Astronomy Cast

Ep. 538: Asteroids: Rubble piles of the Solar System

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Thanks to all the work from Hayabusa 2 and OSIRIS-REx, astronomers are getting a much better look at the smaller asteroids in the Solar System. It turns out, they’re piles of  rubble… but fascinating piles of rubble. Let’s talk about what we’ve learned so far.

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Fraser:                         Astronomy Cast episode 538. Asteroids: The rubble piles of the solar system. Welcome to Astronomy Cast for a weekly facts-based journey to the cosmos where we help you understand not only what we know but how we know what we know.

                                    I’m Fraser Cain, publisher of Universe Today. With me as always, Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of CosmoQuest. Hi, Pamela, how you doing?

Pamela:                        I’m doing well. How are you doing, Fraser?

Fraser:                         Great. I had a chance – I’m writing a review right now, but I had a chance to play with just an amazing piece of technology. And so, I thought our good friends at Astronomy Cast would want to hear about this. It’s called the Stellina, S-T-E-L-L-I-N-A, and it comes from France. And it looks like the gun from Portal. So, it looks like this, you know, are you still there? You know?

                                    And it is about – I guess about the size of a gun from Portal. Anyway, but it’s a telescope. So, what you do is you take it outside, you set it up just anywhere kind of flat, and then it unfolds into telescope mode, looks at the sky, figures out its location on earth from GPS, figures out what it’s saying on the sky, figures out where it’s oriented, takes a picture, autofocus is, and then goes ready, and then you take your phone and then you say okay, let’s take a picture of M51. Let’s take a picture of the Andromeda galaxy. Let’s take a picture of the Great Globular Cluster in Hercules or the Ring Nebula, or whatever you want. And it just does it.

                                    And it just on your phone, every 10 seconds, a better and better version of the image pops up until you’ve had enough. And it is an astonishing piece of technology, just absolutely incredible, absolutely the future.

Pamela:                        And absolutely should not be taken as a carry-on on an airplane, because some TSA person is going to think it is a gun turret from Portal.

Fraser:                         Yeah. Exactly. And you could probably pack it in your luggage, but incredible. And it works. That’s the part – I was really skeptical and ready to just –

Pamela:                        Pan it?

Fraser:                         Pan it. Yeah. But it just – it really totally works. The images are not as good as if you buy all the separate parts and build a proper astrophotography rig, but you know how they say with cameras, the best camera is the one you’re going to use?

Pamela:                        Yeah.

Fraser:                         And this is the astrophotography rig that you’re gonna use. You’re gonna take it outside, you’re gonna set it up, you’re gonna take pictures. And I was really impressed. And I’m looking forward to do my review.

                                    And it’s funny, because people ask me how big is the lens. I’m like it doesn’t matter. How big is the camera? Who cares? Doesn’t matter. Right? I don’t know. It’s bigger than a roll of paper towel kind of but smaller than a stovepipe. I don’t know. Right?

Pamela:                        Okay.

Fraser:                         The point is photographs of the night sky better than anything you’ll ever see through the eyepiece of a telescope show up on your phone and you save them and you share them to social media. And you won’t get on APOD, but you will have taken pictures of any object that you could possibly want. So, I’m quite impressed.

Pamela:                        And I bet you could get on APOD just not for the most beautiful photo. It’s for the most in-the-moment photo.

Fraser:                         Yeah. Yeah. So, I’m quite impressed. And I can’t wait for this modern future where you don’t have to spend all evening apologizing to all your friends because you’re trying to set up the telescope and it’s not working. You can polar align it, and the camera doesn’t work, and you don’t have the drivers. You gotta reinstall them. Anyway. Yeah.

Pamela:                        I’ve been there.

Fraser:                         Of course it costs $4,000. So, you know, which if you’re gonna buy an astrophotography rig, that’s how much you’re going to spend, but then you’re going to get better pictures.

Pamela:                        It’s only the cost of four iPhones.

Fraser:                         Yeah. Exactly. Yeah. You’ve bought four iPhones. Come on. All right. Let’s get on with this week’s episode.

                                    Thanks to all the work from Hayabusa2 and OSIRIS-REx, astronomers are getting a much better look at the smaller asteroids in the solar system. It turns out they’re piles of rubble but fascinating piles of rubble.

