LIFE-SUPPORT TECHNOLOGY
The air inside the International Space Station (ISS) is kept at a similar pressure to Earth’s atmosphere at sea level and is made up of a similar mixture of gases: around 20 per cent oxygen and 80 per cent nitrogen. The astronauts inside are able to breathe as easily as we can down here on the surface of the planet, and they have been doing so on a continuous basis ever since the ISS was first occupied in November 2000. That’s all thanks to the ingenious technology that maintains a healthy atmosphere inside the space station.
A critical component of any crewed space vehicle is its Environmental Control and Life Support System, or ECLSS for short. This is a complex network of machinery, pipes, tanks and sensors designed to provide astronauts with all the air and other life-support essentials they need. Unsurprisingly, the most complex ECLSS of all is the one found on the ISS, which is required to support its unprecedented internal pressurised volume of more than 900 cubic metres.
As well as maintaining a breathable atmosphere, the ISS’ ECLSS includes a water recovery system that captures, purifies and recycles water both from cabin humidity and from crew members’ urine. As gross as the latter sounds, the result is perfectly drinkable. “Today’s coffee was yesterday’s pee,” as astronaut Tim Peake put it. There are other bodily side-effects that ECLSS has to deal with, too, such as the nasty smells from gas and sweat. The offending gases – methane and ammonia respectively – are removed using activated charcoal filters.
Recycling water, as well as constantly replenishing it with fresh supplies from Earth, is important for more than just drinking. It also provides the astronauts’ main source of oxygen for breathing thanks to the fact that every water molecule contains an atom of oxygen. Its chemical formula is H2O – two atoms of hydrogen and one of oxygen. These can be separated using a process called electrolysis, which involves passing an electric current through the water to split it into molecules of hydrogen and oxygen – two of the first and one of the second for every two molecules of water.
After discarding the hydrogen, what’s left is breathable oxygen.
A similar process is also used to generate oxygen on submarines, which have no shortage of water to electrolyse. Submarines also share the ISS’ problem of having to remove excess carbon dioxide – a well-established process known as CO2 scrubbing. Various technologies are available for this, including the use of potassium hydroxide as suggested by Jules Verne in his novel. This – and related chemicals such as soda lime and amines – have a propensity to lock on to CO2 molecules, thus removing them from the atmosphere. The ISS, on the other hand, uses a different method for CO2 removal involving a type of mineral called zeolite. The airflow inside the space station takes it over a bed of zeolite, causing CO2 and water to stick to the mineral while everything else passes through unhindered. The water is then retrieved for recycling, while the CO2 is ejected into space.
When astronauts go outside the ISS to perform a spacewalk, or extravehicular activity (EVA), they breathe pure oxygen rather than the usual oxygen-nitrogen mix. The spacesuits they use are effectively self-contained life-support systems, complete with their own oxygen supply as well as electrical power, a ventilating fan and an in-suit drink bag.