NASA Wants To Redirect Asteroid In Space : Short Wave In movies, asteroids careening towards Earth confront determined humans with nuclear weapons to save the world! But a real NASA mission to change the course of an asteroid (one not hurtling towards Earth), the Double Asteroid Redirection Test (DART), is about to launch.

NPR science correspondent Nell Greenfieldboyce joins the show to talk about what it takes to pull off this mission and how it could potentially protect the Earth in the future from killer space rocks.

Email the show at shortwave@npr.org.

A Mission To Redirect An Asteroid

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EMILY KWONG, HOST:

You're listening to SHORT WAVE...

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KWONG: ...From NPR.

Hey, everybody. Emily Kwong here. So we know that in the past, rocks from outer space have struck Earth and caused destruction, sometimes mass destruction. And the idea of having to deflect an incoming asteroid is a staple of science fiction.

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BILLY BOB THORNTON: (As Dan Truman) It's what we call a global killer, the end of mankind. Doesn't matter where it hits. Nothing would survive - not even bacteria.

STANLEY ANDERSON: (As President) My God.

KWONG: That's a clip from the movie "Armageddon." It came out the same year as the similarly themed yet scientifically more plausible flick "Deep Impact."

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MORGAN FREEMAN: (As President Beck) Now, we get hit all the time by rocks and meteors, some of them the size of cars, some no bigger than your hand.

KWONG: And then there's "Meteor" from 1979.

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UNIDENTIFIED ACTOR: (As character) It's 5 miles wide, it's traveling at a speed of 30,000 miles per hour, and there is no place on Earth to hide.

KWONG: And these three movies all have something in common when it comes to how they deal with this potential threat. NPR science correspondent Nell Greenfieldboyce is here. I'll bet she knows what it is.

NELL GREENFIELDBOYCE, BYLINE: I assume it's nukes - right? - nuclear weapons.

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KWONG: Yes, Hollywood's favorite counterstrike, the nuke.

GREENFIELDBOYCE: Yeah. You know, Hollywood loves to go after asteroids with nukes. But, you know, if people really care about planetary defense, they say that is just generally way too messy and uncontrolled and, you know, suboptimal for all kinds of reasons. You might end up with, like, lots of smaller but still deadly asteroids heading your way.

KWONG: Yeah, it might create a whole other problem. And, oh, Nell, this is the expertise we crave on this show. You're here to talk about ways to really redirect an asteroid and a very real NASA mission that is scheduled to launch this week for this very purpose. My understanding is that this will be the first time, with this mission, that NASA has ever tried to change the movement of an asteroid.

GREENFIELDBOYCE: Yep, it is indeed NASA's first test mission for planetary defense. That makes it a big deal. And it's kind of unusual. Nancy Chabot is at the Johns Hopkins University Applied Physics Laboratory.

NANCY CHABOT: A lot of times when I tell people that NASA is actually doing this mission, they kind of don't believe it at first, maybe because it has been a thing of movies.

GREENFIELDBOYCE: She's the coordination lead for the Double Asteroid Redirection Test, or DART.

CHABOT: So DART is demonstrating asteroid deflection. It is absolutely not asteroid disruption, which is how it goes a lot of times in the movies.

GREENFIELDBOYCE: The plan isn't to blow up an asteroid, but just to give one a little nudge to see how it reacts.

KWONG: OK. So today on the show, what it takes to give an asteroid this kind of scootch and how this subtle maneuver could potentially inform future efforts to protect us from killer space rocks. You're listening to SHORT WAVE, the daily science podcast from NPR.

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KWONG: All right, Nell, Hollywood aside, how much of a threat are asteroids to our planet really? I mean, does NASA really need to test out ways to protect us from space rocks?

GREENFIELDBOYCE: It's enough of a threat that NASA set up a Planetary Defense Coordination Office back in 2016. Its goal is to find potentially hazardous objects out in space, track them, understand what they're like, figure out deflection methods and also to coordinate with departments like FEMA - you know, the Federal Emergency Management Agency.

KWONG: FEMA? FEMA's involved in this, in asteroid disaster planning?

GREENFIELDBOYCE: Oh, totally, yeah. I mean, depending on how much notice the Earth had, the best option might be to just evacuate a geographic area to save lives. So FEMA would be working on that. NASA and FEMA and a bunch of other agencies regularly hold these kind of tabletop exercises, these kind of fictional but realistic scenarios to think about what would need to be done if telescopes ever spotted a rock that was headed our way.

