremely slim. But if one did strike, it could cause widespread damage. Because of this scientists have been looking for ways to deflect asteroids from potential collision courses with Earth.

One scheme proposes attaching a spacecraft to an asteroid and firing the craft's engines to push the space rock away.

The plan presents some practical difficulties, partly because asteroids spin as they travel through space. The task is also made harder because scientists are unsure what the surface of an individual asteroid looks like.

"Landing means dealing with a rough surface with poorly known physical properties and somehow compensating for the asteroid's rotation, which wants to whirl your thrust[ers] … around like a lawn sprinkler," Love said. "Using gravity as a towline frees you from those messy details."

Lu adds that blowing up an asteroid with nuclear weapons or slamming a large spacecraft into it to break it up are "bad ideas."

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