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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
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    $\begingroup$ 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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    $\begingroup$ Finite oxygen??? You need to go back to biology class and study photosynthesis. $\endgroup$ – RonJohn May 11 at 6:44
  • $\begingroup$ that oxygen is released from atmospheric carbon dioxide @RonJohn, not created. so yes, it is a finite (although vast) resource. $\endgroup$ – JCRM May 11 at 14:08
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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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Answer 1

A person converts roughly 1 kg of oxygen per day (1, 2) into carbon dioxide.

One Falcon 9 per day converts perhaps 600,000 kg of oxygen per day.

One F9 per day is like a half-million people.

Earth population is 7.7 billion, growing at about 1.2% per year or 92 million per year or about 3 Hz.

So launching one Falcon 9 per day uses as much oxygen as the number of humans born in the next two days will.

Worried? I'm not, but if you are, the Bill & Melinda Gates Foundation as well as other organizations and experts reminds us that the best way to lower population growth is to improve the dire problems of childhood disease and malnutrition. Bring the world up to some minimum level of existence, and population growth will slow substantially, our collective conscience will rest easier, and we can launch more rockets without worrying about oxygen.

Answer 2

The Biosphere is a dynamic and responsive system. Add more CO2 (as we are doing now, way faster than we are burning rocket fuel) and more trees and vegetation grows in the richer CO2 and warmer environment. That in turn makes more oxygen.

But if we use some oxygen, how will the biosphere respond?

That's a really good biosphere question, and definitely askable in Earth Science SE (or possibly Biology SE if carefully written to suit topicality there).

In the mean time, @Hobbes' answer does address this a bit, and the oceans do function to some extent as a reservoir and atmospheric regulator.

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No.

  1. LOX-LH2 (liquid oxygen-liquid hidrogen) fuel is used mainly to leave the orbit of the Earth. Its main reason is that liquid hydrogen boils on 40K, and there is no simple way to hold it on this temperature too long, not even in the space.

  2. The hydrogen is created mainly by two ways for that:

    • burning methane with cathalitic reactions
    • or by electrolysing water

    As the H2 burns, essentially these reactions are going only into the opposite way.

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  • $\begingroup$ "LOX (liquid oxygen-hidrogen)" is wrong by the way, LOX stands for Liquid OXygen and LH2 for liquid hydrogen. Cryogenic fuel is the term for LOX and LH2. When H2 is burned with LOX, no methane is generated, only water. $\endgroup$ – Uwe Oct 1 '17 at 9:45
  • $\begingroup$ Cryogenic fuel is the term for any propellant combination that has boiling temperatures below 0 ºC. This includes LOX/LH2, but also e.g. LOX/methane. $\endgroup$ – Hobbes Oct 1 '17 at 10:32
  • $\begingroup$ @Hobbes I didn't mention LOX/methane. I wrote, how h2 is produced for the LH2. It has two main ways: 1) cathalytic burning of methane 2) electrolysis of water. I didn't write a single bit about methane fuels. I tried to make this more clear in the post. $\endgroup$ – peterh says reinstate Monica Oct 1 '17 at 16:05
  • $\begingroup$ @OrganicMarble The essence of the answer, that the burning H2 depletes exactly the same O2 which was created from the water by the electrolysis. Thus, it doesn't the O2 content of the athmosphere. If you electrolyse water (while to create the LOX/LH2), and then burn it (as you fire the rocket), the O2 content of the atmosphere will remain the same. Exactly. Until the last atom. Honestly, I am surprised, how can it be so unclear, I think it is very clear. $\endgroup$ – peterh says reinstate Monica Oct 1 '17 at 16:53
  • $\begingroup$ @OrganicMarble If the H2 was created by cathalytic methane burning, then the case is not so simple, because also a lot of CO2 is produced. But I simply can't understand, what is so problematic in such trivial things. I can't understand the voting score. I can't understand this whole reaction et al. $\endgroup$ – peterh says reinstate Monica Oct 1 '17 at 16:56

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