Could DART save the world?

It's made a mint for Hollywood, but is the idea of nuking an incoming asteroid - to avoid an Earth impact that might otherwise wipe us all out - rooted more in reality than science fiction? Last week, NASA's DART mission slammed an impactor weighing half a tonne into a tiny moon orbiting an asteroid to see how practical the approach might be. They estimate, long term, that the collision might move the moon about 1% closer to the asteroid. Now the dust has settled, Open University planetary geologist David Rothery has been taking a look at the aftermath for us...

David - DART was launched a year ago to what's known as a near earth asteroid, called Didymos, which has an even smaller moon. The moon's called Dimorphos. It's 170 metres in diameter. Didymos is about 800 metres diameter. It was deliberately crashed into the moon of Didymos to see what change could be made to the orbit of Dimorphos. The point is, in the future, we might want to change an asteroid's trajectory if it's heading towards the Earth. So this was an asteroid redirection test tried out on a moon because when you've got a moon in the regular orbit, it's easy to see what's happened to the trajectory. You can just measure the orbital period of a moon, see how that's changed by the impact. And there was this almighty collision and the plume thrown up from this collision was seen by telescopes tracking it from the Earth and also seen by a little CubeSat which the DART probe released a few days before impact, which was filming from space and saw the impact as well. And the way the ejecta has been flung out is intriguing people. It's come out in streamers rather than a continuous cone. So that's interesting. But that main science that's going to come from this is how has this asteroid moon been disturbed in its trajectory?

Chris - We don't think, though, that there are any objects which are on an Earthbound collision course at the moment do we? I mean, just to kind of calm people down, this is not because NASA knows something?

David - There are no dinosaur-killers lurking out there! There's nothing that's going to disturb global climate for a decade. But if one of these things hits the ground in a city, it will destroy that city. So you don't want it to happen. So we do want to know how to deflect these things away if we know that there's one coming. And the idea would be to get it several years - several orbital paths - before it's going to hit the Earth, because you only need to disturb the trajectory just a little bit so it misses rather than hitting. So the longer in advance we can do this, the better. But what we weren't sure is, if you slam into an asteroid, is it going to fall apart? The surface of Dimorphos is just made up of massive interlocking very large boulders and I guess smaller stuff in between. Now it's a rubble pile. Was it going to fly apart or was it going to stick together and the whole thing be disturbed in its trajectory as a single mass. It seems that the latter has happened, which is good news. It wasn't blasted into smithereens.

Chris - Because that matters, doesn't it? Otherwise you turn one threat into a thousand threats.

David - Absolutely, yeah. And it's good that that doesn't seem to have happened in this case.

Chris - Are they going to go back and have another look or will all of the observations now be from Earth?

David - There are still data coming back from LICIACube, which is the Italian CubeSat that was deployed by DART itself. But there is a follow up mission as well. It's called HERO. It's a European Space Agency mission, which we launched in 2024 and two years later will arrive. It will be able to see the crater formed on Dimorphos by the impact. And I guess it may refine the orbital period of, and the shape of, the orbit of Dimorphos around Didymos. But we will have that from ground based observations as well. By measuring the orbital period, which you can tell from the Earth, we know how much the moon's trajectory has been disturbed by this impact. But the follow-up from here will add another layer of information into that. So it's going to be a well studied system. They're both interesting shapes. They're both made of rubble, so in their own right they're interesting objects as well.

Chris - And does the planetary geologist in you want to know more about what these objects were made of? Because obviously doing things like this does expose to us what they're made of and they're intriguing things because they date from the very same material that formed the solar system four and a half billion years ago.

David - So I don't know how primitive this object is in terms of what processing has gone on. Some of these asteroids have been damp in the past and water has migrated through them and altered the minerals. It's too faint probably to have a very good spectrum of it. I'm speaking outside my sphere of knowledge now, but, looking at the pictures, the jagged rocks of all sizes on the surface probably give it quite a bit of cohesive strength. What's gone on to process this material so it's just a rubble pile intrigues me. And yet it's a pile of rubble which seems to have hung together pretty well under this sudden impact. If you hit this thing slowly, it might break apart. If you did it fast, the shock of the impact will tend to make these interlocking pieces hold together better. So if you want to deflect an asteroid, you've got to do it at the right speed as well. So there's a lot to learn about how to deflect asteroids. This was just the first well controlled step towards doing that.