                                    Let’s talk about what we’ve learned so far. Pamela, you were right there on the front lines as we were looking at images of –

Pamela:                        Bennu.

Fraser:                         Bennu, thanks to OSIRIS-REx, and it’s just a pile of gravel.

Pamela:                        Yeah. Yeah. So, one of the sad things that we discovered is our rock is only slightly denser than water, which means it’s not one rock, it is a myriad of rocks loosely held together with gaps between the stones.

                                    And I don’t know about you, but I’ve heard of rubble-pile asteroids, and in my mind, rubble-pile asteroids were like take a regular asteroid, hit it, and it falls apart into five or six pieces, and those five or six pieces come back together, no big deal, it’s still mostly a solid object, kinda dusty as a few boulders. No big deal.

                                    And Bennu was just like no. Not going to be that asteroid. Ryugu? No. Not gonna do it. These are two objects that both look like D&D dice that have had the worst days of their lives. They are that kind of hexagonal shape that you’re used to from dice, but they’re made of millions of meter to many-meter across rocks that have come together, very, very little dust, lots of variation in color, and they’re just hanging out going hey, you need to rethink everything you thought about how to deflect an asteroid headed towards earth.

Fraser:                         All right. So, what was everything that we thought?

Pamela:                        Well, so what we knew is when you look out towards the asteroid belt, you see these what we call families of asteroids. There’s the Hygiea family, the Cronus family, the Themis family, the Vesta family, the Ceres family, the Unamaya family, and I’m mispronouncing half of these, and I’m so sorry.

                                    All of these families of asteroids come about from having one big main body. So Ceres, Vesta, to give you a couple that we’ve imaged really well, and over time, these main bodies have been hit over and over and over with other asteroids and junks have flung off.

Fraser:                         Or a spaceship flying through with its little laser going zip, zip, zip, and breaking the asteroids into smaller pieces.

Pamela:                        For those of you who remember playing those video games when you were small.

Fraser:                         Yep.

Pamela:                        And so, this leaves behind a asteroid and a bunch of smaller ones all around it forming a family of stuff that’s made of the exact same composition. And based on seeing this, we imagine that there might be out there some asteroids that had been pretty much completely obliterated but temporarily.

                                    The energy that went into shattering the object wasn’t sufficient to send pieces, or at least send all the pieces out at escape velocities. So, a large portion of the former object or objects gravitationally came back together.

                                    And I don’t know why in my head these objects were always made of giant pieces. I guess in my thinking if they were made of tiny pieces, those tiny pieces would be way easier to fling off in all directions. Think Alderon getting blown apart, and I was so very wrong. So very wrong.

Fraser:                         Yeah. I mean you’ve got to deliver the amount of energy to release the binding energy of the whole asteroid, and whether it cracks into four pieces and they sort of spread apart for a little while and then they crunch back together again or just like a whole bunch of just gravel.

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Fraser:                         So, then I’m imagining you’ve got these two asteroids on a collision course with each other. They smash into each other, and they just obliterate because of the amount of energy that’s involved into just gravel like just rubble. But then the rubble is gravitationally bound to each other, so you’ve got this cloud of gravel that’s floating through space, and it’s got nothing better to do but to re-accumulate under its own gravity into this just whatever is the best shape, and it turns out to be of course it’s Dungeons & Dragons eight-sided dice or maybe 10 sided dice.

Pamela:                        Exactly. And this also tells us something about the energies of what happens when our own earth was slammed into by a Mars-sized object, the two objects essentially melted down, obliterate each other, a giant splash occurred that formed the moon, everything else re-coalesced back into forming the earth, and we went from a Mars-sized and a smaller than current earth-sized object that recombined, reconfigured all of their stuffs, and became two new objects that were solid objects.

                                    But with asteroids, they’re smaller. You don’t have as much of a binding energy to overcome. You’re dealing with shattering rocks basically and not having to worry as much about these objects being gravitationally held together.

                                    So, you overcome the chemical energy. It’s essentially hitting it with a giant space-size hammer, and these pieces aren’t molten. These pieces are just broken apart like a boulder. And this all sounds so very non-exciting until you’re trying to figure out how to land on one of these objects, and until you start realizing that this process essentially turned things inside out and got rid of all of the dust and small objects and just left you with a pile of grossness.

