In our latest podcast, the DART mission's Nancy Chabot reveals the science behind asteroid deflection.



Chris Bramley Hello and welcome to Radio Astronomy, the podcast from the makers of BBC Sky Night magazine. You can subscribe to the print edition of the magazine by visiting Or to our digital edition by visiting iTunes or Google Play.

Ezzy At the end of September 2022, NASA will attempt to make a spacecraft do something that they normally spend millions of dollars to ensure it doesn't: crash. The Double Asteroid Redirection Test or DART for short. Launched back on 24th November 2021 with the goal of purposely crashing into an asteroid to see how prepared we'd be if we ever needed to protect Earth from potentially hazardous space rock. Ahead of the impact, we spoke with Nancy Chabot, the mission's coordination lead from the Johns Hopkins Applied Physics Laboratory. Hello Nancy. Thank you very much for taking the time to join us today. First, can you just tell me a bit more about what the DART mission actually is?

Nancy The DART mission is a very exciting first step for planetary defence. It's a NASA mission that's built and managed by the Johns Hopkins Applied Physics Lab, and it is purposely crashing a spacecraft into an asteroid to change the motion of that asteroid slightly in space. It's important to say straight up, this asteroid is not a threat to the Earth. That's the target for Dart's test. This is just a test. It's just this first step. But it's an important first step to potentially prevent asteroids from hitting the Earth in the future.

Ezzy And what are the mission's main goals?

Nancy The main goal of the DART mission is to test out a technology called Kinetic Impact Technology, which is a fancy way of saying crashing the spacecraft into the asteroid to move it slightly. This is thought to be the most mature way to potentially deflect an asteroid if one was ever discovered to be on course with hitting the earth. It's never been tested in space before. Dart is the first mission to take this step. It's a pretty straightforward concept. Just go run a little small spacecraft into a much larger asteroid. This is about deflection, not disruption. We're not blowing up the asteroid. We're not trying to make more pieces of the asteroid than we had before. Rather, it's something that you would want to do years in advance. Give the asteroid a small nudge, which changes it ever so slightly. But when you do this years in advance, it adds up to a bigger change in time. And then the asteroid and the earth wouldn't be on a collision course in the future and everything would be better.

Ezzy Now, you said earlier that the asteroid DART is going to isn't actually a direct threat to Earth. So what asteroid is it going to and why was that particular one chosen?

Nancy Yeah, the asteroid that is the target for DART's mission is actually a double asteroid, hence the name of the mission, the double asteroid redirection test. There's two asteroids there. The largest one is named Didymos, and it was discovered to have a moon that orbits around it. Hence Didymos actually in Greek means twin. It has got a little moon asteroid that goes around it, Dimorphos. This one's named actually Greek for having two forms because it's going to have one form right now and then another form after DART crashes into this small little moon asteroid. The reason why this asteroid is ideal for this first test for asteroid deflection by kinetic impactor, is that DART is targeting that small moon asteroid, not the larger asteroid. And it's just going to change how that little moon asteroid goes around the much larger asteroid ever so slightly. So this makes it a really smart and safe way to do this first test of asteroid deflection technology.

Ezzy Is the fact that it's a double asteroid are important in any way it does that help do some kind of calculations when working out what affect the impact had.

Nancy The fact that it's a double asteroid is actually key to enabling the mission. Because if you think about it, we want to know how much we deflected this asteroid. But what we're doing is bringing a spacecraft in and crashing it into the asteroid. That spacecraft will be totally destroyed. It's not going to tell us how much we deflected the asteroid. So how are we going to do this? We're actually going to use telescopes here on the Earth that have been studying this double asteroid system for decades, and they've studied it really well during this time. And we know that it takes 11 hours and 55 minutes for Dimorphos to go around Didymos right now, they've been making that measurement. They've made it very, very precisely. So once the spacecraft's destroyed, we're going to turn back to those telescopes that already exist here on Earth to look at that asteroid system again and tell us how much did we change this? It's going to be small, maybe about a 1% change, which may be about 10 minutes or so. So rather than 11 hours and 55 minutes, more like 11 hours and 45 minutes, we don't know exactly. That's going to be one of the key measurements that needs to be made. And what's exciting about DART is that it's going to be made by these telescopes that already exist here on the Earth that have looked at this system for years.

Ezzy Will DART actually be able to see anything during this impact, either before, during or even afterwards?

