LEILA FADEL, HOST:
When a space rock threatens Earth in movies like "Armageddon," the answer is almost always, blow it up with nukes.
(SOUNDBITE OF FILM, "ARMAGEDDON")
STANLEY ANDERSON: (As President) This new one you're tracking - how big?
BILLY BOB THORNTON: (As Dan Truman) It's what we call a global killer. Nothing would survive, not even bacteria.
FADEL: Science fiction, obviously - but a more realistic strategy for planetary defense is being tested for the first time in history today by NASA. The agency will try to push an asteroid off course by ramming it with a spacecraft. NPR's Nell Greenfieldboyce tells us what to expect.
NELL GREENFIELDBOYCE, BYLINE: Have no fear. The asteroid that NASA is targeting is not on a collision course with Earth. It's about 7 million miles away. And officials stress that there's no way their test could send it careening towards us.
TOM STATLER: We're doing this test when we don't need to on an asteroid that isn't a danger just in case we ever do need to and we discover an asteroid that is a danger.
GREENFIELDBOYCE: That's Tom Statler, a NASA program scientist for the DART mission. That stands for Double Asteroid Redirection Test.
STATLER: We are moving an asteroid. We are changing the motion of a natural celestial body in space. Humanity has never done that before. And this is stuff of science fiction books and really corny episodes of "Star Trek" from when I was a kid. And now it's real.
GREENFIELDBOYCE: At 7:14 p.m. Eastern Time, a spacecraft that launched last year will slam into the asteroid, which is called Dimorphos. In terms of size and scale, the scientists say this will be like a golf cart ramming the Great Pyramid in Egypt at 14,000 miles per hour.
Elena Adams is a mission systems engineer at the Johns Hopkins University Applied Physics Laboratory.
ELENA ADAMS: This mission has two parts. The first part is hitting the asteroid. The next part is actually measuring what happens afterwards.
GREENFIELDBOYCE: Everyone can watch. NASA will broadcast images from the DART spacecraft on its website. In the last hour, the asteroid will loom bigger and bigger as the spacecraft gets closer.
ADAMS: Our last image is probably going to be from about 2 1/2 seconds prior to impact.
GREENFIELDBOYCE: And then, crash. Another little spacecraft nearby will take pictures and send them back to Earth in the coming days. Plus, telescopes on seven continents as well as in space will watch the smash-up and its aftermath. This asteroid is actually orbiting another bigger asteroid. The collision should drive it closer to its big buddy and slightly shorten the time it takes to go around. Asteroid experts have long called for a test like this.
Former astronaut Ed Lu is executive director of the Asteroid Institute, a program run by a nonprofit dedicated to planetary defense.
ED LU: Someday, we are going to find an asteroid which has a high probability of hitting the Earth, and we are going to want to deflect it.
GREENFIELDBOYCE: He says he's 100% certain of that because asteroids have been hitting Earth for its entire history. Astronomers who search for space rocks have found most of the really big ones. None threaten Earth. But smaller ones that could take out a city - a lot of those just haven't been seen.
Lindley Johnson is NASA's planetary protection officer. He says more asteroids need to be found and tracked so there's plenty of time to devise a plan - years, decades, even centuries - which is a far cry from the usual Hollywood scenario.
LINDLEY JOHNSON: So you have to make it exciting, and, you know, we find the asteroid only 18 days before it's going to impact, and everybody runs around with their hair on fire. That's not the way to do planetary defense.
GREENFIELDBOYCE: He says, with enough warning, you could just give an asteroid a little nudge, like in today's test.
Nell Greenfieldboyce, NPR News. Transcript provided by NPR, Copyright NPR.
