with objects large enough to be tracked by ground-based radar. Damage control hardware will be deployed, and procedures will be implemented to mitigate the effects of collisions with objects too small to be tracked by radar and too large to be stopped by the ISS shields.

To provide the information needed for effective risk management, NASA has developed models of the meteoroid and debris environment in the orbit of the ISS. Over the past five years, NASA has done a good job of improving these models by incorporating new data and by making reasonable assumptions about areas where data are sparse. NASA should continue to update these models with new data and analyses and make the models available for peer review. Although recent models of the debris environment differ considerably from older models in a few areas, elements of the ISS program still use the outdated models. This is justifiable in some cases, but the ISS program should strive to ensure that the most recent meteoroid and debris environment models are used wherever possible.

In general, the effort to shield the ISS from meteoroid and debris impact appears extensive and thorough. However, some portions of the ISS, primarily in the Russian segment, are currently expected to be much less well protected from meteoroid and debris impact than other areas. The ISS program must strive to improve shielding for areas of the ISS that do not yet meet requirements. Further efforts to improve coordination with the Russian Space Agency on meteoroid and debris issues should be explored to help ensure that all parts of the ISS are adequately protected.

A shortcoming in ISS shield design is that the shields have been designed to protect the ISS against a hazard considerably different in some respects from that currently expected. Recent models show that debris may strike the ISS at a lower velocity and from a wider range of directions than previously thought. Because the actual environment under which the shields must protect the station is still not well known, future shields should be designed to withstand a broader variety of threats.

The ISS program should initiate an accelerated shield testing program to ensure that the currently planned shield designs are effective against the expected threat and to aid in the design of future shields. Increased emphasis should be placed on the lower velocity regimes and on gaining a better understanding of secondary ejecta. The ability of extravehicular activity suits to protect astronauts in the current predicted environment also should be assessed.

To further improve ISS shielding, NASA should consider upgrading its capability to perform computer-simulated impacts, perhaps by working with other national facilities. In addition, the meteoroid and debris AIT should consider holding a workshop to bring in experts from outside NASA to discuss the use of advanced shielding materials for future ISS shields.

The ISS team has been slow to concentrate on damage control issues, but it has begun to develop hardware and procedures to aid the ISS crew in the event of



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