Nancy DART is a really focussed mission, but it has one payload on it and that's a camera. It's named Draco, the Didymos Reconnaissance Asteroid Camera for optical navigation. It's kind of a mouthful, but it's based off the highly successful camera that was built at APL that took pictures of Pluto. If. Maybe you remember seeing some of those for the New Horizons mission so modified off of that design. We have a very capable telescopic camera on DART and it has two purposes. One is to see the asteroid and it's going to actually be streaming back these pictures to Earth one per second during its final moments before this collision with Dimorphos. And these images are going to start off just as a little point of light because the dart is coming in at 14,000 miles per hour. So extremely fast, just this little point of light as it gets closer to Dimorphos. And at the very end, you'll be able to see things at about 10cm resolution. And that's going to be filling the screen on NASA's TV when this whole thing happens on September 26, and then those images will stop. But the real purpose for this camera is actually to ensure that we hit this asteroid, which is a very challenging thing in itself.

Ezzy How far away is the asteroid?

Nancy At the time of impact did the most and do more folks will be about 11 million kilometres away from the earth. Now, that sounds like a long way, but in the grand scheme of space, it's actually pretty close. So did Amazon to more folks go around the sun just like the Earth goes around the sun. But it takes them a little bit over two earth years to go around the sun once. And the timing in September 2022 is actually when the distance between Earth and Didymos is minimised, because we wouldn't be able to use the telescopes here on the earth to see how much we deflected the asteroid, if it was on the other side of the sun very effectively. Right. So we want to be doing this at a time when the telescopes on the earth can get this amazing data of how much we've deflected the asteroid. And that's so at the time it'll be 11 million kilometres very safe, but really great for getting some amazing telescopic data and figuring out how much we deflected the asteroid.

Ezzy So will NASA actually be streaming those images from dot back live as it's crashing or will they not appear to the public until afterwards?

Nancy The plan is to share this whole event of DART's Kinetic Impact event live on NASA TV on September 26th at 7:14 p.m. Eastern Daylight Time (11:14pm GMT) over here and where we are at the Johns Hopkins Applied Physics Lab. And it's really going to be a historic event. The images as they come back from Draco are going to be shown there and we'll all get to witness this event together.

Ezzy Now, you mentioned a couple of times that there are telescopic observations happening back here on Earth, but what actually is this going to achieve and what telescopes are involved?

Nancy Well, we've got a really long list of telescopes around the world, so I won't necessarily go into all the fine details there. But one of the fundamental measurements that they're making is called this light curve measurement. And that's because as seen from Earth, you can't actually a resolved amorphous from didymos. It just looks like a single point of light. So the reason that we know there's two asteroids there is because that brightness changes with time. And so what they do is they measure the brightness of this asteroid, double asteroid system, very, very precisely. And that's how you know that right now this period is 11 hours and 55 minutes. And so doing that same measurement is going to be one of the key things. That said, also radar facilities are going to go take a look. We've got some space space capabilities. Hubble and James Webb Space Telescope are also going to be turning their gaze to to see what happens as well as every other facility. We really are welcoming and encouraging to maximise what we learn from this test. That is our first for asteroid deflection.

Ezzy Is there any chance that people using home telescopes might be able to see anything?

Nancy So the telescopes that we're going to use in order to make this very sensitive, precise measurement of how much we deflected the asteroid, have to be state of the art facilities and the precision that you need to do that measurement is very, very high. That said, the distance between Didymos and the Earth is minimised during this time in the fall of 2022. And so if you have a backyard telescope equipped with a CCD camera or other imaging system, you can turn your gaze towards that sky and you might be able to capture an image of where Didymos is relative in the sky, relative to the star fields that are out there. So definitely encourage people to give that a try if you've got that equipment at home.

Ezzy So coming back to the actual purpose of the DART mission, what are the biggest questions that we still have that DART will be able to help us answer about deflecting these potentially hazardous asteroids in the future?

