CO2 Scrubber (CDRA)

Every breath an astronaut exhales adds carbon dioxide to a sealed cabin, and on a space station there is no open window to let it out. The CO2 Scrubber, formally the Carbon Dioxide Removal Assembly (CDRA, pronounced “see-druh”), is the machine that pulls that carbon dioxide back out of the air so the crew can keep breathing.

Quick facts

• Type: a four-bed molecular sieve (a packed filter whose pores act as a microscopic screen) that captures CO2 and can be reused over and over.
• What does the trapping: zeolite crystals, made of silicon, aluminum, and oxygen. One type grabs water (a desiccant, meaning a drying agent), and another grabs the CO2.
• Capacity: continuously removes the CO2 produced by about 6 people when both CO2 beds are running.
• Target air quality: NASA holds the cabin’s average CO2 partial pressure (the share of the total air pressure that comes from CO2 alone) to no more than 3 mmHg over any hour. Keeping the 7-day average below about 2.5 mmHg holds headache rates under 1%.
• Where: two units on the US side of the International Space Station (ISS) — one in the Destiny laboratory module, one in Node 3 (Tranquility). The Russian side uses its own scrubber called Vozdukh.

How it works

Cabin air first passes through a drying bed, because water vapor would otherwise crowd CO2 out of the trapping spots. The dried air then flows through a zeolite bed whose tiny pores hold onto the CO2, a process called adsorption (molecules sticking to a surface, not soaking in).

The clever part is regeneration. The system runs as two paired sets of beds that take turns, like two sponges where one soaks while the other is wrung out. While one CO2 bed is busy trapping, its partner is sealed off, heated, and opened to the vacuum of space, which drives the captured CO2 back off the zeolite and leaves the bed fresh for the next round. On the way out, the cleaned air flows back over the drying bed to pick the moisture up again and return that water to the cabin. The cycle never stops, and no material is used up — only electrical power for the blowers, valves, and heaters.

This is the key contrast with the older way. Apollo and the Space Shuttle used single-use lithium hydroxide (LiOH) canisters, where the chemistry (CO2 + 2 LiOH makes Li2CO3 + H2O) permanently consumes the chemical, so each canister must be thrown away once full.

Why it matters

Removing CO2 is life-critical. In a sealed cabin it would otherwise build up and cause headaches, fuzzy thinking, and eventually loss of consciousness. A regenerable scrubber is what makes living in space for months at a time practical: a six-month mission would otherwise need an unworkable pile of disposable canisters.

It also enables “loop closure” — recycling resources instead of throwing them away. The CO2 that CDRA captures can be sent (through a compressor that raises its pressure) to a Sabatier reactor, where it reacts with hydrogen to make water — recovering oxygen and cutting how much must be shipped up from Earth. That recycling is a cornerstone for future Moon and Mars missions.

Trade-offs and the next generation

CDRA’s strengths come with costs. It conserves water and never runs out of sorbent (the material that traps the CO2), but it is bulky, mechanically complex, and needs significant power. Over roughly 30 combined unit-years in orbit it has demanded more crew maintenance time than almost any other ISS system, largely because zeolite dust shed by the beds fouls valves and seals. LiOH canisters, by contrast, are simple, light, reliable, and need no power, but are single-use — which is why they remain only as an ISS backup.

To fix the dust and upkeep problems, NASA launched an upgraded successor, the 4-Bed CO2 Scrubber (4BCO2), to the ISS on the NG-16 resupply mission in August 2021. It aims for 3 years with no unplanned maintenance using low-dust zeolite and redesigned beds, heaters, and valves. Other regenerable approaches, such as amine-based scrubbers, are also under evaluation, making the CO2 scrubber a key proving ground for deep-space life support.