The planets of our solar system didn’t have such stable orbits a few billion years ago. The giant outer planets moved around chaotically in their orbits, and Uranus and Neptune may have even switched places. To get a more complete understanding of the full history of our solar system, NASA is sending a spacecraft called Lucy to investigate the Trojans, mysterious small objects that share an orbit of the Sun with Jupiter. Principal investigator Hal Levison of the Southwest Research Institute’s branch in Boulder, Colorado, discusses this exciting mission, launching Oct. 16, 2021.

Learn more about the Lucy mission

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Jim Green:We’re launching the mission Lucy. It’s going to a special place in Jupiter’s orbit, where objects called Trojans are captured. What are they? And what can they tell us about the evolution of our solar system?

Hal LevisonIf you think about it, almost everything is chaotic. The stock market, the weather, everything is chaotic. And so is the solar system.

Jim Green:Hi, I’m Jim Green. And this is a new season of Gravity Assist. We’re going to explore the inside workings of NASA in making these fabulous missions happen.

Jim Green:I’m here with Dr. Hal Levison. And he is the principal investigator of a fantastic mission called Lucy. And he’s based out of Southwest Research Institute in Boulder, Colorado. Lucy will be launching very soon, and will be visiting some of the very special asteroid objects that share an orbit with Jupiter. So welcome Hal to Gravity Assist.

Hal Levison:Oh, it’s my pleasure to be here. I can’t wait to fill you in and let your listeners know about the Lucy mission. It’s very exciting.

Jim Green: It is very exciting. But I want to start, actually, a little earlier. What got you into studying how the solar system evolved? And how it may have even moved around over time?

Hal Levison: Well, when I was a kid, I was always interested in asking the questions where we came from, how we got here, how the Earth got the way it is. And so the study of planet formation, you know, was just filling in one of the niches of that question, right? I mean, obviously, there are many aspects of that question from cosmology all the way up to evolution. Right? But for me, I decided working on trying to understand how the Earth and the other planets got to be, is really what my passion is.

Hal Levison:My whole career has been trying to understand how planets came by building large numerical computer simulations following the orbits of things around the Sun as they accrete to form the planets.

Jim Green:What’s really neat about that whole concept is that it led you to some really exciting discoveries. The concept of how planets form, and then how they interact with each other, and actually move their orbits based on that interaction. That that is a fantastic story, you know, moving something the size of Jupiter and Saturn, Uranus and Neptune. How did that come about?

Hal Levison:Let me take a step back, right, because, you know, the concept of planet formation back when I got into this field, I’m a gray beard at this point in my career, was the planets sort of formed out of a narrow region of what we call the protoplanetary disk, which is made of gas and dust.

Hal Levison:And the planets slowly accumulated objects around that.

Hal Levison:What we did, through my career, over the last 20 years or so, was to understand that that’s not how planets form. Planet formation is actually very violent. The system evolves together, right. So rather than having this little isolated region where the Earth grew, materials handed back and forth between the planets as they grow, the planets are pushing each other around. They’re competing for resources, the ones that are successful grow, and become the planets that we see today.

Hal Levison:And it was that understanding of this evolution from a very quiescent situation to a very violent one that sort of led us to this understanding that the planets had to move around. And in particular, what we noted when we tried to run our models of the outer solar system, is that you can never build Uranus and Neptune where we see them today.

Hal Levison:Jupiter and Saturn, which are closer to the Sun, and thereby, in the standard picture form much faster, much faster, stop Uranus and Neptune from growing at all. So we needed to come up with an idea which allowed the four giant planets to grow together.

Hal Levison:And what, what we came up with was the idea that they formed as a very compact group, as a system of four planets. And then we needed to get Uranus and Neptune out where we see them. Right?

Jim Green: Right.

Hal Levison: And that led to this, what we call it, we call it the Nice model, which is a really a global instability, where the orbits of the planets become really become nuts. And they cross each other and gravitationally scatter each other around — gravity assists, right? Jupiter, Saturn gave Uranus, and Neptune a gravity assist to basically kick them out to the orbits we see today.

Jim Green:So Hal, in the Nice Model, did the order of the planets remain the same? Or did they change places?

Hal Levison:Um, it depends on the details of the model. Jupiter, and Saturn, their orbits remained the same. Uranus and Neptune. It’s about 50-50, which one ends up the inner planet and which one is the outer planet. There’s also a version of the Nice Model, which is actually becoming quite popular at the moment, where there was another ice giant that was actually ejected into interstellar space during this event, so that the solar system originally had nine planets in this idea, and now it’s down to eight.

Hal Levison:The reason why we need this extra planet is because we need gravitational interactions between Jupiter and this planet to get the orbit of Jupiter, right.

