Quantitative assessment of a threshold for risk mitigation actions

Abstract

Many low-Earth-orbit missions have a policy that if a future conjunction with a secondary object such as a piece of orbital debris is detected, a go/no-go meeting will be held to decide about a risk mitigation action before the time of closest approach. Commonly, the policy is that a probability of collision (Pc) above a predetermined action threshold at the time of the meeting means the mission will take action to reduce the risk. The value to which the action threshold is set is a compromise—if it is higher, then there is a higher probability that a collision might occur when action is not taken; if it is lower, then more actions will be taken, increasing the cumulative costs and risks of the actions themselves. This paper shows how a policy using an action threshold affects the overall mission risk of a collision with a large object. We augment this with estimates of action success, expected hard-body radius, and expected covariance to obtain an algorithm for estimating the risk reduction associated with an action threshold policy. We apply this algorithm to the OCO-2 and CloudSat missions as examples, using historical conjunction data for these two missions, and show how this algorithm can guide developing missions in setting an action threshold.