We imagine the asteroid belt as the place where all the rocks hang out in the Solar System, but there are two huge bands of asteroids that orbit the Sun with Jupiter called the Trojans. And soon, we might actually get a chance to see them up close.
Lagrange points (NASA)
Asteroid basics (NASA)
Asteroids v. Comets (Caltech)
Fraser: Astronomy Cast, Episode 574: Trojan Asteroids. Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, publisher of Universe Today. With me as always, Dr. Pamela Gay, a Senior Scientist for the Planetary Science Institute and the director of CosmoQuest. Hey, Pamela. How are you doing?
Pamela: I’m doing well. It’s almost summer. It’s almost summer hiatus, but we’ve got a few more episodes.
Fraser: Three more episodes, including this?
Pamela: I think so.
Fraser: For people who aren’t familiar, of course, across all of the CosmoQuest empire, we take a much-needed vacation over July and August to replenish our energy and mostly just to spend a couple of months not having to find our way to high-speed internet all the time. I find it really recuperative. I’m not sure exactly what’s gonna happen with the CosmoQuest world, but Astronomy Cast Weekly Space Hangout, virtual star parties, and the Open Space that I do, they’re all gonna go on hiatus for the next few months. Some of the other stuff that I still do, the non-live stuff, the QAs, the Guide to Space videos, all of the work we’re going on the Universe Today, the newsletters, all that is still gonna happen, but I just will be able to do that without having to need live, high-speed internet. We’re just a couple of episodes away from everything going on hiatus.
Pamela: And we’re gonna be taking weeks off throughout the summer to do Daily Space, but one of the big things that we have planned is normally in the summer. You and I would be going to various events, traveling all over the place. A couple weeks ago, I was sitting at my computer and I was thinking, “I’m supposed to be in Budapest,” and I clearly was not in Budapest. I was in my pajamas sitting next to my radiator.
Fraser: Hoping the Coronavirus doesn’t find you.
Pamela: Exactly, exactly. And with this lack of all the things that give me the opportunity to see so many people that are amazing talents, that we’re lucky enough to call friends at all of these events, I was like, “We could just do this on the internet. We have ways.” So, we’re looking at, in July, hosting a CosmoQuest-a-Con. We’re gonna do it on Discord and Twitch. We are gonna charge a ticket fee because we also want dental benefits and vision benefits, and my part-time staff doesn’t currently have any health benefits with their job. If we can bring in a reasonable amount of money doing this, I can start offering people healthcare because – well, we’re in America, which isn’t as good as Canada, where you are. We’re calling the event Space for Benefits – or Space with Benefits? I think we’re gonna go with Space with Benefits.
Fraser: That sounds good.
Pamela: Tickets are going on sale next week. You can find links to all of this next week when this episode is live as a podcast over on Cosmoquest.org. We’re even gonna do a masquerade but spelled with a “k” because it’s gonna be masks, like Coronavirus masks, but cosplay. It’s gonna be fun. Come join us, please, please.
Fraser: All right, sounds good. We’ll have more details on all the various places shortly.
Okay. So, we imagine the asteroid belt as a place where all the rocks hang out in the solar system, but there are two huge bands of asteroids that orbit the sun with Jupiter called the Trojans. Soon we may actually get a chance to see them up close. Now, the Trojans – actually, I mentioned Jupiter, but there are Trojans around other planets as well, so before I get all the angry emails, I know that. We’ll get into that in a second, but the Trojans around Jupiter are the biggest, the ones we’re most aware of. What are the Trojans?
Pamela: Trojan asteroids, whichever world they may be with, are objects that orbit 60 degrees ahead and behind the primary thing they share an orbit with. That whole IAU definition of a planet as something that has cleared its orbit is lies. I tell you, it’s lies. Even Earth has a little Trojan rock trailing behind us in orbit. Now, these places 60 degrees ahead and 60 degrees behind in orbit are special, gravitationally balanced places called Lagrange points.
Fraser: Right, this is gonna lead into my next question: Why is the Trojans?
