Europa Clipper Planetary Protection Probabilistic Risk Assessment Summary

Protecting Jupiter's Icy Moons from terrestrial contamination is a key consideration for the Europa Clipper mission, whose goal is to explore Jupiter's moon, Europa, and investigate its habitability. The mission must satisfy NASA's Planetary Protection (PP) requirements specified in the NASA Procedural Requirement (NPR) 8020.12D, stating: “The probability of inadvertent contamination of an ocean or other liquid water body must be less than 1x10^-4 per mission.” The NPR further defines contamination as “the introduction of a single viable terrestrial microorganism into a liquid-water environment". The probabilistic nature of the requirement demands a probabilistic response. Europa Clipper developed an end-to-end probabilistic risk assessment (PRA) to ensure requirement compliance and to inform resulting design considerations. The methodology offered in the PRA conservatively quantifies the unanticipated events required to place the Europa Clipper mission in the state of concern: contamination of an icy body. This requires a model of spacecraft failure scenarios and the potential for a resulting icy body impact, expected geological resurfacing timescales that may introduce terrestrial biology to interstitial liquid water, and an assessment of biological mortality throughout the journey from Earth over a 1000-year period. Motivation for this PRA follows from the desire to ensure the proper amount of bioburden reduction is performed pre-launch such that the required PP risk threshold is met, while minimizing programmatic and mission risk. The goal is to effectively 'solve for' the not-to-exceed bioburden at launch, and to allow this solution to inform microbial reduction planning and hardware selection. This is a feature previous mathematical approaches have failed to offer, resulting in over or under-specified cleaning protocols and greater risk either to the exploration target or to the mission. If one solely considers sterilizing spacecraft hardware to meet NASA’s PP requirement, the initial results are unintuitive: a 12-log bioburden reduction prelaunch, or virtual sterility, would be required in order for Europa Clipper hardware to be sterile in the event of an inadvertent impact. This is an untenable solution; sensitive spaceflight hardware cannot endure such cleaning protocols. The Project therefore seeks to show compliance with the requirement by demonstrating the probability that Clipper inadvertently impacts Europa and subsequently introduces hardware into the Europa subsurface ocean is sufficiently small, not that the spacecraft hardware would be sterile upon impact. The end-to-end model and findings are explored at a summary level in the following paper.