Life Support Dependability for Distant Space Missions

The dependability of a life support system for a long duration mission can be specified using any of several metrics availability, reliability, probability of Loss of Crew (LOC), or Loss of Mission (LOM). The availability, reliability, and probability of LOC LOM are defined and the relations between them are explained. The appropriate dependability metric depends on the mission, but the availability, reliability, and probability of LOC LOM have a common basis. Decreasing the life support system failure rate will improve all these dependability metrics. High reliability life support can be achieved by providing spares to replace failed components. Long duration, short distance missions such as the space station or a permanent moon base can use frequent resupply and rotating crews. Spares can be provided when a failure occurs, or if repair is not possible, the crew can return to Earth. For a long distance, long duration mission such as a Mars visit or asteroid rendezvous, providing emergency spares or on-demand crew return is not possible. All the spares must be on hand at the beginning of the mission. The required additional mass for the spares needed to guarantee very high reliability is approximately equal to the original system mass, provided the original system reliability is not too low. Examples show how high reliability can be achieved by providing spare parts. The life support dependability metrics are computed for Moon and Mars missions.