"If you do that, you better have a darn good idea where all those pieces are going," Lu noted.
With the gravity plan, a spacecraft would not have to dock on the asteroid, but instead hover above its surface. The craft's thruster jets would angle outward to avoid blasting the asteroid's surface and pushing it away.
"You would use this small gravity force between the [spacecraft and the asteroid] as your towline to basically pull this thing," Lu explained.
"We had looked at landing on the surface [of an asteroid] and all the problems associated with that, when we found that the gravitational force is about equal to the force that you plan to be pushing with anyhow," he said. "We realized, Wow, rather than pushing, why don't we just pull?"
The astronauts calculate that, given a lead time of about 20 years, NASA could launch a spacecraft that could safely deflect an asteroid some 650 feet (200 meters) across in about a year of "towing."
Lu notes that early intervention makes the job easier: When an asteroid has a close Earth approach, followed by a later return and possible impact, the gravitational tug needed to prevent the impact would be much smaller if applied before the close approach.
This scenario applies to 99942 Apophis, a 1,000-foot (320-meter) asteroid that will swing by Earth at a distance of about 19,000 miles (30,000 kilometers) in 2029. The flyby will change the asteroid's orbit, and there is a small chance (about 1 in 5,000) that it will hit Earth when it returns in 2036.
"If around 2013 we find out that it's going to hit, we could [initiate] a very tiny change in [the asteroid's] orbit before 2029," Lu said.
Love says he personally doesn't lose much sleep over the possibility of a major asteroid impact, but that it would be foolish to put off making plans for such an event until the last minute.
"In my office, we do not wait until the building is on fire to conduct fire drills," the astronaut said. "Recent large natural disasters, such as the tsunami in the Indian Ocean and the hurricanes in our own country, underscore the value of being prepared for an emergency."
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