The Setting

World consists of humans with primitive technology living within an asteroid space station built by their technologically advanced ancestors. A lot of knowledge has been lost, obviously.

The station has the interior square footage of Manhattan and is home to 300,000 humans. After generations of pilfering the station for materials, the station is mostly bare of technology, little more than a complex of ultra-durable habitats on the side of a rock with a bunch of iron age maniacs living inside. The space station consists mostly of large sun-lit greenhouse areas, originally parks but now used for cultivating food. If there was ever a source of artificial gravity, it no longer works, though thankfully these humans are genetically fortified against the delirious effects of microgravity.

Over time these humans have learned manage their air quality by managing the growth and decomposition of plants, avoiding combustion, and wicking away humidity with ceramic dew collectors*. The sun exposure of this rock can be adjusted post-hoc for whatever would make this scenario most plausible, but you can assume the base is built on Earth's moon as a default. The humans have access to the asteroid's rocky interior, where they might be mining ice (water, nitrogen, ammonia, etc.), tholin (space tar), metals, or whatever else might be useful in answering the question.

Re: plausibility comments. The station is built of futuristic "handwavium" as someone put it, so don't worry about that. These iron age humans started as a very small population in a massive greenhouse structure - managing their air quality would not be an issue until population increased. They would have time to adjust and learn as their situation became more difficult. Humans have learned how to thrive in some extremely hostile environments here on earth, living on ice sheets, deserts with single digit inches of rain per year, on floating rafts, and many other perilous places that require highly specialized survival skills. I have confidence that humans could learn to learn to live in what is essentially an enormous garden. I just need to figure out what that garden would have to look like to give them the best chance.

Question Setup

The air quality issue I've had the most difficulty understanding is temperature. My current understanding is that there is no need to actively heat a space station occupied by humans, because their body heat is sufficient. On the contrary, temperature control on a space station seems to be entirely about removing heat. On the ISS this is done by pumping liquid ammonia between the inside of the station and the exterior to radiate out heat.


How can my iron age human occupants manage the temperature of the station? An active solution, like bicycle-powered pumps feeding through the station's legacy radiators, is not preferred. Would it be possible for the station's original design to include a passive temperature regulation system in case of power failure? For example, could the station's air be channeled through long, finned tunnels of aluminum that created enough surface area with the (near) vacuum of space to cool the air? Perhaps the number of tunnels the air flows through is manually controlled to keep the air at the right temperature. And then what would create the airflow? If the uninsulated tunnels were colder than the rest of the station, could that create a natural passive airflow?


In addition to answering my question, I would appreciate references for further reading on this subject. More in-depth than the articles that come up on google. I don't really remember much physics, but I'm eager to learn if someone can tell me what area I should start looking at first.

*Note that the station does not need to be as dry as 21st century spacestations, because there are no longer any electronics.

  • $\begingroup$ What is the nature of the habitat and green houses? Are the greenhouses on the outside of an asteroid, and gravity provided by some magical means? Or are they on the inside of an asteroid with gravity provided by rotation (for example) and artificial lighting? Is there a day/night cycle? $\endgroup$ Feb 20 at 21:22
  • $\begingroup$ @StarfishPrime Thanks, forgot to include that info! Greenhouses are lit by the sun. There is no artificial gravity - everything is very floaty. $\endgroup$ Feb 20 at 21:34

4 Answers 4


Giant Space Fans

Heat exchange is partially a function of surface area. That's why heat sinks and radiators have so much surface area - the larger the surface, the more space there is for heat exchange. If you give the station a way to deploy giant space fans with a very large surface area, that will give the station a heat sink to dump the heat into space. (As long as the heat sink isn't under direct sunlight, because if it is, then it'll superheat.)

The most inefficient part of this process will be transferring heat to the heat sinks, but if you use very conductive metal frames to extend to the heat sinks, it will help channel the heat. In addition, the fans can be retracted to stop dumping heat into space if you need to retain heat.


