[Interview+] Phobos Might Already Be Destroyed and Reformed. Possibly Multiple Times

It's an exciting time to be an asteroid researcher. Think about all of the missions that have just wrapped up, missions that are in the works, missions that are in the planning stages. You've obviously got Osiris Rex and Hayabusa 2 that have gone and visited RoboPile asteroids. You've got the Lucy mission that's on its way out to Jupiter's Trojan regions. We've got the MFX mission to Phobos. There's a ton of interesting missions that are in the works right now. And what astronomers are finding is that the vast majority of these asteroids out there, the smaller ones, are RoboPiles. That they are just piles of rocks held together by mutual gravity. And of course, one of the most momentous things that humanity has done in the last couple of years is changed the trajectory, changed the orbit of an asteroid with the dark mission. And we avenged the dinosaurs. So my guest today is Dr. Harrison Agris. He's a CNES post-doctoral fellow at the Observatoire de la Côte Azure. And he is researching RoboPile asteroids. Has worked on many of the missions that you're probably familiar with, worked on the dark mission, is working on the Lucy mission, is going to be working on other upcoming missions, and has been analyzing just how these RoboPile asteroids work, how they spin up, how they gain moons, lose moons, migrate material, how we can defend against them, and what they might say about the formation of this sort of entire class of asteroids in the solar system. And it's really interesting. And the paper that drew him to my attention was about the fate of Phobos. You know, we know that Phobos is very close to Mars. It's going to eventually get torn up and crash onto the surface of Mars. But Harrison believes this might happen sooner than we think, and maybe has happened many times before. You're going to want to see the rest of this interview to understand that. All right. Enjoy this conversation with Dr. Harrison Agris. Harrison, both Osiris Rex and Hybusat 2 went to RoboPile asteroids. Did we know these were RoboPile asteroids before we got to them? We probably had a good idea. These missions flew sort of roughly before my time, so I wasn't involved in when these missions were getting selected. But yeah, in the last 20, 30 years, the field has kind of undergone this big revelation that most small asteroids are RoboPiles. So we think that all asteroids formed big at a scale of roughly 100 kilometers. So most things that are below the size scale are collisional fragments from massive collisions. So you have two asteroids collide and they kick off a ton of debris and this debris re-accumulates as it's flying away. And so yeah, everything that's more or less kilometer size or even hundreds of meters, they're probably nearly all RoboPiles. Huh. And I mean, that sort of led to my next question. I was going to like, what percentage of these things out there are probably RoboPile? You're saying like most. I would say pretty much all of them. There's probably outliers. For example, I saw a recent paper. It's like one of these LSST early release papers. Right. I was just going to mention that. They were missioning where they planted this telescope at one point in the sky and took a bunch of images and they discovered a handful of super fast rotating asteroids and they're like almost a kilometer in size. They're like, I don't know, some of them are like five or 700 meters and it's been periods of like, I can't remember, like 10 seconds. These types of things, that's probably just a single shard, a single rock. But in our collisional models, we don't really produce things of that big intact fragments because we're just limited by the computational resources we have. We can't resolve these super small fragments and collisions. So there's a bit of a mystery with those. Are they single fragments or not? And so then if the vast majority of these are RoboPiles, what are the, I guess, the forces that are limiting them and sort of shaping their evolution over the history of the solar system? Yeah. So the dominant thing is just their self-gravity. So they're just very loosely bound debris piles, rubble piles. Other forces at play can be like cohesive forces, right, they hold them together. So if you have a lot of fine-grained material, you know, Van der Waals forces like what the, for example, like the Lunar Regolith has a lot of cohesion. So cohesion could play some small role. But as we found when we've interacted with the surfaces, like the high-booster 2 SCI experiment where they made an artificial crater on the surface or when when those hydrosterex actually took the sample off the surface, when we interact with the surface, we realize that's so much weaker than we expected. So these cohesive forces may not be playing much of a role. And so really, it just gravity and like basic friction sort of sets everything. But then there are external forces, right? There's the heat coming from the sun, there's the rotation of the asteroid, there's interactions with another moon. How does that sort of play into their evolution? Yeah, this plays a really fundamental role. So the reason, it was a big mystery like in the 90s, like why do we have near-Earth asteroids at all? Because we know that their dynamical lifetime of any given near-Earth asteroid statistically is like 10 million years. It's going to be encountering the planets and within 10 million years, it's either going to hit a planet, hit the sun, or get ejected from the solar system. So, and we know the solar system is over 4 billion years. So there should be no near-Earth asteroids left, right? They're coming from the main belt. So you have an asteroid family forming collision, a big collision that makes a bunch of small fragments. And then these fragments feel Yarkovsky forces. So they get the heat up from the sun and they're also rotating. So you know on Earth, the hottest part of the day is like 3pm because you've had all day long to absorb all the sunlight. And even into the afternoon, you're still absorbing some more sunlight. So you get a little bit hotter. So around the 3pm location on an asteroid is where it's hottest. And so that's where it's emitting the most thermal radiation. And we also know full tons carry momentum. And so the asteroid is preferentially emitting some momentum that is roughly parallel to its orbit. So the Yarkovsky force either adds or subtracts a little bit of anger momentum from the asteroids actually