Planetary Radio • Nov 29, 2023

Lucy's first asteroid flyby reveals a surprise moon

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Hal Levison

Lucy Mission Principal Investigator for Southwest Research Institute

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Simone Marchi

Senior Research Scientist for Southwest Research Institute

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Bruce Betts

Chief Scientist / LightSail Program Manager for The Planetary Society

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Sarah Al-Ahmed

Planetary Radio Host and Producer for The Planetary Society

On Nov. 1, 2023, NASA's Lucy spacecraft, which is on a mission to investigate Jupiter's Trojan asteroids, made its first flyby of asteroid Dinkinesh. Hal Levison and Simone Marchi, the mission's principal and deputy principal investigators, join Planetary Radio to discuss the asteroid rendezvous and the surprising discovery of Dinkinesh's moon. Stick around for What's Up with Bruce Betts, the chief scientist of The Planetary Society, as he digests the discovery.

NASA's Lucy Spacecraft
NASA's Lucy Spacecraft NASA's Lucy mission will explore six Trojan asteroids, a unique family of asteroids that orbit the Sun in front of and behind Jupiter.Image: NASA
Dinkinesh and its moonlets
Dinkinesh and its moonlets This image shows the asteroid Dinkinesh and its satellite as seen by the Lucy Long-Range Reconnaissance Imager (L’LORRI) as NASA’s Lucy Spacecraft departed the system. This image was taken at 1 p.m. EDT (1700 UTC) Nov. 1, 2023, about 6 minutes after closest approach, from a range of approximately 1,630 kilometers (1,010 miles). From this perspective, the satellite is revealed to be a contact binary, the first time a contact binary has been seen orbiting another asteroid.Image: NASA / Goddard / SwRI / Johns Hopkins APL
Dinkinesh flyby image sequence
Dinkinesh flyby image sequence The first series of images of Dinkinesh and what looked like a single satellite, as seen by Lucy's terminal tracking camera during its close approach on Nov. 1, 2023. The frames were each taken 13 seconds apart. The apparent motion of the asteroids is due to the changing perspective of the camera as the spacecraft flew past, rather than the satellite’s orbit around the main asteroid.Image: NASA / Goddard / SwRI / ASU
Lucy tracking Dinkinesh
Lucy tracking Dinkinesh An illustration showing how the Lucy spacecraft likely shifted its orientation as it passed Dinkinesh, allowing its instruments to stay locked on to their targets.Image: NASA / Goddard / SwRI
Lucy’s flight path through the Solar System
Lucy’s flight path through the Solar System Image: NASA's Scientific Visualization Studio


Sarah Al-Ahmed: Lucy flies by its first asteroid, this week on Planetary Radio. I'm Sarah Alahmed of The Planetary Society, with more of the human adventure across our Solar System and beyond. NASA's Lucy spacecraft, which is on an epic adventure to investigate Jupiter's Trojan asteroids, passed by its first asteroid, called Dinkinesh. Hal Levison and Simone Marchi, the principal and deputy principal investigators for the mission, join us this week to discuss the surprising results and the spacecraft's health. Then Bruce Betts, the chief scientist of The Planetary Society, joins me for What's Up and some more asteroid awesomeness. If you love Planetary Radio and want to stay informed about the latest space discoveries, make sure you hit that subscribe button on your favorite podcasting platform. By subscribing, you'll never miss an episode filled with new and awe-inspiring ways to know the cosmos and our place within it. NASA's Lucy mission launched in October, 2021 on a journey to explore Jupiter's Trojan asteroids, a group of celestial bodies that share Jupiter's orbit around the sun. To understand why these asteroids stay in place relative to Jupiter, we need to talk about Lagrange Points. Most space fans have heard of them in the context of science fiction, or perhaps when you're learning more about the placement of space telescopes like JWST. Lagrange Points are like sweet spots in space, where the gravitational forces of two large bodies, like the sun and Jupiter, balance with the orbital motion of a smaller object, like an asteroid. There are five of these points, called L1 to L5. Jupiter's Trojan asteroids are mostly found near two of these points, L4 and L5. They're 60 degrees ahead of and behind Jupiter in its orbit, and in all of these years of exploring we've never seen one up close. Lucy is the first dedicated mission to explore Jupiter's Trojan asteroids, which may contain the primordial materials of our Solar System. Just in case you're wondering, the Lucy mission is named for the famous fossilized skeleton called Lucy, which completely advanced our understanding of human evolution. This mission mirrors its namesake's purpose, not by telling us more about human history, but by seeking to unveil the Solar System's origin story. On November 1st, 2023, after two years in the sky with diamonds, the Lucy mission reached its first asteroid, the main belt asteroid called Dinkinesh. Its primary targets are still a few years away because it's a long way to Jupiter, but right out the gate, Lucy's first flyby was a wild success. As the spacecraft flew by Dinkinesh, which some people are lovingly calling Asteroid Dinky, it discovered something surprising. It turns out the Dinkinesh system isn't one object. It has a moon, and that moon is a contact binary. There's a lot to unpack here, but we have the perfect people to tell us more. Our guests today are Dr. Hal Levison and Dr. Simone Marchi. Hal is a scientist at the Southwest Research Institute and the principal investigator for NASA's Lucy mission. His research covers a broad spectrum of subjects. He works on the formation and evolution of Solar System bodies, including terrestrial and gas planets, comets, Kuiper Belt objects, and Trojan asteroids. He's also the co-author of SWIFT, which is a widely used software package for orbit integration in Solar System studies. Broadly speaking, he works to understand the early dynamical evolution of the outer Solar System. Our other guest is Simone Marchi, who you may remember from the October twenty-fifth episode about modeling craters on the metallic asteroid called Psyche. Well, he's back. Simone is the deputy principal investigator for the Lucy mission, and also works at the Southwest Research Institute as a staff scientist. Simone focuses on the bombardment history of our Solar System by analyzing craters on terrestrial planets and asteroids like Dinkinesh. Thanks for joining me, Hal and Simone.

