ELSS

An extravehicular life support system, or ELSS, is a self-contained unit worn by an astronaut during spacewalks. It supplies oxygen, removes carbon dioxide, regulates temperature, and provides communications and power for the suit itself. The ELSS is essentially a life-support backpack that lets a suited crew member survive and work in the vacuum of space for several hours.

The ELSS carries liquid cooling and ventilation garment loops that circulate water through tubes in the suit to shed metabolic heat; without active cooling, an astronaut in direct sunlight would overheat rapidly. It houses carbon dioxide scrubbing canisters (lithium hydroxide or similar absorbents) that prevent CO2 buildup in the helmet, which would otherwise cause hypercapnia within minutes. Oxygen is supplied from high-pressure tanks in the backpack at carefully regulated pressure to maintain breathable atmosphere in the suit.

Two classes of ELSS exist in current spaceflight. Primary life support systems (PLSS) on NASA's extravehicular mobility units (EMUs) can sustain a suited astronaut for up to seven hours depending on workload and suit configuration. Simpler secondary systems, such as emergency oxygen supplies or short-duration chest packs used on some international vehicles, provide only an hour or two of emergency life support to allow return to the spacecraft.

Reliability and redundancy

Failure of an ELSS component during a spacewalk is life-threatening. Multiple redundancies are therefore built in: dual oxygen regulators, dual carbon dioxide scrubbing cartridges that can be switched during a walk, and independent water loops for cooling. Communications and caution-and-warning systems alert the astronaut and ground control to any degradation in system parameters long before a critical failure occurs.

The ELSS sits at the heart of extravehicular activity operations. Without a functioning backpack, no spacewalk happens. Maintenance personnel must certify every component, and pre-breathe protocols before each EVA exist partly to prevent decompression sickness that might stress the life support system during the walk itself. The term ELSS is common in NASA and ESA documentation; Russian systems use different nomenclature but serve the same function.