orbit. So then it then it migrates. It's semi-major axis evolves. And so it can migrate inwards or outwards, slowly over, you know, million year timescales. The big asteroids don't do this because they still feel the same forces, but they're so massive that it's negligible. The small guys, the 100 kilometer, 1 kilometer, these guys, they feel these forces. And so on solar system timescales, they migrate inwards or outwards till they eventually get to like a resonance with one of the outer planets, mostly Jupiter or Saturn. So these can be like mean motion resonances with Jupiter. So then they start feeling these resonant forces with Jupiter that causes their orbits to change more drastically where they become more eccentric. So they become very elliptical. And now their Paris center is now going to start reaching down into the inner planets. You know, once they scatter once or twice off of Mars, they're now, you know, in the Earth space. And so this is how we have a constant steady state of near Earth asteroids, right? We always have, you know, some background level of collisions in the main belt, which are producing small asteroids, which then migrate to the resonance locations, which through Yarkovsky, which then interact with Jupiter, and then they then those start interacting with the inner planets. And so that's how we have this. Can we can we settle this thing with Jupiter once and for all? People say Jupiter is is your friend that is protecting us from asteroids. But would you definitively now say no, it's actually a bully that is sending asteroids inward? In fact, it's not because if you didn't have the planets there, and you just had the if you didn't have the outer planets at all, you just have the asteroid belt, you'd form all these small fragments, and they would migrate through Yarkovsky. So the small ones could actually migrate all the way down to Earth. Right. So you might still have some near Earth asteroids, but but it would take a lot longer and it'd be a lot less efficient. So I'm sort of imagining this sort of orbital mechanics in my mind. So you've got these asteroids, they're being heated, they're rotating, and they're absorbing energy. But then the the point where they start to emit it is when they are like 90 degrees off of where they're receiving it from the sun. And so that's either putting a thrust in the direction that they're orbiting or in the direction against that they're they're orbiting. That's then raising or lowering their orbit, putting them into this, you know, into this this location where Jupiter and Saturn's gravity can get ahold of them and then force them inward, or they can just spiral themselves. You know, I didn't realize that the Yarkovsky effect sort of preferentially put things on to that 90 degree. Yeah, I mean, it's not quite 90 degrees. The forces, one component of the Yarkovsky force is a long track. So it's either, you know, adding energy or subtracting energy from your orbit. So it's not, you know, totally 90 degrees. Some factor of it is pushing you radially outward or inwards. But in orbital mechanics, that doesn't change your orbit all that much. Right, right. But then there's this other sort of effect that that these forces will will cause these asteroids to spin up. Yes, they will spin up faster and faster. Exactly. So there's the Yarkovsky effect. And then a very similar sort of side effect of this is the Yorpe effect, which stands for Yarkovsky, O'Keeffe, Radbiesky, Paddock effect, which are names of the four people that kind of wrote some of these early papers. And this is a similar thing. So it's because all asteroids are somewhat irregular, they're not perfectly symmetric spheres. They absorb all these photons from the sun, and then they re-emit them as mostly thermal photons. But they don't re-emit them perfectly in a spherically symmetric manner. And so this imparts a small torque to their, to the body. So you, the orbit is torqued. And so the orbit evolves, but at the same time, the spin state evolves. And so this Yorpe effect will drive asteroids, it'll drive your spin pole to be either 90 degrees or minus 90 degrees with respect to their heliocentric orbit. So it'll make them perfectly prograde or retrograde spinners. Right. And then also it will spin them up or down. And so, so like, if they're already going up, they're just going to go up faster and faster. And if they're going down, then they'll, they'll, they'll aim for zero and then randomly pick their next direction. And this mechanism is incredibly sensitive to the topography of the surface. In a way where it's like hard, like if you looked at an asteroid, you couldn't, I couldn't tell you, like what's your Yorpe is going to do if it's going to make it spin up or spin down. You, you can take like a, you know, a synthetic like shape model of an asteroid, and you can numerically sum over all the facets and vertices and compute what the Yorpe torque would be. But then if you change like one facet by a small amount, it changes the answer almost completely. So it's, it's super sensitive. So there's also an idea that this could be somewhat like stochastic, like you could spin up until you get a small landslide and a few boulders move on the surface. And then that changes the Yorpe torque and maybe you spin down, you know, it's this, it could be this kind of stochastic process. And so then what role do these, does, do these rotating asteroids play with their existence of their moons? Yes. So this is sort of one of the reasons why we think that a lot of these asteroids form moons through this effect. So if you're an asteroid that happens to spin up, eventually you're going to get, you're going to spin so fast that, you know, self-gravity can't hold you together and you're a rubble pile, right? If you're not a model, if then what can happen is you'll shed mass, kind of like imagine if you're like spinning, you're spinning a pizza, right? Like a pizza dough. It's like this. You, so we see a lot of these fast rotators are sort of like top shape, like a diamond or, you know, a little sort of flattened oblate spheroid and they have a close in moon. And so what we think happens is these things spin up to their spin limit, then they go some type of failure event and you get some maybe landslides and material landslides down and picks up enough speed that a material goes into orbit and then reaccumulates into a moon. Wow. And I also understand that it can, it can migrate. So boulders will say lift off the surface and then