Simone Marchi: Hi Sarah.

Hal Levison: Hello, Sarah.

Sarah Al-Ahmed: It's great to have you both back on the show. I know I spoke with you just a few weeks ago, Simone, but Hal, you haven't been back on this show for two years. It's been quite a while, so thanks for joining us again.

Hal Levison: My pleasure.

Sarah Al-Ahmed: When last we left our heroes, that was both of you, Hal, the last time you were on the show, you had just set your eyes on the Lucy spacecraft for the last time before it was off on its adventure, and now it's cruising through space. So what has this last two years been like for you and the team?

Hal Levison: It's been a roller coaster ride, because we've had a problem with one of our solar arrays, which didn't deploy properly. That has been very stressful getting through that. We think we're through that. And then of course we had a flyby of Dinkinesh, which I can be honest to tell you that I thought would be boring as hell. It's just a little tiny main belt asteroid, and it turns out to be really fascinating and exciting. So it really has been an up and down trip.

Sarah Al-Ahmed: It has been. And I'm glad that you brought up the solar arrays, because I remember that moment. We were all watching the live stream for the Lucy spacecraft launch, and then just a few days later to get that word back that the solar arrays were having trouble deploying. We were all very nervous. So how did you actually manage to try to deploy these, because my understanding is that they unfold kind of like a Chinese fan and that it just didn't latch, so you guys did a lot to try to make that work. What were your attempts?

Hal Levison: Well, the pull harder is basically all we could do, right. The way these things deploy, as you say, it looks like a fan unfolding, and there's a lanyard that's on the end of one of it that's being pulled by a motor. First of all, and I have to say the team was absolutely amazing. We learned about this the day of the launch, and frankly we didn't know whether we could fly the mission for quite a while. And it took about a month, maybe six weeks of a lot of work of very talented people to figure out just from a little bit of data. We just had basically one plot of current versus time on the motor, figured out what happened and came up with a plan of how we could get it to deploy more. Full deployment, because it unrolls, is 360 degrees. We were about 330 degrees. We turned the motor on, run it for short periods of time, let it cool off, run it again, let it cool off. Did that several times during our redeployment attempts, and we now believe we're at about 355 to 357 degrees. The array is not latched, but we've gotten it about as far as we can. And we think the array is going to be structurally sound enough to be able to fire our main engines, which is the biggest stress. The first time we do that is in February.

Sarah Al-Ahmed: That's good to hear, because you've got so many targets you have to navigate between, and the first question in my brain was, even if we get it deployed most of the way, it might get knocked back a little bit if we start firing those thrusters. So fingers crossed.

Hal Levison: Yes. The other aspect of this problem is that these arrays are made out of cloth, so the way you get structural strength is by having them under tension, and the one is not under as much tension as it should be, right? So the spacecraft is floppier than we expected. That led to us having to redesign all the control systems on board the spacecraft. Lockheed Martin has never done that before. So again, that was an amazing effort by talented people to redesign the control systems and do it so we could still do Dinkinesh.

Sarah Al-Ahmed: And this is a big moment. I mean there are so many targets you're going to fly by, but being able to do this first flyby and really test the systems will tell you how healthy the spacecraft is. Simone, you just recently saw the launch of the Psyche mission. You're also working on this one, so it's been a jam packed few weeks, few months for you. What was that flyby day like for you and the rest of the team?

Simone Marchi: I think it was just an amazing experience. Every time we fly by an asteroid, we don't really know what we are going to see, right? As much as we can prepare and try to visualize what might happen, but reality is that we don't know. And sure enough, there is always room for unexpected things to happen. Now, in this case, I have to say that it wasn't just simply fly by an asteroid with the spacecraft that we already used in the past for similar events in which everything has been tested, and so there was lots of uncertainties, and we were doing this for the first time. Also building on what Hal said, we were also concerned some of the performances of the spacecraft, because of the conditions that we have with the solar arrays and such. There were lots of expectations, and not knowing precisely what we will get out of this flyby, and that was really a motivation to add this to our already rich list of targets, really with the idea of testing the system as soon as possible to find out if works as expected. And so there were many things. And it's not just a condition of the spacecraft, also the way the spacecraft kept tracking over the target as a flyby. There were many things that we were interested in testing at once, and so this was really a great opportunity to do all that.

