|MadSci Network: Engineering|
Both submarines and spacecraft provide breathable air to the people inside in similar ways.
Vehicles which are occupied for a short time, like the Space Shuttles or the submersible "Alvin" carry oxygen in containers and release it into the cabin. The Space Shuttles carry containers of oxygen in super-cold liquid form. Alvin just uses bottles of pressurized oxygen gas.
Vehicles which are underwater or in space for a long time, like the International Space Station and nuclear submarines, can't carry enough oxygen for the trip: it would weigh too much and take up too much space. Both the space station and nuclear submarines use "electrolysis" to supply oxygen. Water is made from atoms of hydrogen and atoms of oxygen. When electricity passes through water, it can split the water molecule apart, forming hydrogen and oxygen gases. Space stations and nuclear submarines have plenty of electricity to do this. They throw away the hydrogen, and let the crew breathe the oxygen. A submarine can always get more water from outside, but where does the space station get water from? When people breathe the oxygen, it combines with carbon and hydrogen from the food they eat to form carbon dioxide and water. The water leaves the body through sweat and urine: the space station sends the water through the electrolysis unit to make more oxygen -- it's a closed cycle!
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