they'll find their way up towards the poles and then now they can lose some of that momentum and attach back and then the landslides kick them out to the equator again. And so there's like some kind of flow going on. Exactly. And it's this super, I mean, it's amazing. You would think, naively, like you think asteroids are really boring, you just a rock in space, but they're super active in this sense, like geologically in ways that you don't get on a body with higher gravity, right? It's only in these special super low gravity environments. It really feels like this is where science fiction has completely let us down and failed to catch up. That if you're going to make some science fiction show where people are going to be trying to navigate around an asteroid belt and if you don't have all of the asteroids looking like rubble piles, then you have failed. And of course, obviously, you also should only be able to see one asteroid at a time, maybe two. Right. Yeah. But yeah, super cool. These things just form, they spawn their own moons. Yeah. And will they absorb their moons again? Yeah, it's possible. So there's yet another radiative effect. So once you have a moon, there's another effect called the B-Yorpe effect, B-Yorpe, binary Yorpe. And so the way you can think of it is the satellite is almost like an appendage of the primary. And so it can absorb photons and emit them. And that leads to the satellite's orbit with respect to the primary, possibly migrating. And it can migrate inwards or outwards, and it's very sensitive to its shape. So it is possible that you form your moon. In most situations, ties are going to be pushing the moon outwards through tidal dissipation, but B-Yorpe might overcome that and push the moon inwards. Until eventually, maybe it gets too close to the primary, sorry, and it tidally disrupts and forms a new moon. Or if it has enough strength, like if maybe the moon is just one big boulder, for example, it could migrate all the way in and make contact with the primary, and now it's maybe a contact binary asteroid. Right. And that explains some of the contact binary asteroids that we've seen. It could. Although most of the contact binaries we've seen have a fundamentally different overall shape than the binary asteroids that we've seen. So theoretically, you could have this process where these asteroids are spinning, these rubble pile asteroids are spinning. They are calving off a moon. The moon is drifting away. The Yorpe forces are interacting. It brings the moon back. That it's just evolving in this weird amorphous blob over time. You could come back a million years later and it doesn't have a moon anymore. Yeah, exactly. Or two moons. It's possible, especially for near-Earth asteroids, it's possible. There's no evidence of this, but for example, Bennu and Ryugu both have this classic top shape. They look like the primaries of binary asteroids if they don't have moons. So they could have had moons in the past and lost them somehow. Right. And so with the Dart mission, once again, both Didymus and Dimorphus are rubble piles as well. Yeah. Yeah. So this is exactly how we think those moons formed actually after the Dart impact. I led a paper on this on how the moons formed because now that we've seen one of these binary asteroids up close, there's a lot more geologic context we can apply to these formation models. We see like a Didymus, this equatorial ridge. We also have craters on the surfaces so we can estimate the surface ages. And so it's going to be hard for me to remember numbers off the top of my head, but I think we estimated that the surface age of Dimorphus is something like, I want to say, 100,000 years, but the surface age of Didymus is like a million years. So factor of 10. And so we had to explain how can Dimorphus be so much younger. And the idea is you can spin up Didymus and have it shed mass and you form a moon that's the right size and everything. And this whole process can preserve most of the big craters at the high latitudes of Didymus. So what we showed is you can shed mass, you form your moon without ruining all your craters because the craters record this super old age. And then Dimorphus would be much younger and it hasn't had so much time to form lots of craters. Yeah, yeah. It's really interesting. And so how did, I mean, we're a couple of years now since the Dart mission did its impact. Do we have a sense of what the results were? Were the dinosaurs avenged, I think is the first question. Yeah, I mean, I think we demonstrated pretty robustly that like, yeah, we can hit the thing, we measured how much momentum we transferred to it. So yeah, I think in the future, we could totally deflect an asteroid. It's just a matter of having enough warning time and having the budget to do it, I hope. I hope if we ever detect an asteroid that's Emily gonna hit the earth and we can afford to do a mission to do it. One of the worries was that these things are because of this sort of loose sort of glom of material that you blow it up or you smash something into it and then it reforms itself and continues on its way. So did that part work? Like did we substantially change the orbit? Yeah. So I mean, in this case, even though, you know, if you, what you described, in that scenario, you still would have transferred momentum to it. So momentum is still gonna be conserved, you still transfer all your spacecraft's momentum. But yeah, to answer your question, so we hit demorphos. And we very shortly after the impact, we are able to determine how we change the orbit of demorphos with respect to Diddymos. Because its orbit period is something like 12 hours, and we saw that change by roughly 30 minutes. So that's a really big change. And so it's very measurable. And so we knew that we changed the orbital velocity of demorphos around Diddymos by something like two millimeters per second or something like that, pretty small number. Now, the system, you know, Diddymos is much bigger. So the momentum we imparted changed the velocity of the system by a much smaller amount. And that took several years to accomplish the measurement. And the reason is, because it's so, so small. So there was a recent paper that came out by Makati et al. And they used a bunch of techniques, they used both, you know, the spacecraft images, ground-based observations, and, and occultations, and they determined that we changed the velocity, the orbital velocity of the system by like something like 10 microns per second. So like less than the