Sarah Al-Ahmed: This is such a small target. Of the asteroids we visited, this thing is so small, and you're testing all these systems on board. Were there any unique challenges to try to get so close to an object this small, and what did it tell you about the health of the spacecraft?

Hal Levison: There were challenges, right? In particular, as Simone said, we were trying to test our system that keeps the instruments pointing at the target as we fly by. It's called our terminal tracking system. That was new to Lockheed Martin. This particular one has never been flown before, and unlike our main targets where we'll be able to see the targets an hour away or a couple hours away, this one we could only see it within a few minutes of close approach. And so the system had to be fine-tuned to actually respond very quickly, and didn't have a lot of time to actually... Can I use the word think about? But you know what I mean, about where to point and how to point. And it worked absolutely flawlessly.

Sarah Al-Ahmed: It absolutely did, because the images that we got back were very startling for such a small body. This is really cool that you pulled this off.

Hal Levison: Can't disagree.

Simone Marchi: Indeed.

Sarah Al-Ahmed: What's interesting about this is that it's on its way to go visit the Trojan asteroids. We're not there yet. It's going to take us a few years to actually get there. As you said, I didn't expect this to be the most spectacular test ever, but as you got the images back from Asteroid Dinkinesh, it revealed all these interesting things that we didn't expect. So what was that discovery process like for you and the team as the images came back?

Simone Marchi: Most of them were gathered in the morning when the actual flyby was taking place, down in Lockheed Martin in South Denver. So we were monitoring for what's possible during the real-time event, what the spacecraft was doing. Once we got confirmation that things seems to be fine regarding the spacecraft and we moved back to our office in Boulder, and then we had the science team gathering and looking at the screen. Well, basically there's a white screen in front of you and you're just counting the seconds until the first picture comes up. That basically is what it was. And lots of expectations and trying to guess what it might be, but there is no real way of imagining this. And the funny thing was, the first picture we saw was a little bit from far away, not one of the closest approach, and these objects look weird. I mean it was completely weird shape, and we were really scratching our heads because it didn't make much sense. And things started to clear up a little bit with another picture because a part of the body was actually moving independently from another part. Well, it turns out that was a satellite. You can imagine, oh, this was great excitement for seeing. There is an eventually the highest resolution picture came down, and lo and behold, the satellite was there, but also lots of details on the surface. So we have lots of pictures that were taken during that day with basically our mouths kind of open and admiring what was on the screen in front of us. That's how it went.

Sarah Al-Ahmed: My exploration of it was mostly through images coming off of social media, so as they were revealed one at a time I was like, "Oh, there's a moon. And oh, that moon is actually some kind of contact binary object," and it just got weirder and weirder. When did you guys realize this moon actually might've been two objects kind of smashed together?

Hal Levison: It took overnight actually because the data that showed the close approach images where you could clearly see the contact binary didn't come down until the next day. So we knew there was a satellite. That was very exciting. That was not totally unheard of. There's certainly many objects of the same size as Dinkinesh in the near earth object population that look a lot like this, the top shaped primary and little satellite next to it. So that wasn't totally shocking. It was exciting but not shocking. And then the next day we get down the images post close approach where you could really see the secondary well, and we noticed it was a contact binary. And that, I can't imagine anybody would've expected that, and no one in the room certainly did. I must admit, I still don't know how you make it. But that just sort of blew our minds. There's a great picture, when that moment came down with all our mouths sort of open in shock.

Sarah Al-Ahmed: I haven't seen that picture yet, but I have to look it up because I feel like I did the same kind of shocked Pikachu face as I was looking at that image. Because really though, how did that thing form? Clearly either they all formed together in one system or somehow this moon was captured by Dinkinesh, but at some point they were all completely different objects and that's just... That's a lot for one flyby test.

Hal Levison: Well, we don't know, right? There's certainly a lot of models that have been done that use, invoke a spin up of the object that, due to radiation effects from the sun, that when the object gets up to spin speeds that are really fast, the material will come off the equator and end up forming a satellite. And indeed, these objects all have ridges on the equator, and Dinkinesh does have such a ridge. So you might think that things formed together, I mean formed in that way, right? The challenge is how you get two objects, and if you do why they're the same size. That's saying there's something in the process. Let's say you formed two objects this way and they just came together. That says there's something in the process of the formation that likes a particular size object over other sizes, and none of the models that I'm aware can explain that. So the real key here is we have two objects of the same size, and I don't think any of the models can predict that.

Sarah Al-Ahmed: That's so fascinating, so strange, and already this mission is throwing us for a loop when we didn't even expect it to do it at this point. Did we have any indication from the light curves, from the data that it might've been some kind of binary object before we got there?

