impact that causes structural failure and rapid decompression of the crew module, for example, would be critical. An impact that penetrates the primary structure and explosively ruptures an internal pressure vessel could also be critical. Impacts that penetrate the leading edge of a wing or the lower surfaces of the wing or the fuselage might not be immediately critical—or even detected—but the consequent thermal heating on reentry could have a “blow torch” effect inside the wing that causes loss of flight control or failure of the primary structure resulting in the loss of the vehicle. Major damage to the control surfaces or the hydraulic systems that operate them could result in critical failure during reentry, approach, and landing.
Lesser damage could be survivable but might have a significant impact on the cost and schedule of the shuttle program. For example, an impact on the leading edge of a wing that caused a small hole could result in heating of the wing’s inner structure during reentry that might not cause the wing to fail but would require that a substantial portion of the wing skin, ribs, and spars be replaced. The repair could take 18 to 24 months and could cost as much as $25 million to $40 million (Boeing Space Systems Division, 1997). In addition to the cost, prolonged repairs could have a ripple effect on operations and scheduled modifications of the remaining orbiter fleet, especially if the repairs must be done during ISS assembly.
Impacts of meteoroids and orbital debris could also cause a mission to be terminated early. An impact that penetrates a freon coolant line in the radiators on the payload bay doors, for example, would leave only one operational coolant loop. The remaining loop could perform satisfactorily under reduced power conditions, but, because of the absence of further redundancy in the coolant system, the shuttle flight rules require that the orbiter terminate its mission activities and make the earliest possible return to the primary landing site. A noncatastrophic penetration of a pressurized volume, such as the crew cabin or a Spacelab, would also probably result in early termination of the mission.
Damage from meteoroids and orbital debris impacts could also be costly to repair, even if it is not critical or mission-limiting. Orbiter external surfaces, for example, have experienced impacts from particles on every shuttle mission. Inspections of the windows after each flight have revealed pits that were caused by impacts in orbit. The outer thermal panes of the crew cabin windows have sustained one or more impact pits greater than 0.25 mm in diameter on most flights. Almost 300 pits were reported between 1981 and 1996, and 55 windows were replaced (NASA, 1997b). Windows are replaced based on the design stress conditions and the location and depth of pits.