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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.

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    $\begingroup$ There is also oxygen brought up either as oxygen or in the form of water (which can then be converted to hydrogen and oxygen). $\endgroup$ Feb 25 at 18:45
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    $\begingroup$ I’m picturing it being like hot water for showers in youth hostels: I hope they have lots of quarters! $\endgroup$
    – Jon Custer
    Feb 26 at 2:00
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    $\begingroup$ 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. $\endgroup$ Feb 26 at 19:08
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    $\begingroup$ 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? $\endgroup$ Feb 26 at 21:17
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    $\begingroup$ @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. $\endgroup$ Feb 27 at 17:07

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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.

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    $\begingroup$ 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. $\endgroup$ Feb 26 at 15:47
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    $\begingroup$ 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\$ : $\text{CO}_2$. $\endgroup$ Feb 26 at 16:06
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    $\begingroup$ 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. $\endgroup$ Feb 26 at 16:10
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    $\begingroup$ @DavidHammen Thanks David - love learning something new! $\endgroup$
    – phil1008
    Feb 26 at 16:19
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    $\begingroup$ @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. $\endgroup$
    – phil1008
    Feb 26 at 16:32

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