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Ceres is the largest asteroid in the belt, and in my world building, is a booming mining hub, supplying water-ice and cheap minerals to other destinations in the solar system.

For gameplay purposes it would be great to have a small moon in orbit around Ceres. Something with a diameter of a few hundred meters up to a few kilometers.

What hard-science reasons are there to justify the work and economic cost of moving a rock (from the surface or somewhere else is the belt) into Ceres orbit? You can assume a near future level of tech. No fantasy fusion drives, FTL, or inertial dampening fields.. Perhaps an effective mass driver or other engine on the rock, but the way it got there is less important than WHY it is there. What benefits are there to colonists, traders, orbital shipyards, etc.

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    $\begingroup$ Because 50 isn't enough? "Nevertheless, Ceres is able to capture other asteroids into temporary 1:1 resonant orbital relationships (making them temporary trojans) for periods up to 2 million years or more; fifty such objects have been identified." $\endgroup$
    – Mazura
    Dec 4, 2020 at 4:37
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    $\begingroup$ There would be no hard-science reasons for Ceres to be a minion hub in the first place. Just saying ;-) $\endgroup$
    – Diego Sánchez
    Dec 4, 2020 at 6:23
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    $\begingroup$ Without stricter criteria this question will just generate a list of disparate ideas for why someone would do this. $\endgroup$
    – rek
    Dec 4, 2020 at 6:29
  • $\begingroup$ What kind of criteria would you like to see here? $\endgroup$
    – Innovine
    Dec 4, 2020 at 9:11
  • $\begingroup$ "or somewhere else is the belt" should be "or somewhere else in the belt"? too small an edit at my rep. if fixed or explained, I'll delete this comment. $\endgroup$
    – CGCampbell
    Dec 4, 2020 at 14:38

16 Answers 16

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You don't need to find a way to tow it there. (It was possibly made to generate power, see the end of my answer.)

It's a slag heap!

You have already determined mining on Ceres is a booming, and mining produces unbelievable amounts of slag. It's basically everything you dig up from the mine minus the stuff you want, and consists of rock and non-useful metal compounds. Growing up in a town with one of the longest running mines in the world this was my first thought, as slag heaps are a dominate part the landscape there.

Getting it into orbit doesn't need to be that hard. Part of the solution could be placing some sort of railgun near the equator and throw it up piece by piece. (Edit: I use "throw" instead of "shoot", as you need to fine-tune the firing speed so that the speed of impact is minimised) This, like @NuclearHoagie points out, needs to be combined by some mechanism to redirect the orbits to prevent them from falling back down. At least before the moon's gravity becomes large enough to be of use. Also, you may need to cover the first few billion rocks in something sticky (or Velcro?) to make them stay together, but if your aiming is good enough you could eventually get them to clump together by their own gravitational mass. (Edit: Using similar calculations to this answer) on Astronomy SE you likely need at least a few billion metric tonnes of slag, so I assume we're not in a hurry...)

You then need either a reason why you want a moon, OR a reason you don't want slag lying around. For the former, you could use some of the other suggestions or one of the following.

  1. A moon would stabilise Ceres's rotational axis. This could be useful if you want to build launching sites that regularly point in a good direction. Or maybe an observatory or some communication devise?
  2. A moon is nice to look at and provides some sense of familiarity for those who grew up on Earth. Some populist politician realised this long ago and made the construction a pledge for their campaign; the rest is history.
  3. It's simply a billboard. The moon is regularly visible to all of Ceres and is really the perfect place for advertising. Sure it was a large investment for the company who put it there, but the dedication itself is good PR.
  4. The moon has been built over centuries as part of a yearly celebration. Shooting things into orbit is akin to the Earth tradition of Celebratory gunfire, and various local groups compete to send of larger and more numerous rocks in closer intervals and cooler patterns than their neighbours. To avoid ending up with a dangerous cloud of pebbles that would interfere with traffic, the Moon Act dictates that all fired projectiles must be placed in the same orbit. The larger the Moon became the easier it was to aim at, and today the Moon Festival is a huge event, the height of the Ceres tourist season. Coincidentally, it's also the off-season for tourism TO the Moon.

