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I've got a lunar colony that was set up to be self-sustaining before a collapse of life on Earth. The colony has survived on its own for several thousand years through a period of technological regression. But now they've recovered to roughly their original technology levels, and realize that they've been slowly losing something which will soon threaten their ongoing survival. What could a colony on the lunar surface slowly lose that would ultimately prove fatal?

Some notes on their technology:

  • The colony is a shielded crater about 12km across.
  • They have advanced, but not magical, 3D printing technology.
  • They have large solar collectors for power, but no fusion.

Some thinking so far:

  • The obvious answer is oxygen, but lunar regolith is 40% oxygen so that's easy to replace.
  • My understanding is that lighter elements are more likely to escape even tightly-sealed systems via atomic diffusion or simpler processes like opening and closing airlocks. So a slow loss of hydrogen (rare in lunar regolith) seems likely, especially if there's a mechanism via which it would become unbonded from water.
  • Humans need a lot of trace elements to survive (zinc, magnesium, iodine, etc). Is there some process via which a key one of these would be slowly lost, no matter how carefully things are recycled? (i.e. soil is tilled to recover, air is scrubbed, etc.)
  • I know certain substances exposed to space for long periods of time will degrade or chemically alter (space weathering), possibly in ways that wouldn't be easy to reverse. For instance, the flags planted on the moon are supposedly bleached of color because of ~50 years exposure to UV rays. What substances are most susceptible to this kind of degradation? I have a notion that some of these processes might cause, say, atoms to bind together into molecules that can't easily be separated back into constituent elements, but haven't found good sources with more detail. But if so, the parts of the colony exposed to vacuum/space (solar wind/flares, cosmic rays, micrometeor bombardment etc.) might be a weak point.
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    $\begingroup$ Key question, do they have spaceflight to the asteroids or planets other than Earth? $\endgroup$ – o.m. May 21 at 17:56
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    $\begingroup$ What is the population of this colony, and what has been the limiting factor preventing them from making many redundant colonies at some point in the past several thousand years? $\endgroup$ – Nosajimiki May 21 at 21:23
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    $\begingroup$ Mental health....... $\endgroup$ – Patrick Trentin May 22 at 4:38
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    $\begingroup$ Bones: 66.media.tumblr.com/tumblr_kvk3497Aw31qzbmsz.jpg $\endgroup$ – AJFaraday May 22 at 10:30
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    $\begingroup$ Not enough for a full answer, but Genetic Diversity is the first shortage that came to mind, but if they've been successful for "several thousand years", maybe they've overcome that. Maybe not. Maybe something cultural changes, causing mate selection to become a problem leading to poor diversity. $\endgroup$ – TecBrat May 22 at 19:15

20 Answers 20

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Immunity to Disease

Your lunar citizens live in a perfect environment! They have all the food they need, they have recycling that, frankly, removes most bacteria from their lives. Atmosphere is constantly recycled, which includes scrubbing. Water, too, is recycled, scrubbed and clean for use.

In fact, the colony has been disease-free for, well... forever.

And that is a massive risk. Disease has this nasty habit of hanging around. Stuck in some quiet, little corner of life or living until somebody moves the proverbial old stack of books in the attic out of the way and boom! you're sick.

And then everybody's sick

It isn't like everybody needs to start dying. Oh, they could. Diseases like polio, measles, etc., really aren't gone. We've simply enhanced human immunity through vaccination such that they can't get a foothold on the population anymore. If one of them peeked into the clean and controlled world of your lunar colony, it could wipe out the colony overnight.

But it really doesn't need to be that dramatic. Maybe it's just a newly mutated form of the flu. And suddenly people are having trouble getting out of bed, which means they're not maintaining and operating the closed, controlled environment. Repairs and adjustments aren't being made. And things begin to slip.

The funny thing is that doctors would be trying to fix the problem. At first it would be, "oh, this is an historical curiosity! When's the last time this happened? Here's two aspirin, you'll be all right!" Until one of them notices the problem. "Uh, Frank? You might want to take a look at this...." And then the medical community realizes the ugly truth, "uh, how are we going to stop this?"

Right up until Louise, who has an absolute genius for maintaining the ventilation systems, can't make it to work.

And then a fuse buried deep in the ventilation system blows....

And with his last breath, Louise's apprentice, who'd been buried in technical manuals and user guides for a week, says... "I think I fou...."


