Sustaining a Permanent Environment in Space
Sustaining a permanent environment in space requires things many of us take for granted here on Earth: fresh air, water, food, a comfortable (and habitable) climate -- even waste removal and fire protection. First, let's talk air. We need oxygen so the ISS has several systems for providing it. One system is to have oxygen delivered from Earth via spacecraft. This oxygen is stored in external tanks; similarly, these spacecraft deliver nitrogen gas, which makes the ISS air supply. The ISS also has a generator that makes oxygen from recycled water. Using electricity, it splits water into hydrogen and oxygen by electrolysis. The hydrogen gas gets vented into space and the oxygen goes into the ISS air. And finally, the ISS has a solid fuel oxygen generator that burns oxygen candles to release oxygen into the ISS air. Fans circulate the air within the ISS modules. The circulated air gets passed through various filters to remove particles and microbes. The composition of the air is constantly monitored and regulated throughout the station.
On Earth, plants remove the carbon dioxide we exhale, which is poisonous to us. In the ISS, it's removed chemically from the air by various "scrubbers" that absorb carbon dioxide and react it with other chemicals. In addition to carbon dioxide, we also breathe out water vapor. Aboard the ISS, this excess water vapor gets condensed into liquid and is recycled. But this isn't the only source of water. Like oxygen, water gets delivered to the ISS. In fact, there are various methods to transport supplies to the space station. The Russians have the Progress supply ships, European Space Agency has the automated transfer vehicle and Japan has the Kounotori2 or HTV2. Water is a byproduct of the fuel cells that the shuttle uses to generate electricity and water is even recovered from the crew's urine, which gets filtered and treated to make water for drinking, hand-washing and showers. The water gets stored in bags and containers throughout the station. Food is also delivered to the ISS by spacecraft. And the ISS has a kitchen with a food preparation area, food warmers and a table where the crew can eat.
The electronics aboard the ISS make more than enough heat to warm the station. In fact, the problem is getting rid of excess heat. So there are various methods to distribute heat evenly throughout the station. The temperature control system uses electric heaters, insulation and liquid ammonia pipes (heat distributors) to control the internal temperature. Radiators on the station's outside help eliminate the heat to outer space.
Like any home, the ISS must be kept clean. This is especially important in space, where floating dirt and debris could present a hazard. For general housecleaning, astronauts use various wipes (wet, dry, fabric and disinfectant), detergents and wet/dry vacuum cleaners to clean surfaces, filters and themselves. Trash is collected in bags, stowed in a supply ship and returned to Earth for disposal. Solid waste from the toilet is handled in a similar manner after it has been compacted, dried and bagged.