Solid rocket motor insulation adhesives with sporicidal activity promote planetary protection for deep space missions

To prevent microbial contamination of extraterrestrial biospheres, NASA has established planetary protection requirements for spacecraft bioburden reduction. For missions to land on the icy moons of the outer planets, solid rocket motors (SRM) commonly used as de-orbit and braking stages are of particular concern for planetary protection since microbial contamination may occur during spacecraft manufacturing and assembly and debris from the SRM can spread over large portions of the planetary surface after impact. In concept spacecraft designs for deep space missions, certain SRM regions are not expected to reach temperatures sufficient for sterilization prior to landing on Europa's icy and potentially viable surface. This study evaluated the antimicrobial properties of candidate primers, adhesives, and insulations commonly used in SRM designs. We observed significant reductions in the number of viable spores of Bacillus atrophaeus (75.0%), Bacillus pumilus (88.9%) or Bacillus subtilis (87.6%) by application of the Chemlok® 205 + 6450 adhesive, compared to no-adhesive controls, when applied onto a polybenzimidazole (PBI) insulation substrate. More consistent reductions in viable spores were observed after adhesives were applied to PBI insulation compared to other insulation types tested. An aqueous extract of Chemlok® 205 primer was observed to have sporicidal activity, and LC-MS analysis indicated the presence of multiple water-soluble compounds predicted to have antibacterial activity. The reduction in recovery of viable spores observed in this study was due to sporicidal compounds present in adhesives, the spore-binding capacity of insulation types, and physical damage to spores due to cryogrinding. Compounds within rocket motor primers, adhesives, and insulations can contribute to planetary protection efforts, particularly in missions to land on the icy moons of the outer planets. The combination of insulation and adhesive may be optimized for the purpose of bioburden reduction, and ultimately planetary protection risk mitigation.