Edit: The premise of this answer is that there are people today who believe we wash our hands too much. In other words, we have created an environment today that is so sanitary that we are already losing our natural immunity to many bacteria. An hermetic environment like a lunar colony would have this problem in spades because the bacteria in used resources (you should be thinking, "ewww... sewer...") requires sanitizing — and every resource is "used" (you should be thinking, "ewww... recycled bread...").

It could be said that a lunar colony (or any other hermetic colony) should actually introduce bacteria to ensure the healthy strength of the colonists (you should be thinking, "ewww... you're making people sick!"), which could actually be a rather cool aspect of a story.

If you ever saw John Travolta in The Boy in the Plastic Bubble, based on the life of David Vetter, you'll understand the problem. In fact, go read about how David Vetter died. That's the problem.

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    $\begingroup$ “But there are no bacteria in Mars, and directly these invaders arrived, directly they drank and fed, our microscopic allies began to work their overthrow.” -H.G.Wells: The war of the worlds. $\endgroup$ – Joe Bloggs May 22 at 7:11
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    $\begingroup$ This weakness isn’t specific to a Moon Colony. $\endgroup$ – Michael May 22 at 11:18
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    $\begingroup$ @Michael, the OP didn't require that mandate, only that it could be experienced on the moon. I doubt any limitation could be 100% specific to our moon and nowhere else. $\endgroup$ – JBH May 22 at 14:36
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    $\begingroup$ "it could just be something like a mutated strain of flu" ... or a zombie outbreak! $\endgroup$ – Magic Octopus Urn May 22 at 17:32
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    $\begingroup$ This reminds me of what-if.xkcd.com/117 which talks about how much bacteria is likely still alive on the voyager probe. $\endgroup$ – Muuski May 22 at 23:17
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Materials exposed to vacuum for extended periods often become brittle and/or literally lose mass over time.

Outgassing, cold-welding, decomposition of alloys back to their constituent materials, coronal arcing due to ionization from exposure to ionizing radiation, acceleration of outgassing and decomposition again due to ionizing radiation exposure are all concerns both for metallic elements, barriers and bearings, and outgassing particularly for all plastic, polymer or latex components (seals etc) induce changes in surface interface chemistry and surface finish, and over sufficient time these would impact significant portions of the volume of a material.

Further, scratches and dings in vacuum-exposed surfaces can lead to rapidly-propagating fissures and cracks, due to thermal expansion / contraction.

Bearings will cold-weld, and even failing that, lubricants which survive well in vacuum-whetted conditions are both rare and exotic, and many of the ones which are found to work cannot take direct insolation as they will themselves offgas and change composition. Think airlocks, manipulator arms or waldoes, locking / docking clamps, wheels, rovers, et al.

All these issues can of course be dealt with individually, but en masse it means that absent new metallic components along all vacuum-exposed surfaces, they will be in serious trouble. I don't know if your thousands of years is even feasible... NASA has been studying these material science issues since the 1960's, as you can imagine, and they've a lot to say about it.

Two other issues are sublimation of solid materials as temperature climbs during insolation, and corrosion, which can be very strange and unexpected when surfaces are impacted by ATOX (see ESA paper below) - all of which support my basic contention:

Material thermal stress / embrittlement / volume & mass loss / corrosion on all vacuum exposed surface materials are the single greatest threat / risk to a longstanding Moon colony, and absent re-supply / repair would constitute a rapidly self-reinforcing series of chain-reactions leading to catastrophic enclosure failures.

1961 JPL Paper on Vacuum Exposed Materials
1969 NASA Paper on Vacuum Exposed Materials
European Space Agency Paper on Corrosion in Vacuum at LEO

Hope that helps some.