Nancy Well, this is really where one of the main challenges of this whole thing is just targeting a small asteroid in space that you've never seen before. This asteroid is 160 metres in diameter. That's the size of a sports stadium or the Great Pyramid. And we're targeting it very fast, 14,000 miles per hour when we've never seen this object before. Ensuring that you can do that and targeted accurately is actually one of the main challenges and one of the main technologies that's being demonstrated for this mission. And in fact, because Dimorphos is so close to Didymos, you can't actually distinguish Dimorphos from Didymos until the last hour of the mission. And so the whole spacecraft has to autonomously detect Dimorphos, autonomously fire its thrusters, use those Draco images on board to decide how to target, to ensure that you get this a collision that you need to be as effective as possible. This technology alone is a very important development for planetary defence. If you wanted to target a small asteroid in space with a high speed impactor that you've never seen what that asteroid looks like before. So in itself, that fundamental goal of purposely crashing a spacecraft in an asteroid to move it slightly is one of the main challenges and one of the main planetary defence investigations for DART.

Ezzy Is there any chance that DART might just, Mr. Moore focus entirely?

Nancy Well, I mean, DART we've been testing it a lot. So this on board, Smart NAV, is what we call it, actually the Small body Manoeuvring, Autonomous Real Time Navigation. It's this set of algorithms that are developed onboard the spacecraft in order to ensure that you do do this collision and that they've been thoroughly tested, as you can imagine, we've seen other asteroids. They have all sorts of different shapes and and different materials. And that's all been going into this testing, robust testing of the SMART NAV algorithm. And where we're on track to make this collision. But this is also a test and this is one of the main challenges. And this is why it's important to be doing this test now before we need it.

Ezzy Are there any problems that might arise because we're not 100% sure exactly what kind of asteroids these are. For instance, might do more foes break apart as soon as DART slams into it?

Nancy This is really why the Didymos-Dimorphos system is the ideal target for this first test for planetary defence. And it's because of this double asteroid system and targeting this much smaller moon, 160 metres that goes around didymos 780 metres. So basically Didymos is 100 times more massive than the small moon Dimorphos that goes around it. And what dart is going to do is it's going to change that orbit by only 1%, and it's going to knock Dimorphos slightly orbiting even closer to this much larger Didymos. And so it's a very safe and effective way to do this first test, as well as with cost effective, because we're using the telescopes on the ground to see how much it's been deflected. And so overall, the system really is ideal. And in 2022, the distance is minimised between it and the Earth, which is why now is the time to be doing this first step and to be taking these... Doing this planetary defence technologies now before you need them.

Ezzy Now after DART impacts excessively, provided it does impact successfully, of course. What will be the next steps then?

Nancy Well, it's really important to recognise that DART is just one part of a much larger planetary defence strategy. And key to that strategy is finding the asteroids, knowing where they are, tracking them and identifying them. And so it really goes hand in hand. Something like DART is a technique that you would want to use years in advance, not blowing up the asteroid, just a small push. So you need to know where the asteroids are in order to be able to use that technique. So finding the asteroids, you want to be able to characterise the asteroids. You want to know what they're made of, what are you dealing with? Importantly, also, you want to be collaborating internationally and coordinating all of these efforts because planetary defence is an international issue. We're all on this planet together. Things hit our planet together. And so international cooperation for this international issue is key. And you want to be assessing the situation, understanding what's going on at time, and then if you need to, you want to be able to have some tools that you can draw on to mitigate this, to potentially prevent this from happening if you find you need to in the future. And so that's where DART comes in, taking this first step to potentially develop a way to prevent asteroids from hitting in the future. But you need to be doing this whole larger strategy for planetary defence in order to know where the asteroids are tracking them, coordinating, collaborating, working internationally in order to really be in shape that we have planetary defence in a good situation.

Ezzy So there are a lot of projects trying to find potentially hazardous space rocks. But what happens when we actually find one? Is there a set of criteria that decides what asteroids we can leave alone, which ones we need to keep an eye on, and which ones might be potential targets?

