How do astronauts/cosmonauts manage the atmosphere at the International Space Station as their missions are years long, versus shorter flights?
Do they recycle? Import via cargo vessels? Electrolyze water? Chemical processes?
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7
Several approaches are taken.
Cargo vehicles bring up Oxygen and other atmospheric components (Nitrogen, etc).
The Russian segments life support system works different and independant of the US side, which is a nice redundancy model.
The Russian side has a system that recycles water from urine and moisture that needs to be removed to electrolyze to oxygen and hydrogen (and throws hydrogen overboard).
They have a backup system of 'candles' that 'burn' and release oxygen.
The US side I am not as familiar with specifics. But there is a great article on ECLSS at the ISS. ECLSS is Environmental Control and Life Support System.
There are many interaction points between different systems, using by products of others. So it is not just about 'air' or atmosphere, it is also about water, carbon dioxide, nitrogen, and more.
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10$\begingroup$ Correction: The vast majority of the oxygen is brought up in the form of water. Water is 8/9 oxygen by mass. Both the US and Russian segments have equipment that electrolyze water into oxygen and hydrogen (the latter is vented overboard). Most of the oxygen loss is from humans converting oxygen to CO$_2$, which is concentrated and vented overboard. It's nitrogen that is brought up as compressed gas. All of the nitrogen loss (but only a small amount of the oxygen loss) results from dockings/undockings, EVAs, experiments, incomplete CO$_2$ concentration, and unplanned leaks. $\endgroup$ Commented Sep 5, 2014 at 0:08
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2$\begingroup$ Just how airtight is the ISS? $\endgroup$ Commented Sep 8, 2014 at 19:41
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2$\begingroup$ @MartinSchröder Pretty airtight? Ain't leaking that fast. No doubt it leaks, and probably every EVA plus CBM berthing event (Cygnus, Dragon, HTV) looses some air. I am sure there are some actual numbers, but they are probably mostly meaningless without context. $\endgroup$– geoffcCommented Sep 8, 2014 at 20:27
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6$\begingroup$ @DavidHammen A small correction: humans do not convert oxygen into $CO_2$; the oxygen goes into $H_{2}O$. The exhaled $CO_2$ is created in the metabolism of glucose, without atmospheric oxygen involved. $\endgroup$– mpvCommented Sep 14, 2014 at 21:10
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1$\begingroup$ @mpv Technically true, unless you view the respiratory system as a black box where $O_2$ goes in and $CO_2$ comes out. Of course water also comes out, but that could be viewed as a non-primary function of respiration (evaporation). $\endgroup$– called2voyage ♦Commented Sep 21, 2015 at 16:48