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    $\begingroup$ Burying the base mitigates many of these issues since the base will then no longer be exposed to the vacuum. You still need to replace airlocks and solar panels (and cabling) but that's less than having the whole based exposed to vacuum. $\endgroup$ – ShadoCat May 21 at 23:30
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    $\begingroup$ That's truly amazing that solid metal will sublime like that, to that extent, under vacuum. The phase diagrams definitely don't lie when they have the gas phase down to zero pressure - it's just that the solid phase is metastable, not stable as if under pressure. But the surprise part is that the "meta" is pretty meta. Or perhaps, another way to say it may be that 300-400 K is a lot hotter than you might think it is. Keep in mind that's 3 boilings to get to human temperature from the Universe's temperature floor. $\endgroup$ – The_Sympathizer May 22 at 2:39
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    $\begingroup$ @ShadoCat - you may be missing a simple point here: if the metallic outer surface of the base pressure hull is facing regolith, which is very micro edge sharp and at macro level quite porous, it IS vacuum-whetted - lower insolation, lower general incident radiation, but all the pressure and temperature gradient extremes still apply. Moreover, unless you’re drilling down to the equivalent of bedrock, you don’t get the benefit of thermal mass as you do on Earth with burying in the soil - with terrestrial soil, it’s partially the water content providing the immense thermal stability. $\endgroup$ – GerardFalla May 22 at 3:26
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    $\begingroup$ @aaronius, if you're into this, you also might want to check out a material phenomenon called 'brittle fracture', its mechanisms, and how to prevent it. It would be one of the more exciting failure modes to write about after all the stresses this answer demonstrates. $\endgroup$ – Sean Boddy May 22 at 4:59
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    $\begingroup$ @Gnudiff - indeed, that is useful data - but my essential point above still obtains - though greater depth (beneath the regolith per your comment) does definitely imply higher effective thermal mass and therefore a more favourable hysteresis (greater thermal stability over time) it doesn't alter the fact that the outer surface of the pressure hull would still be considered to be vacuum-whetted, and would therefore undergo a wide range of materials changes over the timescale proposed, due both to pressure and thermal effects; radiation exposure would be decreased, however. $\endgroup$ – GerardFalla May 22 at 18:03
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A non-obvious loss area would be Genetic diversity. After several thousand years of carefully controlled plant growth, and several thousand years of inbreeding, unless care was taken to maintain genetic diversity in plants and animals (including humans), a single mutated bacteria could take out a key component of the self sustaining ecosystem.

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  • $\begingroup$ This was actually my first thought, as it doesn't even need an external factor. Genetic diseases that are carried but not expressed would end up spreading to all the descendants, then with the recessive genes so common, more and more people would be born expressing whatever it is, leading to a smaller and smaller population. $\endgroup$ – Izkata May 22 at 18:04
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    $\begingroup$ This a great approach. Lack of genetic diversity and a single threat can be devastating. The banana industry has experienced it before, and currently is again. en.wikipedia.org/wiki/Panama_disease $\endgroup$ – Cody May 22 at 20:21
  • $\begingroup$ I would assume a higher exposure to space radiation would counteract that with higher mutation rates. $\endgroup$ – jcayzac May 23 at 9:21
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Volatiles like hydrogen, carbon, nitrogen, and noble gases. These elements are necessary for life and useful for industrial processes, while being difficult to replenish. Hydrogen, carbon, and even nitrogen can be used as rocket propellant which will consume these elements without any chance of recycling. They are also liable to loss through slow leakage. It is difficult to replenish these elements because they exist at parts per million concentrations in lunar regolith. This means many tons of regolith must be processed to obtain small quantities. However, we do know that permanently shadowed craters at the lunar poles at least contain water ice. It's quite possible they can contain other volatiles too. These volatiles are probably much easier to extract than the volatiles adsorped into lunar regolith. But it's quite possible that with 1000 years of continued exploitation that these deposits could run dry. It's been estimated that the Moon's north pole could have 600 million metric tons of water ice. While this seems like a lot of water, consider that we produced about 380 million metric tons of plastic in 2018. It's quite possible that with heavy extraction, these deposits could be depleted much faster than 1000 years.

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  • $\begingroup$ These represent strong opportunities for logistical mismanagement or accidents to spin a lunar society into dangerous chaos. Along side simple "Critical replacement parts" due to faults resulting in local manufacturing being unable to keep up: Unexpected failure in supply chains turns into social breakdown, which turns into unrecoverable failure due to chaos/rioting. $\endgroup$ – TheLuckless May 21 at 20:06
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    $\begingroup$ I was going to suggest Helium. Electronics, rocketry, welding, even oxygen cylinders are all broken or more difficult to get working without helium. $\endgroup$ – mcalex May 22 at 9:19
  • $\begingroup$ @mcalex What do they need helium for? Most of what you list merely needs a noble gas--and the moon has plenty of argon. Hydrolox engines need helium to purge the fuel lines on Earth but in space you can just use vacuum. That leaves only supercooling your fuel/oxidizer and I don't think that's an extinction-level problem. Your rockets still work, they just don't carry as much. $\endgroup$ – Loren Pechtel May 24 at 21:07
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The will to live

Survival on a lunar colony is a lot harder than here on planet earth. You are tired at the end-of-day, and the time to have and raise kids is just a little more than you can take. Population decreases until there is just not enough left for the colony to be viable/

