Can moons have moons? :: Project Espa #13

Added: 2026-06-01

[00:00:00]Can a moon have a moon?

[00:00:03]Known as a submoon or subsatellite, a moon of a moon is a hypothetical concept with no observed examples in reality.

[00:00:11]For that reason, submoons in worldb building might be too easily dismissed.

[00:00:16]Ignoring the fact that they are of course entirely possible if the right conditions are met, those conditions are definitely restrained. But larger moons that are far away from their parent planet are probably among the most likely environments for a submoon to exist in. Since today we are finishing the outermost major moon of the Monta system, now seems like the perfect opportunity to put in a submoon to spice up our system. So, let's get into it.

[00:00:55]Hello everyone and welcome back to project espa. My name is Yiji online and in this series I document the process of my worldbuing project Espa. Thus far in the series we have started out with the astronomy of Espa star system. a wide separation trinary star system with the sunlike star Ojour at its center which is being orbited by two red dwarfs at a far distance. Ojour itself has nine planets. The fourth of which is the gas giant Mont the systems main Jovian Shepherd and over twice as massive as Jupiter. Mont possesses an extensive lunar system consisting of six major moons. Over the last couple episodes, we have finished all but one of those. The hypervulcanic world of Idime, the exotic lifebearing world of Torah, the cryovcanic ice world of Sanu, the two-faced magnetic world of Hira, and last video, the dark world of Tolu, which means that today we get to work on the final and outermost major moon of Manta, Ulia.

[00:02:08]When I built the first iteration of the Monta system 9 years ago, Ulia, then called Triton, was by far the smallest moon of Monta, being only 2700 km across and with a mass of just 28% that of the moon, giving it a low density of just 2 g per cm. That's actually realistic. Oh my, I'm so proud of Pasigi. Based very loosely on Ganymede, Triton was a cold, desolate, airless world. At the time, the system was much more compact and no resonances had been established. So, Triton orbited almost a million km closer to Monta. By far its most distinctive feature, though, was the fact that Triton, which is itself a moon, had a moon. A tiny asteroid called Aruti had been captured into an unstable orbit, slowly escaping its gravity.

[00:03:00]Aruti was just a 30 km wide object. Its orbits at just 270 km above the Tritonian surface, taking it around the moon once every 26 minutes. That um was definitely within the rose limit and something we will need to address today.

[00:03:17]But all by all looking purely at the physical aspects compared to the other five moons of Montriune will definitely require the least amount of overhaul. In fact, I think just some finetuning will be enough for today.

[00:03:38]The pattern we have established over the previous videos is that as the moons are further from Monta, their densities will decrease and the amount of their mass being made up of ice will increase. This pattern arises naturally from the mass distribution in the primordial accretion disc from which these moons formed. As the outermost of Mont's major moons, Uluya is set to achieve both the lowest density and the highest mass fraction of ice, making for a unique type of world.

[00:04:07]Furthermore, there is, of course, Ouya's captured moon. Where Aruti used to be only a few kilome across, I think the new version can be scaled up a little. I really like the idea of making it a captured centaur specifically. So, let's aim for that. I'm also going to rename Triton and Aruti to Uluya and Okco mainly just to distance the new version from the old. Okay, goals established.

[00:04:31]Let's get started.

[00:04:39]Ulia sits in a remote orbit some 2.2 million km away from Mont. At that distance, even after 5.8 8 billion years, it has avoided any resonance interactions with the other moons. In fact, it is so far away from Manta that at almost 29 planetary radi, it is really pushing what some would consider to be realistic. I talked about this all the way back in episode 6 of the project, which by now was over a year ago, so don't worry, I'll refresh your memory. But when we are dealing with realistic natural moons, that is moons that form through accretion around the body they orbit, the range in which to place their orbits is defined by the extent of the primordial accretion disc from which they formed. So what we end up with is a reasonably strong rule that major natural moons can only form between roughly 4 to 27 planetary radi from the planet. Of course, that is only the range in which they can form. The range in which stable orbits can exist is actually defined by the hill sphere, which for Monta extends over 66 million km out. Of course, after 5.8 billion years, which is the age of our system, orbits can and do shift around, as evident by the outward migration of Manta's inner moons.

