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I was reading an article which broke down some of the methods of oxygen creation on the ISS. This was in response to my daughter commenting that "air is free," and being curious about counterexamples.

How much does it cost to produce the oxygen needed to support the astronauts on the ISS? I'm assuming this would be in the form of the cost of power generation (solar cells) and the wear and tear of the hardware that needs replacing.

Cort Ammon
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    There is also oxygen brought up either as oxygen or in the form of water (which can then be converted to hydrogen and oxygen). – manassehkatz-Moving 2 Codidact Feb 25 '24 at 18:45
  • related: Can the ISS's NORS tanks be used to recharge spacesuit or spacecraft oxygen supplies? and Supply of liquid oxygen (LOX) maintained on the ISS? Kept cold using "space", or refrigerator? (note: not as liquid) and Isn't electrolysis of water less efficient for a space craft than pressurized oxygen tanks? and How close is the ISS to a closed system, in terms of carbon, hydrogen, and oxygen? – uhoh Feb 25 '24 at 22:11
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    I’m picturing it being like hot water for showers in youth hostels: I hope they have lots of quarters! – Jon Custer Feb 26 '24 at 02:00
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    Also, you may want to take into consideration that (as far as astronauts being able to breathe is concerned, if that is what you actually want to know), removing carbon dioxide is as big the deal as supplying enough oxygen. – Matija Nalis Feb 26 '24 at 19:08
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    Am I wrong to assume all air on the ISS is shipped up?

    Doesn't that mean there's nothing special about air and the shipping costs depends on a combination of weight and volume?

     – Robbie Goodwin Feb 26 '24 at 21:17
  • I think what's special about oxygen and water is that they can at least in theory be (mostly) recycled, so then it becomes an equation of the cost of recycling vs. the cost of shipping. Or the cost difference between recycling nearly 100% vs. recycling say 75% and shipping the rest, since the cost of recycling that last 25% might be really expensive. ISS however is more of a technology demonstrator than an economics demonstrator, because it's cheaper to ship oxygen and water to LEO than to Mars for example, which might tilt the equation in the other direction. – Steve Pemberton Feb 27 '24 at 00:59
  • @RobbieGoodwin All air is certainly shipped up. However, it may be recycled several times, which would make it more efficient (fewer times than I expected, based on the answers!). Also, the oxygen needs to be stored in some container. Pressure containers weigh a lot. Several of the answers I received pointed out that they avoid shipping gassious O2 for this reason, and find other ways (such as water) which can be used for transport. – Cort Ammon Feb 27 '24 at 13:59
  • @CortAmmon The Sabatier reaction combines carbon dioxide and hydrogen to form methane and water. The ISS has deployed multiple experimental devices that use the Sabatier reaction as a means for getting rid of carbon dioxide. The methane is vented while the water is reused. All of these experimental devices have had problems of various sorts, which is why they are still deemed to be experimental. Note that these failures do not bode well for in-situ resource utilization on Mars, where the Sabatier reaction has been proposed as a source for methane as propellant for leaving Mars. – David Hammen Feb 27 '24 at 17:01
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    @CortAmmon Water and oxygen gas and nitrogen gas are regular shipments to the ISS. Water can be used as water, but also can be used as a source for oxygen, with the hydrogen being vented to vacuum. Ammonia could be a source for nitrogen gas, but as ammonia is highly toxic and also is rather reactive, it's deemed better to ship nitrogen as a gas. Nitrogen is needed because the air on the ISS is a one atmospheric pressure mix of nitrogen and oxygen and because the ISS is leaky and is getting ever more leaky as the ISS gets older. – David Hammen Feb 27 '24 at 17:07
  • @CortAmmon Of course much of the air can be recycled and how does that impact the cost of lofting it in the first place? – Robbie Goodwin Feb 27 '24 at 21:02

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The current cost of resupplying the ISS is around $80,000 per kg, although this price has varied over the years. (references)

The pressurized volume of air on the ISS is 1,005.0 $m^3$ (ref). As the density of air is 1.293 $kg/m^3$, this equates to 1300 kg, or 104 million USD worth of air, after considering the launch costs. So, certainly far from free!

The oxygen (O₂) in the air is consumed by the astronauts on the station, converting O₂ into CO₂. Some of the CO₂ can be converted back into O₂ but this process isn't 100% efficient. So replacement O₂ needs to be sent up on ISS resupply missions.

Each crew member requires about two pounds per day of oxygen, which is 14 pounds for a crew of seven, 6.35029 kg per day, 2318 kg per year, or roughly 29.2 million USD worth of O₂ per year, at current resupply costs, if no CO₂ was recycled.

However, the O₂ is sent up as water (H₂O), which makes it heavier by a factor of (18/16 = 1.125).

Some of the CO₂ is recycled. A 2017 report stated...

the state-of-the-art system currently used on the International Space Station recovers about 50% of the oxygen from exhaled carbon dioxide.

The SpaceCraft Oxygen Recovery (SCOR) project is developing novel technologies to increase the recovery of oxygen to more than 75%, with a stretch goal of 100%, reducing the total oxygen resupply required for future missions.

If we include the 18/16 factor for sending up water, and assume that 75% of the CO₂ is recycled, then the cost of maintaining the O₂ can be estimated at roughly 29.2 x 18/16 x 0.25 = 8.2 million per year.

Note: I have not included here the cost of developing and equipping the station with its Environmental Control and Life Support System (ECLSS) or sending up parts occasionally to keep it operating.