Edit: As a final note, launching a moon from a planet(oid) WILL affect its rotation. This provides a fifth reason to do this:

  1. The moon is a by-product from a project to change Ceres' rotation speed or axis. The current real-world rotational period of 9 hours is rather quick, and humans really prefer longer nights to get proper sleep. There are other reasons to slow (or speed up) the rotation as well, perhaps related to spaceship travel or telecommunication. Maybe they'd even want to make Ceres tidally locked? A motivation for the latter could be to get large differences in temperature between the two sides, and use this to generate power using Stirling engines or the like?

I actually really like this last idea, as it would provide a means to generate energy without being dependent on import of solar panels or fuel from larger planets!

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    $\begingroup$ I like the idea of an orbiting slag-heap, but you wouldn't want to just shoot slag up there via railgun. The escape velocity of a moon that size will be minimal, so any impact like that will kick dust and other material loose from the moon in all sorts of random directions, creating a massive safety hazard to anything else in orbit or on the surface of Ceres. $\endgroup$
    – Salda007
    Dec 4, 2020 at 0:41
  • $\begingroup$ Are you sure it is a good idea to have tourists visit a moon that is under kinetic bombardment by slag-spewing railguns? $\endgroup$
    – zovits
    Dec 4, 2020 at 9:20
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    $\begingroup$ Bullet 4 ... except for those intrepid children who 'borrow' their dad's shuttles to compete to be the closest to the landing zone of the firerocks without being vaporized... and Mikey always wins, 'cause he's too stupid to turn down a bwaaack bwaaack dare! $\endgroup$
    – CGCampbell
    Dec 4, 2020 at 14:48
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    $\begingroup$ The problem with the railgun is that the launch site itself is on the orbital path of whatever you launch. Unless you apply another impulse once the slag has reached altitude, it will just fall back down - if you don't fire it fast enough to reach escape velocity, it will come back. It is not possible to launch from the ground into orbit with a single impulse (barring some very complex gravity assist maneuvers). Generally speaking, it's impossible for a railgun (or any surface-mounted source of impulse) to put a ballistic projectile into orbit. $\endgroup$
    – Nuclear Hoagie
    Dec 4, 2020 at 15:57
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    $\begingroup$ @EdvinW You'd need a powered projectile to perform an orbital insertion at altitude. Perhaps you could use the railgun to launch rocket-powered slag capsules, which should need only a fairly small amount of delta-v to circularize the orbit. Even if you were able to launch from very high altitude outside the atmosphere, you'd still run the risk of bombarding your launch platform if you can't change the trajectory after launch. $\endgroup$
    – Nuclear Hoagie
    Dec 4, 2020 at 16:26
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Space elevator counterweight

A tensile space elevator requires a large counterweight at the far end of the cable, which is what holds the cable taut and upright. A captured asteroid could feasibly serve as the counterweight for such a system. If you don't want a space elevator in your setting, perhaps it's in the early stages of construction and the counterweight was just recently put into orbit.

The lower gravity on Ceres makes a space elevator perhaps a less worthwhile investment than on a high-gravity body, but might still be worthwhile for a booming mining colony that frequently ships heavy raw materials off-world. The per-launch savings might be lower, but in the long term, space elevator launches will be cheaper than traditional ones. A space elevator is feasible with current technology (particularly for low-gravity environments), but it is a massive undertaking to construct one.