Hal Levison: Do you want to talk about that, Simone?

Simone Marchi: Yeah, that's a good question, Sarah. Yes, indeed. We had been gathering light curves for our target for quite some time, and in fact, since we made the decision to fly by Dinkinesh, then we started an observational campaign from the ground. And so we had built a light curve, which if you look at it looks similar to many other light curves, and we thought we understood it. So the light curves gives us an oscillation of light that reflected, that was interpreted to be simply due to the shape, uneven shape of the object as it rotates. Now by doing that kind of analysis, you get a period for such a rotation for the object, which was 52 hours, similar to many other asteroids, hundreds of other asteroids that have similar light curves in similar periods. So there was nothing really super peculiar about this curves that was gathered from the ground. And so we were going in with this event having a sense of, I would say security in terms of understanding what was going on. This probably was an object, uneven object with a spinning period of fifty-two hours. It's not that complicated. Now, of course that was terribly wrong in the end. And we started realizing there was something that was not really making sense. In fact, a month prior to the flyby, that is where we started taking light curves data from Lucy instead of from the ground. So that's where we were approaching our target. That is done typically for navigation purposes in order to refine where the position of the object is in space, and so we can have a flyby as planned. And so in taking all that data, we also built a light curve, and the light curve was showing a behavior that would not make much sense with what we measure from the ground. Now, a key difference was our approach was coming from a very different angle that had seen from the earth, and so the fraction of the surface that was illuminated was very different than was seen from earth, and so that could mess up a little bit the interpretation. But still we were really, it was a puzzle. We couldn't reconcile the light curve from the spacecraft in approach with the ground-based light curve. And that is the time where I believe we sort of started thinking, "Well, maybe there is something different going on." Maybe we didn't understand everything, and maybe it's because there's a satellite. I have to say honestly, at some point we were betting what you think of this. Because I was quite convinced. I said, "Well, I'm not convinced that the data shows us there is a satellite, but if I have to bet, I'll say fifty-fifty." And well of course, eventually there was a satellite, and now that we are going back and we're analyzing all the data, I think we have a way to make sense of everything. So I think things are coming together in this regard.

Hal Levison: Let me just add one thing to what Simone said, because it's a cautionary tale. There have been a lot of work by a lot of people over decades taking light curves of asteroids and interpreting them in the way we were interpreting the ground-based observations. And this just shows that those interpretations are wrong. A lot of what we think we know about the shape of asteroids is probably incorrect.

Sarah Al-Ahmed: Given that we had data before and now it's completely surprised us, are there any other targets that Lucy's going to be flying by in the future that we think might be these kinds of binary objects when we thought they were initially just one?

Hal Levison: Probably not, because in addition to taking light curve data from the ground, we've had a comprehensive campaign of doing stellar occultations, and this is going out into the field when one of our targets moves in front of a star, and watching the star blink out. And from that you can actually get real shapes and sizes of our targets, and we've done that for all our targets. We have a lot of data. So we really to zeroth order understand the shapes of our Trojan targets. And so I don't expect there to be a surprise. We may discover a satellite or two. I think there has to be another satellite in our first Trojan encounter called Eurybates, but for various reasons, I think there's another satellite there. But I don't think we're going to be surprised again like this for the Trojans. Now remember, we have a second main-belt asteroid encounter with an object called DonaldJohanson, and it has an extremely strange light curve. And interpreting that is going to be... I mean, I just don't know how to interpret the light curve we're seeing. It's very high amplitude and very long period, and I wouldn't be surprised if we see something similarly weird at DonaldJohanson when we get there.

Simone Marchi: And we already know satellites for some of our Trojan targets. So in a way, we are a little bit ahead of the process, right? Because we have discovered, the science team, Lucy science teams has discovered a couple of satellites. One is for Eurybates, our first Trojans, and the other one is for Polymele, our second Trojan. So we already know that we have, are dealing with satellites. Now, as Hal said that there may be more hidden somewhere, but we'll have to see. At this point what I find most puzzling about this, as Hal said, this light curve was simple and we thought we understood it. And clearly that was wrong in the specific case. And so that brings you to the point of, what really know about other small asteroids. There may be lots of weird things that we can't really imagine because we don't have good data enough.

Hal Levison: The only way to solve this problem... There are two, right? One is to send a spacecraft, which is very expensive. But the other is through doing these stellar occultation campaigns. And so I think this is sort of indicating that perhaps the community should invest more in doing these kind of campaigns, because what we thought we know we clearly don't.

Sarah Al-Ahmed: We'll be right back with the rest of my interview with Hal Levison and Simone Marchi after the short break.

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Sarah Al-Ahmed: We're only at the beginning of doing something like this. I mean, this is the first time that we're sending a mission to a bunch of Trojan asteroids. I'm sure that we're going to learn things that we can't even begin to predict at this point, which is so strange. And I wanted to ask about this too because with a lot of previous asteroid missions, we've gotten there and found that these objects are very rubble pily, very not the texture that we expected. As we flew by Dinkinesh, what did we learn about its composition and maybe how rigid it is?

