Something I've pondered in a work of mine, in which a corrupt authority figure threatens the protagonists with being sentenced to forced labour "chiseling ice off of (insert name here) comet." Assuming they really are masses of ice as scientists now believe, could a comet be large and stable enough to hold a prison colony?
Sending forced-laborers to a comet-colony might be more trouble than its worth, since it'd be easier to just send autonomous probes like Kuck's Mosquito, but if the specific aim of the corrupt ruler is to punish people then i think he might ignore the practicalities. And i think with enough workers in the same spot, he might just break even for costs and expenses.
Living on the surface of, or even inside an asteroid is a thing that people have been seriously considering for quite some time, since asteroids are very resource-rich and often large enough to support structures on them.
Since comets aren't 100% ice anyways, i'd imagine it wouldn't be much more difficult to build on, or inside of them, compared to an asteroid.
It doesn't have to be flashy of course, especially for a penal-colony, but it's a neat idea.
Could a comet be large and stable enough to hold a prison colony?
Comets can get quite large. Comet Bernardinelli-Bernstein for example, which is larger than the state of Rhode Island. Though their gravities are meager milligees. A prisoner could achieve escape velocity by jumping.
As for stability. When comets swing through perihelion, at closest approach to the Sun, things get a little messy as surface volatiles (water, carbon dioxide, methane) and dust get blown off. But comets usually have highly eccentric orbits and spend decades or centuries out in the cold fringes of deep space where nothing happens.
If you've got competent specialists overseeing the operation, hacking off, transporting, and processing chunks of ice isn't inherently dangerous to the comet's stability.
Same goes for building a station near or inside the comet, with competent engineers. Building inside the comet may be preferable because it offers extra protection from cosmic rays and general space radiation. And there's no prison like one trapped under a mile of ice.
Depends on the style of prison
- You could quite reasonably have a setting where comets (or asteroids) are mined for water ice and other elements. Say the lunar water turns out difficult to exploit, then it may be more economical to get water from a near-earth object than from the surface of the Earth.
- Fuel and time to transport people will be a major obstacle in any realistic, near-future setting. If the choice is sending 100 highly-paid workers, or 100 unpaid prisoners plus 10 highly-paid guards, the cost of sending extra guards will surely outbalance any savings from unpaid labor. Moving the prisoners will be a significant drain on the public purse.
(But that is the case in many Western societies. Even if prisoners are made to work, the prison system runs a net deficit as far as money goes. Privatized prisons may profit still yield a profit, but not just from exploiting prison labor. And society may or may not benefit from incarcerating rule-breakers.)
- The prisoners will have to work in a high-tech environment. Any one of them is likely able to kill dozens by just not being careful, let alone actual malice. A chain gang is right out.
To get even close to the realm of plausibility, there needs to be an already-established prison on a celestial body. Comets are terrible for this, because they have very eccentric orbits and spend decades/centuries away from earth, and would be exponentially more expensive to supply and maintain.
You should aim to have your prison on either a moon or asteroid. I suggest Ceres. It's an asteroid so big we call it a dwarf planet, the only one inside of Neptune's orbit. It's big- about the size of Texas, comprising 40% of the entire asteroid belt's mass. It is rich in water (ice) and oxygen, which that make it easier/cheaper to maintain a long-term presence. The low escape velocity makes it easy to launch (or throw) cargo ships of mined minerals to Mars or Earth to slightly offset the ENORMOUS cost of maintaining a life-sustaining ecosystem on a rock in outer space.
Comets are a poor choice for long term settlement
The idea of if a comet can be economically mined is simply a matter of available technology. For a civilization where adequately advanced propulsion exists, to make the journey relatively quick and affordable, then the answer is of course yes. But the bigger question is if one would make a permanent penial camp on one. The thing about comets is that they experience everything from the cryogenically cold temperatures of the Oord cloud to the sizzling radiation and solar winds of passing close to the sun. Plus they are extremely unstable. A bunker you bury 50m under the surface of the comet one year, may the next year be exposed and fly off into space or be crushed by glacial forces of the shifting ice. This means that no matter what other assumptions you make about the civilizations tech level, that you automatically know that the life support requirements for this facility will be much more complex than any facility you could construct on an astral body that follows a less elliptical orbit.