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    $\begingroup$ Hello depression, so we meet again :P $\endgroup$ – Nuloen The Seeker May 21 at 21:48
  • $\begingroup$ I like the one... it's unexpected yet feeds the darkness of my soul $\endgroup$ – WernerCD May 22 at 13:18
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    $\begingroup$ @MagicOctopusUrn these are n-generation colonists, not a carefully vetted group of driven people. $\endgroup$ – Gary Walker May 22 at 17:50
  • $\begingroup$ @GaryWalker oof- I don't know how I missed the first paragraph. $\endgroup$ – Magic Octopus Urn May 22 at 18:04
  • $\begingroup$ Hippies to the rescue? $\endgroup$ – mootmoot May 23 at 15:04
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Light and thus Energy.

This probably will happen as a result of natural phenomena involving Earth's magnetic cycles.

The current dust storms on the moon - referred to as "Moon Dust Fountains" that occur as a result of electrostatic levitation of tiny particles of regolith (as a consequence of being stripped of electrons by the solar wind), are due to get much much worse:

NASA scientists have suggested that Earth's magneto-tail might cause "dust storms" on the Moon by creating a potential difference between the day side and the night side.

  • The Earth's magnetic field (protecting the atmosphere, preventing it being stripped by the solar wind), is diminishing:

enter image description here

Attribution: Wikipedia 2019, CCASA License

Over the last two centuries the dipole strength has been decreasing at a rate of about 6.3% per century. At this rate of decrease, the field would be negligible in about 1600 years.

The direct consequence of this is that the strength of the wind of atmospheric gasses stripped from Earth (and thus projected at the moon during that phase of rotation) will greatly increase and create an even greater charge difference between the dark and light side of the moon (the side facing Earth). (To be fair, the charge will first diminish for several hundred years, then grow alarmingly)

This means that the abrasive dust (albeit ballistic dust that settles) will vastly increase periodically - every 1 Earth month - obscuring solar cells and vision, smothering everything on the surface. Everything will get scratched and worn away much faster - further - it will get carried away in more static-dust storms. There will be less solar energy to mend and replace worn items.

This phenomenon will reach a peak (if the secular variation continues on-trend) 1,600 years in our future. Enough of the Earth's atmosphere may remain to make the planet salvageable, but what about the moon colony?

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    $\begingroup$ So in 1600 years we won't have an atmosphere? Well that blows. $\endgroup$ – Magic Octopus Urn May 22 at 17:34
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    $\begingroup$ @MagicOctopusUrn Fortunately the magnetic fields will regenerate (we hope) before we turn into Mars Mk.II of the solar system, the loss is fast enough to affect the moon's static charge but it should take hundreds of thousands of years (possibly millions) in that state to totally lose the air, which won't happen. Mind you, that depends on the nature of the OP's apocalypse I suppose. $\endgroup$ – Chickens are not cows May 22 at 17:40
  • $\begingroup$ Regenerate...? So it's losing it at a rate of 6.3% but also regenerating it at a rate nearly equivalent to that amount? How's it do that?! Can you link me like 2-5 wikipedia readings for that- whatever is easiest for you? $\endgroup$ – Magic Octopus Urn May 22 at 17:43
  • $\begingroup$ @MagicOctopusUrn More like a bounce than continuous regeneration.: Magnetosphere is the only ref I didn't already include in the answer. The most comprehensive is: Earth's magnetic field. $\endgroup$ – Chickens are not cows May 22 at 17:50
  • $\begingroup$ I completely overlooked the time-dependency part of that wiki page hah-- kind of an odd way to phrase it until I thought about it more. Thanks again! $\endgroup$ – Magic Octopus Urn May 22 at 18:03
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Computer chips.

Computer chips are an interesting case. You can pack millions of high-end microchips into a single 1-ton storage container.

Conceivably you could store enough to provide spares and replacements for thousands of years of maintenance in a fairly small space.

Yet to actually produce more takes a huge industrial base. Chip fabs are vast collections of some of the most advanced technology on the planet with a single Fab costing north of $10 billion to build, employing many thousands of people and requiring extremes of purity of materials all requiring a vast external industrial base to produce all the rare materials, chemicals and solvent with high enough quality to be suitable.

Batteries

The problem with solar collectors on the moon... is that night on the moon lasts 2 weeks.

Sure, you get 2 weeks of solid sunlight but then you need to keep everything running for the next 2 weeks and for that you need to store enough power to run everything for 2 weeks without recharge.