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    $\begingroup$ @Innovine Can an ion engine lift you off Ceres? Do you want to start your high-power ion engine on the surface? I think it could make sense. $\endgroup$
    – Karl
    Dec 3, 2020 at 20:34
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    $\begingroup$ @Innovine As I mentioned, the utility of a space elevator is indeed smaller on a tiny celestial body with low gravity, since launching into orbit is already relatively cheap. You'd need a longer timeframe to reach the break-even point for the investment, but the more you use it, the faster it pays for itself. This seems like a good fit for a colony whose economy is dependent on shipping as much as it can off-world. In the long term, launching via space elevator will be significantly cheaper than traditional methods. $\endgroup$
    – Nuclear Hoagie
    Dec 3, 2020 at 20:35
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    $\begingroup$ The main advantage of a space elevator, regardless of where you put it, is that it doesn't require fuel at all. You can operate it off the Ceres bases' nuclear reactor, or solar panel-arrays, or whatever else you like. Leaving you to save your non-renewable reaction-mass for where it's most needed. That's definitely going to add up over time. $\endgroup$
    – Ruadhan
    Dec 4, 2020 at 9:26
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    $\begingroup$ Lower gravity makes a space elevator less useful, but much, much cheaper. $\endgroup$
    – Emilio M Bumachar
    Dec 4, 2020 at 19:55
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    $\begingroup$ @JustinThymetheSecond You keep coming back to this "sudden" change, but you can make the launch transition as long as you need (you never need to do anything "suddenly"). This is basically the principle behind a crane lifting a heavy load (but in reverse) - you don't suddenly jerk on the cable very hard, since that would result in an apparent weight many times the actual weight. Instead, you slowly accelerate the load. Here, you'd unreel the payload from the launch point, allowing it to accelerate slowly, resulting in a completely smooth relief of tension with arbitrarily low jerk. $\endgroup$
    – Nuclear Hoagie
    Dec 7, 2020 at 19:13
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You needed to build a Skyside Terminal for your spaceport.

Ceres may have only a thirtieth of a gee of surface gravity, but that's still surface gravity, and in most hard science settings, spacecraft that are efficient at moving interplanetary freight are generally not built to sit dirtside, that's what shuttles are for.

So you need terminals. Warehousing. Passenger support. Immigration. Shipyards, entertainment facilities, housing and the like to support the people and cargo that are coming and going, so you build that in orbit over Ceres. Use the rock as building material, and if things get up to the point where you need to build a Space Elevator, you can put it in Synchronous orbit, and use it for that.

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    $\begingroup$ zero-g-only spaceships makes a lot of sense actually $\endgroup$
    – Innovine
    Dec 3, 2020 at 19:52
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    $\begingroup$ I'm not sure I'd trust a shuttle that couldn't take a thirtieth of a g of acceleration. How does it steer? $\endgroup$
    – Cadence
    Dec 3, 2020 at 20:19
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    $\begingroup$ @Cadence I think the answer is proposing that the short-range orbit-to-surface shuttles (probably chemically propelled) can handle it fine, but the big freighters moving from Mars to Ceres and back, say, can't accelerate that fast — those big ones need more efficient high-delta-v engines with lower thrust, like VASIMR, ion drives, fission-fragment... $\endgroup$
    – parasoup
    Dec 3, 2020 at 20:27
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    $\begingroup$ @parasoup Perhaps, but if larger freighters are going to dock with the asteroid, they'll need to be capable of short bursts of relatively high-thrust maneuvering. It would be safer to stay "at anchor" in their own orbits, but then I'm not sure what the asteroid is adding to the equation; shuttles can go Ceres-to-ship and back just about as easily as asteroid-to-ship. $\endgroup$
    – Cadence
    Dec 3, 2020 at 20:35
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    $\begingroup$ There is no economical way to ship ore by space ship. They would bundle it into one huge blob, point it at its destination, and shove it off. No crew necessary, just a remote probe on the stack. It doesn't matter if it takes ten years to get to earth, no incremental costs, once the supply chain is established, one delivery per month on a regular basis. Once it gets close to earth, a tug goes out to capture it and 'bring it home'. Human intervention at each end, absolutely none on the journey, no in-transit hourly costs. It's like absolutely free no-cost-to-anyone shipping . $\endgroup$
    – Justin Thyme the Second
    Dec 4, 2020 at 1:30
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(answer cloned from Nuclear Hoagie, then expanded and somewhat quantified.)

Space Elevator!

If your Ceres is "booming mining hub, supplying water-ice and cheap minerals to other destinations in the solar system", then you want the cheapest and easiest way to lift all those resources off-planet. erm, off asteroid?

Sure the Escape velocity of Ceres is only 510m/s, but that is still delta-v that needs to be paid for. So if you could get it for free, why not? And if you could get some more velocity for outbound cargo containers for free, double bonus!