... but that does not rule out mining comets completely
That said, there is nothing stopping comet mining from being a perfectly valid punishment for prisoners, as long as you rule out the part where it is a permanent prison facility. One of the problems with getting water in space is that nearly all of it is either at the bottom of a powerful gravity well like Earth, it is just a trace molecule that you can only get by melting the stone that contains it like on Luna or it is frozen as hard as rock like on Europa making it very difficult to mine in a low gravity environment. Comets present a unique opportunity because they are not solid balls of ice, but rather more like snowballs. This should make actually gathering the ice much easier, especially if you time your mission just right for when the ice begins to melt, but before it begins to violently boil off.
So, you could have prison ships that go to a comet perform the comparatively dangerous but economically more efficient task of mining it, and then come back.
As for "chiseling ice off of Biela’s Comet.", this phrase could still hold, even if the average person has never heard of Biela’s Comet. It makes since for a Judge or prosecutor to know exactly what comet will the next penal mission will be to, because it is a normal part of sentencing, and the prisoner does not need to know what Biela’s Comet is to know that he is being threatened with a comet mining mission.
Can unskilled labor compete with robotic automation and professional mining crews?
Technically no, but they don't have to. One of the biggest assumptions about penial colonies is that you can only work with people with negligible practical skills, but modern prison labor is more and more often being paired with rehabilitation and education programs. Many Western European prison systems show that if you teach a prisoner a valuable skill, that they are more likely to take pride in thier future and are less likely to re-offend once released.
So instead of just putting a pickaxe in these prisoner's hands on day 1, you force them to study what ever technical and mechanical skills they need during the several months long journey to the comet they are sent too. In this time, they either learn the skills they need to make it a successful mission, or they don't and they get themselves killed. So the threat of "chiseling ice off of (insert name here) comet.", more accurately means you are being threatened with 16 hour shifts in front of a computer screen watching tutorials and lectures cramming in the equivalent of an entire technical college's education that your life literally depends on. And once you get there, you are still faced with around the clock geology surveys, maintenance tasks, rationing, etc... all while sitting in an barge built by lowest bidder government contracts floating near the tail of comet that could at any point be ripped through by an un-detected chunk of ice.
The barge itself could use an auto navigation system that will only activate for a return mission once the water tanks are full or the mission takes so long that the risk of losing the ship outweighs the benefit of topping off. Any crew that fails to educate itself will typically fail to top off and face being resentenced for another mission. Those who educate themselves well can top off before the comet starts to come apart reducing the chance of dieing during the mission, and then re-enter society with a valuable education that could be applied to a wide range of less dangerous jobs that rely on the same general skill set.
In other words, it does not need to be hard labor to still be a very meaningful threat.
Not precisely, but you don't have to be precise.
Comets aren't like asteroids in a lot of ways, but the biggest one is that they aren't a single, big chunk of rock. Even most asteroids are dusty conglomerates, but comets are like pop-rocks, waiting to heat up to the point that the melting ice allows them to release their gasses.
Imagine trying to build an underground bunker in a shelf of natural gas shale. How long do you think it could survive? More importantly, what would be the value of that mining colony when the comet itself is exhausted of material? With a comet, I can't imagine that it would be a good idea to be drilling into the substrate that your habitat is anchored in.
A better idea would be to build a prison ship that visits comets and cracks pieces off of them. Despite how the Expanse starts, you don't need a ship to actually haul spacebergs from one place to another, you just need a tug to get them going in the right direction, and patrols to make sure that opportunists aren't shaving their share off of them on the way.