Ep. 564: Mini Moons

Posted on Apr 2, 2020 in Planetary Science, podcast | 1 comment

Last month astronomers announced that they had detected a tiny asteroid that had been captured by the Earth’s gravity well and had been sharing our orbit for a few years. Today, let’s talk about the smallest moons in the Solar System.

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Show Notes


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Fraser:                         Astronomy Cast, episode 564: Mini Moons. 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 and with me, as always, is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of CosmoQuest. Hey Pamela, how you doing?

Pamela:                        I’m doing well. How are you doing, Fraser?

Fraser:                         Again, with the caveat of the apocalypse, I am doing great. I am staying busy. I am learning Chinese. I am gardening. Our mason bees have come out of their hidey holes and now they’re starting to pollenate our plants. Hanging out, playing a lot of video games. Hanging out with my kid while my other kid is out there on the front lines. It’s surreal and we’re getting through it.

Pamela:                        That’s all any of us can possibly do at this point.

Fraser:                         Yeah, exactly. All right, so last month, astronomers announced that they had detected a tiny asteroid that had been captured by the Earth’s gravity well and had been sharing our orbit for a few years. Today, let’s talk about the smallest moons in the solar system. All right, Pamela, what’s the line? When you’ve got a moon like Titan, the moon, moons, Ganymede; they’re big, they’re round, they’re moons. A piece of dust that is going around the earth, is that a moon?

Pamela:                        No.

Fraser:                         When does it become a moon?

Pamela:                        Well, so the reason I went, “no,” to the piece of dust is because that doesn’t fully count as a geologic object yet. It’s just a piece of dust. I think you need to be at least a few meters in size, but when it comes to a mini moon, what I love about these things, is they kind of fit in your garage. And I think any astronomical object smaller than a house, calling it mini is totally legit.

Fraser:                         Before we go any deeper into this, I need to at least lodge a protest with the name “mini moon” because we, for many years, on Universe Today – it’s been one of our writer guidelines that the opposite of a supermoon is a mini moon. So, in other words, this is a term coined by Dave Dickinson. The supermoon is the –


Fraser:                         So, when you’ve got the moon closest to the earth and the moon is full, then you get a supermoon. And then the opposite of that, where the moon is the farthest and it’s a full moon, that we have been calling a mini moon.

Pamela:                        Yeah, and that’s apogee and perigee; syzygy is related to the sun.

Fraser:                         And so, we were trying to get that term to stick and then, a month ago, everybody started calling this little moon “mini moon” and then we’re done. But I still hold out for Milkdromeda. Seriously, you write for Universe Today, you call it Andromeda Way, you’re gone – fired. You call it Milkomedia, just give me your resignation. That’s just the way I run Universe Today with an iron fist. But I know with “mini moon” I just can’t –

Pamela:                        You lost.

Fraser:                         I just can’t fight this one, yeah. All right, so we’re gonna make that definition – it’s still a moon if it just barely fits into your garage. I think that’s a great definition. If it landed on you, it would kill you.

Pamela:                        That is probably the best definition.

Fraser:                         Perfect, then we’ve decided. If it lands on you and it can kill you, then it is a moon. Perfect.

Pamela:                        And this all goes back to the idea that we called everything going around the earth a satellite before we had man-made satellites and then our moon was the biggest of the satellites. And then, humanity got a little bit screwed up because we started making our own stuff and shoving it in orbit and those were called satellites. And people started to think there’s the moon that’s a natural object, then there’s satellites, there’s man-made objects. And this idea that satellites were just things in orbit got lost, so we needed a new word for these worlds, and mini moon was what folks came up with.

And the one that we found recently isn’t the first one or anything. It’s just the most recent. Back in 2006, the good folks at the Catalina Sky Survey who are out there trying to protect us from falling rocks. They found one of these back in 2006 and, as all good mini moons do, it flew away. It turns out the one that was found this year has also left our orbit as of March 20. So, we are currently without, ever so temporarily, a mini moon that we know about.

Fraser:                         That we know about. But it could very well be that there are many of these – Femto-moons, someone is calling in the chat. I’m gonna do everything I can to not use this term. So, we’ve got these Femto-moons and they are captured temporarily by the Earth’s gravity well and they spend a little time in our vicinity. Now, are they just orbiting around the earth just like the moon does or is their orbit a little more peculiar?

Pamela:                        I’d say chaotic is the word we want. These objects are generally coming in from an orbit that probably originated as far out as the asteroid belt, but they got perturbed along the way and got sent at just the right velocity that they got snagged up by the Earth’s gravity, the moon’s gravity, by the Earth-moon system’s gravity. And they went from being purely orbiting the sun to orbiting in this messy spirograph pattern that wove them around the Earth, sometimes around the moon – depends on the object – in a way that’s constantly changing and precessing as it interacts with the different gravities.