Albedo control

One important concept here is that the overall station temperature is relatively unaffected by how the incoming solar radiation is used. Provided albedo remains constant and no material energy sources (eg fuel) enter or leave, the overall temperature of the station will remain the same whichever of the following is occurring inside:

  • There are no lifeforms or machines, it is a lifeless, inert rock
  • There are machines using solar cells to do manufacturing
  • There are plants growing and decomposing/being eaten without higher animals present
  • There are plants being grown by humans who are eating some of them and burning others to cook food

Unless pre-existing fuel stocks (chemical, radioactive or whatever) are used to increase the temperature, this space station has a relatively simple energy model:

  • Energy in = solar radiation absorbed by station
  • Energy out = radiation to space
  • (Energy in) - (Energy out) = increase (if positive) in temperature

Assume that the station was designed such that Energy in = Energy out as closely as possible and only fine tuning is required. With the technology available to the inhabitants, it would be very difficult to increase the amount of heat energy directly radiated to space. However, given some effort it would be possible to change the albedo of a portion of the "windowed" area to control how much of the sun's energy is absorbed (adding to the total heat budget of the station) or reflected back into space (not adding to the total heat budget).

Note that this strategy requires the "windowed" area to cover a quite wide arc of the station's surface or for the albedo-changing surfaces (see below) to be right up against the window, since this strategy relies on reflecting sunlight back through the window into space. Any sunlight reflected onto an interior wall of the space station becomes part of the "energy in".

Options to change albedo:

  1. Have many big slabs of metal mounted on pivots, with one side as reflective as possible (painted white at a minimum) and the other side painted black. To increase temperature, pivot to have more slabs with the black sides towards the window. To decrease temperature, pivot to have more slabs with the reflective / white side towards the window. This will allow for quick, highly responsive changes in albedo provided there are enough people available to pivot the slabs - which should be relatively easy to move in microgravity - but it has the disadvantage of reducing the space in which plants can be grown.
  2. Plant zones of high albedo plants. This requires a bit of botanical techno-magic from the ancestors of the current inhabitants, since the most successful plants are the ones that can absorb the most energy rather than reflect it. These plants would have been genetically engineered to be reflective and have some mechanism to discourage other plants from competing with them, yet one that does not permanently prevent the areas being used for growing other plants if the temperature needs to be increased. This method would be much slower to change albedo and consequently lower temperature but requires less human effort.

Two final notes:

  • There have been several efforts to build sealed, self-sustaining habitats on Earth. None have succeeded yet, even with today's technology to monitor and active efforts to balance environmental factors. Especially with the low technology available, the space station's habitable area will need to be huge in order to have any chance of becoming a self-contained ecosystem.
  • Unless the windows of the space station are made of handwavium, over time they will be degraded by micrometeor impacts - basically sandblasting in slow motion. Long before the windows are breached they will become less transparent. The direct temperature change will depend on whether this increases or decreases the station's albedo, but the key issue for the inhabitants is that the amount of usable sunlight getting through the windows to grow plants will decrease. So the inhabitants need to regain the technology to conduct major EVA's in order to fix the windows before this gets too bad. Hint: If you want a long period of primitive technology, don't put the station somewhere like a L4 or L5 point where junk and dust tend to accumulate and the windows will degrade faster.
  • $\begingroup$ First of all thank you, lots of very useful information here. As a field of barley is growing, it emits a very small amount of heat. The barley is harvested and threshed. The barley hay is then left to compost, which would produce a sudden increase in heat relative to what was produced before during growth. It seems to me that the temperature can fluctuate in a sealed system, because plants act as energy sinks that can dump their heat very fast. $\endgroup$ Feb 22 at 3:22
  • $\begingroup$ Also I think the biggest Biosphere project failed for strange social reasons, not technological. The construction of the space station - its durability, how airtight it is, how clear the glass stays - can all be future tech handwaving. $\endgroup$ Feb 22 at 3:25
  • 1
    $\begingroup$ @PinkSweetener yes, the local and temporary air temperature will vary, but only slightly in a sufficiently large system. If the system is so small that even having a bonfire is going to affect the overall atmospheric temperature then it is not a sustainable habitat for other reasons (such as being unable to sustain a sufficiently large genetic pool of humans to be a viable population). Which is fortunate, since they need quite hot fires just to maintain Iron Age tech. $\endgroup$ Feb 22 at 5:02

