LYNN NEARY, HOST:

At 2:18 this afternoon, Eastern Time, an asteroid nicknamed Pitbull will zip past us, barely missing the earth by some 25,000 miles. Now that may seem like it will be pretty far away but in astronomical terms, it's practically next-door.

Pitbull was only discovered a week ago but that's better than usual. Normally, there's no warning when an asteroid is about to hit.

PAUL COX: In fact there have only been two examples of these asteroids coming into Earth's atmosphere which were detected before they hit.

NEARY: That's Paul Cox of the citizen astronomer group Slooh. They've partnered with NASA to keep track of thousands of so-called Near-Earth Objects. There are so many, in fact, that the space agency is unable to detect and monitor all of them.

COX: It came to our attention that these things, rocks, you know, nearly a thousand feet in diameter - after going, spending all the money to actually find them, to discover them in the first place, they were getting lost because nobody was following up the initial discovery. And what's required is lots of observations of these things so that we can accurately determine their orbits.

NEARY: The chances of collision from one of these hurling rocks are low. But when they hit, they hit big. Look at what happened when an asteroid exploded over Siberia in 1908.

COX: That wiped out, it devastated, I think it was something like a thousand square miles of Siberian forest. Now if that asteroid had hit three hours later, it would've hit Moscow. That would have changed the face of twentieth century politics, certainly.

NEARY: So more than a hundred years on, how vulnerable are we to such an event? Surely we have more up our sleeves than what we see in Hollywood blockbuster movies like Armageddon where we pulled out all stops to save the planet.

COX: If we had 10 years warning then yes, in theory we can just keep nudging it with very small amounts of energy. But the only thing that we have in our arsenal at the moment if we discovered something, say, that wasn't going to impact for one or two years maybe, the only thing we can do is exactly like Hollywood suggests. Our nuclear weapons are the only defense we have.

NEARY: That was Paul Cox of Slooh.com, a group of amateur astronomers that helps NASA track the thousands of asteroids in Earth's neighborhood. It will broadcast the visit of Pitbull live on their website. Transcript provided by NPR, Copyright NPR.