As such you'd need huge energy storage systems. Batteries degrade over time and need to be replaced. If you can manufacture suitable batteries you still need to keep all the equipment for manufacturing the batteries working long term.

The nuclear material in an RTG

enter image description here

RTG's are incredibly simple and reliable sources of power, good for hundreds or thousands of years depending on the fuel with minimal maintenance needed.

Solar power is great during the 2 weeks of daylight on the moon but then you have the 2 weeks of long-night.

Perhaps batteries and other energy storage systems were considered non-optimal when building the base but the systems needed to survive the 2 weeks of cold and to keep the oxygen recycling are run off of a set of RTG's similar to the ones proposed by NASA.

Plutonium-238 has a half-life of 87.7 years which is a bit short... but Americium-241 can be used as a fuel and has a half-life of 432 years.

So a couple thousand years ago the facility was built with a set of Americium-241 RTG generators that could provide plentiful power throughout the long dark night on the moon... but every 432 years half the Americium decayed away.

Now the generators have about 1/20th the Americium they started with, getting worse with each long night and the facility can just barely keep everyone warm and oxygen flowing and building enough energy storage to provide for the whole colony throughout the night is no minor feat...

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    $\begingroup$ This is, imho, the best answer. A non-magical 3D printer can’t do microcircuitry, and the idea of a silicon chip manufacturing plant on the moon is absurd. $\endgroup$ – Daniel B May 22 at 22:11
  • $\begingroup$ Chips are a good one, but energy storage I think less so; there are many lower tech alternatives to batteries; thermal ones like molten salts are being explored right now, kinetic systems, gravity based - maybe pressure? I think energy storage would be pretty solvable given the necessity. $\endgroup$ – Whelkaholism May 24 at 9:05
  • $\begingroup$ @Whelkaholism for molten salt you need a really large quantity of salt. Even regular plants that only need to last overnight on earth need tens of thousands of tons. You need to run things for 336 hours vs 8 and shipping hundreds of thousands of tons of salt to the moon may not have been practical. Gravity based stuff works far worse in low gravity. kinetic systems like flywheels have very poor energy density and are great for sudden demand but terrible for weeks worth of supply. Though your comment made me think of an alternative option for something running out... $\endgroup$ – Murphy May 24 at 11:00
  • $\begingroup$ You can use that energy to pump something up, like water, and when night comes, you let the thing fall moving a turbine. No need for batteries. And there is water there. $\endgroup$ – Geronimo May 24 at 23:13
  • $\begingroup$ @Geronimo You can. But any such reservoir would need to be vastly larger than the equivalent on earth given the lower gravity. You also now need to heat and/or insulate your massive reservoirs to keep them liquid, making the system even less efficient. $\endgroup$ – Murphy May 28 at 10:58
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Something we didn't know we needed, because we always had it.

The people who built this colony were not fools. They would know plants need boron and humans need sodium. There would be provisions to tap lunar water and generate building materials and oxygen from regolith. Carbon is precious and would be carefully conserved. There will not be holes in the recycling scheme.

Likewise genetic diversity. The builders know about inbreeding. They know that charged particles hitting the moon can cause mutations. There would be giant sperm and egg banks for diversity and molecular genetic remediations.

But what about something humans have always had and so never missed. Something that keeps humans going, but is slowly dying. And when it is gone, we will slowly die as well. I mean Gaia - the Earth.

Maybe the gestalt of all that is alive on Earth is tied to life in some way - something that has been there so long that there is no name for it. And Gaia is not going to go easy. After the humans escape the Earth there is still life; the deep things, the tenacious things. Roaches and mites, the bacteria, the fungi. And maybe this power is great enough to reach the moon - so the humans there and their dogs and corn are sustained even though they do not know that they are.

But if there is a tipping point, and when after thousands of years the last feeble weed finally dies, whatever this power is goes with it.

It would be a sad story.

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    $\begingroup$ The closest answer on this page to saying everything, +1. What wouldn't a self-sustaining lunar colony slowly lose that would ultimately prove fatal? Nothing. Entropy is coming. $\endgroup$ – Mazura May 22 at 2:44
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Electronics

Electronics manufacture require a vast and complex industrial base. There is no way the Moon Base has what it takes.

They have a large store of spare parts, and things don't break all that often, but there is a limit.

Some things can be replaced in non-electronic ways. Solar panels is probably the hardest to replace.

It might be possible to 3D-print some electronics. These are going to be very bad compared to todays electronics. Think really old computers filling buildings and doing very little.

