Ep. 656: Smashing Asteroids for Science! (2)
My favorite of these is the poor little Philae Lander that traveled with Rosetta, and it was in storage for so long. It traveled so far. And then it just didn't quite do its job because it was meant to harpoon the asteroid and attach itself to the surface that way. And its harpoon just didn't quite fire correctly. So, there was an accidental impact.
Fraser: Yeah, the thinking is that it landed, bounced a couple of times –
Dr. Gay: Yeah.
Fraser: – came to rest, fell over, but was operational.
Dr. Gay: Yes.
Fraser: And then its batteries ran out, and then that was that.
Dr. Gay: Now, in addition to the international mission that was Rosetta and Philae, JAXA with their Hayabusa mission has been doing some of my absolute favorite let's fling everything we possibly can, including anti-tank weapons that have been repurposed at the surface of an object. And it all started with Hayabusa 1 at Itokawa, an asteroid that led us to believe that not the entire solar system is made of rubble piles.
And when Hayabusa 1 got to Itokawa, it was carrying with it the first of the Minerva series of missions. And these are little flywheel having robots. They're cylinders. And the idea is that they bounce around looking in high detail at different places on the surface. And they tested all of this out with Hayabusa 1. Hayabusa 1 went and stole a piece of Itokaya. It tried twice. It didn't as much as it wanted. But it demonstrated that all of this is possible and set things up for the more ominous Hayabusa 2 mission.
Fraser: Right. Anti-tank weapon armed Hayabusa 2.
Dr. Gay: Yeah. Yeah, yeah. They referred to it as a “small carry-on impactor.” We all know that the reality is it was basically an anti-tank weapon flung at an asteroid instead of at a tank, which I approve of, by the way.
Fraser: Yeah.
Dr. Gay: And on April 5th, 2019, it attacked. And the idea was the exact same thing that we did with LCROSS, that we did with the original part of Deep Impact. It released materials so that we could see what is beneath the surface. And Hayabusa 2 was at the rubble pile asteroid Ryugu. And it got to allow us to see how craters form within the rubble pile structure. In addition to that, they had a whole series of little flitting, flying –
Fraser: Hopping, jumping.
Dr. Gay: Yeah.
Fraser: Yeah, yeah.
Dr. Gay: There's a whole series of different things they did, including, again, reaching down, touching down to the surface of the asteroid, grabbing a sample – which has made it back successfully to Earth. This mission was remarkably successful in everything it did.
And from there, we also had our own little favorite OSIRIS-REX mission that on October 20th, 2020, during a pandemic, managed to pull off a beautifully successful smash-and-grab of asteroid samples.
Fraser: That was a very gentle impact –
Dr. Gay: No.
Fraser: – in terms of – compared to DART and others.
Dr. Gay: This is true.
Fraser: Yeah.
Dr. Gay: But they still shoved the spacecraft like 30 centimeters into the asteroid.
Fraser: Yeah. Apparently, if it hadn't fired its retrorockets, it would have sunk into the rubble pile, which is kind of amazing.
Dr. Gay: Yeah, yeah.
Fraser: So, let's talk about DART.
Dr. Gay: Okay.
Fraser: What was the purpose of the DART mission?
Dr. Gay: To see if we can move an asteroid sufficiently that we can see a change in its orbit around its parent body. In this case, the parent body isn't the sun. In this case, we have Didymos with Dimorphos going around and around. And depending on how the impact took place, different amount of momentum would get transferred.
The pretty much worst case would be if Dimorphos was hanging out and the spacecraft plowed in and had a fully inelastic collision and became one with Dimorphos. The best possible situation is one where the spacecraft hits the surface of Dimorphos and flings out a vast amount of debris, carving out a crater, and also itself elastically bouncing off. In the case of a fully-elastic with shrapnel joining it, in that case you get the greatest transfer of momentum to the main object.
And we're still waiting for NASA to release the details on exactly what happened. We know that there was that plume. We don't know if the spacecraft stuck, bounced, or anything else.
Fraser: So, at this point, no matter what happened, the change in the orbit for Dimorphos is gonna be so tiny –
Dr. Gay: Yeah.
Fraser: – that it won't be visible until several weeks or even months of constant observation to detect the slightest difference in the orbital trajectory of Dimorphos. And we're not there yet.
Dr. Gay: Yeah.
Fraser: So, right now, we don't know how much of a change that it made. But what does that tell us? I mean, if we see a big change, if we see a small change, if we see no change, what does that tell us about this goal of potentially being able to protect Earth from asteroids in the future?
Dr. Gay: If we see no change, it means that momentum transfer failed and physics is broken. So, I'm pretty sure we're gonna see some sort of a change.