[00:06:02]So for Ulua to have ended up at 29 planetary radi only some 150,000 km further than where it would have formed and after such a long time I don't think is an issue but I wanted to address it nevertheless since I'm technically breaking my own rules here. But as is often said in worldbuilding know the rules before you break the rules.

[00:06:30]Uluia is the third largest moon of Manta, measuring some 4,900 km across.

[00:06:36]It is about 300 km smaller than Tulu and some 600 km smaller than Hira. With just 1.4 lunar masses, that gives Uluan incredibly low density of just 1.7 g per cm. With such a low density, some 2/3 of the moon's mass will be made up of water ice, the highest ice fraction we have had so far. This makes sense because this far from its parental gas giant, icy volatiles like water would have been the primary materials available for its formation. Ulia also has the lowest surface gravity of all Montas moons at 12gs. Its surface bound exosphere exerting barely a few pyrobars of pressure on the surface. With such a trace atmosphere, there's little heat retention on the surface and temperatures usually peak at 170° below zero.

[00:07:35]Hia and Tollu are fully differentiated, that means layered bodies with distinct cores, mantles, and crusts. But at densities lower than 2 g per cubic cm, buoyancy begins to struggle, giving bodies a layered makeup like that. For Ilia, that means it won't have a clear core at all. Instead, its center will be made up of a mixture of heavy metals, rocky material, and compressed ices, which will gradually fade into a mantle of just rocky materials mixed with exotic ices. This transitional layer makes up for the thick of the moon's composition, extending all the way up to just a few hundred km below its surface.

[00:08:15]Due to the low gravity, most of its water will already be mixed into the core and mantle, meaning there will be less available to form its subglacial ocean, which will end up being substantially shallower than onsanu, hira, and tulu. Due to this lesser water volume available, the salenity of this ocean skyrockets and water purity drops to near saturation. With no mechanisms to purify the ocean, its actual water content may be as low as just 2/3, making for a hypers saline environment where the water will act more like a thick salty slush than a liquid.

[00:08:54]Buoyancy will still be strong enough to see to it that the top of this ocean will be more pure than its depths. But that will only mean the crust will grow that much thicker. On Ulia, the icy crust might extend over 150 kilometers down before the subglacial ocean starts.

[00:09:21]When viewed from space, appears almost blackened by millions of craters. Unlike with Sananu, Hia and Tolu, it never had any cryote tectonics to begin with, which means that very soon after its initial formation, its crust became quiescent and craters began to accumulate. After 5.8 billion years, the surface of become fully saturated with crater features with most craters being covered by other craters themselves.

[00:09:50]As the outermost major moon of Monta, Ulia receives the bulk of impactors before they can make their way into the system more deeply and thus has by far the highest impact rate of all Montas moons. And its surface will show this.

[00:10:04]Similar to how it is on Tou, the crater debris has dirtied the originally bright icy crust with darker grains of space dirt and dust, causing Ulua to be one of the darkest bodies in the O Jordan system with a low albido of just 0.19.

[00:10:19]The only bright features on its surface are raid crater systems, which are younger, larger craters that have penetrated deep enough to splash up the original icy material below, leaving behind these bright radio ejector structures of which Ouya has hundreds.

[00:10:37]Other large impactors left behind enormous multi- ringed structures, some of which measure over 1,000 km across.

[00:10:46]Records of truly apocalyptic impacts in the solar system. There are no moons that have moons of their own. That isn't really surprising. Most moons are far too weak in mass to hold on to them. The margin around them where orbits can be stable often being so narrow that it becomes statistically improbable for sub moons to exist. Improbability is not the same as impossible though. And in world building, we are at full liberty to seize upon 0.01% opportunities like this. I must concede though that a natural satellite for Ulua would be so unlikely to have formed, let alone survived for 5.8 8 billion years that even with soft realism, I can't really justify it. But what about unnatural satellites?

