Systematics in the application of natural radiation environments to allowable launch burden and its distribution

A central feature of the advanced planetary protection planning for missions to Europa is the credit for the reduction of the spacecraft microbial burden by the radiation belts of Jupiter. Although the United States National Aeronautics and Space Administration planetary protection program has not yet set requirements for missions to Europa, the Space Studies Board of the National Research Council has published guidelines. Whether the requirements will be the allowable probability of contamination of Europa or the allowable microbial burden at launch, it is reasonable to consider the natural radiation environment in the approach to compliance. The systematics of the analysis for the microbial reduction due to the proton and electron environments of Jupiter's radiation belts include: a "shielding" representation of the spacecraft; the external mission fluence spectra of each of the natural radiation environments for the mission trajectory; a radiation transport analysis in the "shielding" representation for each of the spectra; and established planetary protection specifications of appropriate classes of microbes and the D-values (dose for a one order of magnitude reduction in population) by protons and electrons for each class. The proton dose and the electron dose in discrete regions of the spacecraft, the "nodes" of the "shielding" representation, an intermediate product, is analogous to the design of shielding for the protection of the system electronics. The application of the D-values to determine a lethality factor at each node for each class of microbe is unique to planetary protection. From the outlined procedure, the relationship between the microbial population at launch and at Europa encounter (or after any specific trajectory in the Jupiter radiation environment) may be calculated. Details of this outline will be presented. The precedent of the shielding analysis for Project Galileo and the analogy to the thermal analysis for the Viking lander terminal sterilization will also be discussed.