operations is with the payload bay pointing down (toward the Earth) and the nose not pointing forward. Exposure time with the payload bay pointing forward and with the nose pointing forward while the payload bay points up or out of plane is kept to a minimum. If other attitudes are required by payload or orbiter requirements, they will be used. The flight rules are primarily designed to protect the orbiter windows and radiators (for which the hazard is 16 times greater when the payload bay points forward than when it points down) (Reeves, 1997). Although orientations that present the maximum risk of critical penetration are not prohibited, the shuttle program’s maximum allowable critical risk of 1/200 might force mission planners to minimize flight time in those attitudes.
Operational procedures planned to reduce the risk to astronauts performing EVAs include avoiding EVAs when meteor storms or showers or conjunctions with cataloged debris are predicted. Whenever possible, EVAs are performed in locations that are shaded by the orbiter or (in the future) by elements of the ISS