Why is it so hard to build a closed life support system for humans? There are "ecosystems in a jar" even with fish that are stable for many years. It is clear to me that humans have much higher demands than fish, so it must be much more complicated. But it seems almost "unattainable". The MELiSSA project has been running for 30 years. It seemed to me that it was difficult to make all processes strictly controlled, but even more "natural" things like Biosphere 2 ended in failure.

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    $\begingroup$ Biosphere 2 could have been much better managed, but was not a total failure. humans lived in the sealed habitat for well over a year and a considerable amount was learnt. To build a working biosphere its probably best not to assume we know how to build one from the get go, but just keep adjusting and experimenting. $\endgroup$
    – Slarty
    Commented Jan 2 at 11:31
  • $\begingroup$ (The short story Destroyer Of Worlds by Charles Sheffield takes a sobering if fictional look at these issues.) $\endgroup$
    – gidds
    Commented Jan 2 at 23:27
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    $\begingroup$ You might want to read A City on Mars. They discuss many of the difficulties in creating space habitats. $\endgroup$
    – Barmar
    Commented Jan 3 at 15:21
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    $\begingroup$ It's very easy to build one of those "ecosystem in a jar" doodads, because if something goes wrong and all your fish die you just shrug and start over. $\endgroup$
    – Tacroy
    Commented Jan 4 at 19:57
  • $\begingroup$ Those ecosystems in jars rely on populations reproducing and dying to eventually find an equilibrium. If you had a big enough jar, you could have humans mindlessly reproducing and starving to death until they reach the right population number as well. But that's usually considered inhumane. $\endgroup$ Commented Jan 5 at 0:02

5 Answers 5


To answer another facet of your question that others have looked over, is the question of why it is so easy to do with much smaller "ecosystems in a jar". You would think a larger system with intelligent humans included would be easier to maintain ecosystem balance, which makes it all the more confusing why it seems to be harder. The reason being is that smaller systems have animals that have very short generation cycles, like fish and insects, whose population can rapidly grow and shrink to maintain homeostasis with the plants and fungi in the system, and very rarely include a significant number of mammals.

While these animals like insects do exist in sizeable quanties in the human attempts, the bulk of the "animal load" is in the humans, and we do not accept dynamic changes in the number of alive humans in the habitat to nearly the same extent, to put it lightly. And so all the balancing has to be done on the plant and fungi side, which has to be performed "manually" by the humans, so homeostasis is much harder to maintain.

To put it shortly, animals need to die to maintain balance, but when your main animals is humans you've got a problem.

  • $\begingroup$ This is very interesting and would answer my question. I'll wait to see if anyone else comments on this. $\endgroup$
    – Saturn V
    Commented Jan 3 at 18:05
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    $\begingroup$ Just oversizing the system relative to the humans by a large safety margin in both capacity and time would probably make things a lot easier. $\endgroup$
    – user4574
    Commented Jan 3 at 18:29
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    $\begingroup$ Yes exactly, but that's where the "hard" comes in, particularly the type of hard we call "expensive". The sizes required for a human ecosystem to have a large safety margin require far too much mass we can afford on a space station or volume on a colony, so the challenge Biosphere2 and others are facing is how can we reduce the size of the system to its absolute minimum. That is the challenge. $\endgroup$ Commented Jan 3 at 19:37

It is not so much 'hard' as it is an expensive area of research that is challenging to justify funding for. Biosphere 2 proved a number of things, some of them about humans but but most significantly that anything similar needs to be much larger and more expensive.

In terms of relevance to space there is also uncertainty about big complex systems full of life, since there is not actually a need for tightly closed loop, but rather highly efficient and reliable system with known losses (hence MELiSSA). Lack of clear need is also an issue.

  • For LEO operations the aim is minimal mass to orbit, but what does come up can be complicated (eg per ISS recycle air and water, bring up food).

  • For travel to Mars etc total system mass matters the most (along with reliability) meaning dried food and lower efficiency chemical water recycling may be sensible.

  • For the moon some raw materials are plentiful (Oxygen/Carbon) but light/power may be limited with the 2 week night. System mass is less important, especially for materials that can be extracted in situ.

  • For Mars many raw materials are available but light/power availability may be an issue. System reliability will matter, since anything coming from earth is years away.

Also relevant is astronaut labor. A fully closed loop system for 3 people is no good if it needs 6 full time tenders for the plants etc in it.

All of these mean that most near future design work assumes incremental improvement to technology already in use on the ISS which are largely chemical in nature, needing parts and consumables but having known behavior and reliability.

Most likely the life support for the first moonbase will look less like Biosphere 2 and more like a chemical refinery, using already known designs scaled as required.

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    $\begingroup$ Biosphere 2 got a lot of crap from people with ideological reasons to attack it and from people who didn't really get the concept. I remember back in 1992 it was treated like a big scandal that they were running O2 in from outside. It really wasn't -- I mean the point of the thing was to try to get a balanced system, and they knew air quality was the #1 risk, because science is like that. It didn't work at first and you can't just let everyone inside suffocate! They figured out what was going wrong, opened it back up, fixed some things, and re-sealed it, and it worked okay the second time. $\endgroup$ Commented Jan 2 at 15:54
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    $\begingroup$ It is worth noting that hydroponics/aeroponics is a rapidly advancing subfield of agricultural science. $\endgroup$ Commented Jan 2 at 21:32
  • $\begingroup$ It's true about the chemical refinery, somehow ECLSS already looks like this on the ISS. But if we want to produce food as well, we cannot do without plants. But I understand that it is not primarily about the most closed system, but about the lowest possible weight. $\endgroup$
    – Saturn V
    Commented Jan 3 at 14:45
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    $\begingroup$ @user4574 just plonking some life in a greenhouse is cheap, but if you want to instrument it to find out what is happening you can quickly get into the millions. Life is rough on sensors! Is still certainly possible and various projects of this scale are around. They just do not have the PR machine that Biosphere 2 had, and are more careful on their claims. $\endgroup$ Commented Jan 3 at 23:34
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    $\begingroup$ @SaturnV - "But if we want to produce food as well" -- I think that's the point of this answer; there's not really a demand signal for food production in space. 3 years of MREs for one person weighs in at ~1000kg, which is not that much compared to any feasible closed biosphere. $\endgroup$
    – codeMonkey
    Commented Jan 5 at 17:49

I suspect that the main problem is lack of knowledge concerning all of the possible feed back loops that might occur under different circumstances and all of the possible interactions and competitions between different species again under different conditions.

