Where would space habitats get their oxygen from?

Suppose you want to build a large scale habitable structure in space or on a planet or moon without an atmosphere or any organics, and this would be a long term or permanent settlement, not a temporary location. Where would you get the oxygen for its inhabitants to breathe from? And, if you got it from a different location how would you ship it?

In both cases the sheer volume of necessary oxygen (and other elements used in human-breathable air) would make it very difficult or impossible to build such a habitat, no?

It seems to me that ensuring a constant supply of oxygen for such a habitat is so difficult as to make it impossible to build.

Would the only way to pull this off, be to build habitats in locations that were already connected to some large source of oxygen?

8 Answers
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In both cases the shear volume of necessary oxygen (and other
elements used in human breathable air) would make it very
difficult or impossible to build such a habitat, no?

No, actually; I don't think so.

The accepted atmospheric composition of Earth is 78% nitrogen, 21% oxygen, 0.9% argon, and 0.05% everything else, including carbon dioxide (about 0.04% carbon dioxide and 0.01% everything else, roughly). This is from research by NOAA, Wallace and Hobbs, Vaughan, etc. Lots of research.

To correct one part of your question, while the atmosphere is something like 78-ish% nitrogen and 1% "other elements, the only part of the atmosphere we need and actually use in breathing is the oxygen.

We "only" use 25-35% of the oxygen in any particular breath we take. That's 25-35% of the 21% of the oxygen in the air. So, of the roughly 12,000+ litres of air we breathe every day, we actually consume between 600 and 900 litres of oxygen.

Having said that, the NIH has stated quite bluntly that "Plant-based life support systems represent the only potential for self sufficiency and food production in an extra-terrestrial habitat."

Coincidentally, it also represents the only potential for self-sufficiency and food production on a terrestrial habitat, as well! [;)

Note that it says for an "extra-terrestrial habitat". On Earth it requires 8 or 9 trees to produce enough oxygen to support one person. For a reasonably-sized crew that's a lot of vegetation to be propelling through space in a spacecraft for the purpose of oxygen production.

But there are other ways of of providing oxygen.

We need to have quite a bit of water on the vessel anyway for many purposes, but the main reasons are as a radiation shield as well as a heat sink. Oxygen can be easily electrolysized from that water when topping up is required. You would count on this and make sure that you have enough water on board to account for that need.

The biggest problem with breathing oxygen is the exhalation of carbon dioxide. Humans begin to fare poorly when concentrations begin to exceed 5%. Removing the carbon dioxide is easy, but you have to keep on removing the CO2 throughout the entire flight/mission, then you potentially have to do something with that removed carbon dioxide, and you still need to replace the oxygen which the humans are removing through breathing.

Rather than jettisoning chemical cans every few 100,000km,there are some technological, non-organic methods of separating oxygen from the carbon in carbon dioxide. Intense UV seems to be a proven method.

So; impossible? Far from it.

On a space station or space habitat you use plants, which you also employ in the production of food and the purification of waste water which may include the use of algae and/or phytoplankton. On spacecraft you have compressed or liquefied O2 as backup but primarily convert CO2 back into oxygen, topping up from water stores. That's not to say that you couldn't or wouldn't have some kind of hydroponic shrubbery, but the primary purpose would be as a kitchen garden, not the production of oxygen.

Hope this helps.

According to this lovely image from NASA (article here), the source of onboard oxygen in current spacecraft is mainly water electrolysis. The hydrogen so produced is processed with carbon dioxide to reclaim some of the water and produce either solid carbon waste, or acetylene for propulsion.

This isn't a 100% closed cycle, so you'll have to add more water over time, although you'll have to do that anyway (waste processing isn't 100% closed either).

Current ways to create oxygen in space
There are a few possibilities that can be considered. Currently electrolysis is one way that oxygen is produced in space. This is done by splitting H2O into hydrogen and oxygen. A quick search on the ISS (International Space Station) or electrolysis will bring up more details about this process. Spirulina and other super algae are also being experimented with as a viable option for both creating oxygen and using it as a super food in space. Some Spirulina has already been sent to the ISS. There are also experiments going on that are attempting to produce oxygen via intense vacuum ultraviolet light and CO2. Some such experiments have been slightly successful by producing small amounts of O2 along with the carbon monoxide and carbon molecules.

