Overview of entry risk predictions

Abstract

This paper discusses many aspects of “casualty expectation”, defined as the expected number of people suffering death or injury due to a spacecraft entry event, normally used when planning the end-of-life disposal of satellites. United States guidelines state a disposal via atmospheric entry should be controlled rather than uncontrolled whenever casualty expectation estimates exceed a specified limit. Since this guideline can have serious impacts on the cost, lifetime, and even the mission and functionality of a satellite, it is critical that casualty expectation ( E c) be estimated well, and decision-makers understand all assumptions and limitations inherent in the result. This paper begins with an overview of relevant United States guidelines. The equation the space industry typically uses to estimate E c is presented, along with its sensitivity to typical assumptions, models, and initial condition uncertainties. An alternate quantity is introduced, called “probability of casualty”, or P c. Two examples are given of both E c and P c estimations. Because E c and P c are dependent on variables with significant uncertainty, confidence in the estimates themselves can be challenged. Fortunately, a precise value is rarely needed. Results typically are well above or well below guideline limits to the extent that even if all variable values were certain, the resulting recommended course of action would not change. E c and P c are then ideal to use in relative analyses using a standard tool to assess compliance with guidelines.