Fraser:                         So, let’s talk about this complexity of attempting to land – to find a spot that one might choose to land on the surface of one of these small objects, which of course, you were right on the front lines of helping to figure that out.

                                    So, what are the complications that have been added to the whole process now?

Pamela:                        Well, originally, both the Hayabusa2 team and the OSIRIS-REx team assumed that their objects would look sort of like Itokawa, Eros, one of these other smaller bodies that we’ve imaged before that have vast areas of nice, beautiful, smooth, undisturbed surfaces and that we’d be able to look at our object and find in a nice, easy-to-reach, kinematically not difficult place, one of these nice, large, beautiful, smooth places.

                                    And we wanted a nice big area, because well, these objects are rotating, they don’t have a whole lot of gravity, and trying to get down to their surface requires our spacecraft to match the rotational rate of the asteroid and come into contact with the surface long enough to grab the sample through whatever means the very spacecraft are using and take back off and do all of this without harming the spacecraft both of which have large solar panels.

                                    Now there’s going to be error in coming in for any landing. We have this whole concept of a landing ellipse which we’ve gotten used to from watching Mars landings where we say we expect our thing to land in this area of Gale crater for instance with the Curiosity Lander.

                                    Well, with our landing ellipses on both Hayabusa and Ryugu with our original plans – sorry with Ryugu and Bennu with our original plans, these landing ellipses were designed to keep our solar panels nice and safe, and they were nice and large, lots of wiggle room. And neither object had a smooth, flat surface anywhere.

Fraser:                         Anywhere. Yeah.

Pamela:                        Anywhere. There were none. None.

Fraser:                         Yeah. I sort of imagined trying to find – imagine trying to have a picnic on the big Island of Hawaii’s lava field. And you’re just like where should we set down? Where should we sit down and our blanket and have a nice picnic on this jumble of razor-sharp lava everywhere. And it’s – there’s no place. And this was the challenge.

                                    So, now that you guys have finished mapping the surface of Bennu, how bad is it? How many even areas can they even possibly land?

Pamela:                        Well, we’ve identified four potential sample sites that are now being imaged in even higher detail. And the way this works is we started off with the OSIRIS-REx spacecraft in a nice high orbit. We did general imaging of the entire surface, created a nice mosaic. Using that mosaic, there was much cursing that occurred, because the original plan was use the big mosaic to identify areas for follow-up detailed study. We did the best we could which basically with huh, look at everything.

                                    And so, they dropped the spacecraft OSIRIS-REx into a lower orbit, which meant that we could get more detailed images of the surface. And we looked at all those images looking for an area that was largely free of large boulders and only had small rocks.

                                    And four potentially good-enough sites were identified. And each of the sites thankfully has craters that provide nice fairly smooth areas. It turns out if you have a giant pile of rocks and you hit it at the right velocity with another rock, that area will get mushed down.

                                    And those mushed down areas that look the most promising in many ways.

Fraser:                         Like someone just stuck their finger, their thumb into the eye of Bennu and went pkck. And then that’s where you’re going to land.

Pamela:                        Yeah. So, we went from looking for a roughly 80-meter-across area to looking for a 10-meter-in-diameter area to land our spacecraft.

Fraser:                         And when you think about how difficult it’s been to land spacecraft – you know, we saw what happened earlier this year with Beresheet, what just happened with India’s Vikram lander, these are very difficult things in what were assumed to be the nicest landing zones you could possibly hope for, and now NASA is going to try to land in, as I said, this razor-sharp danger zone.

Pamela:                        Now one of the nice things we have going for us that Beresheet and the – is it Rifka? The Indian lander?

Fraser:                         Vikram.

Pamela:                        Vikram. The thing that we have going for us that Vikram and Beresheet didn’t have going for them is a whole lot less gravity. With both Ryugu and Bennu, our spacecraft are kind of dropping in to bounce off the surface. So, we are not planning to go in and stay. We’re planning to go in and with OSIRIS-REx, we have what I personally have decided to describe as a very angry vacuum cleaner.

                                    We have a hose that comes down. It has little conveyor belt-like bits similar to what you may see on an extension for a vacuum cleaner, and it pulls up bits of the surface that can then be collected.

