Electron beam (10 MeV) irradiation to decontaminate spacecraft components for planetary protection

Lander missions and sample return missions (Class IVa and Class IVb) require the highest degree of planetary protection (PP). To date, dry heat sterilization is the only NASA approved PP technique for meeting the stringent sterility requirements to prevent forward contamination. However, spacecraft and payloads are made of a diverse set of materials and components which are heat sensitive or incompatible with dry heat sterilization such as inflatables, low-temperature adhesive, batteries, paints and polymers. Electron beam (E-Beam) based ionizing irradiation generated using linear accelerators is a routine and effective means for decontamination, sterilization, sanitation, and pasteurization that is widely used in the food, cosmetic and pharmaceutical industries. The salient features of this technology include its penetrating abilities, ability to handle bulk materials, speed and cost. Ionizing irradiation destroys bacterial vegetative cells and spores by means of DNA strand breakage. However, high energy (10 MeV) E-beam irradiation has not been qualified for planetary protection. Studies have shown that doses between 20kGy and 40kGy are sufficient to reduce spore counts by over 12 orders of magnitude depending on the type of Bacillus strain spores. Since normal spacecraft surfaces are known to have spore counts in the range of 10/sup 6//m/sup 2/ , it is anticipated that significantly smaller doses would be sufficient for the PP sterilization application. Microbiology and material compatibility studies have been initiated to develop high energy (10MeV) irradiation protocols specifically for planetary protection purposes. Initial data suggest that different Bacillus sp. spores (previously isolated from spacecraft assembly facilities) exhibit different sensitivities to ionizing irradiation. The data indicates that at least a 6-log reduction of Bacillus sp. spores on aluminum (Ti/Al alloy) coupons was achievable using 30 kGy of E-beam radiation. The availability of different sterilization techniques provides the flexibility needed to treat various type of spacecraft associated materials and components.