This is your quintessential unstable multibody system. To the moon, the Earth is the biggest thing out there, and it’s happy to orbit us from here to kingdom come. Earth-moon-sun, not really a three-body problem because you can reduce the Earth and moon to two bodies that are together chucking around the sun. But, these little rocks were like, “Oh expletive, I see the moon, it has a large effect on me; I see the Earth, it has a large effect on me, and I was already on an elliptical orbit around the sun.” And so, they tend to come in, get caught for a little while, and then end up just sort of going back around the sun.

Fraser:                         Right, it didn’t take a lot of change in energy for it to fall into orbit around the Earth, and it doesn’t take much of a change of energy for it to fall back out of orbit around the Earth and back to the sun. So, this most recent object – how big do astronomers think that it was?

Pamela:                        It was a couple of meters. It was basically the size of a large boulder you might climb at a park without worrying that you were gonna fall off the large boulder.

Fraser:                         A car, a washing machine. Yeah, I’ve heard washing machine, car, something you could give a hug to.

Pamela:                        It was probably a couple of meters, so like two meters on its long axis, maybe. Not big, just big enough to see basically.

Fraser:                         But we are seeing these – objects like this – not just around the Earth, and of course this isn’t the first time that we’ve seen them. We are also seeing fairly small objects that are doing interesting things at other objects in the solar system as well.

Pamela:                        And the idea is that because our solar system went through traumatic times, like the great heavy bombardment that scattered chunks of world in all directions. We have these one- two- three-meter class objects on random trajectories all over our solar system just waiting to get put into orbits – in some cases even around some of the largest moons.

It is numerically possible for Ganymede to capture one of these and have a mini moon orbiting the moon around a big world, in which case, we refer to the mini moon not as a mini moon because it’s now orbiting a full-size moon and it becomes what – in one Earth & Sky article – is referred to as a “moon moon.” And everyone embraced the topic without realizing that “moon moon” was a really silly wolf-related meme.

Fraser:                         We go with toon. A toon.

Pamela:                        I have not heard this one, tell me.

Fraser:                         That’s my favorite term. Again, as an editor, I get to enforce this terminology with an iron fist, so I really like toon. So, you’ve got a moon, then you’ve got a toon. And a toon is a moon that goes around a moon. Should we have clarified the terminology before we even started this show? No, I think this is half the fun of this episode.

Pamela:                        It’s true, we need more laughter in our lives right now and this is the laughter we’re gonna go with. I like moon moon because it means every time I Google “moon moon,” I get lots of dorky pictures of wolves doing really silly things.

Fraser:                         So, the Canadians in the audience will know why I like the term toon, which is that – in Canada, our $2.00 coin – okay, hold on. Our $1.00 coin is called a loonie – a loon – because it’s got the bird on it. So, we call the $2.00 coin a toonie, even though it has a polar bear on it. I love Canadian coins because they feel like gold medallions –

Pamela:                        Real money.

Fraser:                         Yeah, if I hand you a whole bunch of loonies – they’re these beautiful little gold coins – and you’ve got a handful of gold, it’s like you’re a pirate. Yarrr.

Pamela:                        It feels like – for those in the U.S., if you’ve ever had a Susquehanna dollar – it feels like the Susquehanna gold dollar coins.

Fraser:                         So anyway, toon. But a toon – or a moon moon – has never been found, and you would think – I mean, we think about the universe, we think about how you’ve got galaxies going around galaxy clusters, you’ve got stars going around galaxy cores, you’ve got –

Pamela:                        Ida and Dactyl.

Fraser:                         You’ve got planets. Right, and sure, you’ve got asteroids with their own moons. I was going to talk about Didymos in a second as well. And yet, we don’t see – it seems to stop at that point. We don’t see planets with moons and then those moons have moons. Although, is, say, the lunar reconnaissance orbiter currently orbiting around the moon a moon moon? A toon?

Pamela:                        This is where we have to remember that little LRO has to keep correcting its orbit. And this is part of the problem. In order to not experience the chaos of the three-body interaction, these new tiny objects flying in have to snuggle up into one gravity well or the other. So, they can either snuggle in right next to the Earth; they can snuggle in right next to the moon. Whichever one of those two places they go, they’re gonna get lost in well – all the other stuff that’s either there or lost in the glow of that object.

So, if there was one of these little two-meter objects orbiting the same way LRO was – first of all, it wouldn’t be entirely stable because it doesn’t have anything to boost its orbit on a regular basis – but second of all, it would be really hard to see because it would be right next to the super-bright moon. And if the moon isn’t super-bright, then it’s super-close to the sun and it would get lost in the glare of the sky. The same problem happens with other worlds.

