more than 95 percent of the objects that could cause critical damage to the orbiter are not cataloged because they are too small to be reliably detected by SSN sensors.

The capabilities of the SSN to support NASA’s efforts for collision avoidance are eroding, and until recently, NASA had issued no requirements that might have helped to halt this erosion. NASA and the DOD should work together to satisfy these requirements, to identify impending changes to the SSN that will affect debris tracking, and to identify changes that would improve the SSN’s ability to track smaller objects that pose a hazard to crewed spacecraft.

Once NASA has received a warning of an upcoming close conjunction, it must decide whether to maneuver the shuttle to avoid a collision. Two flight rules (A4.1.3–6 and C4.3.2–1) that are relevant to this decision appear to place mission success ahead of flight safety. NASA should re-examine these rules and consider restating them to establish when a maneuver is mandatory for safety reasons. NASA plans to use a new probability-based approach to determine when a collision avoidance maneuver is necessary, but the collision avoidance data currently provided by the SSN is not accurate enough for this new approach to be effective.

RISK MITIGATION

The space shuttle program has developed operational procedures, and is about to implement hardware modifications, to improve the survivability of the shuttle orbiter and crew in the face of the meteoroid and orbital debris hazard. In the future, however, when the orbiter is supporting the International Space Station, many of the operational techniques developed to improve the orbiter’s survivability will not be employed because the shuttle’s freedom to maneuver and control its attitude will be constrained to satisfy requirements for space station power, thermal conditions, and attitude control. The effect of these restrictions on the shuttle’s survivability should be reassessed.

NASA plans to modify the orbiter’s radiators and wing insulation to reduce the risk of early mission termination and critical failure. These modifications appear to be positive steps that will have a minimal negative impact on the program. NASA should continue to investigate potential modifications to the orbiter to improve its survivability against meteoroids and orbital debris. NASA should also reconsider conducting on-orbit surveys to detect exterior impact damage and repair it as necessary.



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