width, less than the width of a human hair per second. And we measured that with meaningful error bars. It'd be like, it'd be like looking at the moon and measuring the speed of like a tortoise walking on the moon, right? It's, it's so precise. And the reason why it took so long to make this measurement is because you can't like, you wouldn't, you can't just look at the asteroid and perceive that it's going at a different velocity. You need years of that slightly changed velocity to add up and manifest in the substantially different position in space. But theoretically, that would turn a hit into a miss if the time scales were large enough if we had the large enough impactor. Yeah, exactly. Yeah. And, you know, if we're, and if we're having to divert a much bigger asteroid, then yeah, we would need more impactors or a bigger impactor, right? So, you know, the, in this is where, remember, we're talking about the change of the entire Diddymos system. Diddymos is something like 800 meters across. These types of collisions are quite rare with the earth, right? What we're more concerned about in planetary defense are these like 100 meter sized impactors, because they're big enough that they do enough damage that we care about it. But they're also small enough that the impacts are frequent enough that we also care about it, like on human timescale, right? Like every 1000 years or so. Yeah. And so we changed demorphosis speed by a much larger number, you know, like millimeters per second, still small number, but not microns per second. You know, I think for the longest time there had been these arguments about what was going to be the most effective way to, to prevent an asteroid hit on the earth. You know, do you blow up a nuclear weapon nearby in a blade material and have it act like a thruster? Do you set up a solar sail or do you paint half of it white, you know, like all of these, you know, trying to boost it up your perfect or whatever. Totally. Does it, does it feel now like just slamming mass into it at the fastest pace that we can seem sufficient? I think so. I mean, maybe I'm biased because I was involved, right? So yeah, but it's the simplest. Like why, why break it if it doesn't, or why you fix it if it's not broken, like yeah, a kinetic compactor is so much simpler. Yes, I think a nuclear device would work or, you know, painting it would work. But it's so much more complicated. And with the nuclear scenario, politically, it's more complicated too, right? So yeah, a kinetic compactor, it works. And, you know, the momentum you transfer, we know is going to be dependent on things like the topography of the surface and the material properties and all these things, which we may not know in advance. But the thing is, you know, if you're designing a mission and you need to save the earth, you're going to design for a beta equals one. So beta is this momentum enhancement factor. So, you know, when you hit an asteroid, you transfer your momentum to the asteroid, but then you also make a crater and you kick off a bunch of ejecta, and that ejecta is going beyond the escape speed and is going in the other direction. So you get a momentum boost. So we know with Dart, you know, we hit it with mass times velocity of momentum, but we actually in the end transferred over two times that momentum to the target, because we liberated all this ejecta at the bush in the other direction. But if I'm trying to prevent, you know, humanity from getting destroyed, I'm not going to design my mission like, oh, will we better hope beta is 2.5, 2.4. No, you say beta is one, and you sent to spacecraft if you need a net, a beta of two, for example, right? Right. But the point being, like, if, you know, if the astronomers say, okay, here's the asteroid, here's the size, here's when it's going to hit the earth, it's now a relatively straightforward math problem to go, then I'm going to need this much mass to hit the asteroid at this speed. Yeah, like, we don't need to do any new, like, science, it just now becomes a massive engineering problem. Like, we got to build this, operate a milanjum, not miss, right? Which is, you know, a huge challenge. But from the basic science perspective, yeah, I think we have a pretty good sense of, you know, we could deflect an asteroid if we really... And I think that was the big, I mean, it feels to me like that was really the big unknown was before the Dart mission did its part. Like, here you got a rubble pile, which is essentially the worst possible scenario. And then, like, does your object just crash into it and just get absorbed like a sponge? Does it, does it punch a hole right through would be the almost the worst case scenario? Yeah, yeah. So then you don't get any of the momentum. Then you don't get any of that momentum transfer. But nope, it's exactly perfect. You get the momentum and you get the bonus of the thrust in the opposite direction. So, so that's all in the right direction. That's great. So the, you came to my attention because you wrote a paper about Phobos and sort of the far, far future of Phobos. You know, we mentioned here on the channel that Phobos is dying. What is, what is the sort of the fate of Phobos? Yeah. So one thing you know about Phobos is it will eventually be destroyed. So it's one of the few moons in the solar system that orbits within the synchronous limit. And so that what this means is the synchronous orbit is the orbital distance at which your orbit period matches the planets spin period. Right. So like geosynchronous satellites that orbit on the earth relative to one position on the earth, they're stationary. Right. So in the case of Phobos, it orbits faster around Mars than Mars rotates. So Mars rotates in something like 24 hours and 30 minutes, something like this is actually very close to an Earth Day. And but Phobos goes around in like seven hours. So it's orbiting much faster. And what this means is that the tidal bulge that Phobos raises on Mars, Phobos is goes ahead of that tidal bulge. So the tidal bulge is pulling Phobos back. So Phobos is slowly deorbiting and spiraling inwards. Right. And speeding up the rotation of Mars. Yes. By very, very small amount. Yeah. But in the reverse of what's happening with the moon, like the moon is beyond that synchronous rotation period. It is drifting away and slowing down the rotation of the earth. Exactly. And Phobos and Mars is doing the opposite. So Phobos is unique because I think it's Phobos and then some of the inner moons of Neptune are the