Pamela: In these special gravitational balancing points, you have a nice hole in the gravitational field that objects can fall into and orbit around inside. When you’re looking at a Trojan asteroid, it isn’t actually so much orbiting the sun as it’s going around this mathematical point in the gravitational field balanced with Jupiter and the sun, and as it’s going around in this point, it’s part of this giant swarm.
Fraser: Right. We’ve done a whole episode on the Lagrange points and this idea that there are the three unstable Lagrange points, the ones that line up between, say, the sun and Jupiter. There’s three that line up between them: one on the other side of the sun, one in between Jupiter and the sun, and one on the other side of Jupiter. They are unstable. You put a rock on one of these and they are going to fall down. They’re gonna roll down the gravity hill and move away from that location.
But these other two Lagrange points, the L4 and the L5, the ones that precede and go behind a planet in its orbit, they are like the bottom of a gravity well that you can roll something – if something is in there, it’s going to stay in there. How did they get in there? I’m imagining you have an asteroid that’s going to fall towards one of these Lagrange points, and it’s still gonna roll in and out of this gravity well, so what gets them to stick around inside the gravity well?
Pamela: Well, once they’re there, they’re stable. How they get there, we don’t entirely know. This is one of the frustrating parts. There’s a bunch of leading theories, and it’s always frustrating when your leading theory, there’s more than one of them. Theory 1 we’ll start with is based on the Nice model of the evolution of our solar system. It looks at a point in the past when we had Jupiter and Saturn flinging rocks in all directions. During this chaotic period, it just worked out that through interactions, Jupiter was able to fill its Trojan asteroid points with asteroids.
Another part of this, a reverse Nice model is how they put it, is at some point either Jupiter, or Uranus, or an additional icy world that we no longer have and just got rid of, came careening through that area and set Jupiter and Saturn out of their resonance, in the process flinging rocks in all directions, filling up a bunch in the asteroid belt. And then, there’s just ideas that they’re things that have, over time, built up through general interactions with things passing through our solar system, and they just had the right energy to get stuck there over time.
Fraser: Right. I can imagine, like I said, if you have a single asteroid that’s gonna pass through this Lagrange point, it’s gonna go in, it’s gonna speed up as it falls down the gravity well, it’s gonna reach the bottom of the gravity well, and then it’s gonna roll back up and exit the Lagrange point with roughly the same velocity that it had before, although it might be slightly changed in its direction. But if you have some kind of three-body interaction where two asteroids smash into each other in this area, or if two asteroids orbit around each other, things can get trapped. Over time, a lot has gotten trapped. How much stuff is in Jupiter’s Lagrange points?
Pamela: This is the thing that, I have to say, remains most shocking to me. There is about the same number of objects 1 km and larger in the two Trojan points for Jupiter as there are in the entire main asteroid belt. You could take the whole main asteroid belt, dump it into two different groups, stick one 60 degrees ahead, one 60 degrees behind, and that’s the Trojan asteroid population.
Fraser: Surprise, the solar system has a second asteroid belt.
Pamela: It has two asteroid clouds, I’d go with. It’s not so much a belt. It’s two asteroid clouds and a belt.
Fraser: Sure, but if you’re imagining the asteroid belt in your mind and the total number of asteroids that are floating around, surprise, there’s another one of those total amounts of asteroids if you add up the two clouds going around Jupiter. In fact, they’re probably more densely packed together because they’re in these clouds. I learned the same piece of information about a year ago, and I just kept going, “Wait, what? No. Really?”
Pamela: Yeah. Why don’t they teach us this in school?
Fraser: I know. I kept digging around to find more information because I could not believe – I’m really glad that you knew this information because if you didn’t, I was just gonna rant about it. So, we know that there is this Trojan around Jupiter. Where else do these Trojan asteroids probably exist?
Pamela: We know that there’s smaller clouds of them associated with Saturn. We have one, probably not there for long, but always getting renewed with the new object associated with our earth. Neptune and Uranus should have them as well. Any object, really, can have them. It’s gonna be less likely to find them when the object you’re looking at is Mercury because getting all the velocities matched is much harder, but this is just a natural thing that asteroids can end up with over interactions with time.