Simone Marchi: Well, it is still early days to have definitive assessments for what we have found. We're still going through the data and analyzing it, but a couple of things are sort of emerging. First is the surface, the overall structure of the object. We're still in the process of building up a shape model of what's needed in order to do detailed investigation of how the shape of Dinkinesh compare with other similar size asteroids. And so still preliminary, but the shape is interesting in a way that at least to me, it doesn't look like a simple rubble pile asteroid. There are structures that we see, there's a chunk that seems to be missing on one side and on the other side there seems to be a big trough going through. And then we also clearly see some craters on the surface, although not many craters, but some are clearly visible. And for instance, if you compare with other asteroids, say Bennu or Ryugu for instance, their shapes are quite different than what we find for Dinkinesh. And also for those asteroids, it was really a challenge to find even a single crater at the surface. Nothing was really super obvious, while we see obvious craters on the surface. So that seems to indicate that there are difference in the bark properties and internal structure perhaps, but it is still a bit early days to find out precisely what the differences are.

Sarah Al-Ahmed: Well, cratering is your whole jam, Simone. So I'm wondering, the fact that it has craters on the surface, does that indicate that it's probably a more rigid body? Because otherwise my brain tells me it might fill in those spaces a little more easily.

Simone Marchi: Well, the first thing, when I saw a crater, I said, "Oh good, so I have something to do." Jokes aside, but it was nice to see craters. I don't necessarily think that it would imply to have, that the object has higher strength because of craters. I mean, you can have craters in a pile of sand with no whatsoever strength or very minimal internal strengths or frictions, and you can have craters that way as well. So I don't necessarily think that craters indicates that. So we'll have to see once we have all the data. And also shape model would be important, for instance, to get to the question of what the shape is of those craters. Or the shape may reveal, for instance, if there is lots of regolith or loose particles on the craters themselves or if it's more, sort of more rigid object. One thing to look for instance in that regard is the depth of the crater with respect to the size of the crater. That's typically used as a measure of how much regolith there is on the surface. So all of that, it's something that needs to be done. So we'll have to see for a proper answer to your question, Sarah, but I think that the fact that we have craters don't necessarily imply it's high strength.

Sarah Al-Ahmed: Well, the data's only just come back. We're only just beginning to learn these things. What other mystery is the team still trying to puzzle through in this data?

Hal Levison: We're really at the point of just looking at the data and getting a first order understanding of what's there. We're not really to the point where we can even speculate. We clearly see this equatorial ridge, we see these cracks and things like that where we're in the process of thinking about, "Well, this feature overlaps that feature and what's that tell you about the history?" But I can say there's a lot of data there, and let's say unlike Ryugu and Bennu. And I think eventually we're going to be able to put together a pretty detailed story of what happened to this object.

Sarah Al-Ahmed: Well, I'm glad. I'm going to need a little 3D model to add to my collection on my desk eventually, when we know what its full shape is. And thankfully, your team has several, well at least two years until your next target flyby. You have time to go through this data. But what does the future hold? What does your next few years look like before we get into the good stuff?

Hal Levison: Well, I mean we have several events before the 2025 DonaldJohanson encounter. As I said earlier, we're turning on the main engines for the first time on the 31st of January. That's going to be a big day for us, because we really have, although we've modeled the spacecraft as well as we can, how it's going to behave in space is still a little uncertain. That is going to be a high stress day, to make sure it behaves the way we expect it to behave. Then in December of '24, we have another earth flyby. Remember, we're covering a lot of space, and we actually are using earth gravity assists to target our Trojans. So there are three over the history of the mission. We had one last year, and we're going to have another one. We're not going to be taking any data, unlike the first one, during this encounter because the geometry just isn't favorable. But we're going to have to work through that detail. And finally, we are I, and I must admit this is getting more intimidating the more we work on it. If you remember how busy the time of the Pluto encounter was with New Horizons, we have four such encounters in 15 months. So the way we're handling that is we're doing all the planning now. There's still a tremendous amount to do in just planning the encounters at our science targets. And so it's going to be a busy time, even though you may not hear a lot from us over the next couple of years.

Simone Marchi: Well, that's really a great point Hal, and I would like to stress that because people outside the team may feel, "Oh, there is all this space between one event and the other all this time, and so you can relax." But I have to say that there is really no time to relax in a sense, because there is always an important activity. It could be the planning, as Hal said, in fact, building the sequence, observational sequence and the planning for future flybys or EGAs, or maybe there is a new asteroid to fly by. All of that is taking all the time we have. And the team has been great at, both the engineering side and the science team, to support all these activities, one after the other.

Sarah Al-Ahmed: Is that a conceivable situation, where you decide on your journey that you might want to get a little closer to another asteroid on the way?