KWONG: So what have NASA and other space agencies found up there?

GREENFIELDBOYCE: Telescopes have found and tracked more than 90% of the really big asteroids that scientists think are out there, so, you know, like a kilometer across or larger, the kind of huge asteroid that could cause extinction events. And, you know, there's no threats like that in the foreseeable future. But we know less about smaller asteroids.

KWONG: OK. How small are we talking?

GREENFIELDBOYCE: Well, like around, you know, 400, 500 feet across. If one of those struck the Earth, it could make a crater that's miles wide, and it would devastate a whole region. It could take out a city, you know, just cause mass chaos on the Western or Eastern Seaboards. Nancy Chabot told me of the ones like that that we know about and have tracked, none are on a path that would threaten Earth in, like, the next hundred years.

KWONG: OK. I'm relieved to hear that. That's some good news.

GREENFIELDBOYCE: But she also told me (laughter) that more than half of the ones that size that scientists think should be out there haven't been found and identified and tracked yet.

CHABOT: And that's why that's such a priority for planetary defense.

KWONG: Yeah.

GREENFIELDBOYCE: Keep in mind, compared to planets or moons, asteroids are really, really small. They can be really dark. It's just hard to spot them.

KWONG: Yeah, not super comforting that there are asteroids of this size that we don't know about somewhere, you know?

GREENFIELDBOYCE: Take comfort from the fact that just this year, NASA gave the go-ahead for work on a new space telescope that's designed to find those kinds of asteroids - you know, the ones that could wipe out a city.

KWONG: OK. Let's say one was found, and it was on a collision course. What could NASA do?

GREENFIELDBOYCE: The basic idea being tested in the upcoming DART mission is that if you can find some kind of threatening asteroid when it's still far out in space, you could do something relatively minor that would alter its trajectory and make it no longer a threat. For example, you could just ram a spacecraft into it.

KWONG: Wait. That's NASA's grand plan here, to just smack an asteroid with a spacecraft?

GREENFIELDBOYCE: The fancy name for it is a kinetic impactor.

KWONG: (Laughter) OK.

GREENFIELDBOYCE: It's a kinetic impactor test. But, yes, they're just going to hit the asteroid with a spacecraft that's going 15,000 miles per hour.

KWONG: OK. No nukes, just good, old-fashioned kinetic impactors. So what asteroid does NASA plan on ramming into with this mission?

GREENFIELDBOYCE: It is not one that's threatening Earth in any way. So they're sending the DART spacecraft out over 6 million miles away to a well-known asteroid system that's got two asteroids together.

KWONG: OK.

GREENFIELDBOYCE: That's the double in the name of this mission. It's the Double Asteroid Redirection Test. And Nancy Chabot says there's this big asteroid that's orbited by a smaller one.

CHABOT: So there's the larger asteroid, Didymos, which is 780 meters in diameter. And there's this smaller moon, Dimorphos, that goes around it every 11 hours and 55 minutes.

GREENFIELDBOYCE: So, you know, 780 meters - for those of you who are not into the metric system, that's about 2,500 feet, so, like, half a mile.

KWONG: OK.

GREENFIELDBOYCE: That's the big one, Didymos. And remember, it's the small one, Dimorphos, that NASA wants to hit. And this asteroid, the target, is about 500 feet across. And the DART spacecraft is about the size of a golf cart. So it'll be like ramming a golf cart into, like, the Great Pyramid.

KWONG: And will that be enough to noticeably change Dimorphos', this asteroid's, movement around its bigger buddy asteroid?

GREENFIELDBOYCE: Yeah. Basically, the DART spacecraft will hit it head-on. And this is expected to shorten the time the asteroid will then take to travel around its larger companion. So maybe its orbit will go from 11 hours and 55 minutes to 11 hours and 45 minutes. It'll change just a little bit, but it should be measurable.

KWONG: Proving this idea. But how are they going to track that 'cause I assume once the spacecraft hits the asteroid, I mean, that craft is gone, right?

GREENFIELDBOYCE: Well, it'll be in pieces, yeah. So let me back up for a minute. I mean, no one has ever seen the little asteroid that they're going to hit.