Fraser: Well, we talked about this, that it could punch right through, right –
Dr. Gay: Yeah.
Fraser: – and not change the physics, and maybe it's still out there.
Dr. Gay: But the plume that came off, the amazing thing about it is, if you have a body and part of it flies away, there's going to be – for all of the momentum that goes away in one direction, it's going to have the same amount of momentum transferred to the parent body that's moving forward. So, because we saw a plume, we have to see a change in orbit, within error bars.
Fraser: Right.
Dr. Gay: And so, we can fully expect to see something. We just don't know did its orbit around Didymos change enough that it's super easy to see. Or is it something that's really hard to see?
Ideally, we want to see the change over three different orbits. But because of where the asteroid is positioned, there's large sections of ocean that make it challenging to get 24-hour coverage. This is a target that is good to see from the Southern Hemisphere, okay to see from the Northern Hemisphere but pretty low down right now. Yeah.
Fraser: So, we really might have to wait for the Hera mission – to the following Hera mission.
Dr. Gay: We should not before then.
Fraser: Right.
Dr. Gay: We should know before then.
Fraser: But it'll do the really precise analysis of the orbit –
Dr. Gay: Yes.
Fraser: – and the consistency and look at the impact crater and all of that. But, I guess, can we learn about this asteroid, asteroids in general? Like, are they balls of rubble the way OSIRIS – the way Bennu and Ryugu –
Dr. Gay: Yeah, yeah.
Fraser: – or are they a chunk of rock surrounded by a think veil of rubble?
Dr. Gay: Yeah.
Fraser: Will the different outcomes of the orbit tell us which of those models is more correct?
Dr. Gay: It should help. The only way to get at what is the structure all they way through is to measure the moment of inertia for that moon. And this is where Hera's going to be able to probably do better observations than we're currently doing with the LICIACube, CubeSat, and definitely what we're doing here from the surface of the planet Earth.
That moment of inertia will tell us is it the same density all the way through, is it different densities going through, and what is that difference. It's the difference between spinning a raw egg, a hard-boiled egg, and a soft-boiled egg. All three of them will spin differently because of their moment inertia.
Fraser: I like that analogy. Totally.
Dr. Gay: Yeah.
Fraser: Yeah, when you spin those three.
Dr. Gay: I mean, how do you check if you actually hard-boiled your egg? You spin it.
Fraser: You spine it. Yeah. Yeah, I like that a lot. I'm gonna use that. I'm gonna steal that. Thank you.
Dr. Gay: You are welcome to it.
Fraser: Right on. So, are there any more comet-impacting missions planned for the future? Or asteroid impacts.
Dr. Gay: There are always people with plans. There are not currently any we-are-going-to-go-out-and-smack-something missions that are in the near future, fully-funded, and we can count on to actually do their job.
But right now, it's really cool to get to see the Deep Impact mission has been repurposed to become the EPOXI mission, which is out there visiting other objects. We're all waiting to see just what does OSIRIS-REx in its new phase do when it gets to Anubis. That is an amazing object. And, yeah, it sure looks like DART discovered another rubble pile out there.
Fraser: Yeah.
Dr. Gay: So, the Lucy mission is hopefully going to show us other examples of asteroids that hopefully aren't rubble piles. I look forward to seeing more Itokawas, like rubbly cashews. We need more of those in our lives.
Fraser: Yeah, yeah. We need to go to all the asteroids –
Dr. Gay: Yeah.
Fraser: – and smash a bunch of them and find out what they're made of. All right, thank you, Pamela.
Dr. Gay: Thank you, Fraser. And thank you so much to all of our patrons who are out there who are supporting us through patreon.com. If you would like to join that community, go to patreon.com/astronomycast.
Each week, I read out some of our patrons. And this week, I would like to specifically thank: Astrosetz, Stephen Veit, Burry Gowen, Jordan Young, Kevin Lyle, Jeanette Wink, nanoFlipps, Børre Andre Lysvoll, J.F. Rajotte, Andrew Poelstra, Brian Cagle, Venkatesh Chary, David Truog, TheGiantNothing, Aurora Lipper, David, Gerhard Schwarzer – I'm gonna go with that – Will Hamilton, Buzz Parsec, cacoseraph, Laura Kittleson, Robert Palsma, Les Howard, Jack Mudge, Gordon Dewis, Jow Hollstein, Adam Annis-Brown, Frank Tippin, and Richard Drumm. Thank you all so much for everything you do.
Fraser: Thanks, everyone. And we will see you all next week.
Dr. Gay: Bye-bye
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