A CeresSynchronous orbit around Ceres is only 722 km above the surface. So if you build a Space Elevator, the center of mass need only be at or very slightly above this altitude. A heavy orbiting rock near this altitude serving as anchor for a space elevator would serve as an excellent base.

Now for real planets Space Elevators are tricky things. You have atmosphere, and slow rotational speeds, and huge gravitational wells. This imposes such demands on the elevator tether material, that you end up having to build your tether out of exotic materials and with a ridiculous taper, resulting is a very heavy and expensive construction.

And the elevator could be made out of ordinary steel, much less any fancy material! With Ceres' feeble gravity of 0.27 m/s² , and a tether length of only 722km, you could even use a Kevlar untapered cable and have a strength margin of more than 2-to-1. A Decently tapered cable (someone else calculate please) should give an even better ratio.

Similarly, you can extend a cable from the anchor rock up to as high as your material science allows you. At least several thousand km. And cargo allowed to whip up the high end cable will exit it with a completely free velocity of a few km/s. You can even use the outward acceleration to generate power, sacrificing some speed for a lot of energy for your system using very simple linear induction motors.

So: Your rock orbiting Ceres is the massive anchor for a Space Elevator to lift bulk cargo from the surface. Its mass provides stability to the system. It provides an excellent zero-g storage, manufacturing and staging area. It also serves as a similar anchor for outgoing cargo slings, and possibly for power generation from these slings. It is effectively a Port City for a rather large country. With all the support infrastructure, housing, entertainment, bureaucracy, crime, and everything else that a big port City has.

And all you lose is the initial construction cost, and a microscopic amount of the rotational inertia of a very heavy (10^21 kg) Ceres.

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    $\begingroup$ Nice expansion of my train of thought. One note, though - I don't think the counterweight will act as a zero-G environment. The counterweight is in a smaller orbit than it "ought" to be given its velocity - this is what keeps the cable taut, because the counterweight is always trying to move further away from Ceres, like swinging a tennis ball on a string. Anything not nailed down to the counterweight will fly off into its own orbit! See the "Intertial trajectory" here: physics.stackexchange.com/questions/99773/… $\endgroup$
    – Nuclear Hoagie
    Dec 4, 2020 at 16:16
  • $\begingroup$ @NuclearHoagie for an elevator that only goes from the surface, the counterweight is well above its expected orbit for that rotation rate. But the one I'm envisioning is an anchor weight at the balance point, elevator from surface to anchor, and launcher from anchor extending out maybe 5-8 times further. This gives potential launch speeds of up to about 3km/s, giving free access to whole asteroid belt. The large mass of the anchor is needed so that one can do multiple trips, launches and up&down movements simultaneously without causing the whole contraption to swing and wobble like crazy. $\endgroup$
    – PcMan
    Dec 4, 2020 at 16:58
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    $\begingroup$ The center of mass has to be at or beyond the geostationary orbit balance point, so the counterweight itself must be beyond that point. An object in geostationary orbit stays there due to gravity alone, so it doesn't provide any tension in the cable. You could use a body at the zero-G balance point as a waystation, but it wouldn't act as the counterweight/anchor, since the G-force felt at the anchor is the only thing holding the cable up. A massive waystation would actually make the real counterweight less effective, as it pulls the center of mass closer to the critical point without benefit. $\endgroup$
    – Nuclear Hoagie
    Dec 4, 2020 at 17:55
  • $\begingroup$ YOu know, i really thought that is exactly what I said? my " the counterweight is well above its expected orbit for that rotation rate" and and your "The center of mass has to be at or beyond the geostationary orbit balance point, so the counterweight itself must be beyond that point" really, honestly seem to be the same statement to me? $\endgroup$
    – PcMan
    Dec 4, 2020 at 21:05
  • $\begingroup$ Right, I think those are saying the same thing. Perhaps I've misunderstood what you meant by "envisioning an anchor weight at the balance point" - I originally thought of anchor/counterweight as different terms for the same object, but I think they're different objects in this scenario? Still, it's not clear to me why you'd want a massive object in the middle of the space elevator cable - given the enormous mass of the cable compared to the climbers, I'm not sure you need yet more mass for oscillation damping. $\endgroup$
    – Nuclear Hoagie
    Dec 4, 2020 at 21:49
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Ice Ice Baby!

