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Nudging Killer Asteroids Off Course

Space scientists say they've come up with a novel way of a preventing a killer asteroid from hitting the Earth -- without laying a hand on it. A spaceship called a "gravity tractor" could pull the asteroid off course just by hovering near its surface.

The gravity tractor is a slight twist on the most popular proposal for dealing with asteroids that wander too near. The conventional wisdom calls for landing on an asteroid with a spacecraft that would use its engines to physically shove the hunk of rock off course.

But Edward Lu, an astronaut at NASA's Johnson Space Center in Houston, says that "the difficulty with that method is that you have to land on the asteroid and attach yourself somehow or other, but you don't know what it's going to look like until you get there. Let's say you assume the asteroid has a smooth surface. And you get there and it's covered with giant boulders. Well, how are you going to land on it?"

Lu says that he and his colleague Stanley Love were recently pondering that problem when they had a strange thought: "Well maybe, you don't even need to land on it at all."

The scientists knew that all objects exert a gravitational tug on each other. So they wondered if the mere presence of a ship near an asteroid could push or pull it off course. What if, say, a 20-ton spacecraft spent a year just hovering above the surface of an asteroid? "When we plugged the numbers in, we were like, hey, it actually works," recalls Lu. "The force of gravity is strong enough, even though it's tiny between an asteroid and the spacecraft, to tow it substantially."

Lu and Love describe their gravity tractor in this week's issue of the journal Nature. And they already have a possible target. It's an asteroid a quarter of a mile wide called Apophis, and it has a tiny chance of hitting Earth in 2036.

This particular asteroid is taking an odd, meandering course through space. So it would take an especially small push to get rid of it. "The hovering technique, what we affectionately call the Ed Lu tractor beam, will be good enough," says Rusty Schweickart, an Apollo astronaut who now chairs the B612 Foundation, which lobbies for more research into asteroid deflection. (The foundation is named after the asteroid featured in Antoine de Saint-Exupéry’s classic, The Little Prince.)

Still, even though Schweickart loves the idea of a gravity tractor, he thinks most asteroids will need a bigger push than it could easily provide. "It would take an extremely large spacecraft to deflect a large asteroid that would be headed directly for the Earth," he says. Building and launching such a huge craft would be a major enterprise, or a smaller ship would have to hover for years and years.

That's why Schweickart still thinks the best bet is to land on asteroids and give them a shove. He says NASA was getting ready to fund a demonstration project on a "convenient asteroid" to prove that this kind of push could be done. But with NASA's recent budget woes, that plan has been put on hold, which Schweickart finds short-sighted. "The Earth is not in any way prepared to defend itself against a devastating natural disaster," he says.

Of course, in Hollywood movies like Armageddon, the answer to a killer asteroid always seems simple: use nuclear weapons to blast it to bits. But most scientists don't see nukes as a viable option. "That's a truly bad idea for a number of reasons," says Lu. "You don't know what you're going to get. It's the blast and hope method. You set off your nuclear device and you hope it doesnt break it up into pieces and you hope the pieces don't hit the Earth and you hope it has the effect you want. As to what you’re going to get, well, you'll find out afterwards, right?"

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Nell Greenfieldboyce
Nell Greenfieldboyce is a NPR science correspondent.