Micrometeoroid and Orbital Debris Hazard Considerations for Space Station-Related EVA

Improved understanding of the orbital debris environment in recent years and the prospect of significantly increased extravehicular activity (EVA) for the assembly and maintenance of Space Station Freedom have resulted in increased focus on the potential hazards to the EVA crewmembers from the micrometeoroid and orbital debris environments. While the hazards associated with any individual EVA remain extremely small, some estimates of the cumulative risks over the anticipated duration of the Space Station Program have been appreciable. These estimates, based on analytical models which treat the micrometeoroid and debris environments as isotropic, suggest the desirability of substantially modifying either the extravehicular mobility unit (EMU) or the planned EVA profiles for space station to reduce these hazards.

This paper presents the results of analyses which extend the prior work to reflect the effects of the true directionality of the micrometeoroid and orbital debris hazards with representative EVA geometries. Analytical predictions of the directionality of the micrometeoroid and orbital debris hazards are compared to data resulting from space operational experience with emphasis on the Long Duration Exposure Facility (LDEF) data base. The significance of this directionality for operational EVA activities is discussed using the STS-49 (Intelsat repair and assembly of station by EVA methods (ASEM)) EVA missions as specific examples. Implications for space station EVA hazard assessment are addressed considering both assembly and operational phases. Approaches to enhance micrometeoroid and debris hazard protection which take advantage of the directionality of the hazards are discussed briefly along with preliminary estimates of their effectiveness. These results indicate that actual hazards may be lower than prior estimates have indicated and suggest protective concepts which can further reduce hazards with limited impacts to the EMU or to space station EVA.