Hal Levison: Well, we certainly did it now, with Dinkinesh. That was mainly put together because as we said, it was the test of our systems. And from an engineering point of view, the best philosophy is if you're going to fail, fail sooner. And although DonaldJohanson is also a test, it's going to be testing different things. But we decided that it was worth going after this. And let me just take a step back. The amazing thing about this encounter is if we had done nothing, we would've flown within 64,000 kilometers of Dinkinesh. That's 20% of the distance from here to the moon. So we were getting close anyway, and so required very little resources. I don't think we have the time to add another encounter between now and the end of the first set of Trojan encounters in 2028. It's just going to be too busy. Even if we found something we were flying by, we just don't have the time to design another science sequence, tweak the trajectory to get there and to be able to do it, particularly if it's Trojan, because all those encounters are very close together. There is the possibility, if we were to find something after we leave the first group of Trojans, which are in the leading L4 swarm, and then we have six years before we get to the trailing swarm. And if we found something there, we might consider doing that. But that's so far in the future that it's not even on a horizon.

Sarah Al-Ahmed: Plus you're already dealing with a situation where the solar panels or the solar arrays didn't deploy perfectly. So conserving energy and conserving fuel when you've already done these tests is probably a good plan. I'm just really glad that of the tests that you did, it was Dinkinesh, because we got this very strange situation there. It could have been the most boring rock ever, but it wasn't. What are the odds?

Hal Levison: Well, I think the way to interpret that is that none of them are boring.

Sarah Al-Ahmed: That's probably the thing.

Hal Levison: And being weird is common.

Sarah Al-Ahmed: Of the targets that you have coming up, once you actually get to Jupiter's Trojan asteroids, which ones are you both personally most looking forward to?

Simone Marchi: I think they're all exciting for different ways, and it's really hard to pick one. But you know me, you know that I like collisions and cratering and smashing things. And so if I have to pick one, I will definitely pick Eurybates, which is the first Trojans that we'll fly by, the reason being, Eurybates is the largest member of an asteroid family. And so we think that the family is formed by catastrophic collisions of a larger asteroids that was blown apart. And then you generate a bunch of smaller ones that are the family, and Eurybates is the largest remnant. And so this will be the first time in which we fly by an asteroid that is the result of a catastrophic disruption. And so I have a great expectation because of that, because maybe can tells us a little bit more about how catastrophic disruptions works. And we know that that's sort of a fundamental process, evolutionary process for asteroids in general. So that makes it very intriguing. And now on top of that, there is also small satellite, Queta, around Eurybates. Well, maybe that is also, the formation of that is also related to the formation of the catastrophic destruction perhaps. And so there's lots of little mysteries like that. And plus, by looking at craters that we find on the surface of Eurybates, we can possibly pinpoint when this catastrophic destruction took place by observing how many craters there are on the surface, which will be also an important [inaudible 00:37:04] for the evolution of the Trojans in general. For all these reasons I would pick Eurybates as my favorite.

Sarah Al-Ahmed: What about you Hal? Is it the same one or a completely different one?

Simone Marchi: No, it's the totally different side of things. My interest has been understanding how planets form and evolve, and that includes forming the first macroscopic planetesimals in the Solar System. And there's reason to believe, and I can go into this, but it'll be a long discussion if you want, that two of our objects have their primordial shapes. One is Polymele, which is our second Trojan... By the way, that only happens like 30 days after everybody, so we're going to have two back to back. And it is shaped like the object Arrokoth, or the large lobe of Arrokoth, in the sense that it is essentially hamburger-shaped. And we can understand that through models of the formation of the first planetesimals, but it's unlikely to arise in objects that have been broken apart. So this indicates, suggests that it's primordial. The other is at the end, in 2033, we're flying by a near-equal mass binary. There's also reasons to believe that it's a nearly primordial object. So studying the objects that are the oldest and leftover from the earliest stages of formation, are the things that fascinate me. And those are the two objects that you can point to in our stable of Trojan targets.

Sarah Al-Ahmed: That would be really interesting to know whether or not those are actually just kind of untouched remnants, because the things that it could tell us about the formation of planets and other asteroids in our Solar System, it's just kind of unfathomable. I feel like we're just at the beginning of this whole new discovery age. But we actually have to get to these asteroids first in order to look at them up close in order to figure it out. So you guys have some really exciting years ahead of you, is all I'm saying.

Hal Levison: I think so. I mean, I think we have to emphasize that, and I say this in my talks, this is really a mission of exploration. These targets have never been seen up close before. And not only that, because of their proximity to the orbit of Jupiter, small things leaving these swarms can't get to the earth. Jupiter will accrete them before they can evolve into an earth crossing orbit. Unlike other small body reservoirs, the main belt, the comets, we don't have any meteorites from these objects. They're really a mystery.

Sarah Al-Ahmed: Do you think there's anything about the Trojan asteroids that might really surprise us? Anything about their composition, or I don't even know how we'd begin to guess at that, but there's probably a lot about those specific asteroids in their formation that makes them very distinct from other asteroids.