Jim Green:Oh, interesting.

Hal Levison:But once Jupiter grabs on to one of these things, the most likely scenario is it gets ejected.

Jim Green: Completely out of the solar system.

Hal Levison: Completely out of the solar system.

Jim Green:Wow, OK.

Hal Levison: So it’s, it’s, it’s gone. It’s history. If this is right.

Jim Green: And isn’t it true that the small bodies tell us this telltale sign of the dynamics of the solar system in its early formation?

Hal Levison:It’s the place to look, right? The way I like to put it is the small bodies are the fossils of planet formation, right? The planets evolved from them by accreting them and growing, right. That’s, by the way, why we named Lucy, Lucy. It’s named after the human ancestor fossil that we know, right? Because these things are really the places to go. Right? If you want to understand the history of planet formation.

Hal Levison:So that’s why NASA and other space agencies have put so much effort into understanding these small bodies, because they tell us about our history.

Jim Green:Yeah, that’s fantastic. And of course, Lucy is going to go to several small bodies. How did you make the decision to go to certain small bodies to uncover this early dynamical period of the solar system?

Hal Levison:What we learned when we looked at this particular period of evolution, is that the Trojan asteroids, which are the this population of asteroids that lead and follow Jupiter’s orbit by about 60 degrees, get captured into their orbits during this evolution. So let me explain a little bit more detail about what we’re talking about. We envision that the giant planets formed in a much more compact configuration. Jupiter, Saturn, Uranus, and Neptune, all forming within let’s say, 12, 13 astronomical units from the Sun right now, Neptune’s out at 30 astronomical units.

Jim Green:Wow, yeah.

Hal Levison:So that gives you how the scale of the solar system changed during this time. And that there was this disk of small bodies, outside the orbit of the giant planets, that extended out through about 30 AU where we see Neptune today. We believe, or this model predicts is probably a better way of putting it, that the stuff that’s in the Trojan swarms now are a remnant of that disk that originally formed outside the orbit of the giant planets.

Hal Levison:And that disk is now gone. Because Uranus and Neptune went through it. Most of it is in interstellar space. And the way to understand what that period of time looked like and what that disk looked like, is found in the Trojans.

Hal Levison:So that’s sort of the theoretical reason to go to these bodies. There’s another aspect of this, right? If you look around the solar system, there are several small major small body populations and the Trojans because they’re sort of at the edge of what we can do with solar power missions are the ones we have yet to go to. So they’re the ones that are really, are not explored. And in addition to that, right, because of their proximity to Jupiter, they’re the only small body population that isn’t supplying us with meteorites. So in a way, we have less information about the Trojans than any other small body population in the solar system.

Jim Green:You know, not all small bodies are created equal, so to speak, you know, we’ve got the rocky asteroids in the asteroid belt, but as you go further out, there’s a lot of small bodies that are in the Kuiper belt. So when we say these objects, Trojan asteroids, are captured around Jupiter, do we believe they’re all from the asteroid belt?

Hal Levison:This model would predict that these objects, the Trojans, formed in this disk beyond the orbit of the planets, but we don’t know that for sure. We need the data. Right? So that’s one reason why we’re doing Lucy is to get the data to test our theories about the evolution of the outer planets also, because according to the Nice Model, these objects formed at different distances from the Sun, they should have different compositions because at different distances, you have different temperatures, right.

Hal Levison:And so as a result, we should be able, by looking at these things close up, determine sort of where they formed, hopefully…

Jim Green:Mhm.

Hal Levison:…how they formed. And we hopefully can put that together with a story of the migration of the planets to figure out the history of the solar system. That’s the overarching goal.

Jim Green:Right. And that’s what Lucy is all about. And it’s launching very soon. So how do you feel about that? (laughs)

Hal Levison:I’m scared.

Jim Green:(laughs)

Hal Levison:But the, I mean, it’s been, it’s been one hell of a ride. We started working on Lucy, in March of 2014. For all these years, until about a year, year and a half ago, it was just Power Point slides, and CAD diagrams, and things like that. And so over only about a year and a half, a very short period of time, it’s gone from something that’s on paper to really, a real spacecraft that’s completed. The beginning of the launch period is October 16. So that’s very exciting.

Jim Green:You’re not just going to visit one object. How many of these Trojan asteroids are you going to visit?

Hal Levison:We are breaking records. So we’re visiting eight asteroids. No other mission is gone that before, seven of which are Trojans. We rattle around the inner solar system for a while and use Earth gravity assists to actually pump up the orbit of the spacecraft so it can get out as far as Jupiter. On the way out, it passes a main belt asteroid, which we’ve named after Donald Johanson, the discoverer of the Lucy fossil.

Jim Green:Right, very appropriate.

