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Natural disasters have been front-page news lately, from hurricanes to a possible flu pandemic. Here's another one to worry about: killer asteroids. Giant asteroids, like the one that wiped out the dinosaurs, only smack the Earth every 10 million years or so. But smaller asteroids, say the size of a football stadium, hit us every few thousand years. Now scientists say they've come up with a new way to knock out dangerous asteroids and knock them off course without even having to touch them. NPR's Nell Boyce explains.

NELL BOYCE reporting:

When you watch a movie like "Armageddon," the answer to killer asteroids seems obvious.

(Soundbite of "Armageddon")

Unidentified Man: Why don't we just send up 150 nuclear warheads and blast that rock apart?

(Soundbite of music; explosions)

BOYCE: Nuclear warheads. Hollywood loves them; scientists hate them.

Mr. ED LU (Astronaut): That's a truly bad idea for a number of reasons.

BOYCE: Ed Lu is an astronaut at NASA's Johnson Space Center in Houston.

Mr. 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 doesn't 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?

BOYCE: Lu and his colleagues say that uncertainty is totally unacceptable and also unnecessary. Astronomers will probably spot a big asteroid decades before the doomsday impact, so there would be time to fly a spaceship to the asteroid, give it a push and send it veering off course.

Mr. LU: 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 have--you assume that 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?

BOYCE: Lu and his colleague Stanley Love were recently pondering that problem when they had a strange thought.

Mr. LU: Well, maybe you don't even need to land on it at all.

BOYCE: The scientists knew that all objects exert a gravitational tug on each other. 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?

Mr. LU: When we plugged the numbers in, we were like, `Hey, it actually works.' The force of gravity is strong enough, even though it's tiny, between an asteroid and a spacecraft--it is strong enough to tow it substantially.

BOYCE: The scientists describe their idea for a gravity tractor in this week's issue of the journal Nature. Lu already has a possible target for his tractor. It's an asteroid a quarter of a mile wide, and it has a tiny chance of hitting the 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.

Mr. RUSTY SCHWEICKART (Apollo Astronaut): The hovering technique the--what we affectionately call the Ed Lu tractor beam--will be good enough.

BOYCE: That's Rusty Schweickart, an Apollo astronaut who now lobbies for more research into asteroid deflection. He loves the gravity tractor idea, but he thinks most asteroids will need a bigger push than it could easily provide.

Mr. SCHWEICKART: It would take an extremely large spacecraft to deflect a large asteroid that would be headed directly for the Earth.

BOYCE: That's why Schweickart believes the best bet is still to land on asteroids and give them a shove. He says NASA was getting ready to fund a demonstration project to do just that, but with NASA's recent budget woes, that plan has been put on hold. Nell Boyce, NPR News.

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