NASA has not conducted a systematic assessment of the survivability of the shuttle with respect to the meteoroid and orbital debris hazard. Similar analyses, however, have been conducted by the U.S. Department of Defense (DOD) to assess aircraft survivability. NASA should improve its approach to calculating the risk to the shuttle from meteoroids and orbital debris by establishing a survivability assessment process and conducting an end-to-end survivability assessment of the entire shuttle orbiter—including all subsystems and components—against the hazard. The assessment should be integrated with assessments of other hazards, such as the risk during ascent and reentry, to create a complete, integrated, peer-reviewed probabilistic risk assessment for the shuttle.

NASA should also continue its efforts to assess in detail the vulnerability of areas of the shuttle orbiter that they predict to be most likely to experience critical damage, mission-limiting damage, or damage requiring costly repairs. This information should be used to refine assessments of the overall risk to the shuttle, to determine which areas require more protection, and to determine whether operational and procedural modifications can decrease the risk.

TOOLS FOR RISK ASSESSMENT

NASA uses computer models to assess risks and guide its efforts to protect the shuttle from meteoroids and orbital debris. A model of the meteoroid and orbital debris environment (ORDEM96) is used as input for a threat assessment model (BUMPER) to predict the magnitude of the risk to the shuttle from meteoroids and orbital debris.

ORDEM96 is arguably the best available model of the debris environment, and BUMPER is probably the best available model for orbital debris risk assessment. However, both models incorporate a number of simplifying assumptions, and the magnitude of uncertainty in their predictions has not been well characterized. NASA should strive to refine BUMPER and ORDEM96 so that their results include appropriate error bars and associated confidence levels. To begin this process, NASA should analyze the sensitivity of the output of both models to changes in the various input parameters.

Because the data are limited and the population of debris smaller than about 5 mm in diameter varies widely, ORDEM96’s predictions of debris fluxes for individual shuttle missions may be highly inaccurate. To predict the short-term hazard to the orbiter from orbital debris more accurately, NASA should expand its data gathering and modeling efforts to better understand the sources (e.g., solid rocket motors and debris wakes) of the sub-5 mm debris population in the shuttle’s orbital regime.

COLLISION AVOIDANCE

The DOD Space Surveillance Network (SSN) warns the space shuttle program of possible close conjunctions with cataloged orbiting objects. But probably



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