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    $\begingroup$ Agreed. On the whole Earth we only have a few factories ("fabs") that make CPUs. In a 12 km crater you couldn't even fit the industrial base for that. $\endgroup$ – Daniel Darabos May 22 at 9:21
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It's not about what gets out, but what gets in. To survive that long, mankind will make many many trips out into the Lunar landscape to gather and process lunar regolith so that they can replace lost air and water supplies. However, regolith is extremely hazardous to human life. It's highly abrasive, nearly impossible to filter, and it builds up in the lungs causing Silicosis. Each time a lunar rover goes out to get more of the stuff, a little bit more of this fine dust comes in on the vehicle's chassis. Slowly it's concentrations build up in the air of the colony increasing the cases of respiratory ailments and early death until eventually, it kills so fast that no one can survive long enough to procreate.

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  • $\begingroup$ Air filtration systems will deal with dust in the air easily enough. This isn't a hard problem to solve. $\endgroup$ – Gryphon May 22 at 16:48
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    $\begingroup$ My understanding is that lunar regolith is not your normal Earth dust. It is small enough to to pass through most filters, and it's razor-like edges quickly destroy any known filtration system. Perhaps a more advanced civilization could find a way to control it when the colony is built, but as time goes on and Earth made super filters have to be replaced with what the lunar colony can scrape together, I doubt they will be able to keep it under control. $\endgroup$ – Nosajimiki May 22 at 18:25
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    $\begingroup$ The problem with regolith is that it's dry. You can clean it up with a wet paper towel, pull it out of the air with a wet-scrubbing system, and so on. The reason that it was such a problem for the Apollo missions is that mass was at such a premium, they couldn't afford the water for cleaning things up. An established colony wouldn't have this limitation -- even a simple shower in the airlock that recirculates its water would eliminate the regolith problem. $\endgroup$ – Mark May 22 at 22:04
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    $\begingroup$ This problem is easily resolved by ensuring nothing that operates outside ever comes inside. Your suit stays outside and opens into the inside. Your vehicle is in a garage outside, etc etc. This also cuts down on the amount of air lost to airlocks. $\endgroup$ – Muuski May 22 at 23:11
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Phosphorus

Always a good candidate when you need an elemental bottleneck to life, phosphorus is needed in minimal quantities, but by pretty much all known life. A closed ecosystem will recycle it pretty well, as long as there is no population growth, so the lunar colony could have been fine with small reserves to compensate the inevitable losses that probably still slowly occur. But now, even without population growth (due to it being a closed environment) they realize the reserves are pretty much gone.

Now, there is phosphorus on the Moon. Probably not much on any given square kilometer, but the Moon is big. And that's where things can get interesting: we may very well be running out of it on Earth, in as little as half a century, according to some - population growth means more fertilizer needed, because of the bigger human (and support) biomass. As of now, we are mining the stuff en masse, and looking for new deposits as the old ones run out. With population growth slowing down and mining techniques getting more sophisticated, maybe we'll be fine. Or maybe not.

If there is not enough phosphorus on Earth, then we'll have to get it where it is. And the nearest source is the Moon: as diffuse as it is, it may at some point become economical to process lunar regolith in bulk for it and varied other elements. Forget the Helium-3 baloney, this is why we could have lunar harvesters.

The problem for the lunar colony is that all nearby sources of phosphorus have long been mined, both for its own reserves and for Earth. So they will have to find new sources.

A short term solution may be to, ah, decrease the colony's biomass in order to recycle its phosphorus. Hey, now we know what Soylent Green is made of...

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  • $\begingroup$ But how can phosphorus deplete if the population is kept constant? $\endgroup$ – Tomáš Zato May 22 at 16:02
  • $\begingroup$ @TomášZato The recycling system is probably not quite perfect, and tiny losses here and there accumulate. Maybe some algae or microorganisms escape in almost negligible amounts, taking phosphorus with them as they die, too widely scattered to be re-harvestable. Maybe some gas containing phosphorus, part of some industrial process, occasionally escapes. And things may have been worse during the technological regression, so the reserve that should have lasted half a million years is now nearly gone. $\endgroup$ – Eth May 22 at 16:11
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Genetic Diversity

With the relatively small breeding population, over tens or hundreds of generations minor genetic defects could be interbred into major genetic defects. It's up to the author to determine what kinds of defects are being introduced. Based on those choices, the colony may be in danger of dying out from a single bad virus or defect induced infertility that forces birthrates too low to sustain the colony.

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There are many things and I would like to list and explain everyone of them (as far as my knowledge goes)

  1. Bone density loss.

In space astronauts have been found to lose bone density despite intense workout sessions.