Hal LevisonYeah, thank you. And that’s actually an interesting object, in and of itself, because it is part of an asteroid family, which formed about 130 million years ago. So it’s one of the youngest objects in the solar system. And then we’re heading out to the Trojan swarms. We are going to do two orbits around the Sun. The first we’ll take us through the L4, the leading swarm. And then we come back to the Earth, do an Earth gravity assist, and go out to the trailing swarm which is called the L5.

Jim Green: Well, how many Trojans are there trapped in the Lagrangian point L4 and L5 at Jupiter, do you think?

Hal Levison:There are, estimates are there are a couple million.

Jim Green:Wow!

Hal Levison:Of these things there. Most of them are really small, right? There are only a few thousand that are what we would call macroscopic big things like the ones we’re going to.

Jim Green:So after you go to L4, and visit a couple of the Trojans, why do you have to come back to Earth and get a gravity assist to go to L5?

Hal Levison:Well, we need to do this, and we need the gravity assist in the beginning, in order to save fuel. Right, putting together a trajectory like this is actually very difficult. And it’s limited by the mass of the spacecraft, which includes the fuel at launch. So if we were going to try to go, let’s say, directly from the L4 to the L5, it would require fuel tanks that are just way too big. And so the trick that we’re using here is to use the Earth as a targeting mechanism. That’s why we have three Earth gravity assists through the entire mission. We’re letting Earth do the work rather than our main engines, and that saves fuel and makes the spacecraft lighter, which saves fuel and money in the long run.

Hal LevisonLet me just give you a little bit more background of Trojans. One of the interesting, and some surprising aspects of this population is when you look at them, they’re very different from one another. Right? This is, this is what leads people to believe that they formed at different locations in the solar system or were captured. But in order to understand what they’re telling us about the history of the solar system, we have to understand that diversity. And so Lucy itself was designed to visit as many of these things as we could. The planets literally are aligning to allow us to do this mission.

Jim Green: That’s right. This is a great opportunity to, to really understand this, this hidden idea of how the solar system came about by studying these small bodies. Well, how long does the total mission take?

Hal Levison:So the mission is roughly 12 years. Our last encounter is with my favorite object, which is a near-equal mass binary. So these are two 100-kilometer size things. They’re almost the same size, in nearly circular orbit around one another. Fascinating. I think they’re leftovers from the formation of the planets, the original formation of the planets. That is on March 3, 2033.

Jim Green:Wow. (laughs) That’s fantastic. Well, what are the instruments that you’re taking on Lucy?

Hal Levison:So Lucy has three basic scientific instruments. There’s a narrow field panchromatic camera called L’LORRI. We put a an L apostrophe before all our instrument names for Lucy.

Jim Green: (laughs) I see.

Hal Levison:So it’s L’LORRI that came out of APL that’s going to do our high resolution imaging for crater counts and looking at geology. And it’s going to do our satellite searches and look for rings and various things like that. We have a thermal infrared instrument spectrograph, which is out of Arizona State University. Right? It is going to allow us to measure the temperature of the object over various locations, that will tell us how the rocks on the surface heat and cool, which will tell us something about the structure of the surface, whether it’s sandy or whether it’s rocky.

Hal LevisonWe have an instrument called L’Ralph, which is out of Goddard, which is actually two instruments and one. It’s a color camera. And it’s a near infrared spectrograph, imaging spectrograph, which is going to give us information about the chemical makeup of the surface. In addition to that, we’re going to use our high-gain antenna to measure the mass of the objects as we go by, as they gravitational tug on the spacecraft, we can measure how massive they are through Doppler shift. And we have a navigation camera, which is a wide field panchromatic camera, which is going to give us its shape. So with the mass and the shape, we should be able to get a dense density, which is a very important diagnostic for figuring out how these things formed.

Jim Green: So as you said, many of these Trojans are different in their spectral appearance. So are we visiting each and every one of the variations of the Trojans?

Hal Levison:Yes, as far as we can tell, right? I mean, the mission was designed to do that. Now a lot of this is luck, but what we set out to do was to visit the extremes, right. So we have one object, which is our first Trojan that we encounter called Euripides, it’s really cool object. And it’s very gray. So we’re going to that one. And then we looked for an object of similar size and a similar orbit as Euripides, but very red.

Hal Levison:And what we tried to do is make them so similar in every other way, that any differences that we see is due to the composition of their surfaces. And so we’re going to be able to do a direct comparison between a gray and red object. And then we just filled in what we could do, and the objects we could find. You know, once we did that, we saw we saw a whole spectrum of objects that fit the diversity that we needed

Jim Green: So Hal, what’s been the greatest challenge in putting this mission together?

Hal Levison: Well, I mean, it’s rocket science.

