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There are asteroids and there are comets. But there’s an entirely separate class of objects called centaurs. But instead of half-human, half-horse, imagine an object that’s half comet, half asteroid but 100% interesting.
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2021 DPS Meeting (AAS)
Centaurs (Swinburne University)
Infographic: What’s the Difference Between a Comet, Asteroid and Meteor? (Universe Today)
Our solar system’s centaurs are half-asteroid, half-comet (Astronomy Magazine)
Studies of ‘amorphous ice’ reveal hidden order in glass (Princeton University)
All is Quiet Among Newly Studied Centaurs (PSI)
Kuiper Belt (NASA)
Oort Cloud (NASA)
There Might Be an Entire Orbit, Filled with Asteroids that Came from Outside the Solar System (Universe Today)
10199 Chariklo (NASA)
Trojan Asteroids (Swinburne University)
New Horizons (JHUAPL)
Is New Finding an Asteroid or a Comet? It’s Both (PSI)
Ten Things to Know About Bennu (NASA)
Asteroid Ryugu: The twirling space rock visited by Hayabusa2 (Space.com)
New study challenges Jupiter’s role as planetary shield, protecting Earth from comet impacts (EurekAlert)
Today in science: The Tunguska explosion (EarthSky)
Five Years after the Chelyabinsk Meteor: NASA Leads Efforts in Planetary Defense (NASA)
Chixcalub Impact Event (LPI)
Transcriptions provided by GMR Transcription Services
Fraser: Astronomy Cast. Episode 614: Centaurs, Comets, and 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 University. With me as always is Dr. Pamela Gay, a senior scientist for the planetary science institute, and the director of CosmoQuest. Hey Pamela, how are you doing?
Dr. Gay: I am doing well. It is fall here in the Midwest. The temperatures have dropped ever so slightly, but all the leaves are starting to drop which is beautiful and horrifying, because we’re gonna have to pick them up.
Fraser: Someone’s gonna have to rake them up, yeah. Yeah. I know that feeling like, “Awe, it’s so pretty,” and then you know with horror that you’re gonna be racking up those leaves, and if you don’t, they choke at your grass and, yeah. We have beautiful fall leaves here as well. They haven’t quite arrived, but they’re probably just a week away and it’s all gonna start turning beautiful yellow and yeah, I love it.
Dr. Gay: It’s a beautiful time of year. Although, observing is getting harder, which is where I’m just gonna sit back and enjoy what has already been done and is currently being presented by folks out at the division of planetary sciences conference, and we rearranged our show order just a little bit much to the frustration of the folks who work with us. We’re gonna talk not about planet nine and potentially ten as planned, but instead about all the active objects that new papers are coming out and talking about past observations stuff.
Fraser: Okay. Whoa, whoa, whoa. Spoiler alert. Let me do my intro before you completely spoil the show. All right. All right. So, there are asteroids, and there are comets, but there’s an entirely separate class of objects called centaurs. Instead of half human, half horse, imagine an object that’s half comet, half asteroid, but 100 percent interesting. All right, Pamela. Definition time. What is a centaur?
Dr. Gay: A centaur is an object that is not a planet, that is orbiting in the part of the solar system that is between the Kuiper Belt and the asteroid belt. More technically, it is generally between Saturn and Jupiter, and these are objects that we are still working to try and understand that mostly seem to be hanging out, happily orbiting, but sometimes like to surprise us by growing tails and other such interesting activities.
Dr. Gay: It’s interesting the way you define that. That it’s in between the asteroid belt and the Kuiper Belt, and then sort of said, “Well, in between Jupiter and Saturn,” because I guess the asteroid belt really almost makes its way out to Jupiter and many of the objects in Jupiter’s trojan area are very asteroid belt-like. Then, Saturn and its moons – its moons are very Kuiper Belt-like and then of course you’ve got Uranus and Neptune, which are living in the outer fringes of the Kuiper Belt. So, there’s actually a fairly small area that defines those two regions in our solar system.
Dr. Gay: And that’s one of the wild things about centaurs is these are transition objects. The orbits between Saturn and Jupiter are not entirely stable and anything that ends up in there is not going to stay there forever. Instead, at some point it’s going to get flung in or out, and when that happens, it’s either going to become a comet, it’s gonna become an asteroid, or it’s going to disappear into the outer solar system because all things are possible.
Fraser: Yeah. Even just the main asteroid belt itself – Earth is constantly at risk from near-Earth objects and where do the near-Earth objects come from? They come from the asteroid belt. Why do they come from the asteroid belt? Because Jupiter is constantly kicking them out, and then they’re drifting into the inner solar system, and smashing into the Earth and killing all our dinosaurs. The same thing is happening with these interactions between Jupiter and Saturn. That keeps everything really dynamic across the solar system.
