Since the dawn of the Space Age engineers have struggled to work out how to eliminate carbon dioxide from the air inside spaceships. Rocket builders saw straight away that it wasn’t enough just to give the cosmonauts and astronauts a constant stream of oxygen. They needed to get rid of the carbon dioxide produced by the crew. It was just one of the many unprecedented problems that came up when humans decided to go into space. The submarines of the era solved the problem quite simply by using tubes or snorkels that rose out of the sea, pumping fresh air in and bad air out. But this was not a workable solution in space. Oxygen accounts for about 21% of the air we breathe, but the human body can survive perfectly well even in concentrations of 15-17%. Anything below that threshold will bring confusion and physical weakness. Just 0.04%, or 400 parts per million, of gas in the atmosphere is carbon dioxide. We may not notice it if the level of oxygen in the air we breathe falls by just a little bit, but we do if CO₂ goes up even by a trifle, because our bodies react by breathing faster.
The CO₂ cold war
That’s why, besides oxygen canisters to replace the stuff the astronauts breathe in, all spaceships are fitted with systems for catching and doing away with carbon dioxide –scrubbers. Each of these must be able to eliminate the CO₂ that every member of the crew breathes out, which is about a kilogram per head every 24 hours. In its Mercury, Gemini, Apollo and Shuttle programmes, NASA used chemical scrubbers. They pumped cabin air into porous baskets filled with lithium hydroxide crystals. The CO₂ reacted by forming lithium carbonate and water. The clean air was enriched with oxygen from pressurised cylinders and put back into the cabin. The problem was these filters had to be replaced regularly when the lithium hydroxide had converted entirely into carbonate.
The Soviets made their space conquests with a different system. For Laika in Sputnik 2 and Gagarin in Vostok 1, their oxygen was stored not as a gas under pressure, but in its solid state as potassium superoxide (KO₂). The stale air was pumped into the KO₂ container, prompting an exothermic reaction that captured the water, in turn releasing oxygen and potassium hydroxide (KOH). The oxygen the cosmonauts breathed out was replaced with more oxygen, while KOH reacted with the CO₂ to form potassium carbonate. Thus carbon dioxide and water from breathing were eliminated, instruments were kept warm and the necessary oxygen was replenished. All that was needed for pumping was electricity, there were very few moving parts liable to break and there was no gas under pressure. The system worked well, what with such minor sizes and weights, which is why it was used again for Voskhod and Soyuz.
The problem of how to build efficient scrubbers reared its head again with the first permanent space stations. They abandoned the chemical systems used in spaceships and came up with new ones reliant on adsorption. These contained a porous solid very similar to carbon dioxide, which got stuck to it as it was pumped over the top. Once this molecular sellotape is saturated with CO₂, all you have to do is close the valves linking it to the cabin and open the ones leading out of it, so the CO₂ flows out into space. Now the material is ready for a new adsorption cycle. The same system extracts the water breathed out by the crew. Scrubbers were installed at the Skylab space station and then in the American section of the International Space Station (ISS), based on silicon dioxide and aluminium crystals called zeolites. They’re molecular sieves with carefully sized holes, specifically cut to fit molecules of certain dimensions as tightly as possible. Specifically the Americans use zeolite 13X (perfect for absorbing water) combined with zeolite 5A (perfect for absorbing carbon dioxide).
Two of these systems, both of them called Carbon Dioxide Removal Assembly (CDRA), are used at Node 3 and the Tranquillity laboratory in the American section, and are enough to keep a crew of four people plus a few guinea pigs alive. The Russians adopted a different system, which they christened Vozdukh. Installed in the Russian Orbital Segment’s service module, it adsorbs H₂O and CO₂ thanks to the basic properties of three different amine beds. The technology is simpler and has no moving parts, apart from valves. It’s based on the experience the Soviets had on the six Salyut space stations and then on MIR. The latest version, installed on board the ISS, can support a crew of six indefinitely, removing 3,000 litres of carbon dioxide a day. It makes the ISS’s overall system redundant. Vozdukh is the main system but CDRA intervenes when Vozdukh is undergoing maintenance. Should both systems fail at the same time, then Soyuz craft, which are always linked to the ISS, have old-school chemical scrubbers on board. These old models can still sustain a team of three for 15 days. After Apollo 13, the ISS didn’t want to leave anything to chance, so until 2011 their shuttles also had adaptors on board which could link American scrubbers up to Russian baskets and make them work.
The International Space Station in orbit
The first space colonies
Another giant leap that space explorers will have to make is creating permanent space stations or colonies, beginning on the moon and Mars, with purification systems to maintain a liveable environment indefinitely. On the moon, where there’s no atmosphere, you could use the two-stroke scrubbers made for the ISS, but on Mars it’s more of a problem, because the Red Planet’s atmosphere is made almost entirely of CO₂. An adsorbent bed exposed to the elements on Mars would never recover; it would be completely poisoned under the abnormally high pressure from the carbon dioxide. This is why the MIT and the Niels Bohr Institute installed the Mars Oxygen In-Situ Resource Utilization Experiment (MOXIE) on board their rover Perseverance, which they launched on 30 July 2020 and which should land on the Red Planet on 18 February 2021 as part of the MARS 2020 mission. This experimental system should show that it’s possible to convert CO₂ from Mars’ atmosphere into carbon monoxide and oxygen.
Another system, Sabatier, has already been tested on the ISS. It transforms the water expelled by a team using an electrolyser, so the oxygen it produces can be breathed back in by them or be used as combustion fuel in their craft’s propulsion system. Hydrogen is combined with the carbon dioxide that comes out of the crew to get methane and water through a catalytic process based on nickel. This is a completely local catalyst, because there’s enough of it in the rocks on Mars. The methane can then be mixed with oxygen in the propulsion system. Stoichiometry tells us there’s also a small percentage of extra oxygen left over, which can be used to support the crew. All these technologies can be used while building a colony, but once that colony is fully up and running it needs to be autonomous and not rely on refuelling from earth. Our best bet for air regeneration technology is probably biological systems. Once they’re set up, they’ll have the advantage of not needing extensive maintenance, as the organisms within them multiply and repair themselves naturally.
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