"The Glacier" is nearly pure water ice, uncontaminated by brine or ammonia. It has a characteristic taste but you can drink it straight. Water is valuable stuff in the Belt. The Glacier was towed here once its nature and value was realized.

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A few possible reasons -- feel free to pick and choose as you like

Rotational Management

At some point it was determined that the period or axis of Ceres' rotation needed to be adjusted, and whomever made the determination decided that the most effective/safe/cost-effective/religiously-acceptable/your-reason-here way to do it was to bring in another rock, set it in a very precisely defined orbit, and then let the gravity of that moon gradually adjust Ceres to the desired spin.

Security Platform

The moon was brought in to act as an orbiting military/security base. This can be both/either to protect Ceres from outside forces or to defend certain factions on Ceres from others. It's a barracks, docking port, surveillance station, artillery platform, and the ultimate high ground for any action happening on Ceres' surface. Why not just put the forces on Ceres itself? Maybe there are some treaty or other legal restrictions preventing the operators of the moon base from doing so, or they just want the additional security of being on their own separate rock.

Separate Jurisdiction

Something isn't permitted on Ceres. Someone with more money and spite than political capital wants the thing anyway. They decided that the most expedient solution would be to just drag a rock into orbit around Ceres and declare it to be its own separate jurisdiction where the prohibited thing was, in fact, legal. Of course, they then imposed some perfectly-reasonable taxes/fees/what-have-yous on the thing so that they can recoup their investment.

It Seemed Like a Good Idea at the Time

Back in the hey-day of early interplanetary settlement however many years ago, there was a lot of enthusiasm for impressive-sounding projects with dubious revenue streams(*). Someone had the "brilliant" idea to put a moon in Ceres' orbit and drummed up enough support and resources to get it done. They then proceeded to lose their space-shirt when the theorized space-bucks failed to roll in. Fast-forward to the time of your game and, of course, the moon is still there. It's not like anyone is going to pay to put it back!

(*- this is definitely not a reference to the 2001 dot-com bubble. Not at all.)

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  • $\begingroup$ Rotational management - wouldn't that also have the net effect of pulling the moon closer and closer to Ceres? $\endgroup$
    – Justin Thyme the Second
    Dec 13, 2020 at 2:12
  • $\begingroup$ Not necessarily - the Moon is causing the Earth to gradually rotate slower and is moving away in the process. $\endgroup$
    – Salda007
    Dec 13, 2020 at 18:06
  • $\begingroup$ Okay, there are several things happening in that scenario, the Sun's pull being one of them. So, yes, not necessarily, not until all factors are considered can the net effect be determined. But I wonder, would not the effect of huge quantities of mass being removed from Ceres also have to be factored in? Perhaps the moon was intended to balance that out? Maybe. With a bit of handwaving, pull the moon in and out along the tether to counter-balance things. $\endgroup$
    – Justin Thyme the Second
    Dec 17, 2020 at 21:40
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It was the bootstrapper

Space is dangerous for squishy life like us; radiation, solar flares, micro-meteoroids, etc. A hollowed out asteroid is one possible avenue for "low-tech" long-term space habitation. And building a mining colony on Ceres is definitely going to be a long-term task.

When the decision was made to colonize Ceres, construction of a gigantic, well protected ship to send out and stay there for the years before Ceres is habitable was simply not possible/too expensive. Instead, they sent out an unmanned probe with a big engine to capture a smaller asteroid, push it into Earth orbit, and mine it out locally (with modern conveniences like "food", "water", "air", and Earth's magnetosphere).

After construction was completed, a small colony of workers board the "spacecraft", containing all the ingredients necessary for indefinite life in space. The asteroid is pushed out into solar orbit by a built-in engine, and after a few years of travel, is decelerated by the same and put into a stable orbit of Ceres.