Dr. Gay: And the wild thing about centaurs is while they seem to be icy objects, they don’t all act identical. So, trying to understand why some of them have these tails, and jets, and comas, and others of them are just hanging out being completely quiet, becomes difficult, because if you assume they all have kinda the same composition. It can’t just be thermal heating from the sun that is causing some of them to be active if others of them aren’t.
Fraser: And so, you talked about this idea of these perturbations coming from Jupiter and Saturn, and I guess lesser some of the other outer objects. So, is Jupiter driving asteroid-like objects out into the centaur area? Or is Saturn pushing cometary objects into the centaur area? Or both?
Dr. Gay: It’s even weirder than that.
Fraser: Oh, man. Okay.
Dr. Gay: So, things come migrating inward for a whole lot of different reasons. You can have interactions between multiple small, icy objects in the outer solar system that lead to objects getting flung out way. You can have perturbations from Neptune, Uranus, Saturn. You can have all of these different things happening at different times, and you can even have things getting flung in from the Oort Cloud as we’ve recently observed. So, we get objects trapped in orbits between Saturn and Jupiter through a whole lot of different, well, interactions. At the end of the day, that’s the only thing we can really say is in common is all these things interacted with something, and that interaction led to a journey inwards.
Fraser: Right. So then, if we were to go and fly out into this region and take a look at these objects, what would they look like? I mean, at this point, I think we’ve got a lot in our mind we can imagine. The flybys of some of the asteroids that have been done. The fly by thanks to Essa’s Rosetta missions. We’ve seen comets up close. We’ve seen asteroids up close. If we got to a centaur and looked at it up close, what would we see?
Dr. Gay: This is actually more of a mystery than we’d like. On one hand, they’re gonna look like snowballs that happen to have some gravel mixed in, and some organics. They’re dirty snowballs, but just like snowballs can be badly made and fall apart in mittens or end up nice, and round, and compact, or just all sorts of crazy shapes based on well, what gloves or mittens you happen to be wearing. Those interactions in the outer solar system can lead to, we presume, these objects having all sorts of different shapes. The one thing we think we know is that they’re not perfectly round. These things, we think, have topography.
Fraser: Right. I mean, you say these things we think have topography, but we saw the photos from Hyabusa2, right? And Osiris-REx. Even tiny, little ten kilometer across asteroids have weird topographies. It’s kinda amazing how much more bizarre these objects are than anything we ever expected, but again, when you say topography, are we talking mountains, craters, strange little diamonds?
Dr. Gay: So, what we’re talking here is slopes that allow them to have landslides, and one of the leading theories on why some of these things become active periodically, is they have landslides, and it is this action on the surface of the object that reveals fresh ices and allows them to temporarily become active. Exactly what causes the landslides? Well, that’s another question, but the leading thinking is you have these objects; they can settle into nice happy orbits for millions of years and while on those orbits, they periodically get a little closer to the sun, a little further from the sun, and during their initial passes to their closest point, they –well, melt just enough to have these landslides, and just enough that we see this activity.
Fraser: So then, this idea that you’ve got these objects, they’re getting impacted, there’s some kind of landslide activity, and it’s revealing fresh material that is then gonna make it act like a comet?
Dr. Gay: Right. So, you can have trapped in minerals, trapped in matrices, all sorts of volatile gasses, and it’ll happily stay trapped there until you do something to expose it to, well, sunlight. And if you have a landslide, that cane expose some of these previously buried volatiles and allow that, well, surface to become active.
Fraser: It’s really interesting. When I envision a comet, you sort of imagine this pristine object that is from the outer solar system and it makes it’s way in and for the first time, all of this outer area of – well, whatever volatiles are able to be reached by the radiation from the sun start to gas out and blast off into space, and then you’re left with the dirt part of it that is now, sort of encasing the comet, and no more of it can escape.
The longer a comet spends in the inner solar system region, the more and more of it is gonna be gone, but if these things are having on going activity, which now that I think about it, like, “Duh –” of course they are, that they’re constantly being resurfaced and refreshed. So, are they then just kinda getting smaller and smaller over time and getting more concentrated to become more rocky?
Dr. Gay: Well, the wild thing about centaurs, is they become temporarily active, and then never again. So, you can have a centaur that is on an orbit, that on its first few passages, when it gets close to the sun, it gets warmed up enough that it actually goes through a phase change that most of us don’t think about.
When we think of ice, we think of ice as being crystal, but it turns out that in the deepest parts of solar system, you can actually end up with what’s called amorphous ice. At a certain temperature below standard liquid water freezing, you can end up with the amorphous ice undergoing to become crystalline ice. In the process of that change, which isn’t enough to generate the kind of tails and things that we see with the standard comets we’re use to, these objects out past Jupiter, can change so that they release a bunch of gasses and other things and allow us to see them temporarily as active. It’s a phase change from amorphous ice to crystalline ice that’s taking place. Not your standard, well, more melty melting that we’re used to seeing.