Your people on lunar colony have been inside a dome for 1000s of years where gravity is 1/6th of what their bones have evolved for. Hence the bone density loss will soon make their bones too brittle and render their bodies useless.

  1. Genetic diversity loss.

The reason humans have survived is genetic diversity. In the dome you built the people will obviously inbreed for 1000 of years.

Assuming average age of 60 years and age at which people make offsprings to be around 33 there will be around 30 generations from inbreeding.

Effectively finishing their genetic diversity which means a single bacteria is enough to wipe out your colony.

  1. Trace metals loss (Zn, Mg,etc)

No matter how good you are recycling you always lose some of things (to be recycled) in the process. Trace metals are useful in our day to day uses and survival.

Their loss can't be compensated until your colony finds a vast reserve of these metals on moon.

  1. Hydrogen and nobel gases loss.

The abundance of these gases on earth makes us too ignorant of their use.

Nobel gases are extensively used in medical fields like Radon for treatment of cancer, and arc welding uses helium,etc.

Hydrogen is maybe the cleanest source of energy after solar energy systems you have installed in the colony.

In case of any future expedition they will need hydrogen in bulk, the loss of hydrogen means they are confined to the colony forever. (Unless they use solar sails which is pretty much impossible as they have same technology as we have today).

All points I could think of.

Edit- I would like to thank everybody in comment who has corrected me. :)

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    $\begingroup$ The gravity on Earth's moon is low, but at about 1/6 of that on Earth, I wouldn't call it "microgravity". $\endgroup$ – a CVn May 22 at 7:22
  • $\begingroup$ @aCVn I get your point thanks for suggestion. :) I have made necessary changes. $\endgroup$ – Berserker May 22 at 9:17
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    $\begingroup$ Your calculation of generation time is incorrect. One generation is the time from birth to procreation, not to death. Therefore there are 40 generations per 1000 years assuming average age of reproduction is 25, and 30 generations if it's 33. $\endgroup$ – dissemin8or May 22 at 14:25
  • $\begingroup$ @dissemin8or Thanks for correcting me I will make changes as soon as possible. :) $\endgroup$ – Berserker May 22 at 15:19
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    $\begingroup$ Excellent point = long-term consequences due to reduction of gravity compared to the development environment of Earth. How about "childbirth becomes riskier due to mother's skeletons being weaker" leading to more caesarian births which are medically more complex, leading to a birth rate below the maintenance level and a gradual decrease in population. $\endgroup$ – Criggie May 23 at 0:42
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Information.

All this time they still used the original networked computer system. The centralized datacenter had massive redundancy, but not quite infinite self-repair. During the technological regression, the redundancies failed one by one. Even at recovery, no one wanted to learn the complex system, and there was no case for building another.

I had been up forever, with not one user-facing glitch.

One fine day, that last backup failed. The system went down, permanently.

They still have computers, and are quick to crudely network them back together. But they lost their archive of information. A lot of files were recovered from the laptops, and a lot of information still existed in peoples heads, but the bulk of it was lost.

Most crucially, the manuals for the life support systems. They cannot maintain them for long without them, even at the same tech level, any more than a smart engineer could maintain the International Space Station without any reference material at all.

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  • $\begingroup$ That doesn't sound like a slow leak so much as an explosive decompression. $\endgroup$ – Muuski May 22 at 23:02
  • $\begingroup$ @Muuski The life support systems are still working perfectly for now, we just lost most of the capacity to maintain them. Slow degradation is a very plausible outcome. $\endgroup$ – Emilio M Bumachar May 23 at 13:20
  • $\begingroup$ I meant that the OP was asking for something the colonists could lose slowly (leak) whereas a server shutting down is sudden (explosive). $\endgroup$ – Muuski May 23 at 16:06
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Prior answers have focused on the genetic diversity of humans. We have more bacteria in our guts than cells in our bodies. There are many bacteria on our skin. Part of our immunity and well being is due to the interaction of our bodies with those bacteria. Hunter-gatherer societies have far more diversity of bacteria. Losing genetic diversity in those bacteria will cause strange problems.

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Social unity and pacifity (not sure if the latter is a real world)

Basically, such a small colony relies heavily on any conflict being within society, and between two groups that are willing to discuss it out or seek peaceful options only.

If you split into two societies, or have any other conflict (between more than a couple individuals) whhich escalates to violence on both sides, your colony is doomed if the conflict/split can't be resolved ASAP.