[00:11:43]Like previously mentioned, as the outermost major moon of Manta, Ouya is the first moon to confront foreign objects wandering into the system. So, it's definitely possible one of these objects came in at just the right angle and was captured. Having a captured object be Ulyia's sub moon is definitely the safe option in the context of the systems age. Such an object needs not have a stable orbit at all. It can be transitory. It just needs to be stable enough to be there in the window of time there will be a civilization on Espa. A couple hundred,000 years at most. The blink of an eye on astronomical time scales. Last video we had a look at the origins of the Martian moons Phobos and Damos in which I briefly mentioned Phobos is on a decaying orbit around Mars and will be destroyed in some 40 million years when it crosses Mars rush limit. That is really cool lore. So let's put Ulua's moon on a similarly decaying orbit, setting it up for an eventual crash into Ulia in the notsodistant future.

[00:12:54]After a planetary system finishes forming, it's unlikely all the Mars available will have been depleted.

[00:13:01]Leftovers from the initial accretion disc will form rich belts of material at the verges of the inner and outer system. We refer to these as the asteroid and Kyper belts respectively.

[00:13:12]Videos on the Ajoran asteroid and Kiper belts are definitely pending. But for today, when it comes to what kind of object Ouya can capture, we essentially have three options. An object flown outward from the asteroid belt, an object flown inward from the Kyper belt, or if we're feeling really quirky, an interstellar object coming in from, well, who knows where those things originate from. Interstellar objects aren't really a good choice here. Only three of these have ever been confirmed.

[00:13:41]So, they remain a poorly understood class of object, but from what we do know, it's safe to say capturing an interstellar object seems to definitely be the least likely option. Likewise, due to the complex nature of orbital mechanics, it will be challenging for Ulua, even aided by Mont's massive gravity well, to pluck a body from the Ojoran asteroid belt. But the Ojoran Kyper belt is much less restrictive. In the outer solar system, there's a type of object known as centaurs. No, not the horsemen. Centaurs are actually former Kyper belt objects whose orbits have been disturbed to the point of them wandering inwards towards the center of the system. In our solar system, the number of centaurs is not precisely known, but their numbers may range from several tens of thousands to several tens of millions. Some centaurs can even be quite large with a number of them being over 100 km across. Naturally, these bodies will also exist in the Ojoran system and I think under the right circumstances, one will be perfect as Ouya's captured moon.

[00:14:57]Let's construct a sizable low mass centaur originating from the Ojoran Kyper belt and dub it Okoboo, a cool anagram. Let's give Okoboo a mass of 530 pag and an average diameter of 100 km.

[00:15:12]That's sizable for a centaur, but still far smaller than the largest known centaur Shereeklo at 260 km. Despite its size, Okoboo will be too small to pull itself into hydrostatic equilibrium.

[00:15:25]Fancy speak for being steroidal.

[00:15:27]Instead, it will be an irregularly shaped body, the first we have built so far in Yojoran system. With that size and mass, Okco will have an extremely low density of just 98 g per cm, which is not that much denser than pure ice at just 91 g per cm. This low density suggests a porous composition with internal voids where Okabo's mass is something like 96% pure water ice. For comparison, that's roughly the same purity as a seawater with just 4% impurities mixed into the ice. Despite being so icy though, Okco's albido is extremely dark, reflecting back just some 5% of the incoming light. This is actually pretty normal for Kyper Belt objects as despite their high ice content, there are trace amounts of nitrogen and methane on their surfaces which have been exposed to cosmic rays and ultraviolet radiation to form a thin layer of complex organic molecules called tolins on their surface. Tlins typically appear very dark and absorb light most strongly in the blue and green spectra which will give a very slight deep reddish tint.

[00:16:50]Because Okco is a captured object instead of a natural one, the range in which its orbit can exist can be calculated very precisely and is given by UA's rush limit on the inner end and Uya's hillphere on the outer end. These can be calculated with the following formulas. Note that the rush limit is dependent on the density of Okobo, unlike the hill sphere, which is the same for any object regardless of its properties. Using these, we find that the range for Okaboobo's orbit lies between 72 and 43,000 km from UA center.

[00:17:23]An incredibly narrow zone just 36,000 km wide. On one hand, that reveals why moons having moons is so rare. But on the other hand, we can definitely fit Okubo inside there. Let's put the capture event further away in time than the impact event. So leaning closer to the rush limit than to the hillphere.

[00:17:43]Placing Okabo semi- major axis at some 8,000 km will mean it's just a couple millennia away from its destruction and several hundred,000 years away from when it was captured. At this distance, it will fly just 5600 km above UA's surface and thus, despite its small size, still appear roughly twice the size of Earth's moon, having an angular diameter of just over one arc degree.