When considering life support systems the chemistry used is usually rudimentary such as scrubbing carbon dioxide, the Sabatier reaction, electrolysis and similar.

But the chemical complexity in a biosphere is more complex by many orders of magnitude. Biochemistry is very complex and predicting what effects what in what way is not always easy. Have a look at the link below to see what I mean, use your arrow keys to scroll around...



Answer: Because the biochemistry of life is really, REALLY complicated.

Spacecraft engineering, by comparison, is really easy. Engineers can design systems which are isolated from each other and are only allowed to interface in a controlled manner. Failure modes can be anticipated. Chaos is diligently avoided.

As an example, here is a portion of the first chart (in a series of charts) from Roche illustrating the biochemical pathways in a simple cell (like an algae)

enter image description here

Imagine you are the systems engineer who has been informed of an upcoming EVA to repair solar panels. Oxygen production output must be increased in anticipation of a period of reduced power availability for the algae tanks. How do you manipulate your tanks of goop? The organisms involved have complex, inter-related regulatory systems stretching back to DNA transcription for enzyme production. How do you over-ride these control systems without producing unanticipated effects?

How about the effect your manipulation has on cobalt concentration. Cobalt? What has cobalt got to do with it? See that Co++ in the center of the ring in the lower chart? What if your manipulation affects Cobalt levels? Or the increased oxygen changes its oxidation state?

Multiply this by the thousands of other reactions in the algae cell and you have a very large number of failure modes.

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    $\begingroup$ Hm-hm. I'm not convinced. While the inner workings of organisms on the sub-cellular level are, as you demonstrate correctly, very complicated and intricate, the "outer interface" is not. As an example, one may ask "why can my mother not operate her senior cellphone". As an answer, I might point to the 150,000 lines of code that power it. But is that a good answer? Sure, there are a million things that can theoretically go wrong; but usually, the operation is fairly robust against various inputs, and the outer interface is simple compared to the inner workings. Isn't that the same with cells? $\endgroup$ Commented Jan 4 at 2:49
  • $\begingroup$ @Peter-ReinstateMonica ... Two cavemen find a laptop. One pokes it with a stick and the screen turns blue. “Look, Thag. Me get it.” Biological organisms are incredibly complicated. Look at how thick a medical textbook is to get a feeling for how many things go wrong. Ecologic systems are another level of complexity. They work on Earth since evolution makes ecosystems self-healing. Every disaster is followed by a new, different equilibrium. $\endgroup$
    – Woody
    Commented Jan 4 at 5:14
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    $\begingroup$ @Peter-ReinstateMonica ... life support ecosystems in space don't have that luxury. A "new equilibrium" which includes decomposing corpses fails to meet performance criteria no matter how happy micro organisms are with the new arrangement. $\endgroup$
    – Woody
    Commented Jan 4 at 5:16
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    $\begingroup$ Sure; it's not easy to find "homeostasis", or a dynamic equilibrium. But that is independent of knowing the entirety of cellular chemistry -- that's a red herring. And of course, even an ecosystem as large as Earth has a history of mass extinctions, runaway effects and what not. But you don't need to know the entirety of cellular molecular biology to understand an ecosystem. $\endgroup$ Commented Jan 4 at 5:39
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    $\begingroup$ Or from another angle: Yes, biological systems are incredible complicated. But the "external interface" of my potted plant is incredibly simple: Water, air, soil, light. It is also very robust. It tolerates a wide temperature range, a wide humidity range, a wide range of soil composition, a wide range of light composition, intensity and duration. I know next to nothing about its molecular functioning and am still able to successfully keep it alive. In fact, the same is true about any other organism I know, including myself. Why do I need alcohol, coffee and cigarettes? No idea! ;-) $\endgroup$ Commented Jan 4 at 5:46

It's not a question of difficulty, it's a question of accepting results. Last experiment to handle closed Ecological experiments was Biosphere 2 in 1990. enter image description here There were oxygen drops, CO2 buildups, various productivity levels of food. The hard part is proper seal to keep the environment strictly sealed. The Biosphere was too small. The tree's grew too high too quickly enclosing the system from added sunlight so they cut and trimmed it making more CO2 and anaerobic methane. Microorganisms and plant/animal die offs occur in ecosystems, by human standards this is unacceptable if it's needed solely for food. Better strategy in closed system is to plan for greater volume. Carry additional organic nutrient waste (manure, food scraps, fresh produce)

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    $\begingroup$ Suggest fact checking, including the claim that there has been no further work since 1990 en.wikipedia.org/wiki/Biosphere_2#See_also in particular en.wikipedia.org/wiki/Yuegong-1 - statement may be correct for closed environments intended to fully feed humans. $\endgroup$ Commented Jan 3 at 5:17
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    $\begingroup$ @GremlinWranger: Answer is not as bad as it looks. It can be improved greatly but it found the core problems indeed. $\endgroup$
    – Joshua
    Commented Jan 3 at 21:09

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