Fraser: The size of these Trojans is just going to depend on the mass of the planet. Jupiter is going to have bigger ones; Earth will have little ones.
Pamela: Also, what’s the total mass, so it’s unfair to say Jupiter’s gonna have larger asteroids. Sure, it actually does, it turns out, but the total mass it’s able to grab hold of, that’s what’s gonna be bigger. And just like with the main belt asteroids, the asteroids in these Trojan clouds, they do knock each other around and make each other smaller over time. We do see these compositional families of Trojan asteroids.
Fraser: That was my next question. How does the composition – what did these things look like they’re made out of? Maybe that can tell us a bit more about their source.
Pamela: So far, they seem to be made of the same kinds of stuff as main belt asteroids. At the beginning of the day, everything formed, and then it got shuffled. The same kinds of objects that got shuffled in the asteroid belt got shuffled into the Jupiter Trojan clouds. The composition distributions are gonna be different, and one of the things that’s kinda cool is, because it’s so much colder where the Jupiter Trojans are, we occasionally will spot activity with these asteroids. Recently, there was one that actually stayed active for an entire year.
Fraser: What do you mean when you say “active”? What does that mean?
Pamela: A little Trojan asteroid decided it wanted to dress up as a comet and grow a tail.
Fraser: That’s amazing. It was sitting in the Trojan orbit with Jupiter, and yet it was throwing a little tail out. That’s incredible. What would cause that?
Pamela: Objects have ices, and in general, it’s thought that this one probably had those ices coated in something. The ruddy color that is associated with Trojan asteroids makes us think that they’re coated in what are called tholins, which are complex organic molecules. This coating may have otherwise protected the ices beneath, but either an impact or a landslide or something caused an area on the surface to get opened up, and those volatile materials were able to grow a tail for at least a year so far.
Fraser: That’s typically the distance from the sun that we will see a comet start to form that tail is out around the orbit of Jupiter. That’s when the light pressure of the sun gets strong enough that it’s able to start knocking material off of the surface. I guess you can imagine that all of the other objects have already had their surfaces blasted away. It’s just that, as you said, one had a little landslide or something, and that was enough to reveal some of its surface to form a tail. This is all great in theory, and thanks to Hubble Space Telescope and ground-based telescopes. They’ve taken some really great images, but we really want to see them up close. Let’s talk about a mission that’s going to go up close.
Pamela: Oh, Lucy. Lucy in the sky with asteroids. Every headline is going to somehow integrate a Beatles song in the next few years, I figure.
Fraser: But the spacecraft’s named after the ancestor to humanity, which was named after the Beatles song.
Pamela: Exactly. It is Beatles all the way down in this particular case. Lucy is a mission that just keeps adding new targets. It’s led by the Southwest Research Institute. These are the same folks that brought us the New Horizons mission. They are planning to head out to one of the Trojan swarms and just go from object to object to object getting a good old close look as they go. In the lead-up to this mission, they’ve been using the Hubble Space Telescope to refine their targets. They even discovered that one of these Trojans has its own little moon, and they’re gonna go get better images with Lucy.
Fraser: They thought they were gonna be taking an image of just one asteroid, and it turns out it’s gonna be two.
Pamela: We can’t, due to the Coronavirus, any longer say with certainty when anything is going to happen. This mission is currently scheduled for next year. As it so often does to get to the middle parts of the solar system, it’s going to take it a number of years to get out there. Since we don’t know a specificity if things are going to launch on time, changing of launch windows changes the orbits that are necessary to get to different places in the solar system. I’m not gonna make up a timeline for you, but this is one of those cool, next great explorations that we can look forward to.
