How does ISS gets oxygen?
Six astronauts use about 840 grams of oxygen per person per day. In total, that’s about 5 kilograms a day.
How does the ISS get this oxygen?
Sending containers with compressed air into the sky is one solution, but a very, very expensive one. Launching costs tend to be in the order of 95 thousand dollars per kilogram of supply. That would result in daily costs of almost 500 thousand dollars for sending pure oxygen. Sending compressed air, which only contains 23 percent oxygen by mass, would result in costs of more than 2 million dollars per day. Even the “cheap” SpaceX rockets will result in sky rocketing costs. So, it should still be obvious that a cheaper system is desirable.
So in July 2006, the OGS – which stands for Oxygen Generation System - was launched into orbit. By using a chemical process called electrolysis, water can be transformed into oxygen. It works as following: As you may know, a water molecule is a combination of 2 hydrogen atoms and 1 oxygen atom: H2O. By adding 1.23 volts of electricity, water molecules will get split and form hydrogen and oxygen molecules. Completing this chemical equations shows that 2 of these water molecules will result in 2 hydrogen and 1 oxygen molecule. And since the weight of each molecule is known, we see that 5.7 kilograms of water is needed to provide for the 5 kilograms of oxygen. This water is obtained by recycling wastewater and urine.
As a side effect, 700 grams hydrogen gas will also be formed with this chemical reaction. This gas is highly flammable and also explosive, so that should be.. taken care of. In an ideal situation, plants and other flora would be used to generate oxygen out of CO2. This would be called a bio regenerative life support system. Although several dozens of squared meters of plants for every crewmember is needed, this would be a very sustainable way of generating oxygen. Experiments on this matter have already been performed, but had severe problems with the rate of growth under micro-gravity. So for now, we are stuck with the OGS but that could change in the upcoming years.
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