only moons in the solar system within this situation where they're spiraling inwards rather than outwards and they'll eventually be destroyed. Right. And literally, if you just measure the orbital height of the moon, you know what the day period of the planet is, that'll tell you whether that moon is going to crash or whether it's going to drift away off into space. Yeah. Exactly. Yeah. So we know it's going to happen. The question is exactly how long will this take. And the mechanism for exactly how it breaks up is still a bit of a question. But yeah, it's a certainty that it's going to happen. Okay. And so what were you trying to understand then? So I was curious about, yeah, what is the distance at which it will really break up? Right. So if you take your classic high school or college physics problem where you calculate the Roche limit, where you just calculate the acceleration on the surface and find the point where the tides and self-gravity cancel out, you might get something like one point, gosh, I can't remember now, like 1.2, 1.5-ish planetary radii or something like this. Another calculation you can use, I don't want to get too much in the weeds, but it's called like a Drucker-Proger yield criterion. The idea here is you calculate the stresses inside the body, right? So all the forces, so you have gravitational forces pulling you inwards, you know, tides trying to stretch you out, centrifugal forces because you're spinning, you know, causing the body wants to, I guess, spread out too. You calculate all these stresses and you check, like, do I exceed some criterion and this criterion is based on material properties of the body? And if these stresses get high enough and I exceed that criterion, that means I break, I've been destroyed by tides. So that's as far as the math gets you. So, you know, we've been previous studies that have estimated the tidal disruption distance should be something like 1.6 to 1.8 Mars radii. And this is based on assumptions about the material properties of Phobos that it's actually would be a very highly cohesive body. It looks like a big potato, like a big rock. It looks like a potato, but I think it's a rubble pile. I think it's just going to be a rubble pile. So, there's two reasons for this. One, you know, Phobos is 20 kilometers in size. It's right in that size range. Well, let me back up a second because I should give some context. A big question about the Martian moons is their origin, right? So, there are thought to either be captured asteroids or, like, re-accumulated moons left over from a giant impact on Mars. I think in both cases, it's reasonable to assume they might be rubble piles. So, if they're captured asteroids, most asteroids are rubble piles. And Phobos is in this size range, it's 20 kilometers, it's probably in that size range where it's small enough, it's a re-accumulated rubble pile. If Phobos and Demos are re-accumulated from a giant impact, they probably, you know, there's a sort of a debris disk around Mars full of a bunch of, I don't know, maybe meter-sized fragments and these things gravitationally, fragulated and formed the moons. And then, they're rubble piles in that case. So, in that case, I think if there's not a whole lot of cohesion, then they're a lot weaker. And so, what we did is we did some simple, like, analytic calculations and then some computer simulations to back those up, where I just simply calculated the accelerations you would feel if you're standing on the sub-Mars point on Phobos. So, if you're on Phobos at the equator, at the location that's physically closest to Mars, where Mars would be, like, straight overhead, right? And so, you have tides pulling you away off the surface of Phobos. You have centrifugal forces due to the rotation pulling you away and you have self-gravity pulling you down to the surface. So, based on Phobos's, and I account for, you know, the regular shape of Phobos, it's not a sphere. So, the gravity is not so simple. But anyway, these forces sum up to zero. So, they cancel out when Phobos gets to, like, 2.2 Mars radii. So, today it's at, like, 2.7. So, this is, like, half a Martian radius farther away than you would have predicted based on models of, like, the material failure. So, the idea I found is that you don't actually, you can get to this point where, like, gravity at the surface is zero without the body formally, like, undergoing material failure. So, you can have, if there's, like, loose boulders on the surface of Phobos, you know, once you get to that point where gravity and all these forces cancel out, this material is just free to fly away without Phobos undergoing this violent, you know, tidal disruption. So, the thought was, like, oh, well, this material is going to leave the surface. It goes into orbit around Mars, just like Phobos. And it's free to come back and collide with Phobos again at some later point in time. And these collisions would occur at much more than the escape speed of Phobos. So, it's like the boulder gets stripped off the surface. Let's say, like, a month later, it reclides with Phobos. It's going to be colliding fast enough that collision will be erosive. So, this one boulder reclides, it might kick off two or three or five more boulders, and then they do the same thing. And so, the idea is that you don't need to migrate all the way in and go and undergo this big tidal disruption. You just need to get to the point where you can strip a few boulders and trigger, like, a runaway, collisional erosion where, like, Phobos is just destroyed by itself by boulders that it originally had. And then all of that debris is slowed down by the atmosphere and rains down on Mars over the next some period. Some of it would hit Mars. But I think, you know, actually, I haven't done, like, the calculation. I think some would get hit Mars, but also, I think a lot of it, you know, you'd collisionally destroy Phobos until there's sort of nothing left and you'd be left with, like, a debris ring, which could then reaccumulate into a new moon. So, that's a possibility. And so, this that that idea is join demos. Yeah, so that idea is not something new. So, it's been proposed that Phobos is like a nth generation satellite in this, like, a, I'm trying to think of a good, like, a, like a rain recycling process. Right. Okay. So, like, like, one of the weird coincidences that we happen to be present in the solar system at a time when Phobos exists, exactly, what you're saying is that if in fact, this is a cycle, then, then