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.
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.
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.
Part of this water is used for drinking and washing (which is achieved with a dampened cloth, since a shower like we take on Earth would be problematic in a zero-G environment, what with the water floating about everywhere in little globules).
The rest of this water is used to create oxygen. The NASA Oxygen Generation System and the Russian Elektron system aboard the ISS uses a process called electrolysis to split the water into its component atoms of hydrogen and oxygen. This involves passing an electric current -- supplied by the ISS solar panels -- through the water from a positively charged anode to a negatively charged cathode; this in turn separates the atoms and recombines them as hydrogen gas and oxygen gas.
This is actually pretty similar to the process of photosynthesis, where plants break down water into hydrogen and oxygen. The hydrogen is combined with the carbon dioxide to create carbohydrates that feed the plant and the oxygen, as mentioned earlier, is expelled.
In the case of the ISS, the hydrogen generated from the electrolysis process is fed back into the space station's Sabatier System. A catalyst is used to combine the waste hydrogen with the waste carbon dioxide exhaled by the astronauts at high temperatures to create water (H2O) and methane (CH4) -- the process is described by the exothermic reaction CO2 + 4H2 → CH4 + 2H2O + energy.
The water produced is then fed back into the water reclamation system, while the methane is vented into space.
In all, this process produces around 2 kilograms of oxygen per day. According to NASA, the average person needs around 0.84 kilograms of oxygen per day to survive and the International Space Station typically has three astronauts aboard at any given time. At time of writing, NASA astronaut Terry Virts, ESA astronaut Samantha Cristoforetti and Russian Air Force Colonel Anton Shkaplerov are on board.
These means that oxygen needs aboard the ISS -- 2.52 kilograms per day, according to NASA's calculations -- outstrip supply from the oxygen generation sources.
It's for this reason that the ISS has two other methods of receiving oxygen. It is provided from Earth whenever the ISS receives a supply shuttle and pumped into pressurised tanks mounted outside the airlock; and by a backup solid-fuel oxygen generator called Vika, or SFOG, developed by the Russian Federal Space Agency for the Mir space station.
The Vika system isn't ideal for several reasons. The first is that they use canisters that need to be shipped from Earth. These canisters contain a mixture of powdered sodium chlorate and powdered iron. This is ignited, which heats the iron to a temperature of 600 degrees Celsius (1112 degrees Fahrenheit), which creates the energy required to break down the sodium chlorate into sodium chloride and oxygen gas.
These temperatures can be hazardous. In 1997, a Vika canister aboard Mir malfunctioned and caught fire, which melted the canister and launched globules of fire on to the bulkhead. Additionally, 1 kilogram of material produces only about 6.5 man-hours of oxygen. With an estimated cost of tens of thousands of dollars per kilogram of cargo shipped to the ISS, this is no small matter. That's why Vika is a backup system, rather than primary source of oxygen. The astronauts' day-to-day oxygen needs are adequately filled by electrolysis oxygen generation and supplies from Earth.