Water heating and cooling

This method involves using the asteroid's ice supply to create cooling liquid, water is the best material for absorbing heat energy, so a series of small streams flowing throughout the space station should take away all the heat energy, as well as redistributing the energy to colder areas. The water would end up flowing into a giant tank that is only seperated from space by a thin sheet of metal, the metal would conduct the heat energy into space, cooling the water and allowing it to repeat the cycle. This has actually been attempted before in the Spanish palace Alhumbra, more information here: https://omrania.com/inspiration/water-management-why-the-alhambra-palace-was-ahead-of-its-time/

  • $\begingroup$ I like this idea, but flowing how? There's no gravity! $\endgroup$ Feb 22 at 22:18

NOTE: There's a comment from the OP stating that there's no artificial gravity. You can ignore my answer — everybody's dead. I have a hard time believing an iron-age technology could grow food (much less solve technical problems) without gravity. Unless the station's fuselage is riddled with rings they can tie ropes to, there's nothing to keep them in place to work anywhere (it's all floating). But, just in case the OP recants...

Also, I'm working on the (obviously false) assumption that the vast majority of the station is open-space covered by a dome. They'll need every square inch to have the food and oxygen they'll need to survive. If, instead, the station is a rat's maze of corridors and rooms then they'll run out of food. Farming requires space — or better than an iron-age technology.

You're not quite right

The ISS does use liquid ammonia to wick away heat - but it's not just heat from the occupants. It's heat from the equipment, heat from the sun, heat from all kinds of things. And it's not helped by a design that better insulates than other craft historically.

Like the Apollo series craft, which had to have heaters.

That's good news for you, because you get to decide what kind of problem to solve. It's absolutely true that your station would have had environmental controls and it's not beyond reason that something that large and complex may have had both heaters and heat-removal systems for the same reasons buildings today have both heating and cooling systems.

And for the record, I like that complexity. Your iron-agers might not just need to vent heat, they might need to care later when something changes, like the station's orbit taking it further from a star, when suddenly they need every joule of heat they can get.

But let's focus on heat removal

When push comes to shove, all space-based systems rely on one passive component: large vanes not at all dissimilar to the heat sinks you'll find in any large computer system to radiate heat into the void. All kinds of active systems may improve the efficiency of moving the heat from the inside to the skin, but eventually you need to move the heat from the skin to the void... and that means having something to move the heat to.

That's an important issue depending on how "realistic" you're trying to be. Space isn't empty — but it's darn close — and heat isn't an object (like a photon or a particle of mass), it's actually a characteristic of an object. When you say you want to get rid of heat, that means you're trying to move energy from one object to another.

And you have to have another object to move it to.

To be fair, this might not be a problem you want your iron-agers to deal with

To consider a tangent for a moment, I'm not convinced that an area the size of Manhattan could absorb enough CO2 and generate enough oxygen to accommodate 300,000 people. If we ignore the value of Earth's ocean in this regard, Earth has only about 12 million square miles of arable land (land that can be used efficiently for re-oxygenation). 12e6/8e9 = 0.0015 square miles for each individual or 450 square miles and Manhattan is only about 23 square miles. No matter what the "we have too much population!" people want you to believe, the Earth can cater to a much larger population than we have today, which would reduce that ratio. But if we assume Earth's at its practical limit, your population needs almost 300% more land if plants alone are used to solve the problem.

But it's more likely that the space station has a very active ventilation system that scrubs CO2 and re-oxygenates. You need that before you need heat removal and from an engineering point of view, it's more likely they're integrated than not. Which means if heat removal goes down, the ability to breathe went first.

But that's boring, let's ignore it.

What can we do?

Iron-agers. The iron age ended around 550 B.C. (or 800 A.D. with the Vikings, it depends on who we use as the reference), so the tech is sometime before that. They can manipulate metal, which means they can work with parts they have. They could figure out bolts, but they couldn't figure out modern welding and they couldn't figure out modern alloys. They certainly couldn't figure out things like carbon fibers and couldn't understand aluminum, which wasn't discovered until 1825. No knowledge of electricity... precious little knowledge of hydraulics and pneumatics....