you're always going to see some combination of moons. It's not special. That's interesting. Because, like, yeah, our solar system is four and a half billion years old. And Phobos is going to be destroyed in, like, 40 million years, which is very short in solar system timescale. So, it's such a weird coincidence. So, if you're uncomfortable with that, this is an appealing theory. So, yeah, here's the idea. I just felt very appealed. Yeah. So, the idea is, you know, if you're within the synchronous limit, you migrate inwards, you get to the roast limit, you're destroyed by tides or whatever, or this process I'm talking about, this collisional erosion thing, that actually doesn't matter too much. You get destroyed, and now you're a ring, right? And remember, you're still within the synchronous limit, right? The ring of material spreads out or viscous spreading. So, some material migrates inwards and re would reach Mars, and some migrates outwards, and it can migrate beyond the roast limit, where it can re accumulate into a new moon. Remember, tides want to pull you in, but you still, while you still have this ring here, the ring actually supplies a torque to the moon and causes the moon to migrate out, and that can overpower tides temporarily until the ring dissipates and goes away. And once the ring is gone, your moon migrates back in, gets tightly disrupted, forms a new ring, and so this process might have occurred, you know, five or six or seven times or something like that over the age of the solar system, and so Phobos is just the last remnant we see today. Oh, it's interesting. And so, there would have been more material in the beginning, exactly, after the first impact, and then you've got this sort of cyclical process, and maybe Deimos is the leftovers that make it, because Deimos is farther than that synchronous rotation rate. Yeah, so Deimos, the synchronous limit is at like six Mars radii. Deimos is at like something like seven Mars radii. So, it's migrating outwards very, very, very slowly, right, but it's beyond the synchronous limit. So, Deimos has probably been there the whole time to witness this. Capturing all the refugees. Yeah. Oh, that's really interesting. So, that's what sort of motivated me to look at this, because these previous studies on this recycling model go off this assumption that, you know, you undergo tidal disruption like 1.6 Mars radii. And the closer you are to Mars when you disrupt, the more mass makes it to Mars, and so you lose lots of mass every generation. And so, to explain Phobos today, you have to invoke a much more massive moon in the past. But if Phobos gets destroyed, or in like previous generations of Phobos get destroyed in this ballpark of like 2 or 2.2 Mars radii, then more mass makes it to the next generation, and so you lose less mass every generation. So, it might just change the parameter space of like what moon, size moon had to be there for this model to work. Wow, that's really interesting. And I wonder, I mean, you must be wondering if something similar ever happened to the Earth. Yeah, I will. In the case of the Earth, you need this synchronous, you need your moon to migrate inwards. So, it actually requires a slow, relatively slow rotating planet so that you can have something orbiting it close in that is going faster. In the case of the Earth, the Earth moon system has a ton of angular momentum. So, if you rewind the clock, and you know, when the moon used to be much closer to the Earth, the Earth was also spinning much faster. I can't remember exactly the numbers, but I think the Earth was probably when the system first formed, the Earth was probably spinning more like maybe 6 hours in a day or something like that. So, there's no room in like the orbital space to stick a moon that's within the synchronous limits. So, it can't migrate and it's always going to be pushed out by tides. This probably didn't happen for the Earth, like, but it could be happening for Neptune, for example. Neptune's got some like, some rings, some moons, the moons are migrating inwards. And then again, what were the chances that we would see moons in Neptune? Yeah, but we don't know a lot about Neptune, this is so far away, we don't know a lot about all these moons, but they're super interesting. So, I think one place to apply this will be... Interesting to see, sort of imagine what it would look like if Mars had rings to see that, you know, the ring system around Mars. It'd be super cool. Yeah, yeah. Well, I'd like to shift gears then and talk about your work on the Lucy mission. Yeah, okay. So, what role are you playing in the Lucy? This is the mission that is on its way out to Jupiter's... I'll give some a little bit of context first. So, yeah, the Lucy is doing a tour of the Trojan asteroids. So, the Trojans are a group of small bodies in the two stable Lagrange points around sharing Jupiter's orbit. And the idea is when the solar system was first forming, the giant planets underwent what's called the giant planet instability. And so, they're scattering off each other until they eventually found their way to their current stable orbits. And so, whenever Jupiter underwent its last big, like, close encounter with another planet and made a big jump in its orbit, whatever stuff was around when the planets were forming that happened to be at the right place at the right time and the Lagrange points got captured. And they've been stuck there ever since. So, we think all these... all this material has been there for, you know, four and a half billion years. So, the idea is... and we also think the material that made it there was scattered inwards by the ice giants. So, we think that all the asteroids in the Lagrange points are representative of the outer solar system, right, like Kuiper Belt objects. And so, the idea is you can go see all these objects in a much shorter time scale and a much smaller budget than actually going to the Kuiper Belt, right? So, you can... that's sort of the motivation to do a Trojan tour is you can go see the diversity of what was going on in the Kuiper Belt. Now, these objects have been there for four and a half billion years, and so, they've under been undergoing collisional evolution for four and a half billion years. So, a big question is, okay, when we go visit these things, how do we know this is really primitive versus something that's really collisional-y processed? So, one thing I'm working on for this mission is to model castor of eclisions on these asteroids. So, the