Things used to be so simple. Comets were snowballs from the outer Solar System, and asteroids were rocks from the inner Solar System. But now everything’s all shades of grey. Astronomers have found asteroids that behave like comets and comets that behave like asteroids.
In this episode we mentioned donations. Click to learn more!
- Asteroids (Wikipedia)
- Asteroid overview (NASA)
- What Is an Asteroid? (NASA’s Space Place)
- Asteroid Belt: Facts & Formation (Space.com)
- Trojan Asteroids (Swinburne Astronomy Online)
- Comets (Wikipedia)
- Comets overview (NASA)
- Comet Facts (NinePlanets.org)
- Comets, Meteors & Asteroids (Ask an Astronomer)
- What’s the difference between asteroids, meteoroids, meteors, meteorites and comets? (Sun.org)
- What is the Difference Between Asteroids and Comets? (Universe Today)
- Cryovolcanism (Wikipedia)
- Cryovolcanism in the Outer Solar System (Science Direct)
- Cryovolcanism Helped Shape Dwarf Planet Ceres (Planetary Science Institute)
Transcriptions provided by GMR Transcription Services
Astronomy Cast, Episode 545
Weird Issues: Are Comets Asteroids or Are Asteroids Comets?
Fraser: Welcome to Astronomy Cast, our weekly fact-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. I sense the word science a lot there I just noticed.
Pamela: Yeah. It’s true. I’m all about that science.
Fraser: It’s really all about science, yeah. Any news, Pamela? Or do we get right into the episode?
Pamela: I will be at QCon which is a software conference in San Francisco next week. I’m probably going to do some sort of a meet-up if I can figure it out. I haven’t figured it out yet, so if anyone has suggestions, hit me up. I can be found as starstryder pretty much anywhere on the internet.
Fraser: Let’s all hope you can figure it out. The people who watch Letter Candy will get that reference. All right. Things used to be so simple. Comets were snowballs from the outer solar system, and asteroids were rocks from the inner solar system. But now, everything’s all shades of gray. Astronomers have found asteroids that behave like comets, and comets that behave like asteroids. Pamela, make sense of this for me.
Pamela: It’s a continuum of objects that simply want to mock with our desire to name everything. Let’s face it. Human beings like to put things in boxes. And this is a problem for us when it comes to the solar system because we used to think it was so easy. We had terrestrial worlds. We had gas giants. We had asteroids. We had icy things. And it’s all fallen apart since we’ve discovered that well, somewhere between the icy things and the asteroidy things is a continuum, and there’s no discreet gap or anything.
Fraser: Now, is that the technical term? Icy things and asteroidy things?
Pamela: Well, technically it’s comet or centaur, or Kuiper belt object depending on where they are. And then, of course, there’s the Oort cloud. We need to stop naming things based on where they’re located.
Fraser: Right. Or just like trying to classify them.
Pamela: Don’t judge. That’s the ultimate answer.
Fraser: Don’t judge, yeah. You’re just gonna drive yourself crazy, and then they’re gonna set off a debate on the internet where people are gonna yell at each other about whether Pluto’s a planet or not. And we’re still gonna have to hear about it now, however many years after.
Pamela: Pluto planet classic. We’re just gonna go with Planet classic.
Fraser: Planet classic. Okay, all right. So then, back in the olden days of yore, what was an asteroid?
Pamela: It was a dry, rocky object, like killed the dinosaurs and exists in the asteroid belt that in general is made of metals, of carbons, of silicates, and is dry. That was where we started. Asteroids were dry.
Fraser: Right. And we’ve got the ones that are near the Earth. We’ve got the ones, as you said, in the main asteroid belt. And then we’ve got the Trojans. Interesting side note, did you know that there is roughly, in the same order of magnitude, as many objects in Jupiter’s Trojan asteroid area as in the main asteroid belt?
Pamela: No, I didn’t. That’s awesome.
Fraser: I didn’t know that, either. Yeah, I know – until I was doing an episode about it. In other words, the solar system has a second asteroid belt and you didn’t even know about it. I mean, we all knew the Trojans were there, but we didn’t know that they were quantity the same –
Pamela: And they’re not even a belt. They’re really just two clouds that leave and trail Jupiter like a parade on procession around our solar system.
