A Primary (Portable/Personal) Life Support System (Subsystem) (PLSS), is a device connected to an astronaut or cosmonaut's spacesuit, which allows extra-vehicular activity with maximum freedom, independent of a spacecraft's life support system. The PLSS is generally worn like a backpack. The functions performed by the PLSS include:
- regulating suit pressure
- providing breathable oxygen
- removing carbon dioxide, humidity, odors, and contaminants from breathing oxygen
- cooling and recirculating oxygen through the pressure garment, and water through a Liquid Cooling and Ventilation Garment or Liquid Cooling Garment.
- two-way voice communication
- display or telemetry of suit health parameters
- telemetry of an indicator of the wearer's immediate health (e.g. heart rate)
The air handling function of a PLSS is similar to that of a diving rebreather, in that exhaled gases are recycled into the breathing gas in a closed loop.
When used in a microgravity environment, a separate propulsion system is generally needed for safety and control, since there is no physical connection to a spacecraft.
Technologies being considered for application in future PLSSs include Pressure Swing Adsorption (PSA), a process by which CO2 can be separated from gas more efficiently, and through a repeatable process, as opposed to the current LiOH canisters, which become saturated with each use, and are limited to around 8 hours. By regenerating the sorbent (A sorbent is a material used to absorb or adsorb liquids or gases) during EVA, the size and weight of the sorbent canister can be greatly reduced. PSA accomplishes this by venting CO2 and water vapour into space.