tape 0.02 inches thick will be added to the length of the coolant loop. In addition, NASA has authorized installation of an isolation valve at the radiator freon inlet and a check valve at the radiator outlet to isolate a leak and limit the loss of freon. The crew will have to close the valves when a caution and warning alarm signals that a change in freon pressure has been detected. An automated system to close the valves is currently under review. The modifications, which will add 30 kg to the orbiter’s mass, are scheduled to be installed on all four orbiters during routine maintenance between February 1998 and February 1999 (Ouellette, 1997).

The modifications to the RCC leading edge of the wing will be made in the region of the wing where the shock wave from the nose of the vehicle and the shock wave from the wing intersect during reentry. Existing insulation inside the wing could not tolerate the heating rates and heating loads from the plasma flow that would result from a penetration of the wing in this area. Several layers of Nextel, a high-temperature ceramic fiber, are being added behind the current insulation. This modification should allow the orbiter to maintain structural integrity during a reentry with a 0.63 cm diameter penetration in the lower side of the RCC. The modification, which will add approximately 77 kg to the orbiter’s mass, will be installed during planned modification periods and inspection cycles (Loftus, 1997b).

By constraining the on-orbit attitude of the shuttle orbiter, NASA can significantly reduce the risk of significant damage. One issue with the current approach is that it forces mission planners to make trade-offs between (1) protecting the crew cabin windows, (2) reducing the probability that the mission will have to end early, and (3) minimizing the risk of critical damage. During some Mir missions, for example, attitudes chosen to reduce the predicted risk to the radiators have increased the predicted risk of a critical impact. Without a mechanism for making trade-offs, NASA runs the risk of treating minor hazards that are well known (e.g., window pitting) as more important than more serious hazards that have not yet damaged the orbiter.

When ISS operations begin, NASA plans to constrain shuttle attitudes to satisfy ISS power, thermal, and attitude control requirements, rather than to minimize risks from meteoroids and orbital debris (Reeves, 1997). The planned configuration for docking the orbiter to the completed ISS, for example, leaves the orbiter in an orientation that maximizes the predicted risk of critical penetration. Missions to the ISS will also limit mission planners’ ability to modify the launch schedule or orbital altitude to reduce the risk from meteoroids and orbital debris.

Front Matter (R1-R12)

Executive Summary (1-3)

1 Introduction (4-6)

2 Risk to the Orbiter and Crew (7-18)

3 Risk Management Strategy (19-26)

4 Tools for Risk Assessment (27-35)

5 Collision Avoidance (36-41)

6 Risk Mitigation (42-50)

Acronyms (51-52)

Appendix A: Statement of Task (53-55)

Appendix B: Biographical Sketches of Committee Members (56-58)