Then, construction begins on the permanent home of Ceres, with shuttles being used to get down to the "surface" and back. The massive engine was designed to be disassembled for parts after they reached their destination, and now, decades later, the bootstrapper asteroid floats quietly in space, unable to move on its own.

In the mean time, of course, technology has advanced to the point that "low-tech" solutions to the many dangers of space are no longer necessary, allowing ships to come and go as they please with little notice of the "moon".

Depending on your intentions, this also has some flexibility - maybe the asteroid was abandoned as its tech aged and became unreliable, leaving a juicy derelict ruin for exploration/salvaging/etc. Maybe the asteroid has been maintained, and there is political tension between the "powerful" government of Ceres and the "weaker" government of the moon.

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  • $\begingroup$ A couple of really great ideas here, thanks :) $\endgroup$
    – Innovine
    Dec 4, 2020 at 9:08
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It's where the miners live

Humanity that has colonized the Asteroid Belt has naturally mastered O'Neill Cylinders, deep-space habitats which provide a mostly Earthlike environment, including spin gravity, and which require only materials that can be sourced from asteroids and 20th century construction technology. Higher technology allows the construction of larger and nicer habitats.

A problem with Ceres is that its gravity at .03g is much too low for long-term habitation without health problems, but high enough to interfere with the construction of rotating habitats providing good spin gravity. Instead of building on (or under) the surface, then, they put their habitat in orbit, where the conditions are more favorable.

While it's possible to build a habitat that's shielded from cosmic rays and debris impacts, the best shielding is really just an adequately thick layer of rock and ice. The colonists build an oblong blob of rock and ice (held in place by a semi-rigid scaffolding net) from locally sourced mining waste and leftover construction materials, which surrounds the habitat in which they live. This is also an excellent place to build warehouses and spaceports, as other answers have mentioned.

This combined structure could also serve as a counterweight for a space elevator, providing fuel-free transport between the surface and the habitat. Most of the colonists probably wouldn't use it very often, though. Only those colonists who are actually miners (as opposed to logistics staff, lawyers, spaceship pilots, doctors, children, etc.) and who can't work by telerobotics would have to actually set foot on Ceres.

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  • $\begingroup$ 0.03 g really wouldn't prevent you from building rotating habitats for gravity. It means a vertical axis cylinder would have floors that appear to slope by 1.7 degrees. $\endgroup$
    – Christopher James Huff
    Dec 4, 2020 at 22:04
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Frame challenge: There doesn’t have to be a reason.

On today’s Earth somebody can have a 4 ton boulder transported from China to the top of a 3km mountain in Austria for 50k€ without any reason or purpose: https://www.express.co.uk/news/weird/181516/Man-carries-four-ton-rock-up-a-mountain-and-calls-it-art

People have also built countless monuments for thousands of years without any purpose and for no reason at all.

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Mining

The rock was captured from the asteroid belt and moved to orbit in order to mine rare minerals from it. The mined ores are then processed on Ceres and later sent to appropiate places in the Solar system.

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  • $\begingroup$ Please feel free to take the idea and turn into a more elaborate answer. Sadly I don't have time to elaborate more, but I find this the simplest idea. I'm surprised noone mentioned it yet. $\endgroup$
    – Neinstein
    Dec 4, 2020 at 9:28
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Because it wants to be there

Once you've got mining and manufacturing in space, plus a whole lot of material already there, it probably becomes kind of trivial to change the orbit of an asteroid. If you're willing to wait a long time it should barely take any energy at all - just a little push here, a gravitational assist from another asteroid there - as long as you're not trying to do something crazy like take it out of the asteroid belt, you should be able to put pretty much anything wherever you want it. Because of this, there doesn't really need to be much of a reason - it's easy to do, so why not?

Others have suggested that the 'moon' is rich in some resource that's needed at Ceres, and I think that's probably the most realistic reason to do it. But here's another:

The 'moon' is actually a colony in its own right. Possibly based on mining or possibly something else - the important thing is that a bunch of people live there. Since they have the ability to move their home around at will at very little cost, it makes sense that they might decide to live near the bustling metropolis of Ceres. Perhaps there's a good economic reason, or perhaps they just want to enjoy the nightlife in the big city, or maybe they just had a vote and that's what they ended up with.