KWONG: Oh.

GREENFIELDBOYCE: No one knows what it looks like, if it's smooth or has big boulders or what. What telescopes do is watch the bigger asteroid and see it dim as the little orbiting one passes in front of it. So, you know, they'll be able to watch this asteroid system in the wake of the collision, and they'll be able to see if the timing of this dimming changes. So, you know, they're only going to get pictures of, you know, this target asteroid in the final seconds of the mission that the DART spacecraft will be sending back and sending back and sending back. They'll get all these pictures, and then the pictures will just stop because the spacecraft has crashed into it.

KWONG: That is Hollywood-level dramatic. OK. But you just said, Nell, that asteroids are hard to see. And it seems like this one they're trying to hit is pretty small. Is there any chance they're going to, like, miss the bull's-eye?

GREENFIELDBOYCE: I talked to Elena Adams about that. She also works at the Applied Physics Laboratory. She's the mission systems engineer.

KWONG: OK.

GREENFIELDBOYCE: And she says this spacecraft will use the kind of automatic navigation and tracking systems that were developed for missile defense, which...

KWONG: Oh.

GREENFIELDBOYCE: ...You know, is another situation where you're trying to hit a small moving object at high speeds.

KWONG: Yeah.

GREENFIELDBOYCE: She told me, in the last four hours of the mission, the spacecraft will be flying itself under its own control.

ELENA ADAMS: During the first three hours of its autonomous operations, it only sees Didymos, the bigger asteroid. And so the whole time, it's just watching this larger asteroid and trying to steer itself towards it. And by the time it detects the smaller asteroid, Dimorphos, it's going to switch. It's going to say, now I'm guiding myself into the smaller body.

GREENFIELDBOYCE: And she and the other mission managers and scientists and engineers will have nothing to do at that point but just watch and wait.

ADAMS: It is four hours of watching paint dry, but kind of terrifying.

KWONG: Yup, nerve-wracking. And I know, Nell, the mission's launch window is about to open up this week, but when will the DART crash into this asteroid?

GREENFIELDBOYCE: Yeah, so after launch, it takes, like, 10 months to get to the asteroid system.

KWONG: OK.

GREENFIELDBOYCE: And that means the impact will occur in late September, early October next year. It depends just on exactly when it launches.

KWONG: And if it hits, how soon will they know the mission was a success in shifting the position of the asteroid?

GREENFIELDBOYCE: So it'll take weeks of telescope time, probably, to do telescope observations. You know, they might be going on for months. They'll also be able to get some photos coming back from a small cube satellite that'll be sort of riding along with the spacecraft for a while. It'll carry this with it, and then it jettisons it about a week before the impact. So this little, tiny satellite will be sort of separate, and it'll be able to spy on the impact and send back pictures of how much debris got kicked up and all that sort of thing. And then in 2024 - so, you know, in a few years - the European Space Agency is actually sending a mission called Hera to this asteroid system. And that should be able to, you know, see the impact crater and take all kinds of measurements.

KWONG: And, Nell, not to be nervous, but is there any chance that the upcoming DART test could send the asteroid careening off - I don't know - towards Earth, towards us?

GREENFIELDBOYCE: I actually asked Nancy Chabot that exact question.

CHABOT: There is absolutely no way that the DART test is a threat to the Earth.

GREENFIELDBOYCE: It's far away. I mean, you can just rest assured on that one. It's not going to come for us.

KWONG: OK. All right. Will you come back and see us next year, and we can talk about what happens with this mission?

GREENFIELDBOYCE: Oh, absolutely.

KWONG: All right.

GREENFIELDBOYCE: Absolutely. Assuming the launch goes OK and the solar panels unfurl and, you know, the spacecraft works.

KWONG: Way to be positive, Nell.

GREENFIELDBOYCE: I've just seen a lot in my years of covering it, so no assumptions till it's on its way.

KWONG: A hundred percent. Nell, thanks for coming on the show with us.

GREENFIELDBOYCE: Thanks for talking asteroids.

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KWONG: This story was produced by Eva Tesfaye and Thomas Lu and edited by Gisele Grayson. Margaret Cirino checked the facts. The audio engineer was Patrick Murray. I'm Emily Kwong, and you are listening to SHORT WAVE, the daily science podcast from NPR.

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