Orbital debris poses a real and growing danger to the Space Shuttle and the International Space Station (ISS). Estimates, noted by some to be optimistic, provide an on-orbit risk of approximately 1/200 of a vehicle loss due to a debris strike for the Space Shuttle and a probability of abort of 1 in 120; the ISS is harder and estimates are an 81% probability of no penetration (PNOP) over a 10 year period. (1,2) The hazards associated with space debris continue to increase due to the number of sources and few sinks. The Space Shuttle, on every mission, provides evidence of collisions with very small particles (Figure 1), but the result of a collision with a larger object can result in loss of life or mission, so two strategies exist-protection through debris shielding and avoidance through debris detection. At the present time, we are able to characterize debris in excess of 10 cm radar cross section with a high probability of collision avoidance. The preponderance of debris on-orbit, however, is smaller than that readily detectable for avoidance. Estimates of debris populations for particles smaller than 10 cm are developed through observation and modeling. Comprehensive tracking of particles less than 10 cm for avoidance will require enhancements to our detection and tracking system. In addition to avoidance, newer space systems are beginning to leverage technological advances in debris shielding technologies, similar to the ISS. Spacecraft debris protection is limited to small particles (typically < 1 cm) and can be difficult to implement for vehicles like the Space Shuttle due to spacecraft mass and flight control constraints. As a result, the added requirements for debris shielding can be mission limiting but also do not protect across the entire range of debris strike risk. Because of this, engineering compromises are made and spacecraft like ISS are protected from collision with particles less than one centimeter in size, whereas the Space Shuttle has little if any protection other than the material strength of its skin. For the Space Shuttle detection and avoidance may be the more practical solution with an added benefit for ISS on orbit crew safety enhancement. This document provides information on the origins of the debris, our ability to detect and map risks and encounters, empirical evidence of debris effects, protection options, and long-term effects on crew safety.
