A mathematical model for assessing the probability of contaminating Europa

The Europa Clipper mission must comply with the NASA Planetary Protection requirement in NASA Procedural Requirement (NPR) 8020.12D, which states: “The probability of inadvertent contamination of an ocean or other liquid water body must be less than 1×10-4 per mission”. Mathematical approaches designed to assess compliance with this requirement have been offered in the past, but no accepted methodology was in-place to trace the end-to-end probability of contamination: from a terrestrial microorganism surviving in a non-terrestrial ocean, to the impact scenario that put them there, to potential flight system failures that led to impact, back to the initial bioburden launched with the spacecraft. As a result, hardware could presumably be either over-or under-cleaned. Over-specified microbial reduction protocols can greatly add to the cost (and schedule) of a project. On the other hand, if microbes on hardware are not sufficiently eliminated, there is increased risk of potentially contaminating another body with terrestrial microorganisms - adversely affecting scientific exploration and possibly conflicting with international treaty. Discussed here is the Icy Bodies Planetary Protection Probabilistic Model, a mathematical model developed by the Clipper Project to demonstrate compliance with the NPR requirement above: Given an initial bioburden launched with the Clipper flight system, the model calculates the probability of inadvertently contaminating the Icy Bodies Europa, Ganymede and Callisto. For purposes of the paper, discussions are focused on contamination of Europa specifically. This model first calculates the probability of the hypothetical event that Clipper fails to perform a planned maneuver and impacts Europa. Then the model assesses the conditional probability at least one viable microorganism survives the impact and reaches the Europa ocean. An initial bioburden launched with the Clipper flight system is provided to the model from the Planetary Protection Equipment List (PPEL). Since there is uncertainty as to which trajectory Clipper would follow to Europa if a maneuver is missed, tens-of-thousands of trajectories are sampled and weighted by their probability of occurrence, given that a particular maneuver is not performed. Flight system reliability and exposure to micro-meteoroids, as pertains to maneuverability, are included in this calculation. Microorganism survival probabilities are calculated along each of these potential trajectories by quantifying microorganism mortality when exposed to the known lethality factors of space such as radiation, temperature and vacuum. The concept of a bio-region is introduced, a critical notion to this modeling which allows us to understand how lethality depends on the flight system design and configuration. This paper will briefly address each of these topics, driving assumptions and areas of uncertainty and conservatism. Preliminary model results are summarized.