Synergistic Interactions among Vacuum, Solar Heating, and UV Irradiation Enhance the Lethality of Interplanetary Space

Abstract

A Planetary Atmospheric Chamber (PAC) was used to create simulations of interplanetary conditions to test spore survival of three Bacillus spp. exposed to interacting conditions of high-vacuum (VAC), simulated solar heating (HEAT), and simulated solar ultraviolet illumination (UV). Synergism was observed among the experimental factors against all three Bacillus spp. tested that suggests increased lethality of HEAT and UV when concomitantly exposed to VAC. The most aggressive biocidal effects were observed for assays with VAC + HEAT + UV conditions run simultaneously over short time-steps. Results were used to predict the accumulation of extremely rapid Sterility Assurance Levels (SALs; def., ‒12 logs of bioburden reduction) measured in a few 10s of min for interplanetary spacecraft. Furthermore, results were extrapolated to predict that approx. 1 x 104 SAL exposures might be accumulated for external surfaces on the Europa Clipper spacecraft during a 3.5 year residence time between Venus (0.7 AU) and Mars (1.5 AU) during a series of Venus-Earth-Earth gravity assists (VEEGA trajectory) to Jovian space. The results are applicable to external spacecraft surfaces exposed to direct solar illumination during transits though the inner Solar System. [Submitted to Scientific Reports for peer review on 08-12-23.]