Fraser:                         Yeah, and I think you really hit the nail on the head there, which is that LRO, which has been nicely in orbit around the moon for quite a while now is having to continuously re-boost itself; even though it’s not moving through any kind of atmosphere, it is getting its orbit changed over time through the interactions of the earth and the moon. Without those changes, it would just be a matter of time before it crashed into the moon like all the others have.

Pamela:                        Exactly. It becomes hard to find one of these tiny objects in the light of Ganymede on the other side of our solar system. It becomes hard to even find them in the light of our own moon. So, they could be out there and we’re just not seeing them, just as they’ve always been around our Earth. It’s thought that we pretty much always have one or more of these meter-class objects orbiting our world and the moon and/or the moon for a few years at a time here and there. And we’re only just starting to find them.

Fraser:                         I’ve heard that the best place to search for objects like this is actually Neptune. Neptune is a slow enough mass and Triton is a large enough mass compared to Neptune and far enough away that, in theory, Triton could have its own moons and be relatively stable for long periods of time without the interactions.

Because the same problem that we’re having with the Earth’s moon and it trying to have any kind of satellite around it is that the Earth is really dominant compared to the moon and if you had Jupiter – yeah Jupiter has tons of moons – but any moons orbiting its moons are going to be pushed around by the gravity of Jupiter. But the way the gravity well works out at Neptune, you could actually have that.

But let’s imagine that there was no sun, that it was just, say, a planet like Jupiter or a planet like Neptune just floating, a rogue planet out there in space. And it had its own moon orbiting around it far enough away. Would a moon moon toon be more stable at that point?

Pamela:                        It’s still a third-body problem, so it depends on exactly where it snuggles in. So, with Triton, which as a reminder to everyone out there is bigger than Pluto which has not one, not two, but a bunch of moons, of which Charon is the largest – with Triton, it’s big enough that it can, as you say, have things far enough away from its surface to have nice stable orbits. And it’s far enough away from Neptune to also have stable orbits, but those objects would have to be reasonably close for this three-body problem to not be a special form of hellscape.

You have to essentially reduce the three-body problem by negating the gravity of something because it’s just negligible compared to the thing you’re primarily orbiting. International Space Station is good; it does not see the gravity of the moon in a way we need to worry about; it’s nice and low, even geosynchronous, not that bad because our moon is so far away from us. Now, the issue is our moon isn’t generally huge and I have to admit I don’t know what the comparison between the moon and Triton for mass is. Do you happen to know that?

Fraser:                         The answer is .29, so Triton is 29% the mass of the moon.

Pamela:                        So, Triton would still have the problem with things not being entirely stable around it unless they were very precisely put there.

Fraser:                         Right, and I think that’s the point, that Neptune’s mass isn’t overwhelming in the way – you would have to do some very serious solar system re-architecture to make everything get pushed around. But the other thing that I find, people always ask me this question on my YouTube channel. They say, “What if we removed a planet? How would that destabilize the solar system?”

Pamela:                        It wouldn’t.

Fraser:                         It wouldn’t, it would be better. It would improve everything, right?

Pamela:                        Yeah, you have fewer things that are tugging and pulling things around. So, Neptune is just 17 times the mass of the Earth and because it is physically so much larger and Triton is just barely held onto, it’s thought that Triton is a captured Kuiper belt object and lots of planetary scientists are desperate to get a spacecraft out there that is capable of turning that hard left to go into orbit around Neptune. Triton should be geologically as active or more active than Pluto. We want to go see it and it’s its own little world, but it was caught up in Neptune’s gravity.

Fraser:                         Now, you mentioned this earlier on in the episode and I pushed right past it, which is this idea that, for a place like the Earth, having tiny moons be in relatively stable orbit is very difficult with the three-body interactions. But, when you look at places like tiny asteroids, they can have small moons that are in perfect stability.

Pamela:                        And that’s not a three-body problem anymore. That is, as far as Dactyl is concerned as it orbits Ida, the sun is dead to it, the same way our own moon is pretty much ignoring the sun because the Earth’s tug is the primary one that matters. So, Dactyl is nice and happy orbiting around Ida, no big deal. It’s as objects get bigger and as they come together such that they have disparate orbits such that they both start out orbiting the sun, that there’s this moment of “Ahh! I don’t know what I’m gravitationally attracted to.” And they may temporarily dance, but then it all falls apart because the sun is still a dominant player and the velocities aren’t right and three bodies are ugly.

Fraser:                         There’s a mission that I’m really fascinated by, and this is NASA’s DART mission, which is the – I forget the term – asteroid impact and deflection assessment, anyway they’re gonna send –

Pamela:                        Double Asteroid Redirection Test.

Fraser:                         There we go, Double Asteroid Redirection Test. The plan is that NASA in 2023 or 2021 –

Pamela:                        Don’t believe that folks.