Fraser: I really like the trajectory that this spacecraft is going to take because it’s totally different from anything that we’ve ever seen before. It’s going to fly out from the Earth, launching, as you say, in late 2021. It’s gonna fly out towards Jupiter’s Trojan belt – one of its belts – pass through the asteroid belt, get a shot of an asteroid as it goes out, then fly up into the Trojan belt, curve back around – imagine that we threw a ball from Earth out to Jupiter, and then at the very height of its trajectory, it will then fall back down to the Earth, do a gravitational slingshot around the Earth, and then back out, this time out to the other Trojan belt. It’s going to take 12 years. It’s gonna be six years getting out to and back to Earth, and then another six years out to and back to Earth. And then, hopefully, it will be able to keep doing it.
Pamela: This is one of those things where it’s just like, how do you pull that off with orbits?
Fraser: Incredible space program.
Pamela: Yeah. It’s a really remarkable orbit figured out by a remarkable team. And the Trojan asteroids, because they’re there due to numerous interactions, they’re both always getting refilled and also always throwing new stuff at us. This is one of the cool things is, we know that Trojan asteroids can be rich in water and calling them asteroids can actually be lies. There is, as we’ve talked about in other episodes, this continuum between rocky, dry object and mostly ices, very little rock object. And we’re used to thinking of comets as predominantly water ice, dry ice, other ices, a little bit of granular material, a little bit of organic material – basically, gravel and corn syrup is the way to think of it. And then, we think of asteroids as rock. But the reality is, most asteroids have ices, and most ices have rocks, and there’s stuff that’s 50/50 in the middle. Trojan asteroids, when flung into the inner solar system, become Jupiter family comets.
Fraser: As to that trick of one of them forming a little tail, if they made the journey into the inner solar system, they would almost always form that tail.
Pamela: And this is because beneath that tholin crust, we suspect that many of them are rich in ices just waiting to become comets. They’re just inactive most of the time because of that organic coating. Now, we do see compositional differences. We do see these families of objects like we see with the asteroid belt. Things do vary, but how they vary, this is why we need Lucy.
Fraser: That dark coating of material, that’s actually kind of similar to the structure of some of the objects from the Kuiper belt, right? It’s surprisingly similar to – some of them are very much like main belt asteroids, like you said, and then others are very similar to the Kuiper belt. It feels like you’ve got two sources of objects that are flowing into the Trojan asteroid field.
Pamela: Exactly, and there’s even some papers that predict that, along with capturing Kuiper belt objects, along with capturing main belt asteroids, there may be at any given moment also a couple of extra solar objects –
Fraser: that are stuck with us.
Pamela: Yeah. These are essentially the storage rooms of the solar system where random debris gets kept until we’re ready for it or not ready for it. This is why we have so many telescopes keeping an eye out for incoming objects.
Fraser: There’s an interesting mission that I’ve seen, an interesting idea that if you want to study a lot of different kinds of asteroids, you can send a spacecraft into, say, the Earth’s Lagrange point, the Earth’s L4/L5, into the Trojan region. Now, it’s not quite a target-rich environment because I think we kind of imagine the Trojans as a globular cluster of asteroids that’s all just buzzing around like a bunch of bees, but they’re not. It’s this huge, enormous, wide range of area. Wouldn’t it be cool if you could send a spacecraft out to, say, Jupiter’s Lagrange point but have it go into orbit into the Lagrange point and then just constantly be making relatively close fly-bys of object after object? It would just be a really rich hunting ground for science.
Pamela: It’s a huge hunting ground for science, and that’s the trick. The main belt asteroids, they don’t have as large a radius as Jupiter’s orbit. When you scale things up, you’re looking at volumes of space for the asteroid belt and for the two swarms of Trojans that aren’t all that different.
Fraser: It’s interesting to think, as they calculated that orbit of Lucy, of course they would’ve tried to have them fly by as many asteroids as they could, but they were only able to get, I think, four.
Fraser: Well, four, but a bunch of them have moons.
Fraser: I think it’s four or five on that second part of the flyby. And I’m sure they examined every possible orbit, just looked at the interactions of every single object and said, “The best we can do is four or five. That’s it.” You’ve got to decide, how close do you want to get? Do you want to get super close? Are you willing to be 50,000 km away? Do you need to be 100 km away? You can imagine how difficult it must have been to do the orbital fitting of that.
