What was the impact of NASA's DART mission?

Interview with Steve Chesley, NASA’s Jet Propulsion Laboratory

Part of the show Immune reprogramming for cancer, and squeaky shoe science

Back in 2022, NASA carried out the first real-world test - if “world” is the right word to use - of whether humans could deflect the path of an asteroid: a mission known as the Double Asteroid Redirection Test, or DART. The target was Dimorphos, a small moon that orbits a larger parent asteroid called Didymos. Scientists deliberately crashed a spacecraft into the Dimorphos moon to see whether the impact would alter its orbit, which it did. A bigger question, though, was whether it also altered the orbit around the Sun of the parent asteroid, Didymos, and by how much. This is crucial if we’re to understand our odds of using a technique like this to protect the Earth from an incoming asteroid in the future. Here’s one of the team, Steve Chesley, a senior research scientist at NASA’s Jet Propulsion Laboratory…

Steve - The backstory, of course, revolves around the DART mission to the asteroid Didymos, which is a binary asteroid system with a satellite named Dimorphos. And in September 2022, we smashed into that on purpose and we hit the satellite in order to deflect it and show that we can deflect satellites.

Chris - When you say satellite, this is like a mini moon, isn't it?

Steve - If you want to know the sizes, the primary body, Didymos, is about 700 metres across and the satellite's around 170 metres across.

Chris -And the purpose of this was to, what, try to understand the practicality of causing things that orbit each other to distort or deform their course?

Steve - Right. Asteroids do hit the Earth from time to time. When they do, it's a serious problem.And so we'd like to be able to know how to deflect them should the need arise. And so the two key questions here are, can we hit it? Because that's a technology problem. You're travelling at hypervelocity and you have to hit a target that's just a few tens of metres across. The other question is, what's the asteroid going to do after you hit it? You know how big the spacecraft is and how fast it's going, but there's also a recoil effect from all the debris that gets blown off from the impact. Boulders and gravel being thrown backwards into space adds to the effect of the spacecraft. This is what we wanted to understand.

Chris - You said you did this in 2022. So that was a little while ago now. Is the intervening period the time when you've been analysing all the data? And how did you get the data? What did you actually measure when you slammed an impactor into that satellite?

Steve - So what we were trying to figure out is not how did the satellite's orbit change when it got hit? We know that it changed by about a half hour in orbital period. So it went from about 12 hours to 11 and a half hours. That was done and settled back in 2023 from the analysis. What we're trying to do now and what we have done is find out what happened to the system's orbit about the sun. So because the system is so much more massive than the satellite, the effect is much smaller. And so we had to have first a really good orbit before impact for this system. And then again, a really good orbit after the impact. And then we can compare the two and see what's different and see what the change in velocity of the system's orbit about the sun was. And that's really what this paper is about.

Chris - How were you measuring that though? How did you keep tabs on it before and after the impact to make those sorts of measurements and that analysis possible?

Steve - I would have to say it's kind of incredible the precision of the observations that we were able to get. The amount of change in velocity that we found amounts to 1.7 inches per hour. And that's for this asteroid system that's hurtling through the solar system at tens of kilometres per second. And that was because we had these very precise stellar occultation observations that were collected by a cadre, a very dedicated cadre of amateur observers all around the world. Basically, they set up their telescopes at some remote location after driving sometimes for days, and they are ready when the shadow of the asteroid cast by a star falls on their telescope. They report the position of the asteroid with milli-arc-second precision. And let me tell you, a milli-arc-second is a thousandth of an arc-second, which is the 3600th of a degree. It's a very tiny angle. This one milli-arc-second accuracy is an angle that if you held up a human hair 20 miles away, that's how accurate these observations are.

Chris - These people sound more like professionals than amateurs to give them their due. I think that's absolutely right.

Steve - When we say amateur, we put that in quotation marks, because these are very dedicated and very serious folks.

Chris - What does this mean though? Now you've got this, you've got this deviation that you've it's an inch. I mean, we're talking imperial here, so a few centimetres in new money. But now you've got these measurements, what does this enable us to do? How do we take that forward and use that?

Steve - Yeah. So one of the things we wanted to understand from the impact experiment was how much of the boost do we get from the debris that gets blown off? And the estimates were the total change in velocity could be anywhere from two to five times that we get from the spacecraft alone. And that's a lot of uncertainty. This work shows that it's actually around two for this particular case. It also tells us that the ejecta is actually providing a similar boost to that of the spacecraft. They both are about the same level.

Chris - What does it tell us perhaps a bit more critically about the practicality now we constrain things in this way?

Steve - Well, with our work, we've actually, you know, for the first time moved a celestial body by human interaction. Everything works as expected. We were able to hit the asteroid. We were able to understand the effect of the asteroid. And that leaves us in an excellent position going forward to do this again if we need to for a real world planetary defence problem.