A Comparative Look at MBU Hazard Analysis Techniques

The flux of radiation particles encountered by a spacecraft is a phenomenon that can largely be understood statistically. However, the same cannot be said for the interactions of these particles with the spacecraft as they are far more challenging to grasp and guard against. The ultimate impact of a radiation particle’s interaction with a spacecraft depends on factors that often extend beyond the purview of any subject matter expert and typically cannot be represented quantitatively in systemlevel trade studies without the acceptance of numerous assumptions. In this paper, many of the assumptions associated with the probabilistic assessment of the systemlevel effects of a specific type of radiation-induced hazard, a Multiple Bit Upset (MBU), are explored in light of MBU events during the Gravity Probe B, Cassini, and X-ray Timing Explorer missions. These events highlight key problems in using probabilistic, quantitative analysis techniques for hazards in highly complex and unique systems such as spacecraft. As a result, a case is made for the use of system-level qualitative techniques for both the identification of potential system-level hazards and the justification of responses to them in the system design.