Pamela: It’s essentially the same thing we went through with the New Horizons mission, just a little bit closer and little less Sagittarius involved to bother everyone. For those who don’t know, while they were trying to find the Kuiper belt object the New Horizons mission would visit, they were trying to find things that were on the sky lined up with the constellation Sagittarius, which is towards the center of our galaxy, so there’s a lot of stars in the way.
Fraser: It’s a mess, yeah. What would it take, then, to find stuff in the Earth’s Trojan asteroid belt?
Pamela: Watching. It’s a little bit harder to do from where we are because we are looking off towards sunrise and sunset. It’s well after twilight that we’re looking, but still we’re looking off into the morning and evening sky. And then, trying to see the motions is just a little bit trickier because of the orbital mechanics. Spotting things that are either passing inside of our orbit where we’re getting the maximum velocity relative to the stars, or outside of our orbit, again, maximum velocity relative to the stars, that’s way easier. The geometry works against us to try to find them in our own. We can do it, but our own gravity, we’re not gonna be holding on to these kilometer-sized objects with the same readiness that Jupiter is. Even Bennu and Ryugu are smaller than a kilometer across.
Fraser: Right, so we’re looking at objects which are in the tens of meters category, which would be interesting scientifically, but they’re objects which are tens of meters across and they are separated by, in some cases, tens of millions of kilometers away from each other. They’re all located in a part of the sky that’s super difficult for us to see because it’s always in sunrise or sunset. It’s a tricky thing. Often, a lot of these things make the most sense, just view them from Earth because you can have the Hubble Space Telescope. It’s better to have the Hubble Space Telescope at Earth than to put a tiny telescope out at Neptune, right?
Pamela: It’s true, it’s true.
Fraser: Well, I cannot wait for the Lucy mission. I think it’s gonna be a sleeper hit.
Pamela: I kind of agree with you.
Fraser: Most people don’t even know this mission exists, but when it actually does launch and the news starts to really ramp up on it, I think people are gonna be pretty excited about what it’s gonna turn up.
Pamela: As always with space, it’s a waiting game. There’s gonna be those years before it’s –
Fraser: Twelve years.
Fraser: On the one hand, I always feel so sad about all the cool stuff that’s gonna be coming down the pike that I probably won’t even live to see.
Pamela: I know. We’re starting to hit that point in our lives where you have to do a, is it even worth me even trying to figure out this mission?
Fraser: On the other hand, if I just look at what’s happening this year, next year, the year after that, they’re coming fast and furious, so I’ll be constantly entertained all the time until I die. Pamela, do you have some names for us this week?
Pamela: I do. As always, this show is made possible thanks to the generous contributions of people like you out there in our audience. And there’s lots of ways you can support us in addition to giving money. Just go write a review somewhere and help share us out to others. But those of you who give through Patreon.com/astronomycast, you allow us to pay the folks that keep everything going behind the scenes. We have Richard Drumm, who does our engineering, Ally Pelfry, who’s handling all of our video, Beth Johnson, who’s handling all of our web content, you let us pay them.
Thank you for that, and I specifically want to thank Nile Bruth, Karthik Venkataraman, Steven Coffrey, Elad Avron, Thomas Chaneker, Brent Kreinop, Eron Sagev, Rachel Fry,Justin Proctor,Paul Heyden, Claudia Mastroianni,Thomas Tubman,David Gates,Fredrick Sjoge,Eric Farenger,Dwayne Isaac, Faudi Tennabau, Gabriel Galfin, Shannon Humber,Benjamin Davies,Kristin Brooks,Ryan James, Nala, Glenn McDavid, Brian Nevsen, Dean,Kenneth Ryan,Russell Peto, Smatski, Matthias Heyden,Martin Dawson,Bart Flaherty, Jason Graham,William Jones, Dana Nourie,Anitusar, Dean McDaniel, Andrew Stephenson,and Donald E. Mundis. Thank you all.
Fraser: Thanks, everyone and thanks, Pamela. We’ll see you next week.
Pamela: Sounds great. See you all later.
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