Transporting and/or maintaining materials for oxygen in biospheres
Experiments are still being done to determine whether using photosynthesis via algae and other plants would actually work. We know they work in concept, however radiation and changes in gravity may effect this process in space. The results of some preliminary experiments done on the ISS were supposed to be released in April, but I haven't found anything reliable about these results yet when searching online other than they were returned via the Dragon in April for analysis. The most realistic option at this time supposing that a biosphere was placed at some location in space that isn't near a source of oxygen would be putting it near an asteroid belt or other potential source of water. This would allow the asteroid(s), moon(s), planet(s), etc. to be mined for water, and in turn this water could be used for electrolysis. This would also mean a source of both water (of course), and fuel in the form of hydrogen.

From rock.

This is something surprisingly few people know: The main component of rocks
is

quartz which is silicium dioxide SiO₂ and therefore contains oxygen.

feldspar which also contains oxygen. Feldspar exists mostly in form: <alkali metal><aluminium>_2/3<silicium>_2/3<oxygen>₈.

And yes, moon rock and therefore also rock from other planets and moons contains oxygen, so no problem there.

So if you have energy available you can not only extract oxygen, but also important materials like alkali metals, aluminium or silicium.

Does water exist on your planet? If it does then electrolysis of water could provide the oxygen needed.

But even if the planet is completely devoid of water oxygen should be the least of your worries as it would be abundant in the surface rocks of any normal planet as the inorganic oxides of silicon, aluminium, magnesium, calcium and iron (among others – that’s what most rocks are). Extraction would not be easy but would be possible using a lot of energy by electrolytic processes as described here:
https://phys.org/news/2009-08-scientists-oxygen-moon.html

It would also be possible to react waste organic material from the habitat with molten rocks containing oxides to produce carbon dioxide and steam for recycling into the habitat. The carbon dioxide could be fed to plants to produce further oxygen.

The key points to consider are what elements are present on your planet? If the elements are absent or in very short supply (like hydrogen is on the Moon) then the element will need to be imported or the base located where the few deposits do exist. If the element is present but chemically locked up in a different form as oxygen is in rock, then chemistry and energy can be used to extract it.

If the element oxygen is totally absent from your planet then you have to consider what is the planets crust actually made of? And you would have to import all of the oxygen you need and ensure that it was recycled very efficiently.

Can you bring organics with you? Because if so, a greenhouse is the answer. The plants you grow take the CO2 you breathe out and water, and provide oxygen and food. It'll be hard to get a perfect equilibrium, so you will probably want to grow plants based on your food needs and balance your atmosphere with other equipment to pick up the slack. A furnace if you're producing too much oxygen, an algae tank if you're making too much CO2.

I'm making it sound much easier than it really is, no one has ever made a totally self-sufficient, air tight habitat. The ISS gets food, and water, and sends trash back to Earth via Soyuz. Nuclear submarines get oxygen from seawater via electrolysis. They pick up food every time they're in port.

One of the first places I'd go looking is Comets. The main reason for this is that one of the known principal constituents of their nucleus is ice. This is one of those 'kill two birds with one stone' situations because the ice gives you water (which you need), and that water can be broken down into oxygen (which you need for breathing) and hydrogen (which can be used as a fuel). Yes, you need O2 to oxidise the hydrogen to burn it, but you can ship excess, unburned H2 back to Earth if you have to where they have O2 in abundance.

So, go comet hunting. This gives you access to water, breathable air, energy and an export market for your 'waste products'. Just saying.

You don't need to get or even create oxygen out of thin air (sic).

Your space habitat will be a closed system (or it better should be one...). This means that you will re-use every last molecule of matter that you have at your disposal. There are no known processes except fission (only relevant to radioactive material, not everyday atoms like oxygen) or fusion (applicable to certain everyday atoms, but you would have completely separate energy/matter cycles for that) to change the actual atoms you have in your enclosed space.

This leaves you with finding technically feasible processes to rip any molecules apart into their constituents (e.g., waste from your humans). This, in turn, is basically a solved problem, which simply requires energy. Lots of it, in some cases, but still only energy.

So you only need to place your space habitat in a location which gives you unlimited amounts of energy. This can be simply a sun; or if you're so inclined some fashionable black hole or whatever where your inhabitants can harvest matter with some future processes.

You obviously need to get enough starting material (harvest enough from some planet while building your habitat), and minuscule amounts that inevitably get lost due to unavoidable inefficiencies in your outwards facing doors/walls. But those you can replenish by getting any kind of raw material (from asteroids, planets, etc.) and putting them into your fancy recycling apparatus.

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