For example, a colony might start under martial law, and then only slowly transform away from it. This could leave some privileged and unprivileged groups (military descendants vs. civilian-descendants), which so far managed to talk out their differences, but not some worker teen found some anarchists scripture in the archives and motivates other people to escalate the conflict. Maybe after some unifying figurehead dies, or just over time.

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The bane of self-replication: Imperfect copying

How is this facility being maintained? By the factories 3D printing new components. Okay, so far so good. How are the factories being maintained? The factories 3D print new factory components. In particular, how are the 3D printers maintained? Replacement printers are themselves 3D printed.

The specifications on these things will not be 100% the same. This means that the capabilities of the factory will drift over time, as the collection of machinery is constantly cycled through new "generations". Effectively, you have a population that mutates over time. The 10th generation of "descendants" from the original manufactories were probably basically the same. The 1000th generation, imperfect copying repeated over and over and over, is a hunk of junk in comaprison.

This wasn't a big problem before, when the people running the show knew what they were doing. They would inspect the new equipment and run it through a body of tests. If it deviates too much, the new unit is scrapped. If a bad unit somehow gets through the body of tests, they'd remove it soon after installation.

But once the collapse happened, they stopped really understanding this. They got lax. Let's suppose that much of the maintenance documentation and expertise was lost in the Great Whatever. The survivors wouldn't be able to tell that subtle problems were creeping up on them. They said 'eh, this seems to be working fine' and the level of quality gradually degraded over time. Such a long time it tooks thousands of years for anyone to notice.

When they look, they find that (for instance) the expected shelf lives of a new (say) door jamb or air filter has decayed to such a large degree that components that used to last for 10 years now have to be replaced twice a month. It just got worse so slowly nobody ever caught on.

After they notice things have gotten so bad the facility is nearly constantly in danger of catastrophic decompression (and the like) it then takes them awhile to figure out what the problem really is.

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  • $\begingroup$ Maybe the equipment is BETTER: There is selection pressure to discard defective lineages and to give resources to effective lineages. $\endgroup$ – Geronimo May 24 at 23:16
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Genetic Integrity

Earth has the magnetosphere to keep harmful solar radiation out but the moon doesn't. This means the luna colony needs radiation protection but even with such protection, everyone if going to receive increased doses of radiation than on Earth. Every time someone goes outside, they'll receive more radiation.

All this radiation will eventually lead to genetic degradation which can accumulate in the colonists offspring leading to increased genetic diseases.

Bone Density

The reduced gravity leads to calcium loss and weaker bones and finally osteoporosis. It's would be hard to stop people from breaking bones regularly.

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  • $\begingroup$ Over several thousand years, the human genome would adapt to a low gravity environment to avoid the problems with bone density (And the attendant mineral storage issue). $\endgroup$ – JohnP May 23 at 17:59
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Their minds.

We've spent tens of thousands of years evolving to expect the sun to rise once every 24 hours, providing electromagnetic radiation filtered through our nitrogen/oxygen atmosphere, to hear the birds chirping in the morning, to smell the wind, climb the trees, and wade in the sea.

There are so many thousands of little things that we don't notice at all on Earth, simply because they're so fundamental, and seem so irrelevant. Once we've measured all the needs our bodies have, and provided that to the lunar colony, we might consider ourselves done, and not realise we missed something important.

Until the murders start.

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The Earth has a frequency of about 7.8Hz. The humans and animals are kind-of calibrated to this. On the Moon you will have different frequency, and this might have either on long-long term or not that long term serious effects, e.g. in physiology or mood.

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  • $\begingroup$ Why on Earth a -1? Some reasoning, please. $\endgroup$ – user9 May 24 at 12:13
  • $\begingroup$ "The Earth has a frequency of about 7.8Hz" is nonsense. The frequency of Earth's rotation is around 11.6µHz. The frequency of the tide is around 23.2µHz. The frequency of Mt St. Helens erupting is about 2nHz. All of these things are on Earth. $\endgroup$ – Adam Barnes May 24 at 16:46
  • $\begingroup$ @AdamBarnes Just for clarity: I was NEVER talking about rotation. The 7.8Hz or more precisely the 7.83HZ is the Schumann resonance frequency of Earth. I thought it is obvious in this forum, duh. Just check: en.wikipedia.org/wiki/Schumann_resonances, or nasa.gov/mission_pages/sunearth/news/gallery/… Anyhow, many thanks for the feed-back, and hope this clarification makes sense to you $\endgroup$ – user9 May 28 at 7:17

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