A primary (or portable or personal) life support system (or 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. A 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.