Fraser: Yeah. And yet, there are as many objects, roughly, as in the main asteroid belt. So, anyway, interesting thing I just discovered. So, okay. So, asteroid – rock, metal, dry. What’s a comet?
Pamela: A comet is a blob of mostly ices. So, here you have water ice, nitrogen ice, dry ice – ices, things that can be liquids. Things that will become gassy tails. And we knew that mixed in with that was some gravelly bits, but it was considered to be mostly ice. And –
Fraser: Nicole Gallucci does a great – she does a great science show where she will make a comet with you.
Pamela: Yes. And this is something that the astronomers have been doing for ages because you can get all of the ingredients to make a comet in a bowl pretty much from your local grocery store if your local grocery store happens to have dry ice. And it’s just a little bit of corn syrup, little bit of molasses, whole lot of dry ice, some water, some ammonia, and then you grab a handful of dirt from the yard.
Fraser: Right. And that’s a comet. And so, it’s pretty clear – like obviously. One is a chunk of rock and metal, rock or metal. Rock and metal – rock or metal. The other is a chunk of water with maybe a little bit of dirt thrown in meatballed in volatile ices. When did this categorization start to go off the rails? What were some discoveries that were made that made this clearly problematic?
Pamela: Well, one of the problems was simply the story we had told ourselves about our solar system turned out to be wrong. And one of the stories that we told ourselves about our solar system was that all of the water we have, all of the Earth’s oceans came to us through the bombardment of comets on the surface of our world. But when we started looking at the composition of comets, the waters that comets are made of don’t match the water that we have here on Earth. So, there was this sudden “WTF, water? What’s wrong with you?” And a realization that all the water we’ve got had to come from somewhere else.
Now, it didn’t end there. It never ends that simply.
Fraser: But just like – just before you move on, so we know that based on whatever the deuterium, tritium –
Pamela: Yeah. It’s the ratios of what are the kinds of hydrogen that are involved in making water. The ratios are wrong for comets.
Fraser: Right. And so, if you sample the water in the Earth’s oceans, and then you look at the samples that have been done in looking at comets, they don’t match perfectly. I mean, obviously, they both have water in them, but it’s like the initial recipe doesn’t seem to match. So, the comets couldn’t have been the source of all of it.
Pamela: And in fact, having looked at multiple comets, if our water came from the as we so far have seen, the typical comets, it still doesn’t work. So, it can’t even be like the smallest fraction of our water came from comets. So, yeah, it’s not comets.
Fraser: Okay. So, then we have a – but that doesn’t mean anything, right? It just means that the water came from somewhere else. Maybe it was formed in place as the Earth formed, and then maybe it was part of underground and then welled up over time as the Earth cooled down.
Pamela: And that is one of the models that’s out there. But that’s no longer the dominant model because we have another attacker that has been identified. It’s been realized that asteroids have water. In fact, they appear to have a lot of water. And big asteroids like Ceres may even have subsurface seas of water.
Fraser: Right. So, then how were these discoveries made?
Pamela: Well, it’s been a whole combination of looking at nearer-by asteroidy looking things and realizing “oh shoot, these are actually active.” They actually periodically spray material outwards. And thinking well, some of them we’re just gonna call dead comets. We’re gonna refer to these as objects that clearly used to be comets. Their orbits changed. They got crusted over with stuff. We’ll blame them on being former comets. And that works to a point. But then, we started doing things like sending probes out to visit asteroids. And when we got to Ceres, we saw cryovolcanism going on.
There are massive features on Ceres that appeared to be former volcanoes that have now become to slump and there’s at least one, what appears to be still active, volcano in place in the bottom of a crater. And this is salty stuff that’s getting sprayed up and then settling down and crusting over that volcano that it is.
Fraser: And so, when you think about say the structure of Ceres, observed from the Earth, it looked like a comet. It’s a member of the asteroid belt. It is this side of the frost line in the solar system, right? So, it should be baked dry by the sunlight –
Pamela: Vesta is on this side of the –
Fraser: Vesta is on this side, sorry. Yeah. Ceres is on the far side of the frost line.
