A New Era For Planetary Protection: The Probabilistic Approach

Abstract

The primary aims of planetary protection are to ensure that: 1) scientific investigations of possible extra-terrestrial life forms, precursors, and remnants are not jeopardised during planetary space missions; 2) Earth is protected from the potential hazard posed by extra-terrestrial matter carried by spacecraft returning from an interplanetary mission. The concept of planetary protection has received increased attention over recent years due to the emergence of new spacefaring countries and the growing involvement of commercial actors. The international standards for planetary protection have been developed through consultation with the scientific community and the space agencies by the Committee on Space Research's Panel on Planetary Protection, which provides guidance for compliance with the Outer Space Treaty of 1967. To date, there are five categories of requirements, which are defined based on the mission's target, type, and scientific rationale. The categories outline the recommended measures to be applied to a mission. As the mission target increases in relevance to habitability and/or the origins of life, the stringency in hardware cleanliness requirements increases. Initial guidelines were guided by a probabilistic approach. This approach uses mathematical models to calculate the probability of the initial microbial contamination from a spacecraft contaminating a target body. Post-Viking, bioburden limits/ spore counts were introduced to the policy for target bodies like Mars, as it was concluded that Mars was less hospitable than initially believed. Yet, the probabilistic approach is still applied to Category III and IV (e.g., Europa Clipper) and Category V (e.g., Mars sample return) missions. This approach could benefit more complex missions where there is a need for a more advanced approach to planetary protection. For this to be reliable, further scientific knowledge is required, e.g., our understanding of cleanroom contaminants and the biocidal impact of the mission environment, and the mathematical models need to be constrained. Ongoing research by space agencies and the scientific community is working towards addressing these knowledge gaps. The COSPAR Panel on Planetary Protection will continue to review this approach as a plausible alternative to bioburden limits to enable the next generation of missions.