A Cruise-Phase Microbial Survival Model for Calculating Bioburden Reductions on Past or Future Spacecraft Throughout Their Missions with Application to Europa Clipper

Abstract

During transit between the Earth and planetary destinations, spacecraft encounter conditions that are deleterious to the survival of terrestrial microorganisms. To model the resulting bioburden reduction, a Cruise-Phase Microbial Survival (CPMS) model was prepared based upon the Lunar Microbial Survival model, which considers the effects of temperature, vacuum, ultraviolet (UV), and ionizing radiation found in the space environment. As an example, the CPMS was used to determine the expected bioburden reductions on the Europa Clipper spacecraft upon arrival at Jupiter under two different transit scenarios. Under a direct trajectory scenario, exterior surfaces are rapidly sterilized with tens of thousands of lethal doses (LDs) absorbed to the spacecraft exterior and at least one LD to all interior spaces of the spacecraft heated to at least 240 K. Under a Venus-Earth-Earth gravity assist (VEEGA) trajectory, we find substantially higher bioburden reductions resulting from the spacecraft spending much more time near the Sun and more time in transit overall. With VEEGA, the exterior absorbs hundreds of thousands of LDs and interior surfaces heated above 230 K would absorb at least one LD. From these simulations, we are able to generalize about bioburden reduction in transit on spacecraft in general, finding that all spacecraft surfaces would sustain at least one LD in ≤38.5 years even if completely unheated. Temperature and vacuum synergy dominates surface reductions out to at most 3.3 AU (for gold multilayer insulation), UV irradiation and temperature between 3.3 and 600 AU, and past 600 AU the effect of vacuum acting alone is the primary factor for all exterior and interior surfaces. Even under the most conservative estimates, if the average interior temperature of the Cassini spacecraft exceeded 218 K, or the Galileo spacecraft interior exceeded 222 K, neither spacecraft would have likely had any viable bioburdens onboard at disposal.