Nancy Absolutely. So Earth is hit by asteroids all of the time. This has been going on for billions of years and will continue for billions of years into the future. This is not new and small stuff hits the earth very regularly. You know something that's about four metres hits about every year. A lot of times that will just burn up in the atmosphere. Maybe it'll make a meteorite, which is a scientific treasure trove that I love to study in the lab personally. But the larger size can cause damage. A lot of times when people think about asteroids, they think about the dinosaurs being wiped out, for example. That's something that's more like ten kilometres, and that would be an extinction level event. The good news is that all of the near-Earth asteroids that are a kilometre and larger, over 95% of that population have been found, are being actively tracked by NASA and other agencies around the world and are not a threat to hit the Earth for the foreseeable future. So these sort of extinction level events are not in our foreseeable future, which is really amazing, awesome news that we're always happy to share. When we get to these things that are a few hundred metres in size, like Dimorphous Target, this is where you're talking about regional devastation. So this would be devastating for tens to hundreds of kilometres area. If this happened over a populated area like a city or A state or a small country. You could imagine the devastation that this would cause. Now, the ones that are a few hundred metres in diameter or so, we've only found less than half of that population currently. Of the ones that have been found, none are a threat for the foreseeable future. But this is why we need to be finding those asteroids and tracking them and identifying them to understand what the threat is, as well as taking steps to develop the technologies to be ready when we want to prevent this in the future. So there's absolutely criteria, but we're still very much at the level where we need to be finding all of these asteroids so that we really can assess this situation.

Ezzy Is there any risk that dart crashing into Dimorphos might actually make it become a potentially hazardous asteroid to us here on Earth?

Nancy No, there's absolutely no danger of that from DART's test. Again, this is one of the beauties of using this double asteroid system, crashing into Dimorphos, which is so close to Didymos and just knocking it to orbit ever so slightly closer to the much larger Didymos. It doesn't change how that asteroid system goes around the sun in any measurable manner, and hence there's no danger to the Earth. We've modelled the ejecta that will come off from this collision as well, and it falls back onto Dimorophs or to Didymos. And so it's a really smart and elegant way to do this first test where you're using this double asteroid system that's perfectly positioned in 2022. So you can use the telescopes here on Earth. And also a safe way to be doing this first test where you're just changing how an asteroid, a moon goes around a much larger asteroid.

Ezzy There was originally supposed to be a European Space Agency ESA mission to fly alongside dart. That's now been reconfigured into the Hera project. Can you tell me a little bit more about what that is and how it will complement Dart?

Nancy The DART and Hera collaboration is really fantastic. We've got measured members of the Hera team actively participating in DART right now, and we're very excited that both of these missions are on track to fly because together they will accomplish much more from planetary defence than any one will do on their own. So DART impacts in September 26th of 2022 this year. Yeah. So Hera arrives in the Didymos-Dimorphos system in 2026 and it will orbit that system and it'll stay there. So this is very exciting compared to DART, which just has one focus crashes into the moon Dimorphos. But Hara will stay and they will really characterise the system. They'll be able to get the massive Dimorphos. How much, how big, how massive is this, actually, asteroid will be able to measure its size to a certain extent, but we'll get much more characterisation from Hera. Hera will also be able to see the impact crater made by the DART spacecraft in Dimorphos. This will give us really insights into the material properties of Dimorphos and how this body is evolved and then what that means for future planetary defence applications. I think also it's important to mention LICIACube. So even though the original ESA mission, AIM, wasn't able to happen, DART is carrying LICIACube, which is contributed by the Italian Space Agency. It's the Light Italian CubeSat for Imaging of Asteroids, and it's going to take some spectacular images of DART's collision and the resulting ejecta and send those back to Earth as well.

Ezzy So there will still be something close by watching over the impact, even though DART itself can't.

Nancy So LICIACube is deployed off of the DART spacecraft about ten days prior to DART's impact, and it makes its closest approach flyby about 3 minutes after DART's collision. And so it will capture some pictures of DART actually colliding with the asteroid and then the resulting ejecta that happens. But LICIACube is also just a flyby mission, so it will capture its pictures as it flies by and then it'll leave the Didymos-Dimorphos system. So really the beauty of Hera is that it's a rendezvous mission rather than a flyby mission. So even though it will get there a few years later, there'll be a lot to learn from that system when they get there in 2026.

Ezzy Well, that certainly sounds very exciting, Nancy. I, for one, can't wait to see those images coming in. I think that's really exciting watching a spacecraft as it crashes. And I'm sure a few of the people at home will be thinking the same as well. So thank you very much for taking the time out of your day to talk to us.

Nancy Oh, happy to.


Chris BramleyThank you for listening to this episode of the Radio Astronomy Podcast from the makers of BBC Sky Night magazine. For more of our podcasts, visit our website at or head to Acast, iTunes or Spotify.


Elizabeth Pearson
Ezzy PearsonScience journalist

Ezzy Pearson is the Features Editor of BBC Sky at Night Magazine. Her first book about the history of robotic planetary landers is out now from The History Press.