Simone Marchi: Yeah, I think that's right. There's lots that we don't know and it's hard to imagine what it might look like. But composition is one of the things that I would say is least understood for these objects, for various reasons. So Hal said earlier, we did occultations of our targets. And occultations give you a good sense of shape. Certainly it's not high resolution shapes like we would like, but at least they give you a sense. But when it comes to composition, these objects are for the most part, and I clarify what that means, but for the most part are featureless. So if you take a spectral data, you do not see clear absorption bands, like for instance you would see on Vesta or Ceres or many other main belt asteroids. And if you don't have those absorptions in the spectra, then it's hard to say what they're made of. And so we don't really know much about their composition because of this. Their spectral properties are relatively featureless, plus they're very far from the sun. They're small, and so they are dark, meaning the magnitude, it's limited. And so also the spectral data, most often than not, it's very noisy. It all builds to the fact that we don't really know much. But these objects likely formed in the outer Solar System. So you would think that in a way or another, they might contain ices, maybe water, co₂ or other species. They might contain organics. So all of that mixture helps that, precisely what's the make-up? We don't know. We hope that as flying by, with our spectrometers, we can definitely get a better sense of that. We may start seeing absorption features on these objects that could tell us the composition. And so I suspect that that is going to be one of the areas of investigations that will be particularly important for the Lucy mission.

Sarah Al-Ahmed: It's just really beautiful that we finally have a spacecraft that can begin to explore these things. And I'm sure it's going to open up so many more questions than we're even prepared to answer at this point. So I'm sure that you're going to have some really, really exciting years, and hopefully in a decade or more when we finally have information back from all these, I'll hit you guys up again and bring you back on the show if you're willing.

Hal Levison: It would be our pleasure.

Simone Marchi: Yeah, that'd be great.

Sarah Al-Ahmed: Well, good luck to you and your team. I know it's going to be all of us kind of waiting on the edge of our seats for years, not getting to see all of the craziness behind the scenes, but it sounds like you're all going to have to put in 110000% to get this all done. So good luck to you and everyone. And seriously, congratulations on this flyby. This was beautiful.

Hal Levison: Thank you.

Simone Marchi: Thank you.

Sarah Al-Ahmed: I'm serious though. If you find a 3D file for Dinkinesh or its adorable mini moon, please email it to me. I'll put it on my desk at Planetary Society HQ, right by my model of Comet Churyumov-Gerasimenko. Okay, now let's check in with our chief scientist, Bruce Betts, for What's Up? Hey, Bruce.

Bruce Betts: Hey there.

Sarah Al-Ahmed: I love this, that we thought we understood what was going on, but we clearly knew it could surprise us. And then we got there and as usual, it surprised us. And even after learning that this asteroid had a moon, it just keeps getting weirder. And I wonder what we're going to find as Lucy continues to travel out there. Probably weirder and weirder things.

Bruce Betts: Yeah. But it definitely just [inaudible 00:44:07]. I often discuss our Shoemaker NEO grant winners that are really advanced amateurs, sort of. I mean, they're semi pro... They're just really good. And part of what they do is put the telescope time into doing light curves that will sometimes show you that something's a binary. Because if something's coming to earth, you kind of want to know that, even the little guys a couple hundred meters I think, which will really ruin a lot of people's day if it came this way. Which that one won't, but we need to find others. Sorry, got off on my planetary defense kick.

Sarah Al-Ahmed: No, but that's the important thing. We need to preserve and protect our planet. We need to make sure that we're not going to get completely beamed. And there's only so many asteroids that we've actually visited so far. So this is going to up the number by quite a bit. Who knows what we're going to discover?

Bruce Betts: It's great. They've turned it up to 11. They actually are making the joke themselves. I love that. Because they started, they had nine, and then now they've got eleven-ish, so they turned it up to 11. But that's amazing. That's unprecedented. And also they'll be showing us the Trojan asteroids for the first time, Jupiter's Trojans that are in front and behind it. And I mean, they probably look like asteroids, but hey, let's find out.

Sarah Al-Ahmed: But what kind of asteroids, right? Even as a kid, I thought they were solid chunky objects. And now it turns out most of them are these rubbly piles of just debris, kind of loosely chilling out with each other, which is not what I expected as a kid at all.

Bruce Betts: Well, not many people expected that, not that long ago. And then we started thinking that there would be these so-called rubble piles. And I'm hoping for a fluff ball, which some people theorize. And then you got your pretty solid metallic metal ones. And there's a lot of variability in the asteroid population, which is another reason it's good to study it, both as many as you can get remotely and then do these missions out there to them. Because it's interesting science, because what the heck's going on? And then also for planetary defense.

Sarah Al-Ahmed: What are the fluff ball ones? Are they kind of more like particulate kind of conglomerated together, like fluffy?

Bruce Betts: Yeah, just smaller fluff instead of boulders.

Sarah Al-Ahmed: Cool.

Bruce Betts: But I mean, they're not at all like cotton candy, but I think it's more fun to picture them that way. And pink.

