Astronaut Andr Kuipers experimenting with a bubble of air inside a blob of water aboard the ISS. European Space Agency

Of all the issues with making space habitable for humans, the most important is something you can't even see -- something you rarely even think about: breathing. A constant supply of fresh, breathable air is absolutely vital. For the International Space Station, in orbit since 1998, this is especially important since shipping oxygen into space is an expensive and cumbersome option.

Here on Earth, the air we breathe contains a mixture of 78.09 percent nitrogen, 20.95 percent oxygen, 0.93 percent argon, 0.039 percent carbon dioxide, and traces of other gases. Each breath we take into our lungs takes the oxygen from the air, distributing it through the lungs' spongy material into capillaries, where it's diffused into the bloodstream.

Meanwhile, blood on its way back towards the lungs releases its waste carbon dioxide, which we exhale with each breath; and exhalation contains, on average, 16 percent oxygen and 5 percent carbon dioxide. On Earth, this works because plant life require carbon dioxide for photosynthesis, releasing oxygen as its own waste. It's a perfect symbiotic relationship.

Rice (left) and arabidopsis (right), grown in both gravity and microgravity conditions. Professor Takayuki Hoson/Osaka City University

There are plants on the International Space Station, but they're not for the production of oxygen and the eradication of carbon dioxide. There simply isn't enough room on the station for a viable floral air recycling plant, for one. The plants are on the space station so that researchers can figure out how well plants grow in zero-G. For example, lack of gravity means that water doesn't wick well into the soil -- meaning, in turn, that root systems can suffocate.

So relying on plants to produce air in space aboard the International Space Station is clearly not a viable solution.

Luckily, we have had a perfect technology for the development of air production and recycling. It's not always practical for submarines to surface in order to ventilate; which means that technologies for the generation of breathable air have been around for decades -- and in an airtight, sealed container to boot. The system used by the ISS is very similar to the system used aboard submarines.

European Space Agency

It consists of two components: the Water Reclamation System and the Oxygen Generation System; the latter can't operate without the former. The WRS reclaims water aboard the ISS -- the astronauts' urine, humidity condensation on the walls and windows, and Extra Vehicular Activity waste. All this fluid is then purified to very stringent standards so that it can be reused aboard the ISS. To be clear, this recycled water can't make up the entire amount of water the ISS requires, but it does reduce the amount of water that needs to be shipped from Earth.

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Breathe deep: How the ISS keeps astronauts alive