Risk assessment sensitivities for very low probability events with severe consequences

Modern aerospace systems are typically designed to satisfy numerous very stringent performance requirements. The risks posed thus encompass very low probability events with severe consequences. Long-term operational experience with these systems may produce no occurrences of the events, yet the risk of the severe consequence remains and good decision making requires understanding of the risk. With no observed events, classical methods cannot be used to assess these risks quantitatively. Conditional methods may however be used very effectively to produce assessments of the risk when no actual events have been observed, without the use of questionable assumptions. This report develops a quantitative risk assessment for astronaut bone fracture during an extended microgravity exposure using these conditional methods. The microgravity environment experienced by astronauts in space is well known to reduce astronaut bone mineral density, approximating osteopenic and osteoporotic conditions that significantly increases risk of fracture for the elderly here on earth. The risk of bone fracture for astronauts working in a microgravity environment is believed to similarly increase. At this point, it is entirely unknown what the effects of a broken bone would be in a space environment, whether healing could occur or not, and how life threatening a fracture would be in the space environment. To date, no astronaut has ever broken a bone during microgravity space operations. Conditional methods are used to develop a quantitative risk assessment for consideration of extending mission durations for the International Space Station, and for risk assessments for long duration missions to Mars. The effect of a hypothetical bone fracture is further investigated as to how a single event can affect the risk assessment.