I'll be honest with you. At this point in my answer I'm more than tempted to switch this to a Frame Challenge that claims it's impossible for you to have iron-agers on a failing space station that survive. I'll try to trudge on, but finding a solution that an iron-ager could deal with....

  • Open a window. A tiny window. An iron-ager would think in terms of letting the heat out, not in terms of exchanging the heat. They'd die, probably quickly, but it's the first option. Punch a hole in the station. They'd have the tech to do that (maybe, but it's suspension-of-belief-able.)

  • Make it rain. Assuming the station has multiple ways or some redundancy to remove excess heat, another solution is to pump water to the ceiling and spray it around, causing rain, causing a drop in temperature. This has the effect of pulling the heat to the floor of the station, where hopefully there's some tech to move it further away.

  • Plant more plants. Very simplistically, plants are endothermic. They consume energy to grow and thrive. If we ignore the details of reality and strive for suspension-of-disbelief, then the more plants there are, the cooler things are because they're absorbing the heat.

  • Let the smoke out! If the station's cooling system still exists (even if it's not operable), our intrepid iron-agers may find a pipe or two that are cold to the touch, and in a manner similar to opening a window, think that the solution to the heat is to let the cold out of the pipes. Yeah, liquid ammonia, dead iron-agers... but they don't know that, and it would look like smoke as the fluid violently evaporated from the pipes.

  • Push more air through the ventilation system. Assuming that the heat exchange system is still (at least basically) working, then moving more air through the vents could (believably!) dump more heat. This means using hydraulics to drive gears that spin fans.

And that's all I have

An iron-age society would not be capable of fixing anything remotely complicated in terms of a space station. That's a pretty ruthless limitation. Personally, I'd avoid anything other than the most simple systems going down. Focus instead on things that don't depend on knowledge they don't have to resolve (like the replicators failing. Easy fix. Cannibalism.) or external problems that they can address with their tech like a small hole from a micro-meteor (pound a nail into it! Fold some oily cloth over the nail first!). Half the fun would be walking through the trial-and-error-before-they-die solution process.

OK, a bit of a Frame Challenge

Finally, remember one thing about your iron-age condition: it's actually really, really, really, really, really hard to lose knowledge. The station would have libraries (digital or otherwise), technical manuals, even class rooms for children, given its size. People would have all kinds of knowledge that would be more likely to be passed on than forgotten. The more specialized the knowledge, the more likely it would be lost. But iron-age level? It's more likely they'd regress back to the late 1800s and stop. There's certainly nothing stopping you, the worldbuilder, from simply declaring it to be so... but beware trying to explain it. It's a lot harder to explain realistically than you might think.

  • $\begingroup$ The BIOS-3 Biosphere project found that 8 square meters of chlorella algae was enough to recycle air for one person. Manhattan has enough algae growing space for 7.4 million people. Crops might be less efficient than algae, so I still have to do more research here, but I feel 7.4 million is enough wiggle room for me to at least assume 300K for now. Station is indeed composed of domed areas. Humans get around using large railing structures, like scaffolding that's spread out in the habitats. $\endgroup$ Feb 22 at 3:39
  • $\begingroup$ The space station was made by advanced people with future handwavey material science. Don't worry about it. There is of course a story to explain how knowledge was lost and it's wild that you thought to even get into that. I framed my question very specifically, you had a little nugget of (genuinely useful) information to share at the start, and then got on a tangent about a bunch of things that weren't on the table. Your answer is badly framed, not my question. $\endgroup$ Feb 22 at 3:44
  • $\begingroup$ @PinkSweetener How can my iron age human occupants manage the temperature of the station? Answer: They can't. I gave you a number of realistic solutions that an iron-age society could be expected to consider. If you think my answer is badly framed or not valuable, down vote it. Your question is OK, but not framed nearly as well as you think. Worse, this community has a long history of challenging the premises of querents. It's very common. You'd better get used to it. $\endgroup$
    – JBH
    Feb 22 at 4:04

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