Sarah Al-Ahmed: That is more fun. I used to make cotton candy at fairs when I was in Girl Scouts, so now I've got a real clear image. Thank you. We did have a question that I wanted to put to you from one of our commenters. I know a lot of people are in this mental space where they've only recently learned that the International Space Station is going to be coming down at some point, and a lot of people want to know what we can do to help preserve the International Space Station. And specifically one of our listeners wrote in, his name is Victor Carr. And he wanted to know what it would take to try to boost the International Space Station to maybe lunar orbit or something like that, and why we shouldn't try to accomplish that. So I wanted to put that one to you, since you are the chief scientist. Why should we not attempt that?

Bruce Betts: That would require some serious rocket power, because you've assembled this over tens of missions to get all this stuff up there and put it together. And so the amount of mass, which isn't on the top of my head, top of my brain, tip of my tongue, something, it would take a lot of rocket power or other techniques, but rockets probably the only practical one on a reasonable time scale. And so it would cost a lot of money. It would cost a great deal of effort. You'd have to design new rockets. They'd have to not damage the station, which is not designed to undergo stresses of sticking rockets on it. And so just generally, especially boosting it to a lunar orbit would be, fundamentally, it would be incredibly technically challenging, incredibly expensive, even if you could figure it out. And remember, we have a limited budget, unfortunately. Well, I mean realistically for space stuff. So if you boost the International Space Station to the lunar orbit, you don't do a whole lot of other things. Yeah, it's a tough call though. I mean, we have that every program we get and people fall in love with the hardware, and then the hardware, you move on to something else. And because the budgetary limit, typically you have to end one, especially hugely expensive program like ISS, to be able to do hugely expensive other programs, particularly in the human program. And they deteriorate over time so they're not as fresh. And you don't get that new space station smell when you go to it. I mean, not even now. God knows what smell you get now.

Sarah Al-Ahmed: Oh man. I was reading a paper the other day about the weird bacteria and the particulate buildup in the air in there. I bet it smells funky in there.

Bruce Betts: Yeah. Okay. Maybe I won't go. Okay.

Sarah Al-Ahmed: Whereas I bet the Tiangong space station that the Chinese National Space Agency sent up, I bet that one's like very nice. Looking at the images, it's very crisp and clean in there. It's beautiful.

Bruce Betts: Probably. But that new space station smell goes away surprisingly fast. You can buy those little air fresheners, but it's just not the same.

Sarah Al-Ahmed: When we create the lunar gateway, we should take a sample of the smell and then make a scratch and sniff of all the different space stations.

Bruce Betts: Wow. Space station scratch and sniff. That is disgusting. I like it.

Sarah Al-Ahmed: Love it. All right, now we can do a random space fact.

Bruce Betts: Random space fact. So this is truly random because it's just something I do that helps me when you hear the degrees of latitude on a planet or a moon or you're coming up with distances. And so a degree of latitude on earth defines nautical miles, to use unfortunate but practical units for people off doing nautical things. One minute of latitude, a 60th of a degree, defines a nautical mile, and there's 60 nautical miles on Earth for a degree. And that's about sixty-nine statute miles. But if you're just doing, you're just like doing some rough calculation, 60, 69, whatever. Well, my being like that, why would I focus on miles on earth? My gosh, what is wrong with me? Because the interesting thing to me, being a Mars head, is there's 60 kilometers roughly per degree on Mars. There's both the number 60, so it's easier to remember. And then if you're on the Moon, divide that by two, and it's 30 kilometers per degree latitude, again, approximately. So 60 miles-ish for Earth of a degree of latitude, 60 kilometers on Mars, 30 kilometers on the moon. There you go. Something truly random has helped me over the years of staring at maps and figuring out how far things are without getting into details.

Sarah Al-Ahmed: That's actually really useful. I mean, it's not like we can use it to navigate the seas of Mars anymore, but if you want to figure out how to navigate a map on the moon, or if you're one of the people looking for the one piece on Earth, you can use that.

Bruce Betts: You're talking pirate anime again, aren't you?

Sarah Al-Ahmed: I am. I am.

Bruce Betts: Yeah. Okay. All right, everybody. Go out there, look up at the night sky and think about whether the term dry land is redundant. Thank you, and good night.

Sarah Al-Ahmed: We've reached the end of this week's episode of Planetary Radio, but we'll be back next week to discuss the largest Mars quake in recorded history. I've been looking forward to this conversation for a long time. You can help others discover the passion, beauty, and joy of space science and exploration by leaving a review or a rating on platforms like Apple Podcasts. Your feedback not only brightens our day, but also helps other curious minds find their place in space through Planetary Radio. You can also send us your space thoughts, questions, and poetry at our email at [email protected]. Or if you're a Planetary Society member, leave a comment on the Planetary Radio space in our member community app. Planetary Radio is produced by The Planetary Society in Pasadena, California and is made possible by our members around the world who want to know more about how our Solar System formed. You can join us as we celebrate every space moment at Mark Hilverda and Rae Paoletta are our associate producers. Andrew Lucas is our audio editor, Josh Doyle composed our theme, which is arranged and performed by Pieter Schlosser. And until next week, ad astra.