Fraser: Okay. We’ve got to spend a little more time on the term amorphous ice. If I could look at amorphous ice up close, hold it in my hand, put it in my scotch, what would it look like?
Dr. Gay: Extremely cloudy. Your scotch would probably be frozen, because you’re at a much lower temperature than we’re use to being at. So, putting it in, might be interesting, but it doesn’t have a crystalline structure. The water molecules are not politely aligned. Instead, they’re in all different angles and within this randomized structure, there’s gasses trapped that can then escape during the restructuring to be crystalline ice.
Fraser: Right. So, again, when I sort of think about regular ice, as you said, it forms these crystals when you break it, shears in various ways. I can imagine this very cloudy material that if I dropped it, it would just smash into powder, because it didn’t build that crystalline structure, and it would be quite readily hiding all of this material, these other gasses and stuff inside of it. It’d be really weird to see.
Dr. Gay: So, what we’re finding with these centaurs is we have this phase change from amorphous to crystalline, then is making them appear active. We have landslides that are revealing pristine materials beneath that are making them active. Just the yanking and pulling of interacting gravitationally tightly with these worlds can make them active, but all of these are one off temporary things.
So, at any given moment, you can look at a centaur and it’s sitting there going, “Hi, I am doing nothing. I have nothing interesting to see,” and there’s actually a researcher, Eva Lily, that went out looking for active objects and found none. None, because the objects she looked at were in stable orbit that they been in for too long. So, all the activity had died away.
Fraser: So, if we could roll back the history of the solar system into the ancient past, can you sort of describe, I don’t know –the lifetime of a centaur? Where did it form and how did it shift around, and what is its final outcome?
Dr. Gay: So, centaurs started out in the outer Kuiper Belt, the Oort Cloud, the space in between. They were formed in the outer cold, cold parts of our solar system through interactions within one another, with the gravity of passing stars, through some sort of dynamical change. Their orbit flung them inwards and one interaction at a time, they made their way to that space between Jupiter and Saturn and had their orbit somewhat circularized through these interactions.
Now that they’re settled in between Saturn and Jupiter, it’s a temporary journey for them. They’ll be there for millions of years, maybe less, and while they’re in there, they might get impacted. They’re gonna end up with landslides and all these other things, and then Jupiter will either fling them inwards or outwards.
Fraser: Now, obviously I’m a little familiar with centaurs. We’ve reported on them quite a bit on Universe Today. Are you familiar with Chariklo, which is one of the centaurs? The one with the moon and a system of rings?
Dr. Gay: Yes. Yes. I had no idea how to pronounce its name, so when you said it out loud, my brain was like, “I do not recognize that.” I learned something new. I learned something new.
Fraser: I could have mangled it, too. So, don’t put all your eggs in my pronunciation basket this time around.
Dr. Gay: Right. So, this is an object that appears to have a slightly brutalist past. There was something that went on that caused it to end up with a moon, an orbit around it, a plane of dust and particular material that forms a ring. It shows us that as empty as space may be, things still find a way to hit one another, but if you have enough gravity, you can hold on to your bits even after they fling off your surface.
Fraser: Yeah. To think that there’s an asteroid with rings, is crazy.
Dr. Gay: Is kinda awesome.
Fraser: Yeah, it really is. All right, so, we’re familiar through your telescope. Tell me about all the plans to send missions to visit these centaurs.
Dr. Gay: So, missions to specifically visit centaurs are not entirely in the works, but we come close with Lucy. Lucy isn’t gonna go out to the centaurs. It’s instead going to the trojans which are a family of objects that may be very similar to the centaurs, but gravitationally dynamically, they are trapped in the leading and trailing lagrange points orbiting with Jupiter around the sun. Lucy is gonna fly out and visit a bunch of them and do these crazy figure eight orbits relative to Jupiter. It’s going to get to go see a regular asteroid and it’s launching, we hope, in about a week.
Fraser: Now, you’re breaking your rule, Pamela.
Dr. Gay: I know. I know. So, normally we don’t talk about spacecraft before the have successfully launched and gotten to their destination, simply because it seems wrong to count your eggs before they hatch. As some of you know, I did have a mission I had counted on for my dissertation fail to function, and I had a telescope that I counted on for my dissertation fail to be fully functional on time. So, around Astronomy Cast, we’re pretty good about not talking about things.
Fraser: So, now I understand. Now, I think I understand where the personal pain comes from. See, for me, talking about stuff that’s going to happen is my bread and butter. Something I’d rather talk about, but out of respect for the devastating losses that happened to you, we’ steer clear of this until these spacecrafts are firmly in orbit and have sent back the data.