Jim Green:(laughs)

Hal Levison:There’s been many challenges. Some we expected, some we didn’t. Lucy, another record that we’re breaking with Lucy is we’re going further from the Sun than any solar powered spacecraft in history.

Jim Green:Oooh.

Hal Levison:So we have very large solar arrays. And we’re in order to save mass for we used, I wouldn’t say the new design, but it’s certainly never been used on deep space missions before. And we had to scale them up, because they’re big. And so typically, these things are two, three meters in diameter. Ours is 7.3 meters in diameter. Scaling those up turned out to be a real challenge.

Jim Green: Well, you know, I just say, making these solar arrays very large, that’s one thing. But you’ve got the other problem of folding them up and how you get them into a fairing to launch and then bringing them out and fully extending them.

Hal Levison: Yes. And then there’s the challenge: They have to be lightweight.

Jim Green:Right.

Hal Levison:So these things have fold up and unfold like oriental fans. And although the solar cells are, are not made out of cloth, the entire supporting structure is made out of cloth.

Hal Levison:I encourage your listeners to go online and see some videos of our arrays. They are really amazing. And it makes the spacecraft really large. Lucy from wingtip to wingtip is about 50 feet.

Jim Green:Wow, okay.

Hal LevisonSo it’s big. most of it is solar arrays.

Jim Green:Rocket science at its best.

Hal LevisonRocket science at its best. So, we’ve been in contact with Donald Johanson the discoverer of the Lucy fossil, through this whole thing, which has been, he’s a fascinating guy.

Jim Green:Yes, he is.

Hal Levison: But he said something to me, I think, is insightful. He said, what makes human beings human beings is our ability to communicate and collaborate, to be able to do more than an individual person or creature can do. That’s what makes us human. And while he’s so into what we’re doing here, is this is sort of the ultimate example of doing that. Going to space and building a spacecraft. Right? It really is rocket science.

Jim Green:Really exciting. Well, Hal, I always like to ask my guests to tell me that event or person, place or thing that got them so excited about being the scientists they are today. And I call that event a gravity assist. So, Hal, what was your gravity assist?

Hal Levison:I would say there were two events, right. Like I said, I’ve always been interested in where we came from. What got me interested in the astronomical side of things, was, you know, I grew up in the 70s. And actually, at the time, we were putting a lot of money in the public schools for teaching science and that kind of thing. And my high school had a planetarium, with a planetarium director, his name was Scott Negley. And I, when I showed up at high school, I took a little class from him and got hooked. I spent my high school years working in the planetarium, going out teaching, teaching elementary school kids and things like that.

Hal Levison:So that was also combined with, what NASA was doing at that time. That was the time of Pioneer and Viking and Voyager. So a lot was going on, the initial reconnaissance of the outer solar system, for example, that got me hooked.

Hal Levison:I remember, in particular, the Pioneer plaque.

Jim Green:Yeah!

Hal Levison:Really inspired me, right. There’s a Plaque on pioneer, that sort of a message to aliens that can pick it up some time. But really, it made me understand that we’re really part of the galaxy, we really are part of the universe, right? We are part of the solar system, this idea, most people sit around and say, “Well, here’s us, and then there’s space, right, and space is separate from us.”

Hal Levison:And it’s not true, we are embedded in it, We are part of it. And that’s kind of a lesson that these kinds of plaques and things send to people and indeed, Lucy has a plaque on it.

Jim Green:Ah!

Hal Levison: It’s different, because Lucy will end its life in orbit around the Sun. Our calculation showed, if no one goes and picks it up, it’ll spend almost a million years just orbiting between the Earth and Jupiter. And so what we did is we put a plaque on Lucy, with messages to our descendants, rather than messages to alien civilizations. So we’ve asked some cultural leaders within our community to contribute quotes that are on the plaque. And the plaque was put on the spacecraft a few weeks ago, and we’re gonna launch it to the, to the planets.

Jim Green:Wow, that’s fantastic. Indeed, I remember the Pioneer 10 and 11 plaques and, and that they were made, very simply showing here are the planets and here’s where the spacecraft came from. And here’s a man and a woman and the size of the spacecraft next to it, some really elementary images that helped understand the origin of our first two spacecraft that are leaving the solar system. Well, Hal, thanks so much for joining me and discussing your fantastic career and I wish you the best and the launch of Lucy.

Hal Levison:It’s going to be an exciting day. And a beautiful launch because it’s a nighttime launch. So it’s going to be really beautiful to watch.

Jim Green:Well, join me next time as we continue our journey to look under the hood at NASA and see how we do what we do, I’m Jim Green, and this is your Gravity Assist.

Credits

Lead producer: Elizabeth Landau

Audio engineer: Manny Cooper