Or maybe they just want to get good ping times from servers hosted on Ceres. I can imagine this being a fairly compelling reason actually, since all communications are limited by the speed of light, and if you're not near another population centre any communication will have several minutes' delay. But the point is there are many possible reasons, and any reason will do.

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    $\begingroup$ Agree that interplanetary mining suggests an abundance of low-cost energy that should make moving an asteroid not a huge a problem. But worth noting that anything entering Ceres orbit will have a velocity of about 500m/s, which would require energy on the scale of large nuclear bombs to slow down enough to capture (for a km-scale rocky object). This might be "barely energy at all" in a relative sense in this setting, but perhaps not in an absolute sense. $\endgroup$
    – Nuclear Hoagie
    Dec 7, 2020 at 19:30
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If this is a one km. moon orbiting 940 km. Ceres, you have a problem explaining the attraction, the orbital speed, the orbital distance, the escape velocity, and such.

Seems to me the best solution you have is to make it a tether, with reaction mass drivers on it. This would for sure tidally lock the moon to Ceres, and Ceres would end up rotating on its axis at the same rotational speed as the moon is orbiting Ceres.

I would suggest a counter-weight moon tethered on the other side of Ceres, to prevent Ceres from wobbling all over the place, bringing the center of gravity back towards the center of Ceres.

In fact, the purpose could be to move a lop-sided center of gravity of Ceres more towards the center, to stabilize its rotation for gravitational purposes (build the habitat so that the gravity is OUT, not IN.) That is, tether a counter-weight to Ceres to balance its rotational spin, like they balance car wheels.

I would suggest they could also put the reaction mass drivers on this moon, with a very strong rigid (compressive) tether, so the reaction mass drivers would cause a rotational torque on Ceres, and cause it to spin, like a pinwheel. The length of the tether perpendicular to Ceres should give some mechanical advantage to the applied rotational force. However, it would again be very adviseable to put a counter-rotation moon and tether on the opposite side. Otherwise, I suspect the wobble would make Ceres uncomfortable to live on - like being on a planet with a continuously shaking movement.

Whiplash, anyone?

Maybe they could market it as the newest and best ultimate amusement ride on a gargantuan scale. Maybe spin the moon around the axis of the tether, as well, to give an even better discombobulating experience. Rein in the moon along the tether, then let it 'free-fly' out to its length, again and again, spinning all the way. For the penultimate adrenaline rush barf experience, before you die, on the last trip, you get ejected into space!!!! Game over!!!! Is that wild, or cool?

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    $\begingroup$ Ceres has enough gravity to orbit, so there's no problem "explaining" the attraction, escape velocity, etc. (In fact, I don't know why you'd need to "explain" escape velocity at all.) It certainly wouldn't make it "wobble". It would move the center of gravity of the whole system, but that wouldn't cause it to shake. $\endgroup$
    – HiddenWindshield
    Dec 5, 2020 at 16:42
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    $\begingroup$ I think you're confusing center of gravity with barycenter. I also think we're using two different definitions for "wobble". Seen from the outside, Ceres would appear to orbit the barycenter of the system. If the barycenter were beneath the surface of Ceres, then, yes, I think the resulting motion could be called wobble. But it certainly wouldn't wobble "all over the place", it would have a well-defined periodic motion, and someone standing on Ceres or the moon wouldn't feel a thing. $\endgroup$
    – HiddenWindshield
    Dec 6, 2020 at 17:06
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    $\begingroup$ Also, whether or not Ceres is a perfectly homogeneous sphere or not would have absolutely no effect beyond determining which specific orbits are possible and which aren't. $\endgroup$
    – HiddenWindshield
    Dec 6, 2020 at 17:08
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    $\begingroup$ As you correctly point out, two massive objects tethered together are functionally one object, rotating around its center of mass. Thus, neither object experiences gyroscopic precession, and the precession of the system as a whole is no greater than what Ceres already experiences. $\endgroup$
    – HiddenWindshield
    Dec 8, 2020 at 0:37
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    $\begingroup$ Wow, how condescending can one person be? Your AskAMathematician link was to a gyroscope on a stand; i.e. has a lot of forces on it that Ceres wouldn't have. Your HackADay link (which was broken btw, I had to Google the page) just says scientists don't know exactly why Earth wobbles, but it's most likely pressure fluctuations in the ocean or magma flowing in the interior -- both of which, I would note, are things that can change the center of mass, which changes the axis of rotation. But, hey, if you can find a physics textbook that supports you, I'd love to see it. $\endgroup$
    – HiddenWindshield
    Dec 13, 2020 at 3:40
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It's Vesta.