Dr. Gay: Exactly.
Fraser: So, who knows what’s gonna happen, but Lucy is fascinating if it may or may not launch. May or may not get its data, but we’ve had a couple of other cracks at things that are centaur-like.
Dr. Gay: Yes. So, new horizons – Kuiper Belt is fairly similar because Kuiper Belt objects get flung inwards to become centaurs. So, there’s that.
Fraser: Yes, and Saturn’s moon, Phoebe, is thought to be fairly similar to a Kuiper Belt object. Sorry, a centaur. Cassini has taken some wonderful, close-up images of the moon, and it is so weird looking.
Dr. Gay: And this is where we have to recognize it’s a continuum of objects out there. Just this week a paper came out highlighting the fact that there is an object that we thought was an asteroid in the asteroid belt where asteroids belong, and that object is not an asteroid. It is a comet that is on an elliptical orbit through the asteroid belt that is sometimes further from the sun, sometimes closer from the sun, and will grow a tail, which is not how asteroids are supposed to behave.
This continuum of objects, it’s all a matter of where did it form. If it formed close enough to the sun, all the ice and stuff got blasted away. If it formed further from the sun, it’s still got the rocks and dust in it, but it kept all the ice and gas, and became an icy, gassy, frozen object. It’s a continuum.
Fraser: The thing that I’m finding really exciting, and this is one of those stories that I think has transformed as we’ve been doing Astronomy Cast over the decade and a half period of time, when we started, asteroids were asteroids and comets were comets, and we’ve since discovered that there really is a blend between what is an asteroid and what is a comet. Even things that are very firmly in the asteroid camp. Things like Bennu and Ryugu clearly have a lot of volatiles just under the surface.
It’s pretty exciting that obviously there’s a store of the early history of the solar system, but also resources that we can use when we become our solar system spanning civilization. That the water and the volatiles that we require are there, and to see these objects that are firmly right in the middle, both I guess in the solar system, but also of just the classification, gives us a really great way to sort of see this transition between what is a comet and what is an asteroid. As always, things are a lot more complicated than we ever expected.
Dr. Gay: One of the biggest changes since we’ve started recording this show, is the understanding that Jupiter is an equal opportunity rock and ice thrower.
Dr. Gay: Research from our friend Kevin Grazier has showed that it’s gonna fling things both inwards and outwards. Both aiming at us and protecting us, and this dichotomy was just absolutely not known.
Fraser: Yeah. This idea that Jupiter protects us, you see this a lot on the internet and such, and it really grinds my gears because it is not our protector. It is both protecting us and harassing us. All of the near-Earth objects that are coming our way have been kicked into the inner solar system thanks to Jupiter. Yeah. What kind of protector is dropping Tunguskas on us? Or dropped the –
Dr. Gay: Chelyabinsk?
Fraser: The Chixcalub. Yeah. Well, the Chixcalub event, right? That killed – yeah. Thanks, Jupiter, for killing all the dinosaurs. So anyway, I think it shows just the level of dynamics, and there’s always a chance that Jupiter can kick one of the larger planets completely out of the solar system, because it can if it wants.
Dr. Gay: Yeah, and our solar system did start with many more planets than we have today. In our next episode, we’re gonna be talking about the possibility that there could be a Mars-sized object, according to a review paper by Kat Volk. There could be a Mars-sized object, still lurking, flung out into the outer parts of the Kuiper Belt.
Fraser: Or many more.
Dr. Gay: Yeah.
Fraser: Awesome. Well, thank you so much, Pamela.
Dr. Gay: Thank you so much, Fraser, and thank you to all of our patrons who made this show possible. We are here thanks to your generous contributions. This week we would like to thank Thomas Sepstrup, Mountain Goat, Jordan Young, Burry Gower, Kevin Lyle, Jeanette Wink, Andrew Poelstra, Brian Cagle, Venkatesh Chary, David Truod, TheGiantNothing, Aurura Lipper, Joe Hook, David, Gerhard Schwarzer, Jean-François Rajotte, cacoseraph, Just joe, Laura Kittleson, Bill Hamilton, Robert Plasma, Les Howard, Joshua Pierson, Jack Mudge, Joe Hollstein, Sean Martz, and Gordon Dewis.
Thank you all so much, and if you aren’t contributing, well, we’re gonna be having our Hangout-a-thon which is how we make up the difference between what we bring in through patron donations and what we actually need to pay all of our bills. We’re gonna be doing that on October 23rd and 24th. So, show up, and well –
Fraser: Hang out.
Dr. Gay: Show us you love us.
Fraser: All right. Thanks Pamela. We’ll see you next week.
Dr. Gay: See you later. Buh-bye.