Pamela: But we still didn’t expect subsurface oceans. And so this gets to the point of now we’ve started looking at nearer Earth objects, like Bennu. And Bennu is evil incarnate. This is an asteroid that took one look at the Osiris Rex spacecraft and said “I don’t like you,” and threw rocks at it. It probably didn’t actually think anything. It is, after all, a rubble pile.
Fraser: Yeah. It just throws rocks all the time.
Pamela: Right. And so, it now appears that we have inner solar system objects that have the volatiles – these are the ices that can become gas – that have volatiles mixed in with their rocks and are still periodically out-gassing. And in the process of doing this, they spray rocks.
Fraser: Okay. So, explain this mechanism here. So, you’ve got Osiris Rex approaching Bennu. Now, this is firmly in the main asteroid belt, right? This should be a pile of gravel –
Pamela: This is a near-Earth object. This is even closer to us than the main belt.
Fraser: Right. So, this should be dry as a bone. It is a pile of gravel that gets jumbled up from time to time, mixed up. It should have had all its volatiles, all of its water blasted away off into space. It should just be just rock and metal, but mostly rock, gravel.
Pamela: Yeah, yeah.
Fraser: And yet, as Osiris Rex was approaching it, it was as if it was like driving down a gravel road and gravel was whacking into the front windscreen. So, what was the process that was going on that was generating this gravel?
Pamela: As near as we can tell, there are some sort of gases – whether it be dry ice, water ice, nitrogen ice – probably dry ice or water ice – those are the most common – that was mixed into the gravel on Bennu. And for whatever reason, it’s still near enough the surface that periodically things shift, resettle just enough to trigger that volatiles doing their “I’m going to be volatile” thing, and sublimating in that rapid expansion from ice to gas, threw rocks. It’s still throwing rocks. It still has unspent volatiles. How this happens we’re still figuring out.
And what we’re finding is our solar system is this amazing mix of stuff that has been rearranged multiple times, which we’re gonna get to in the next episode. And because this stuff formed at all different distances from the sun, and different parts of our solar nebula would have had different compositions, you have things forming with a variety of compositions. And then, our solar system flings things all over the place so that we have this collection of asteroids out near Jupiter – the Trojan asteroids. We have this collection of comet-like material, the centaurs, that’s inside Jupiter’s orbit that create the Jupiter family comets.
And then, we have asteroids all over the inner solar system that don’t follow the “here’s the frost line” inside of this is dry, outside of this is wet rule that we came up with. It’s like we can come up with all the rules we want. The universe is not required to follow them.
Fraser: Right. And now, I know that asteroids have been found with tails.
Pamela: Yes. And this is again consistent with us knowing that there are dead comets that we call asteroids and asteroids that appeared to be more comet-like. It’s a continuum of objects where some of the meteor showers that we experience are actually driven by asteroids that behave more like comets.
Fraser: And so, we’ve talked a lot, I guess, about asteroids that behave like comets. So, are there comets that behave like asteroids?
Pamela: This starts to get into the really awkward “how do you discuss it?” And the way we normally phrase this is “active asteroids” instead of comets that behave like asteroids because if it’s a comet behaving like an asteroid, how are we ever gonna know that. So, there –
Fraser: A chunk of rock way out beyond the orbit of Pluto. How are we gonna find that? How are we gonna know what it’s doing?
Pamela: Exactly. We can’t get there from here. And we do struggle with finding things out in the Kuiper belt that appear to be dark in color. And is that ice that’s covered in organics, or is that a rock that somehow found itself into outer solar system? We don’t know. But what we do know is that there is a whole variety of active asteroids. And David Jewett is one of the people who’s been doing a lot of research on this. And we find things like the periodic asteroid 2010 A2. We find things like Phaethon, which appears to have mass loss over time.
And in constantly looking for these and constantly finding new ones, what we’re starting to figure out is thermodynamics will, over time, as these asteroids heat and cool, heat and cool, cause them to crack. Now, if you have, inside of that asteroid somewhere, trapped ices, that cracking can expose them to sunlight they might otherwise not have had. And so that cracking process can release the volatiles, making them appear active. And so, when we see active asteroids, what we’re seeing is something that revealed its volatiles to sunlight.
