An Environmental Control and Life Support System — ECLSS — is the hardware that keeps a human alive inside a sealed metal can in vacuum. It removes carbon dioxide, generates oxygen, recovers water, controls temperature and humidity, manages cabin pressure, and processes waste. It is the most critical hardware on any crewed spacecraft, and the least glamorous. For any mission longer than a week or two, it is also the hardest.
The Basic Problem
A human being at normal activity consumes roughly 0.8 kg of oxygen, 3 kg of water, and 1.5 kg of food per day, and produces roughly 1 kg of CO2, 1.6 kg of water vapour and sweat, and a kilogram or so of solid and liquid waste. A crew of four over a year therefore needs about 4 tonnes of oxygen, 4 tonnes of food, and 12 tonnes of water — if you resupply nothing and recycle nothing.
The entire design problem of long-duration life support is reducing that mass through recycling. Every kilogram of water you recover and reuse is a kilogram you did not have to launch. For a Mars mission, where the delta-v cost of a launched kilogram is several times a launched-to-low-Earth-orbit kilogram, that compounding matters enormously.
ECLSS on the ISS
The International Space Station has one of the most developed ECLSS in the world. Its current systems recover about 98 percent of water — from urine, humidity condensate, fuel-cell effluent, and hygiene waste — and about 50 percent of oxygen, mostly by electrolysing recovered water and cracking CO2 in a Sabatier reactor to pull oxygen back out of metabolic byproducts.
The main functional subsystems on the ISS are:
- Oxygen Generation System (OGS). Electrolyses water into oxygen and hydrogen. The oxygen goes into the cabin; the hydrogen was originally vented, but since the Sabatier reactor was added, much of it is reacted with CO2 to produce methane (vented) and water (recovered).
- Carbon Dioxide Removal Assembly (CDRA). Uses a molecular sieve to pull CO2 out of cabin air. Regenerable; the sieve is heated periodically to release the captured CO2, which is then fed to the Sabatier reactor or, historically, vented.
- Water Recovery System (WRS). Distils urine under vacuum to recover water, then filters and polishes it through multi-stage treatment. The recovered water is used for drinking, hygiene, and feeding the electrolyser.
- Trace Contaminant Control Assembly (TCCS). Removes trace organic contaminants — solvent vapours, ammonia, chemical outgassing from plastics — that accumulate in the sealed atmosphere and would eventually reach toxic levels.
- Temperature and Humidity Control. Condenses excess humidity (which is then routed to the WRS) and maintains cabin temperature against thermal loads from equipment and crew.
The Russian segment has parallel systems serving similar functions, using different technologies (Elektron electrolysis, Vozdukh CO2 removal, Rodnik water tanks), with crew cross-supporting if one side has a failure.
What Can Go Wrong
Every long-duration ISS expedition deals with ECLSS anomalies of some kind. The most common failure modes:
- Clogged distillation assemblies in the urine processor, usually from mineral precipitation when recovered brine gets too concentrated. Fixing this has historically meant shipping replacement assemblies up on Progress or Dragon.
- Valve failures in regenerable CO2 scrubbers, which can leave the crew having to switch to lithium hydroxide canisters while repairs happen.
- Accumulation of trace contaminants when the TCCS filters saturate faster than planned. Symptoms are non-specific — headaches, eye irritation — and tracing the contaminant source often takes weeks.
None of these have ever been catastrophic, but all of them would be much worse in a scenario where no resupply is possible. On the ISS, the worst case is always "we can deorbit the crew in hours." On a Mars transit, there is no off-ramp.
What a Mars Mission Needs
A Mars-class ECLSS has to do everything the ISS system does, but with much higher reliability, much higher closure, and no ability to ship up replacement parts. Key differences:
- Closure near 100 percent for water and near that for oxygen. The ISS can afford to lose a couple of percent of water and refill from resupply; a Mars vehicle cannot.
- Minimal consumables resupply. Everything the crew needs for a two to three year mission has to be either launched with them or generated from local resources at Mars.
- Crew-repairable. No ground team can send a technician. Every subsystem has to be maintainable with the spare parts and tools that fit on the vehicle.
- Redundancy at the component level. Multiple CO2 removal paths, multiple water recovery paths, and the ability to operate on reduced performance indefinitely rather than pushing to a fix-or-die endpoint.
NASA has been developing the building blocks for such a system for years. The Advanced Closed Loop System (ACLS) rack on the ISS is a testbed for higher-closure technology; other research is focused on biological components (algae or higher plants as supplementary oxygen producers and food sources) that could eventually add resilience and food generation to the same hardware.
Why This Matters
Every ambitious human-spaceflight plan — Artemis Base Camp, crewed Mars, orbital colonies, lunar settlements — runs on the same assumption: that ECLSS will scale and keep closing the loop. That is an assumption, not a demonstrated fact. The ISS has shown that 98 percent water recovery is achievable, but the remaining 2 percent matters enormously at the multi-year scale, and oxygen closure is still well behind. Life support is the invisible spine of crewed spaceflight. When it works, nobody notices. When it fails, everybody dies.