first target we're visiting is the one I'm the most interested in. It's called Uribitis. Uribitis is the largest member of one of the only collisional families among the Trojans. So, a collisional family is a group of asteroids that we know used to be part of one asteroid and underwent a collision. And the reason why we know that is they're clustered together in orbital element space. So, they're semi-major axis eccentricity inclination. So, they're not physically close together, but they're the shapes of their orbits sort of all match. So, we know that they used to be somewhere at the same place at the same time and underwent some collision. So, Uribitis is something like 70 kilometers in diameter. And if you add up all its family members, you estimate what the size of the original parent body was, you get something like 100 kilometers, which is this sort of textbook size at which we think planetesmos forms. So, the idea is like Uribitis was like a pristine planetesmo captured in the Trojans. And then sometime after that, underwent this big collision to form this family. Uribitis also has a satellite, has a very small distance satellite, which also probably came from this collision. And so, what I want to model is the actual the collision itself, right? This is the first time with the spacecraft, we're going to see the largest remnant of a family forming collision. We've never seen one up close, right? There's lots of them in the name belt, we've never seen it with a spacecraft. So, it's a really unique opportunity to constrain our impact models, right? For example, when Lucie flies by, if it sees like a bunch of volatile elements and ices, we need to explain how we can form the family and retain the ices, right? Without heating them too much and losing them. So, these are the types of questions I'm interested in looking at. And so, what do you think? I mean, I think about some of the asteroid flybys that have been done so far, you know, at the rubble piles. We also had Ares, say, with the near mission, which looked a little different. Although, now that I think about it, the surface was very rubble piley. Yeah, although Ares, some people refer to it as like a fractured shard, which, you know, we have a problem with nomenclature. We need to figure out what we're talking about. So, I think some people think Ares is still kind of a rubble pile, but less rubbly, I don't know, like maybe bigger fragments. It doesn't look like a 10-sided die in the way, or an eight-sided die the way the other ones do. Yeah. But, and so, what do you think, like, what are you sort of expecting? Like, someone's going to put the picture in front of you and go, okay, we got our Yerbide's picture and you're like, yep, that's what I was expecting, which of course is not going to happen. Yeah, of course. You're going to be shocked. No matter what, I'm going to be wrong and we're going to be surprised. Yeah. We know that the parent body was something like 100 kilometers. Yerbide's today is 70 kilometers. In terms of mass, that means Yerbide's lost half of its mass, right? Because, you know, the mass or volume scales is the size cubed. So, a small change in the size is a huge change in the mass. So, Yerbide's lost half its mass. So, it underwent a catastrophic collision, a big collision. And so, I think Yerbide's itself is mostly re-accumulated, right? It's not like it just like you hit it and you split off a chunk in that fluid way. Like, the whole thing was disrupted and then came back together, I think. What that's going to look like? Maybe just like a big rubble pile. Like, you just take a picture of like Bennu and just like, you know, stretch it, make it bigger. And I don't, yeah, I don't know. It's going to be really interesting. Yeah. I even forgot, they've already done a fly by. Yes. The Lucy mission already did a fly by of what's up, Daniel Henson? Yeah, that was super cool. I mean, really interesting object that has this contact binary shape. Yeah, it was awesome. I was there for the encounter. I actually got to meet Donald Johansson himself. Oh, that's great. It was super cool. So, that was also, I think, the first time in history we visited an asteroid and the person who's named after got to like see the first images. And then actually before that, Lucy, before I joined the Lucy team, they did another fly by in the main belt in 2023. They flew past the asteroid Dinkinessh and as a satellite, Salam. And the satellite itself is a contact binary. Right. All right. And so we think that system formed similar to the Europe, you know, the Europe spin-up like we talked about earlier, although it seems like the satellite, it might have been a triple system at some point in the, that the two satellites merged. Yeah. It's funny that, I mean, there are whatever, hundreds of millions, billions of asteroids, caprialt objects or clad objects in the 10 trillion, you know, like there's a large number of objects in the solar system and each one is its own special. It's crazy. And, right. It's a mission to everyone. The case of Dinkinessh, Salam is crazy to me because, well, first of all, it was never like on the main mission. It was actually a colleague of mine was the one who found it and realized like, oh, we, we would basically do it, get a fly by for free and get an extra chance to just practice like the image sequencing. So we're fully prepared for the Trojan. So they, they convinced NASA to do it. And this is like a, Dinkinessh is like an S type silicate asteroid. It's like less than a kilometer and the main belt. Like, I remember people telling me beforehand, like, oh, this is going to be super boring. Like, this is just like the most boring generic asteroid ever. And, you know, one thing you knew about it is that I had like a 50 hour rotation period. And I was just like, okay, this is going to be totally boring. They get there. If it was not a 50 hour rotation period, that was the orbit period of the satellite that they were seeing in the light curves. So really at a faster rotation, and they just didn't see it. And so like this whole time, like everyone thought, oh, this is going to be super boring, we're just going to do it because it's good practice. And then it turned out to be like scientifically the most the coolest object we've seen in a while. And it was in, and that's the interesting thing about it is we just picked a nondescript small asteroid in the main belt, you know, flew by it in it, not only happened to be a binary, but happened to have a con the satellite