Microbial contaminants in the interiors of spacecraft components

The hazard from microbiological contamination in the interiors of components of planetary-impacting spacecraft has been generally recognized, but techniques for experimentally evaluating the significance of this hazard have been unavailable. A goal in sterilizing early Mars-impacting payloads is the elimination of the possibility that a microbial stowaway will cause the extraterrestrial-life detection techniques falsely to indicate existence of Martian life, particularly if none exists. Another goal is the prevention of changes in Martian ecology. Several promising life-detection techniques were evaluated in an experimental study of methods to detect microbial populations inside spacecraft-type materials after pulverization or being dissolved. The type of material inspected has a greater effect on the sensitivity of the detection method than any other parameter studied. Through culturing abraded particles, a bacillus inoculum of 100 spores per milliliter can be detected in solid propellants. Fifty percent of an inoculum can be recovered from some solids soluble in nontoxic solvents. For many of the solids studied, however, the culturing techniques used were unable to detect inoculums of 10(6) spores per milliliter of solid. The techniques other than culturing did not discriminate between living and dead cells. As with products of the food and pharmaceutical industries, assurance of sterility in the final product never rests in a negative sterility test on a copy of the product. A negative sterility test is certainly most meaningful for a product for which a very sensitive sterility test has been developed. When a sensitive final-product sterility test is not available, assurance of sterility depends more heavily on other information such as the manufacturing history of the product.