[00:18:18]Like has become common with the moons of Manta, there is of course also the radiation to discuss. Monta's powerful magnetic field subjects its innermost moons to truly apocalyptic radiation doses. But by the time we get as far out as Ouya's orbit, the radiation levels will have dropped significantly. In Uluia's remote orbit, 2.2 million km away from Manta, the decay factor N will approach its maximum value of 15.

[00:18:45]Inputting that into our trusty formula yields a radiation dose of roughly 2 to four micro severs per hour. At only 60% that of Kalisto, that's an incredibly mild radiation dose for a Jovian type moon, making Ouya the prime target.

[00:19:01]Should future Espen explorers wish to set up a base or spaceport anywhere in the Mont system, for example, as a hub to explore deeper into the outrojoran system from there. Nevertheless, two to four micro severs per hour is still eight to nine times the common background radiation on Earth, and any Espen astronauts venturing here for a long time would require some shielding to mitigate health effects. But without a doubt, Ouya would be the easiest moon to set up shop compared to the others.

[00:19:41]Ulia is the sixth and final major moon of Manta. It is located in a distant orbit over 2.2 million kilometers away from the gas giant going around the planet once every 13 1/2 days. In this remote orbit, it has never been part of the intricate resonances at play between the moons further in allowing for it to become tectonically quiescent pretty much immediately after formation. Ulia has a diameter of some 4,900 km, comfortably ranking it as the third largest moon of Monta with a size roughly comparable to real life Kalisto.

[00:20:16]Its mass constitutes roughly 1.4 lunar masses, giving the world a low density of just 1.7 g per cm, indicating that almost 2/3 of that mass is water ice.

[00:20:28]Because of that, Ulua has failed to fully differentiate its interior, and to this day, it remains a quarter world. On the surface, it has a gravity of just2gs and no atmosphere to speak of with surface temperature stable at 170° below zero, while Mont looms its sky some 7 times the size of the full moon.

[00:20:52]Against all odds, Uluia has managed to capture a wandering centaur into orbit, giving the moon a moon. Having been captured a few hundred,000 years ago, Okobo is a tiny 100 km wide object originating from the Ojoran Kyper belt.

[00:21:08]After having wandered inwards, it was captured into a transitory orbit by Ulua. Today, flying just 5600 km above Uluya's surface. It goes around Uluia once every 15 hours. Being so close, when viewed from Ilia's surface, it still appears twice the size of the full moon. This closeness, however, betrays instability. Okobo's orbit is gradually decaying with it getting closer to Ulua every time. In the next 10 to 20,000 years, it will fall through Ula's rush limit and be destroyed, making for a spectacular future event.

[00:21:50]All right then, everyone. It's time to get to the comment of the day, which this time is by Musen 9, who says, "We need to continue project Espa. It's been 2 weeks since you posted your latest video. We need to get that next and final moon. First of all, I am really happy so many of you are excited for the Project Espa episodes, which are essentially just me fumbling around with astrophysics. So, it means a lot to me that there's actually an audience for that. But yeah, they do come out slowly and that's because I do it all by myself. The research, the world building, the scripting, the narration, the editing, and that's a lot of fun.

[00:22:26]But it does mean making a Yi video takes a lot of time. Of course, I also have the regular science videos I make, which I try to alternate with the Espa videos, so there is something for everyone. At least for me personally, that has done wonders for my workflow because the science videos get to inspire my Espa videos and vice versa. And ever since I got back from my burnout, I've been highly motivated to work on my next video each time. As a creator, I just feel like I have found a workflow now that really works for me. So, I hope you guys can be patient enough for the episodes. Anyways, thank you guys so much for watching another Project Espa episode. Let me know in the comments what you think about Submoons and if you'll be making any in your own projects. Okabo is of course just one specific example and I'd love to hear what other concepts you guys have worked out. With all the major moons of Manta now done, that just leaves its minor moons, which I think we'll be able to tackle all at once next episode. So, make sure you're subscribed for that. It really helps me out massively as a small channel, and I hope to see you again next episode.

[00:23:53]Heat.