                                    But it’s just a touch and go. It’s not there to stay. And this gives us some benefits where since we are already planning to pretty much bounce, as long as we don’t clipped those solar panels on anything, we’re pretty good.

Fraser:                         Yeah. Just don’t snag a panel and like anyone who’s ever gone fishing, you send your line down and then it gets stuck to the bottom on the hook.

Pamela:                        And it’s the clipping of the solar panels that is in some ways the most terrifying potentiality, but we think we’ve figured out – we hope we’ve figured out how to not let that happen, and we’re doing yet another round of imaging. The spacecraft has yet again been dropped into an even smaller orbit, and we are once again taking images of even higher resolution.

                                    And some of the members of the CosmoQuest community are going to be asked to once again look at these images, and they’re going to be on the mission critical path with the rest of the science team to figure out okay, of these four regions, where in these four regions is actually the best scientifically and the safest for the mission. We have to balance both those facets, and then hopefully will be able to well, go grab a rocks.

Fraser:                         So then let’s talk about the chilling implications of this for us attempting to prevent an asteroid impact. So traditionally the plan is you’ve got your asteroid coming to hit the earth, and say it’s gonna be 10 kilometers across. That’s a city killer. That’s a continent ruiner. You fire the earth’s collective armada of nuclear missiles at this asteroid, and you explode them on the surface.

                                    What happens with your rubble pile?

Pamela:                        Well, with a rubble pile, they basically just sort of fall apart and then come back together gravitationally. And like yeah? So? And?

Fraser:                         I’m imagining the liquid metal from the Terminator. And so, you just shoot it, and then it all just comes back together, and sure you’ve reorganized it, you’ve jumbled it up, but it was already jumbled. You have shuffled a shuffled deck of cards.

Pamela:                        And even a bit more disturbingly, the way I view this in my head is it’s like trying to push a leaf pile across your lawn. You can’t just push in the center. All it’s going to do is make a bigger mess. One of the other more reasonable than nuclear weapons ways of deflecting an asteroid is to go plant some engines on it, fire up those engines, and give it a push in a new direction.

                                    Well in this case, when you fire those engines, you might just be burying those engines inside of the asteroid, and that is also not useful.

Fraser:                         Right. Imagine you think that you’ve put an engine on the side of this asteroid to push it around, and all you’ve done is your little rocket is going to be just drilling itself into the middle of the asteroid and that’s that. Yeah. This is a nightmare.

Pamela:                        Yeah. And luckily Bennu and Ryugu are both kind of small. Ryugu’s the bigger. It’s about 800 meters across. Bennu is about 500 meters. This makes them capable of just hanging out on a nice city block. Any of you could play Wizards Unite. You could walk across the diameter if you could walk through the asteroid, which again, might be possible.

                                    It’s twice the distance Wizards Unite expects you to walk every single day. These are tiny. But tiny can still destroy a lot of things, and –

Fraser:                         What was [Inaudible] [00:23:15]? About a 15-meter rock?

Pamela:                        Yeah.

Fraser:                         Right? So, that’s the size of a house. These are the size of the building. These are city killers. These are tsunami creators. These are bad days for a lot of people if they hit.

Pamela:                        And because they’re so fundamentally different, it also means we need to redo all of our models for things hitting the earth. Our models are based on a variety of different factors and generally span from hey, we have a really metallic object coming to attack us to hey, we have a carbonaceous chondrite, which is basically a big pile of soil coming to attack us.

                                    But our models don’t generally say hey, there’s this loosely-conglomerated-together-we-don’t-know-how-much-energy-it-takes-to-disrupt pile of boulders coming at us. And this means we need to start rethinking well, what do the tidal forces of the earth due to an object like this? Is it going to string it out into a larger number of rocks that wraps his way around our planet? Is it going to explode in the atmosphere as any of the vapor and gases inside the planet suddenly undergo a phase change and explode things outward?

                                    We know Bennu has volatiles, because it’s throwing rocks at our spacecraft. This changes how we have to consider incoming rocks essentially.

Fraser:                         Yeah. Well, then let’s talk about the good news, which is – you mentioned volatiles. It is for our bold future of living in space and attempting to mine asteroids, it feels this got a lot easier. You no longer need to hard rock drill into the side of a gigantic asteroid. You really just have to scoop and pick stuff up and get it off into space, and you can find volatiles mixed in with the boulders that you can use for all kinds of purposes.

