CO2 scrubbing and the lithium hydroxide cans

You’re sitting in a tin can hurling through a vacuum at seventeen thousand miles per hour. The views are incredible. The coffee is freeze-dried. But every breath you take is a quiet transaction with death. Without a machine the size of a tackle box, you would drown in your own exhaust. That machine relies on chemistry so simple it feels almost primitive. It’s called a lithium hydroxide canister, and it is the unsung reason no human has asphyxiated in orbit.

Let’s get the basics straight. You breathe in oxygen, your cells burn it for energy, and you breathe out carbon dioxide. CO2 is not toxic in small amounts, but in a sealed spacecraft, it builds up fast. At concentrations above one percent, you get headaches, dizziness, and impaired judgment. At four percent, you’re unconscious. At eight percent, you die. The International Space Station cycles through about four pounds of CO2 per person per day. That waste gas has to go somewhere. It cannot simply be vented into the void because the cabin is a closed loop system designed to conserve atmosphere. So engineers reached for a chemical trick discovered over a century ago.

Lithium hydroxide powder reacts with carbon dioxide to form lithium carbonate and water. The reaction is exothermic, meaning it produces heat, but more importantly, it is irreversible at standard spacecraft conditions. You pack the powder into sealed cans containing a granular bed. Air from the cabin is blown through the can, the CO2 molecules stick to the lithium hydroxide lattice, and the cleaned air circulates back. It is a one-way chemical sponge. Once the canister is saturated, it gets tossed into the trash, or more accurately, stowed for disposal. The NASA Apollo missions used these cans, the Space Shuttle used them in portable emergency units, and modern suits for spacewalks still rely on them for short-duration CO2 control.

Why not just use plants? Because biology is slow, heavy, and finicky. A single astronaut produces as much CO2 as about fifty houseplants can scrub. Growing that many plants indoors would take up volume, weight, and water budget that is better spent on propulsion or science. Mechanical carbon dioxide removal systems exist on the ISS, using a process called adsorption beds with zeolites that heat up and vent CO2 into space. But those systems are complex, power-hungry, and prone to failure. Lithium hydroxide is dead simple. No moving parts. No electricity draw. Just a chemical reaction that works until the powder is gone. It is the emergency backup that keeps the big systems honest.

The trick is managing the cans. They have a finite life. On the ISS, the primary system regenerates, but the backup lithium hydroxide cans are swapped out manually. Each can lasts about one person-day, meaning a crew of six goes through six cans per day. In the early days of the station, this was a non-issue because supplies were abundant. But as commercial resupply missions tighten their margins and long-duration missions to Mars become real, the logistics get ugly. A round trip to Mars takes roughly three years. You cannot pack three years worth of single-use chemical cans. The weight penalty would be astronomical. That is why NASA and private companies like SpaceX are investing in regenerable systems that use heat or electrical currents to release captured CO2 into space or process it into oxygen and methane.

But for now, the lithium hydroxide can is the workhorse. It is the reason the Apollo 13 crew survived after their oxygen tank exploded and the primary systems failed. The command module’s lithium hydroxide cartridges were square, but the lunar module’s receptacles were round. Ground engineers MacGyvered a solution using plastic bags, duct tape, and cardboard to make the square cans fit the round hole. That improvised scrubber kept the crew alive for four extra days. It is one of the most famous engineering hacks in history because it was simple, ugly, and it worked.

What does this mean for you, the guy who just wants to understand how we aren’t dead in space? It means that life support is not glamorous. It is not warp drives or laser cannons. It is a white powder that grabs waste gas and turns it into rock. Every canister you see in a spacecraft photo is a silent chemical lifeguard. It does not complain. It does not need updates. It just sits there until you need it, then it sacrifices itself.

As we push further out, the demand for smarter, lighter, recyclable CO2 scrubbers will increase. But do not underestimate the humble lithium hydroxide can. It is the reason we have not suffocated in orbit yet. And that is the whole point of life support: not dying is the baseline for every other achievement. The can does its job so you can do yours.