a serious meteoroid or debris impact. The team has developed a software tool, MSCSurv, to assess the relative merits of various damage control procedures and devices. NASA should continue to refine this program and to update it to reflect failure modes associated with all critical and high-energy systems, toxic gas releases, nonpenetrating impacts, and equipment and system failures caused by impacts.

The ISS program should accelerate its efforts to plan for damage control and repair. As part of this effort, NASA should intensify its work with the Russian Space Agency to identify and resolve differences in damage control hardware and procedures. The program also needs to study the failure modes of shielded pressure walls and to assess the capability of the ISS to continue safe operations with damaged wiring, piping, and other systems.

The ISS program plans to maneuver the space station to avoid debris large enough to be tracked and cataloged by the U.S. Space Surveillance Network (SSN). (Because of the limited maneuvering capability of the ISS, onboard sensors will be ineffective in providing collision avoidance services.) The ISS program expects the SSN to alert the space station several hours in advance when a close encounter is predicted. If it appears that the ISS is in the path of an oncoming object, the station will maneuver out of the way.

The current debris environment model suggests that the ISS can expect to receive about 10 warnings per year that may require an avoidance maneuver. The ISS program should work on reducing the number of false warnings, perhaps by increasing the accuracy of locating threatening objects. The ISS program currently has no plans for maneuvering the station during some phases of the assembly sequence or when the shuttle is docked, due to concerns about the structural integrity of some ISS configurations under acceleration. The ISS program should work to ensure that maneuvering capability is always available.

The risk that the ISS will collide with untracked debris could be lowered if more objects were tracked. The number of objects being tracked could be increased by improving the sensitivity of the tracking radars and by using optical sensors, but this would require significant effort. The future capability of the SSN, however, may actually decrease due to sensor shutdowns or other actions caused by budgetary pressures. NASA should work closely with the SSN at the highest level of authority to determine what support the network will be able to provide the ISS over its lifetime.

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