To conclude, the initial cost of the air was ~104 million USD and it costs ~8 million USD per year to maintain the O₂ in it.

phil1008
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    The first machine that was intended to convert $\text{CO}_2$ back into oxygen via the Sabatier reaction was shelved due to repeated failures. The next generation machine, which you mentioned, has fared a bit better, but is still somewhat dubious. Most of the oxygen breathed by ISS crew is still suppled via resupply (at $80k per kg), either in the form of oxygen gas or water. BTW, this does not bode well for planned human missions to Mars. – David Hammen Feb 26 '24 at 15:47
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    I see that you have used $CO_2$ as a way to represent carbon dioxide. This does not visually appear quite as intended because of the way MathJax interprets those symbols. Unless qualified otherwise, symbols in LaTex (and hence MathJax) are individual letters and are printed in italic, oftentimes with weird spacing due to LaTex/MathJax treating the $CO$ as $C\times O$ but with an implied multiplication. There are multiple ways around this strangeness. A simple one is to tell MathJax that the item in question is text: $\text{CO}_2$ : $\text{CO}_2$. – David Hammen Feb 26 '24 at 16:06
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    An alternative is to use operatorname instead of text: $\operatorname{CO}_2$ , which yields $\operatorname{CO}_2$. This can also lead to weird spacing. The mhchem LaTeX package, which some stackexchange sites have adopted, does a really nice job. I'm not sure if this particular stackexchange supports mhchem. – David Hammen Feb 26 '24 at 16:10
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    @DavidHammen Thanks David - love learning something new! – phil1008 Feb 26 '24 at 16:19
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    @DavidHammen Yes this was an interesting question to me because I had no idea that we had such a long way to go here to catch up with plants. But it might not matter how efficient we are on Mars because there's lots of CO2 there. Just need to send H2 or find water. – phil1008 Feb 26 '24 at 16:32
  • just a note .. $80k/kg seems high to me. Just checking https://en.wikipedia.org/wiki/Space_Shuttle is referencing a launch cost of $450M for 16t to ISS (or 14t return) which is more like $30k, and supply rockets (Soyuz, .. ) should be a fair bit cheaper because they have no need to throw 80t of Orbiter to ISS altitudes. Anyway, the difference may be in how costs are counted, funny, price per kg google search results vary wildly.. – Apfelsaft Feb 27 '24 at 00:38
  • @Apfelsaft Yes, 450M was the NASA quoted average cost over some unknown period of time. That value does not correct for inflation since ~2011? If you look at the graph in the referenced article, the space shuttle's cost was sometimes lower and sometimes higher than 80K-per-kg, after correcting for inflation. It was higher when the two failures caused the flight rate to dip significantly. – phil1008 Feb 27 '24 at 01:04
  • I think it would help if you only use the word "air" when you referring to the mixed gas atmosphere on ISS and not use it interchangeably with oxygen, which I think is how I am reading some of your statements. For example when you say "new air needs to be sent up on ISS resupply missions" and talking about the cost per kg of "air", I assume that you are referring to oxygen? Then in other cases I think you are referring to the overall cost for maintaining the "air" (i.e. a breathable atmosphere) I guess including LiOH. Possibly I'm misreading it but I think maybe it could be clearer. – Steve Pemberton Feb 27 '24 at 02:29
  • Yes, good suggestion. Changes made. Isn't LiOH currently used as a backup CO2 removal system on ISS? I thought CDRA is the primary CO2 removal system. – phil1008 Feb 27 '24 at 03:04
  • @DavidHammen (Or if you can enter the relevant Unicode SUBSCRIPT 2 character directly — e.g. via a Character Viewer or other widget, depending on your OS, or copied-and-pasted from elsewhere — you can simply enter ‘CO₂’ directly.) – gidds Feb 27 '24 at 13:19
  • @StevePemberton The ISS does not use lithium hydroxide, except perhaps as an extreme backup should the US CDRA and Russian Vozdukh systems fail. The problem with LiOH is that its reaction with $\text{CO}_2$ is permanent. Both the CDRA and Vozdukh system use desiccant beds to remove / restore humidity and absorbent / desorbent beds to absorb $\text{CO}_2$ from the breathing air and desorb it to vacuum. While the reaction with $\text{LiOH}$ and $\text{CO}_2$ to form $Li_2\text{CO}_3$ is not reversible, both the desiccant and absorbent beds are. – David Hammen Feb 27 '24 at 16:38
  • @DavidHammen - good info, some of which I was not aware of. Although I was responding to the comment about "air" being shipped from the ground and I was trying to steer the answer into being more specific about what is actually shipped to replace what is not recycled. I didn't realize that LiOH is no longer used occasionally, for some reason I had thought it was still used as an occasional supplement, I must have been thinking of Mir. – Steve Pemberton Feb 27 '24 at 17:44
  • @DavidHammen - interestingly (and coincidentally) I came across something in the Columbia CAIB report which pointed out that Li2CO3 heated to 1250 F in a vacuum will convert to LiO. They theorized that depleted LiOH canisters on Columbia could have been placed in a hot area of the payload bay where some of it might have converted to LiO. Realistically that likely would not have helped in the Columbia situation, however I can imagine on the Moon or Mars they might use parabolic solar heaters to heat up Li2CO3 and recycle some of it. – Steve Pemberton Feb 29 '24 at 04:38