Before the Challenger accident in 1986, NASA management did not encourage or seem to understand the use of PRA, as reflected by the accident investigator and Nobel laureate Richard Feynman’s statement, “It appears that there are enormous differences of opinion as to the probability of a failure with loss of vehicle and of human life. The estimates range from roughly 1 in 100 to 1 in 100,000. The higher figures come from the working engineers, and the very low figures from management.”5 After the Columbia shuttle accident, the accident investigation board again urged NASA to enhance its risk analyses.6 This lack of attention to probabilistic risk assessment by NASA management had resulted in the MMOD programs finding it difficult to become part of any overall risk assessment associated with mission design and operations, since there was no agreed upon procedure for doing so. This is less true today: NASA management has become increasingly aware of the necessity for risk management, as reflected in NRC studies concerning MMOD with regard to the space shuttle7 and the International Space Station (ISS).8

The initial goals of NASA’s MMOD efforts were to characterize the risk to humans in space, beginning with NASA’s crewed spacecraft programs, more than 50 years ago.9 The primary tool for characterizing risk has been what could be called a Poisson Consensus Model,10 which has the purpose of consolidating theory, measurements, and assumptions into an average event rate where Poisson statistics apply. This approach requires the integration of various statistical distributions (such as velocity and angle of impact) by techniques that were established early in the MMOD programs.11 The history of these consensus models predates the beginning of the space program,12 and they have since been used and their accuracy improved over the years by the international community.

Over time, NASA’s efforts have expanded to include the characterization of risk to uncrewed spacecraft and the addition of the orbital debris population as another source of risk. This addition quickly led to the conclusion that risk could be reduced by minimizing the growth in the orbital debris population. In addition, just as international interest has increased in minimizing the risk to Earth from natural collisions with comets and asteroids, the NASA MMOD programs have also expanded to minimize the risk to people and assets on the ground from reentering orbital debris. As a result of the 2010 National Space Policy,13 which directs NASA to consider the issues involved in the active removal of large derelict debris from orbit, the goal of minimizing risk on the ground is likely to have increasing priority, and trade-offs between reducing the risks to Earth and the risks to spacecraft in orbit may be required. In addition, the risk posed by MMOD has now expanded to include the possibility of catastrophic damage to a spacecraft resulting from colliding with a tracked object in orbit. Other changes to NASA’s mission could occur in the future, for which NASA may need to be prepared.

The hazard from the MMOD environment represents only one component of the total risk to any system or program. It is up to NASA’s program managers to identify systems critical to their mission and manage the risk to those systems. The responsibilities of MMOD programs include determining the probability of failure of any critical system as a result of being hit by either a meteoroid or orbital debris object. Failure for crewed critical systems is defined as loss of the vehicle or loss of life. In some cases what constitutes failure is obvious, such as the penetration of a pressurized container. In other cases a cause of failure is not as obvious; examples include events that could lead to an electrical failure and be interpreted as such (for example, spraying high-speed ejecta


5 R.P. Feynman, Personal observations on reliability of shuttle, Appendix F in Report of the PRESIDENTIAL COMMISSION on the Space Shuttle Challenger Accident, Volume 2, June 6, 1986, available at

6 Columbia Accident Investigation Board, History as cause: Columbia and Challenger, Chapter 8 in Columbia Accident Investigation Board Report, Vol. 1, NASA, August 2003, available at

7 National Research Council, Protecting the Space Shuttle from Meteoroids and Orbital Debris, National Academy Press, Washington, D.C., 1997, available at

8 National Research Council, Protecting the Space Shuttle from Meteoroids and Orbital Debris, 1997.

9 B.G. Cour-Palais, with the assistance of an ad hoc committee, Meteoroid Environment Model¾1969 (Near-Earth to Lunar Surface), NASA Space Vehicle Design Criteria (Environment), NASA SP-8013, March 1969, available at

10 M. Drouin, G. Parry, J. Lehner, G. Martinez-Guridi, J. LaChance, and T. Wheeler, Guidance on the Treatment of Uncertainties Associated with PRAs in Risk-Informed Decisions Making, NUREG-1855, Vol. 1, U.S. Nuclear Regulatory Commission, March 2009.

11 D. Kessler, A Guide to Using Meteoroid-Environmental Models for Experiment and Spacecraft Design Applications, NASA TND-6596, NASA, March 1972.

12 A.C. Lovell, Meteor Astronomy, Oxford University Press, Oxford, U.K., 1954.

13National Space Policy of the United States of America, June 28, 2010, available at, accessed July 6, 2011.

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