ECLSS (ISS Life Support)

Every breath an astronaut takes aboard the International Space Station, and nearly every drop of water they drink, comes from a system that quietly recycles the air and water around them. That system is ECLSS, and without it, the station would be uninhabitable within hours.

Quick facts

• Full name: Environmental Control and Life Support System (ECLSS) – the suite of machines that keeps the ISS cabin safe to live in.
• Cabin air: held at about 101.3 kilopascals (14.7 psi), the same pressure as Earth at sea level, using a normal mix of nitrogen and oxygen rather than pure oxygen, for fire safety.
• Oxygen source: made on-orbit by electrolysis – splitting water into oxygen and hydrogen using electricity.
• Water recycling: running since 2008-2009; for years it recovered about 93-94% of water, and in June 2023 NASA demonstrated a record 98% overall recovery.
• Daily need: each crew member uses roughly 1 gallon (about 3.8 liters) of water per day.
• Built by: NASA Marshall Space Flight Center with UTC Aerospace Systems, Boeing, Lockheed Martin, and Honeywell.

What it is and how it works

ECLSS is organized into three regenerative blocks – meaning they renew air and water instead of just consuming supplies – plus supporting subsystems.

The Oxygen Generation System (OGS) runs electricity through clean water to release oxygen into the cabin and capture hydrogen. Russia’s Elektron unit does the same job, while chemical “candle” generators (Vika/SFOG, which burn lithium-perchlorate canisters) and stored oxygen serve as backups.

The Air Revitalization System cleans the air. The Carbon Dioxide Removal Assembly (CDRA) and Russia’s Vozdukh use reusable molecular-sieve beds – filters that trap and then release gas – to strip out the carbon dioxide crews exhale. The Trace Contaminant Control Subassembly (TCCS) uses activated charcoal and a catalytic oxidizer (a unit that burns off pollutants) to remove off-gassed contaminants like ammonia and methane that come from electronics, plastics, and the human body. The Major Constituent Analyser (MCA) watches the gas mix.

The Water Recovery System turns waste back into drinking water. The Urine Processor Assembly (UPA) distills urine under near-vacuum, using a spinning centrifuge to substitute for the gravity that would normally separate liquid from vapor. The Water Processor Assembly (WPA) blends that distillate with cabin humidity, pushes it through multifiltration beds and a catalytic oxidation reactor, then checks purity with electrical-conductivity sensors before storing it. A newer Brine Processor Assembly (BPA) wrings still more water out of the leftover brine. Think of it as a kitchen that keeps reusing the same ingredients rather than throwing scraps away.

Closing the loop further, the Sabatier reactor combines exhaled CO2 with leftover hydrogen to make water (recycled) and methane (vented to space). Other subsystems manage temperature, humidity, ventilation, and fire detection and suppression.

Why it matters

ECLSS is what makes a permanently crewed station possible. The ISS has been continuously inhabited since 2000 because this system produces breathable air and clean water faster than cargo ships can deliver them. Every liter of water and breath of oxygen made on-orbit is mass that never has to be launched from Earth, which sharply lowers the cost of keeping the station running. Just as importantly, ECLSS is NASA’s main testbed for the closed-loop life support that future missions will require – the Artemis lunar program, the Gateway station, and crewed trips to Mars, where resupply is impossible and pushing water recovery toward 98-100% directly determines how long a crew can stay away from home.

Notable examples and trade-offs

• Oxygen Generation System (OGS): operational since July 12, 2007; acidic-water trouble in October 2010 was resolved in March 2011 with a repair kit delivered by the STS-133 shuttle mission.
• Elektron (Russian segment): repeated failures (2004, 2005, 2006, October 2020) showed why redundant oxygen sources are essential.
• Brine Processor Assembly (BPA): the upgrade that pushed total water recovery to a record 98% in June 2023.
• ESA Advanced Closed Loop System (ACLS): delivered on HTV-7/Kounotori 7 in September 2018; uses a Sabatier reaction to regenerate oxygen for about three crew and save roughly 400 liters of water per year.

The trade-offs are revealing. The station deliberately uses sea-level mixed air instead of low-pressure pure oxygen to cut fire risk, accepting the need for nitrogen and stronger pressure control. Recycled water from urine sounds unsettling but is purified cleaner than much municipal tap water. And the UPA’s urine recovery had to be dialed back from an 85% target to about 70%, because microgravity bone loss raises the calcium in urine, which combines with the treatment chemicals to form crystals that can clog the distiller – a case of biology constraining hardware. The loops that remain open, like vented methane and brine, are the very problems being solved before crews can reach Mars.