We have frequently heard of spacecraft delivering cargo and crew to the International Space Station. One such cargo craft Dragon was delivered by the SpaceX’s Falcon 9 nearly a month ago. Among everything else, there has never been mention of oxygen. How do people on the ISS survive such long periods with the limit of the cargo that can be delivered there? Is it carried in heavy gas cylinders like the ones used on Earth?
Oxygen is not transported to the ISS; it is generated on the space station. A Russian device called Elektron is used to make oxygen using water through electrolysis. The produced oxygen is then stored in a pressurized storage tank while the hydrogen is dumped into space. The electrolysis is powered by the massive solar arrays of the ISS. The newer way of producing oxygen on the ISS with the 1,500 pounds Oxygen Generation System (OGS). It works in a way similar to the Elektron and produces 12 pounds of breathable oxygen per day.
Another way of transporting oxygen to the space station are the SFOG (Solid Fuel Oxygen Generator) candles based on lithium perchlorate. Canisters of lithium perchlorate are carried to the ISS, and each can support one crew member for a day.
The US segment of the ISS also carries NORS (Nitrogen Oxygen Recharge System) along with other cargo in the cargo craft like the Japanese HTV, OrbitalATK’s Cygnus, and SpaceX’s Dragon.
In addition to exhaling large amounts of carbon dioxide, humans also produce gases like ammonia and acetone in small quantities. Life support systems on the ISS are very complex because they have to prevent the accumulation of these gases while maintaining the pressure of the cabin. Before astronauts go out on spacewalks, they breathe in oxygen through a mask for hours to eliminate any oxygen in their blood, which would be trouble in the low-pressure spacesuits that they need to wear. This is done using Recharge Oxygen Orifice Bypass Assembly (ROOBA), the oxygen reserve of the space station.