The major spaceflight risks are launch, life support, special operations such as Extravehicular Activity (EVA), and reentry. Life support has had slightly greater risk of causing a crew fatality than either launch or reentry. The long duration of a Mars mission greatly increases the risks of life support and special operations. The life support risk for Mars could be very large. Ultra reliable life support is needed in deep space because providing resupply and spare parts or a quick crew return are not possible. Mars mission planning usually assumes that high reliability recycling life support systems will available when needed. Providing spare parts to repair systems can improve reliability but cannot eliminate common cause failures due to specification and design errors. Diverse redundant systems are needed to prevent common cause failures. The required Probability of Loss of Crew and the system reliability together determine how much system redundancy is needed.
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