Dragged (as an act of independence and to a great disappointment of governments of the Earth) into the Ceres orbit in order to ease the production of something that needs ingredients from both places.

Some mass (as well as some people) lost in the process.

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Separate Jurisdiction

otherwise illegal services

As mentioned first here, to serve the Ceres population with something forbidden by the local laws. For example, in the real world we have boats providing abortions and gambling barely outside of the territorial waters of jurisdictions where they would be illegal.

a spying outpost

Too obvious? Well, the thing is there's no need to hide it at all. Here in the real world, some countries assert the right to operate their spy planes barely outside the territory of their targets, or over disputed areas. E.g. this.

a military outpost

In case things get hot, or to deter things from getting hot.

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Big Spacecraft Factory

A giant spacecraft is being built, and needs convenient access to the refined metals from Ceres, but not the gravity. So it's orbited at some distance, perhaps 100km above Ceres' surface, where the orbit is not so close that mass concentrations disturb the orbit, but it's still visible and convenient to fling raw materials to.

There were some earlier comments about slag heaps. I counter that Ceres is the BEST place to put slag, because humans need places with higher gravity. Mine out some spots to significant depth, then cover with slag from other operations. Slowly increase the whole mass of Ceres.

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To increase Cere's spin rate:

In order to provide artificial gravity within subterranean (subceresean?) habitats. A long cable is dropped from a passing asteroid and securely connected to Ceres, whipping it around in a long orbit that is slowly tightened to increase Cere's spin rate.

This is the opposite of the orbital energy process that occurred with Earth's moon and slowed Earth's rotation. As tidal forces acted on the moon energy was removed from earths rotation and added to the moons velocity, increasing the size of it's orbit.

Pulling on a tethered moon would reverse this, as the moon drops into a lower orbit the additional energy is transferred into the rotation of Ceres.

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    $\begingroup$ An apparent gravity force that pulls people outwards of Ceres would mean the same force applies on every piece of rock on Ceres. The whole asteroid would fall apart. $\endgroup$
    – Neinstein
    Dec 4, 2020 at 9:30
  • $\begingroup$ Are you talking about tidal forces? Those wouldn't be strong enough to pull apart an asteroid unless it was extremely close to Ceres, and the asteroid composition was loose enough, such as a coniferous asteroid $\endgroup$
    – SafeFastExpressive
    Dec 5, 2020 at 19:19
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    $\begingroup$ No. If I understand well, you want to turn Ceres into a rotating space station to create apparent gravity force. This force will point outwards of Ceres and will apply on everything, including stones and dust. If your people experience an outwards overall force, so will the stones. $\endgroup$
    – Neinstein
    Dec 6, 2020 at 8:22
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    $\begingroup$ Yup. With rotation to give 1 g at the surface, objects on the equator would actually be moving at about 4 times escape velocity. The tensile strength of the loose surface material is effectively zero, the ice mantle isn't much better, so Ceres would just break apart long before you got useful amounts of spin "gravity". $\endgroup$
    – Christopher James Huff
    Dec 6, 2020 at 16:46
  • $\begingroup$ @ChristopherJamesHuff! Thanks for destroying my world building idea! I got idea straight from The Expanse, and made the (bad?) assumption tensile strength of frozen Ceres could take it. Oh, well maybe if we seal Ceres in a high strength plastic coating? $\endgroup$
    – SafeFastExpressive
    Dec 7, 2020 at 16:27

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