Fraser:                         I know, I know. All dates are just out the window now, but they’re gonna be sending a mission to this double asteroid called Didymos, which the main asteroid is – I forget the mass, it’s not much –

Pamela:                        So, DART, this double asteroid reconnaissance test –

Fraser:                         .78 kilometers.

Pamela:                        DART, this Double Asteroid Redirection Test is going to Didymos, which is a binary asteroid where the primary one is 700 meters in diameter and the smaller one is just 163 meters in diameter. And they’re about a kilometer apart, and DART’s gonna take that smaller asteroid, which is still like 10 times bigger than the mini moon going around the Earth. It’s going to take that tiny asteroid which is still big enough that you really don’t want it hitting the Earth, and it’s going to see what it can do to move it. Now, the nice thing about this mission is Didymos, this binary system, is not an Earth-crossing asteroid, so if they screw up, we are not in danger.

Fraser:                         Or super successful.

Pamela:                        Let’s not think about that.

Fraser:                         If they’re incredibly successful and really push this tiny moon out of –

Pamela:                        Directly at us?

Fraser:                         Yeah, but can’t, right? So, no matter how successful they are, they can’t actually make this system impact the Earth. But the point is that they’re going to take a 170-meter-across chunk of space rock and significantly attempt to change its velocity and calculate what that impact is. It shows us for the first time – we’ve been talking about methods of mitigating asteroid impacts for –

Pamela:                        Decades.

Fraser:                         For almost the entire time we’ve been doing the Astronomy Cast. What are all the different ways to move an asteroid? But nobody’s ever tried one. So finally, someone’s gonna try one.

Pamela:                        I really wish they’d go out and spray paint it though. Because really, that’s the best way in terms of silliness.

Fraser:                         They should take like a Banksy stencil; go out there and spray paint the asteroid appropriately. So, do we think that some of the larger moons would have formed starting with these mini moons and then they built up over time? Or are the larger moons in the solar system like a separate formation event?

Pamela:                        The moons, in general, came about in three different ways in our solar system. You have the ones like Triton, like Deimos and Phobos, like many of the outer Saturnian and Jovian moons that …were kidnapped. These were objects that happily formed on their own, weren’t necessarily big enough to hit hydrostatic equilibrium and become spheres and they had their own lives going before they gravitationally got captured up and turned into moons.

Now, the other way that you can get a moon is the way we got our own Earth. Two things collide, something goes splush, and that splush comes back together to form a moon. And then you have all the moons that probably formed in situ around the giant gas giants: Jupiter, Saturn. These particularly large ones could have, we now believe, formed in the – well, planetary nebula isn’t really the best of terms, but I’m not sure what other phrase to use – within that swirling cloud of dust and gas that was forming the gas giants in its core but still had eddies and vortices of material going around it that went on to form those moons.

It’s only with these giant planets that you can have the moons forming in situ. Otherwise – well, they can still form in situ if you break a couple things up like the Earth and Theia did to make our moon – or you just have to steal things.

Fraser:                         Right, and of course, we go back to the Vera Rubin telescope which is, again, opening dates –

Pamela:                        It’s the Vera Rubin Observatory; the telescope has a completely different name and it’s gonna stay LSST to me.

Fraser:                         Right, yeah, so the Vera Rubin Observatory opening – now, who knows when, of course – will be a really powerful tool for finding these objects, especially around the Earth. I’ll bet you as soon as that thing opens up its eyes, we will suddenly discover that we have many of these little objects buzzing around us all the time, and we can watch them come and go with almost certainty, which is sort of an amazing idea.

Pamela:                        And until then, when it can, the Catalina Sky Survey is gonna be out there trying to keep us safe and at least having fun finding silly things we give sillier names to.

Fraser:                         Pamela, thank you. Do you have any names for us this week?

Pamela:                        I do. As always, we are here thanks to the generous contributions of folks like you. And right now, in these crazy times, we’re particularly grateful for what you do as our patrons. Thank you for still being there to help us get through these strange times.

This week, our thanks go out to Brian Nelson, Kristin Brooks, Eric Feringer, Martin Dawson, Kensleyeth Penfilianko, Dwayne, Isaac, Frodo Tennebau, Shannon Humber, Justin Proctor, Thomas Tubman, David Gates, Rachel Fry, Iran Segev, Fredic Sejorg, Claudia Mastriani, Nuderdude, Paul L. Hayden, Omar Del Riviero, Brent Krenop, Tim Garish, Arthur Lats Hall, William Andrews, Jack, Mark Grundy, William Lauer, Jeremy Kerwin. And thank you, all of you, for everything you do that allows us to do what we do.

Fraser:                         All right, we’ll see y’all next week.

Pamela:                        Buh-bye everyone.

Narrator:                      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 info@astronomycast.com, 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 Sorrel, and the show was edited by Suzy Murph.

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