over a circuit board, severing an electrical wire, or creating a plasma). In these more complex failures, additional hypervelocity testing may be required to determine failure mechanisms.
A solution to reducing failure rates from such collisions can be to add shielding, which sometimes must be customized to minimize the added weight to the spacecraft. Where uncertainty exists, safety factors are sometimes added. In other cases, the risk can be reduced by redundant systems or changes in operations, such as orienting the spacecraft in a direction that minimizes the risk (as has been done for the space shuttle).
It is the responsibility of the MMOD programs, usually through the Hypervelocity Impact Technology Facility, to coordinate with the program managers and offer the best solutions for their mission. The MMOD programs have put together a handbook to aid in selecting spacecraft protection options.14 As stated in the handbook, the definition of “failure” has a significant influence on the resulting risk. For example, failure may be defined as a penetration of a critical item that could lead to either loss of the function of the item, or loss of the crew. Such a definition could lead to a significant amount of hypervelocity testing and shielding development, as was the case for the critical items identified for the ISS.15 Alternatively, the failure criterion could be as simple as the depth of the pits on a window pane that might lead to loss of the window during launch, which was one of the critical items identified for the space shuttle.16 In this case a sufficient amount of hypervelocity testing had already been conducted to identify the frequency with which such pits would occur, and all that was then necessary was to operationally plan to examine the windows after each flight and have enough spare windows on hand as replacements when craters were found that exceeded the critical depth.
The focus on collisions and risk from penetration does not, however, fully cover all of the risks involving orbital debris and interplanetary meteoroids. Other risks are discussed more fully in other chapters. In some cases the difficulty in assessing risk is not the result of poor analysis but is the result of lack of data; for example, as is pointed out in Chapter 4, the risk analysis to be performed is sound but suffers from the lack of measurements in the interplanetary environment.
Finding: NASA’s MMOD risk assessment processes have evolved beyond focusing primarily on the damage to spacecraft from collisions with debris that are too small to track, to incorporating a more complete range of risks. More remains to be accomplished, however, including the need in some cases for more measurements as parameters for risk analyses. As gaps are filled, NASA’s MMOD efforts can progress toward ever more integrative risk assessment in which all sources and types of risk are modeled and assessed.
Recommendation: Although NASA should continue to allocate priority attention and resources to collision risks and conjunction analysis, it should also work toward a broad integrative risk analysis to obtain a probabilistic risk assessment of the overall risks present in the MMOD domain in which all sources of risk can be put in context.
Communication of Information About Uncertainty
NASA’s work on reducing the threat to spacecraft posed by orbital debris and meteoroids faces increasingly challenging problems stemming from the complexity of physical changes in space, changing spacecraft designs, increased international use of space and contributions to debris, and private and public sector initiatives in space. An intrinsic challenge also exists in creating models that fully capture the uncertainties and the phenomena being modeled. Examining the sources of the uncertainties, how to reduce uncertainties, identifying those that cannot
14 E. Christiansen, J. Arnold, A. Davis, D. Lear, J.-C. Liou, F. Lyons, T. Prior, M. Ratliff, S. Ryan, F. Giovane, B. Corsaro, and G. Studor, Handbook for Designing MMOD Protection, NASA TM-2009-214785, NASA, June 2009.
15 E. Christiansen, K. Nagy, D. Lear, and T. Prior, Space station MMOD shielding, Acta Astronautica 65(7-8):921-929, 2009.
16 K. Edelstein, Orbital Impacts and the Space Shuttle Windshield, NASA-TM-110594, NASA, Washington, D.C., 1995.