                                    This seems like good news for our future of space mining and resource utilization.

Pamela:                        And especially with Ryugu. It tells us that the kinds of materials that we normally would’ve expected to see in the center of an asteroid might instead be on its surface.

                                    There are two main models for understanding Ryugu. When you look at Ryugu, it has boulders of two very different colors that are mixed together. And the thinking is that either something came along and clobbered Ryugu and caused a differentiated asteroid that has a different structure in the center and a different structure on the outside to get knocked apart and essentially get turned inside out a bit with some of the former interior materials now being turned into boulders on the surface.

                                    Or Ryugu is two different asteroids that clobbered each other and mutually came back together to form a new object again with these two different colored materials.

                                    In either way, you’re finding the same stuff on the surface as we presume you find on the inside. So, you don’t have to worry about different layers of strata containing different materials. It’s all right on the surface.

                                    Now with Bennu, we still don’t know all of the details. Bennu has this really weird, super shiny stuff scattered in dribs and drabs all over the surface, and we’re still trying to figure out what this stuff is.

                                    And so again, here you have multiple compositions mixed together on the surface and, because Bennu throws rocks, proof of volatiles that can melt, and there’s your potential fuel, your potential water, all right on the surface.

Fraser:                         So, for those of you who are watching from home, when will we see the next big moment in the OSIRIS-REx mission? What’s the next big – I mean they’ve got to collect the sample. So, when’s that going to happen?

Pamela:                        Right. So, I love to be able to say we are going to be able to announce the exact places where we’re going to go and collect the samples next week, in three weeks, and four weeks. We had a beautiful mission timeline. We really, really did, and the actual asteroid is so much harder to figure out. So, I can’t tell you –

Fraser:                         So, we don’t know.

Pamela:                        – precisely – well –

Fraser:                         When they’ve looked at the next round of pictures, when they’ve figured a place that they think is safe, they’ll announce when they’re going to make that attempt, and it’ll happen sometime in the future. Probably not tomorrow, and –

Pamela:                        No. But it scheduled –

Fraser:                         – probably not 100 years from now.

Pamela:                        It’s currently – we’re looking at this winter. We’re looking at December probably, but I’m just going to go with our timeline kinda got thrown out the window, and we’re doing okay. We’re doing okay, but well, this object is so much more complicated than anything we imagined, and were holding to this orbit at this time, this orbit at this time, but the decision-making processes are taking extra time when they need to.

Fraser:                         Yeah. Well, good luck everyone working with the mission, and good luck OSIRIS-REx. We watched you launch. Now we want to watch you collect that sample and return safely home.

                                    Pamela, do you have some names for us this week.

Pamela:                        I do. And the people I’m going to list are people who are supporting us over on Patreon.com/AstronomyCast. You are the people that really allow us to be functional in producing all of this content.

                                    Susie gets paid thanks to you, and when we need to, this is what allows us to travel to things like the OSIRIS-REx takeoff.

                                    So, this week, I would like to thank – sorry, scrolling. This week I would like to thank Les Howard, Paul Jarman, Jess Cunningham, Emily Patterson, Dana Norrie, Joseph Hoy, Frederick Haganeke von Jensen, [Inaudible] [00:29:51], Ed, Gordon Dewey, Bill Hamilton, Frank Tippin, Greg Thorwald, Richard Riviera, Alexis, Thomas [Inaudible], and Steven Shearwater.

Fraser:                         Thank you so much, Pamela. We’ll see you next week. Thanks everybody.

Pamela:                        Bye-bye.

[Music]

Susie:                           Thank you for listening to Astronomy Cast. A nonprofit resource provided by the Planetary Science Institute, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at Astronomy Cast. You can email us at info@AstronomyCast.com, Tweet us at Astronomy Cast, like us on Facebook, and watch us on YouTube.

                                    We record our show live on YouTube every Friday at 3:00 p.m. Eastern, 12:00 p.m. Pacific, or 1900 UTC. Our intro music was provided by David Joseph Wesley, the outro music is by Travis Surow, and the show was edited by Susie Murph.

[End of Audio]

Duration: 31 minutes

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