was a contact binary, which tells me there could be lots of other asteroids in the main belt that we like are totally overlooking. Like, we take a light curve and we say, oh, like, that's a 20 hour or 30 hour spin period. Okay, it's nothing really that interesting. But that could be the orbit period of a binary that we're seeing in the light curve in a signal from the primary spin is just not as big. So we don't notice it. So it sounds like we need to visit more asteroids. Yeah, like, and think more carefully about when we like analyze other asteroids with ground base and space space observatories, like we might be totally overlooking things because, you know, what we thought was, you know, 10 or 15% of all asteroids are binaries, but like, just the fact that we just picked a random one and it was like crazy, tells me, unless we got really lucky, and now it's an outlier, but I don't think it is. So it's, yeah, it's amazing. And then of course, we've got the the psyching mission, which is going to take us to a metal asteroid. So yeah, there's some really exciting missions coming up. Harrison, a question I always ask my guests, what are you obsessed with right now? Gosh, what am I obsessed with right now? I'm actually obsessed with cycling. So I was never a big like exercise person. And but like in the last year, I've gotten super into it. I live in Nice, France, which is to a lot of people considered like the cycling capital of the world. I see like, when I go cycling, sometimes like all pro cyclists will pass me going so fast all the time. So I've gotten super into that. So I cycle to work every day up the observatory in Nice up the hill. So it's really good exercise. So that's my obsession. I'm obsessed with all the mechanical aspect of all of it. Yeah. Yeah. My only advice on that is watch out for your hip flexors. I totally wrecked mine. I've been spending years recovering from from mountain biking uphill to then do a downhill. Okay, I'll keep that in mind. Be careful. Yeah. I'm starting to get to the age right. Yeah, I need to be serious about stretching and just like feel the tightness in your hips. Then you're probably starting to crank your hip flexors up. Yeah. Yeah. But yeah, it's great. It must just be beautiful. How high up is the is the elevation of the observatory? The observatory is like, I want to say like something like 400 meters. So it's not super high. Yeah. Because it's like very close to town. Like, you know, the telescope said these observ at the observatory, we don't do science with really anymore because you have light pollution and everything. But at the time it was built in the late 1800s, I think it was the highest elevation observatory ever built. It wasn't even at high. But before that, like you wouldn't be able to haul materials up to like the top of Montaguea and build a giant telescope, right? Right. At the time, you know, took what you could get. So, yeah. Yeah. But even so, if you get a good night, it must be a great view up there. Yeah. Yeah. Yeah. And it's really nice. It's an awesome view. And that's the, I mean, the home of the NIST model. Yes, exactly. A lot of kind of places where my recent work down there. Yeah. Like what I talked about with the Trojans and Lucy, the principal investigator of Lucy, Hal Levison, he came here in the early 2000s for like a sabbatical. And where that's like when the NIST model was developed. So, in one way, like the Lucy mission is like, we're going to validate it, basically. We're going to check one of the prediction from the NIST model is that the Trojan should be this diverse sample of all these types of objects. So, they should all be super unique and different because you're sampling, you know, plant-esmo formation at different regions of the disk. But they all got scattered and captured at this one point in time. Oh, really cool. Yeah. Yeah. Yeah. It's kind of amazing that we're going to see some practical. Like you have a theory, you make predictions, and now a mission is flying to the Trojans, to the Lagrange points. Like this again, my audience is obsessed with the Lagrange points. Okay. Yeah. And so, the fact that a mission is going to actually explore two of Jupiter's Lagrange points is pretty great. And the orbit, the orbit they designed to hit the Lagrange points is so cool too. Yeah. Yeah. Well, Harrison, thank you so much for taking the time to chat. Good luck with your research and we'll keep an eye on Phobos. Great. Yeah. Thanks for having me. I hope you enjoyed this conversation with Harrison and Grisa. I'm going to give you some final thoughts, but first, I'd like to thank our patrons. Thanks to Abe Kingston, Andrea Pardretti, Bailey Groofing, Brian Bode, Kieredwin, Chuck Hawkins, Commander Baleck, Darkfinger, David Guilton, and David Mass, Evan Dot Bro, James Clark, Janice Smith, Jeremy Madder, and Jim Burke, Jordan Young, Josh Schultz, Marcel Smiths, Michael Purcell, Nord Space, one step for animals.org. Please follow my nephew at Vbrick6994, Rick Kaidu, Richard Williams, Sean Sargent, Stephen Foundemoney, Team 49, Telseps Canada, Vlad Chippolin, Wolfgang Klotz, and Zeldemore Galactic Defender who support us at the master of the universe level. And all our patrons, all your support means the universe to us. So this was a great interview for me. It filled a lot of holes in my brain about the sort of the nature of these rubble pile asteroids. And it's funny when you hear about this stuff after the fact, after the hard work, the discovery, all of the various factors have been thought about and considered. And you get to this place where the whole system is starting to make a lot more sense in sort of its dynamic process that in fact, all of these small asteroids are probably rubble pile asteroids that they are shed from the impacts of larger objects, planetoids, things like that. And that there is this sort of constant cyclical nature of how these things operate, the way they spin up, they spin down, the way they cav off moons, absorb their moons and how they migrate within the solar system. And it makes sense that this could also happen on on Phobos as well. And it just shows you how much even just getting a glance at one of these asteroids tells us a lot more about it and how important it is for us to have a lot more missions. And then when you consider even the larger picture, the fact that there are interstellar objects moving into the solar system that each one of those could be studied. And so there's going to be a lot of work for asteroid.