The chance of a spacecraft colliding with meteoroids or orbital debris increases with the size of the spacecraft and the time it spends in orbit. The International Space Station (ISS), a multibillion-dollar crewed orbiting laboratory, will be the largest spacecraft ever built and is expected to remain in orbit for at least 15 years. Due to its large size and long operational lifetime, the ISS will face a significant risk of being struck by potentially damaging meteoroids or orbital debris. This report is the National Research Council assessment of ISS program efforts to protect the space station from meteoroids and debris.
Both the overall ISS risk management process (which primarily addresses cost and schedule risks to the program) and the ISS safety office (which focuses on threats to crew safety) monitor and react to the meteoroid and debris hazard. An analysis integration team (AIT) staffed by the National Aeronautics and Space Administration (NASA), Boeing, and the international partners (Canada, Japan, Russia, and the member nations of the European Space Agency) has been established to study the threat from meteoroids and orbital debris, develop and evaluate countermeasures, and provide input to the risk management processes. Although the ISS risk management approach appears sound, the unique nature of the meteoroid and debris hazard has made it difficult for the ISS risk management schemes to properly weigh the risk from meteoroids and debris against other risks to determine whether action should be taken. The ISS program needs to ensure that the findings of the meteoroid and debris AIT are communicated clearly to program managers.
The meteoroid and debris AIT has developed an approach to reduce the hazard posed by meteoroids and debris to the station and crew. The team plans to shield the ISS against smaller objects and to maneuver the ISS to avoid collision
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
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.