And for that particular object, this may be a special point in time, and most asteroids probably have the potential to do this. And we only see a small fraction of them doing this because it’s only a small fraction that at any point in time are being cracked and revealing their volatiles.
Fraser: Now, you have always been a huge fan of drilling into asteroids and building your space colonies inside of them.
Fraser: This has gotta be great news for the future of space exploration – to know that there’s this much of these precious volatiles inside these asteroids.
Pamela: And this tells us that our future mining hopes can go both terribly right and terribly wrong. And what I mean by this is if we periodically see asteroids out-gassing because they’ve cracked and volatiles have been revealed to space, imagine the poor miner who accidentally cracks into a bunch of volatiles.
Fraser: This is like a movie.
Pamela: Yes, yes.
Fraser: Imagine if this asteroid was the size of Texas and it was going to crash into planet Earth. Anyway, just spit-balling.
Pamela: So, one of the things that I particularly love is I attempted NaNoWriMo a number of years ago, and I didn’t make it very far because grants are due in November, which is unfair. And I’d actually started a story that was based on hitting a pocket of volatiles in an asteroid. And so, it’s just like the science is true. This is a thing. I need to finish this novel some NaNoWriMo when I’m not writing grants. But, yeah. This is one more potential hazard and one more potential good thing because this represents a source of materials that can be broken apart. And it’s gonna take a ton of energy and processing to do this. But this is refining. We know how to refine stuff.
Fraser: There are some really fascinating concepts for missions. There’s a test mission – I think it’s coming from Honeybee Robotics – and their plan is to make a little bag on a spacecraft. And it’ll go and find like a really small asteroid, something that’s just like a meter across, and then grab it with its little claws, and then wrap the bag around it and then bake this asteroid to get all of the volatiles – water, ammonia, methane, hydrocarbons – whatever it can get out of it – and then use that as a proof, but also later on to take that material and be able to supply it to say a future space mission. Right?
Because right now, pulling that stuff from Earth, you’re looking at $10,000 a kilogram. Maybe you could bring those prices down to $3000 a kilogram on a Falcon 9 to carry just water to space. But if you could actually find the stuff in these tiny little asteroids in space, bake out the water, transfer it to your spacecraft, now you’re saving money. So, it’s pretty exciting.
Pamela: Now, while active asteroids are super cool, potentially super dangerous, and throw rocks at our spacecraft, we can kind of, sort of, mostly get a handle of how this happens. What’s more confusing is have you heard of active centaurs?
Fraser: No. I’m sorry. Just refresh my memory. What are the centaurs?
Pamela: So, centaurs are the icy objects that are out near Jupiter and are believed to be the source of Jupiter family comets, which is abbreviated as JFC, and makes reading these articles far more amusing than it should be. And out at the distance of Jupiter, we don’t expect icy objects to be active, but we nonetheless see icy objects at the distance of Jupiter that are active. The object that is best known is 29P/Schwassmann-Wachman. This is an object that was discovered November 15, 1927. And even though it stays out between 5.7-6.2 astronomical units, it still has a coma and a tail – a tiny, tiny, tiny, tiny tail – but it has them, which means –
Fraser: How long has it been there? It’s crazy for it to still have a tail, however many billions of years it’s been orbiting around the sun, even at that distance.
Pamela: Right. And this is an object that’s 60 km across, so 40 miles for those of you who like to think in other units. And dynamically, objects that are in the centaur part of our solar system aren’t entirely stable. And they should only be able to last there for about a thousand years before they get sent cascading into the inner solar system or flung outward, which means that if this object gets sent inwards, we’re going to have a 60 km diameter comet headed into the inner solar system, which will be like naked eye out at the distance of Mars. And I want this to happen now.
Fraser: Yes, please. I was just – another video that we’re working on right now is just this idea that comets could pass through the Earth’s upper atmosphere and then carry biological material back out into space. And so, it’s thought that it happens about once every 100,000 years. So, when you think about say Hyakutake or Hale-Bopp, we saw them at like 15 million kilometers away, like 0.1 astronomical unit. But can you imagine watching a comet go right through the upper atmosphere of the Earth at say 100 km altitude?
