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When we use our rockets in space, are we using up a fraction of the finite oxygen here on earth?

I'm assuming that it's a negligible amount at our current rate of space travel, but could this potentially become an issue in the long run, assuming we increase our space travel frequency?

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    $\begingroup$ You need to read Isaac Asimov's short story The Martian Way. $\endgroup$ – Nathan Tuggy Sep 29 '17 at 9:38
  • $\begingroup$ In what way "using it up"? Expending propellant into space, never to return to Earth, or burning to produce carbon dioxide and/or water? Burning isn't "using up" a finite resource; there are natural and/or artificial processes to perform the reciprocal chemistry, requiring only the input of energy. Most of what a rocket burns to lift off and achieve even a low Earth orbit is either expended directly into the atmosphere or ends up in a less than orbital trajectory, to fall back into the atmosphere anyway. BTW, oxygen is about the most abundant element on Earth, if you include all the rock. $\endgroup$ – Anthony X Oct 2 '17 at 0:58
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Let's start with a Fermi estimate:

The atmosphere has a mass of about 5.15×1018 kg, 20% of that is oxygen. A rocket launch uses on the order of 106 kg of oxygen. To use up all the oxygen (and assuming no oxygen is replenished by plants) requires 1012 rocket launches.

Of course, rockets are only a tiny part of all oxygen consumption. We burn 4×1012 kg of oil every year, and 7×1012 kg of coal. This requires on the order of 1013 kg of oxygen.

Oil/gas/coal/wood burning has a measurable effect on our atmosphere. CO2 concentration was stable at 280 ppm until the year 1800, then it began to rise to the 400 ppm we have today. The total plant mass on Earth is no longer able to convert all the CO2 we create back to oxygen. This also means we have to consider the total oxygen use by the rocket, not just the fraction that is used outside the atmosphere.

Assuming all oxygen is converted to CO2, the oxygen content has changed by the same amount, from 209,580 ppm in 1800 to 209,460 ppm now.

For the rocket industry to have a similar effect on our atmosphere would require 106 rocket launches per year.

The history of our atmosphere gives some hints of what might happen when the oxygen content drops. The O2 fraction was held at 10% until the soil and oceans were saturated. I suspect this means oxygen trapped in the soil and oceans will start to be released once the O2 fraction drops below 10%.

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    $\begingroup$ Shouldn’t we only count the upper stage? $\endgroup$ – Antzi Sep 29 '17 at 11:14
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    $\begingroup$ @Antzi: It depends. If we're building and stockpiling all those hypothetical $10^{12}$ rockets for a single massive launch effort, then most of the oxygen will end up in the first stage tanks. Of course, lack of oxygen will be the least of our environmental issues at that point. On the other hand, if we're assuming that the $10^{12}$ launches are spread out over a long time. then any oxygen consumed by the lower stages will indeed stay in the atmosphere as water and CO2, and eventually get recycled by plants. Actually, so will most of the upper stage exhaust too, at least for LEO launches. $\endgroup$ – Ilmari Karonen Sep 29 '17 at 11:54
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    $\begingroup$ To put this further into perspective (since 10^12 is "just" a number): at 100 launches/year that we approximately have currently, you would need 10^10 years, which is about twice as long as the earth is old. $\endgroup$ – Polygnome Sep 29 '17 at 12:07
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    $\begingroup$ @Sid: Due to tyranny of rocket equation, only a little fraction of the launch propellants mass is used up above the atmosphere. Meanwhile, Earth loses vastly more to solar wind tearing away outermost reaches of the atmosphere, and gains a similar amount, as various oxides from meteors that burn up in the atmosphere. $\endgroup$ – SF. Sep 29 '17 at 12:41
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    $\begingroup$ @RussellBorogove Annual worldwide iron ore production is around 2e9 kg, and that is probably conservatively 10-20% oxygen. Ordinary smelting operations can probably release the oxygen used by 100 rockets a year. $\endgroup$ – chepner Sep 29 '17 at 20:13

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