DERIVING EVENT THRESHOLDS AND COLLISION PROBABILITY FOR AUTOMATED CONJUNCTION ASSESSMENT AT MARS AND THE MOON

It is well known that conjunction assessment is necessary in the Earth orbiting environment to prevent spacecraft collisions and reduce debris in orbit. Alt-hough there are many fewer spacecraft in orbit around Mars and the Moon, the number of missions to these bodies is increasing rapidly and the consequences of creating debris riskier due to the inability to track debris objects in those en-vironments. The Multimission Automated Deepspace Conjunction Assessment Process (MADCAP) is used at the Jet Propulsion Laboratory (JPL) to perform conjunction assessment at Mars, the Moon, and Sun/Earth libration points. Previous papers have described this process and its development. Conjunction assessment of spacecraft trajectories produces many close approaches which must be screened in order to inform users when a specific event may be of concern. Probability of Collision (PC) is the main attribute used for screening conjunc-tions in the Earth orbiting environment. Trajectory uncertainty information in the form of covariance data is required to calculate the PC. However, covariance data is not as easily obtained for objects in the Lunar and Martian environments. Instead, various attributes of the close approach must be calculated and com-pared against thresholds based on typical expected orbit uncertainty. These thresholds are best informed by the individual spacecraft navigation teams who are most knowledgeable about the spacecraft’s orbital uncertainties. This paper will explore the development of the thresholds used in MADCAP and provide a guideline for navigation teams to derive them from their orbit determination analysis. A method for the derivation of close approach attribute thresholds from covariance data, when it is available, is also described. Finally, collision probability calculations are examined, with exploration of methods for approximating probabilities when covariance data are limited.