Pamela: So, we haven’t seen it at quite 100 km altitude with the nuclei, but we did fly directly through the tail of a comet recently enough that there’s actually a song about it that I’m trying to look up the details on. The 1934 jazz standard Stars Fell on Alabama is based on a meteor shower that was generated by flying through the tail of a comet. This was the Leonid meteor shower that was observed in 1833, and is referred to the night that the stars fell.
Fraser: And you can still see this.
Fraser: I mean the Leonid’s occasionally. The one back in 2000 and – I always forget – 2003 I think it was – 2001 maybe?
Pamela: It was 2001.
Fraser: Okay, 2001 was stunning. Just zip, zip. Right? That’s how fast you were seeing comets. I was seeing many a minute. And I was in the middle of Vancouver at the time.
Pamela: And the degree to which we see meteors is due to essentially three things – how recently was the stream refurbished by the comet or asteroid passing through that region; did the planet Earth pass through the densest region; and the third is the rare “oh, we happened to go through the comet tail while it was there,” which is what generated this song.
Fraser: Now, I think you mentioned asteroid Phaethon earlier. So, did you know that it is the one source of a meteor shower that is an asteroid? So, mostly they come from comets, but the Geminids come from an asteroid.
Pamela: Yes. And I actually was born during the Geminids, so –
Fraser: Oh, really?
Pamela: Yes. I am the child of a meteor shower. Take that how you will.
Fraser: Asteroid-based meteor shower.
Pamela: And this is one of those objects that is discussed as being active because of cracking and revealing the volatiles inside, leading to massive amounts of mass loss.
Fraser: I wonder if – I mean, we saw what happened as Osiris Rex approached Bennu. It got smacked by chunks of gravel as it approached, but if we pass through the tail of these outbursts – if we pass through the tail of Bennu, it would look like a meteor shower, wouldn’t it?
Pamela: So, on any given night, the Earth passes through a whole lot of debris. And so, the question becomes is it that much denser in the region around Bennu than in any other region of space if you’re not there in the moment? So, it’s one thing for a spacecraft to be orbiting just a couple of kilometers up and going “oh, shoot. Rocks flung at me,” versus going through that patch of solar system a year later when all of those rocks are just part of the background collection that’s everywhere. Because let’s face it, there’s active asteroids everywhere.
Fraser: Yeah. And as you said, you go out any night, watch the sky long enough and you will see one of those chunks of gravel thrown off one of those asteroids out there somewhere impacting the Earth’s atmosphere, and those are the small ones.
Pamela: So, if you wanna read about these active asteroids or these active centaurs, again, David Jewett is the researcher to talk to. And it’s a continuum of objects. They get moved around. So, when you look up, always be prepared to be surprised.
Fraser: And next week, we’re gonna talk about how things all formed and moved around, and that will help kind of explain where they might’ve come from. Thanks, Pamela. Do you have some names for us this week?
Pamela: I do. We are supported as always by you. The best way that you can help us out is to follow us over on Patreon, where we will work to well, get things to you,. And thanks for everything that you do as well. The people I really wanna thank this week, and every week we have a new set of patrons so that over the course of a month, we’ll thank everyone at the $10 level and higher. And this week, I wanna say thanks to Justin Proctor, Jordan Young, Burry Galwin, Romja Aminonthu, Andrew Polestra, Brian Cagle, David Trog, The Giant Nothing, Laura Kitelson, Robert Polesma, Corey Duvaly, Les Howard, Joss Cunningham, Paul Jarman, Emily Patterson, Warp Factor 9, and A Blip in the Universe.
Fraser: Awesome. Thanks, Pamela. We’ll see you next week.
Pamela: See you later.
Thank you for listening to Astronomy Cast, a nonprofit resource provided by the Planetary Science Institute, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at Astronomy Cast. You can email us at firstname.lastname@example.org, tweet us @astronomycast, like us on Facebook, and watch us on YouTube. We record our show live on YouTube every Friday at 3:00 p.m. Eastern, 12:00 p.m. Pacific, or 1900 UTC. Our intro music was provided by David Joseph Wesley. The outro music is by Travis Searle, and the show was edited by Susie Murph.
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Duration: 30 minutes