Application of the ATP assay to rapidly assess cleanliness of spacecraft surfaces: a path to set a standard for future missions

The National Aeronautics and Space Administration (NASA) measures and validates the biological cleanliness of spacecraft surfaces by counting endospores using the NASA standard assay (NSA). NASA has also approved an adenosine-5'-triphosphate (ATP)-based detection methodology as a means to prescreen surfaces for the presence of microbial contamination, prior to the spore assay. During Mars Science Laboratory (MSL) spacecraft assembly, test, and launch operations, 4853 surface samples were collected to verify compliance with the bioburden requirement at launch. A subset of these samples was measured for microbial cleanliness using both the NSA (n = 272) and ATP assay (n = 249). NSA results revealed that similar to 8% (22/272) of the samples showed the presence of at least one spore, whereas ATP assay measurements indicated that similar to 15% (35/249) of samples exceeded the threshold cleanliness limit of 2.3 x 10(-11) mmol ATP per 25 cm(2) used by MSL. Of the 22 NSA samples with a spore, 18% (4/22) were considered above the level of acceptance by both techniques. Based on post launch data analysis presented here, it was determined that this threshold cleanliness limit of 2.3 x 10(-11) mmol ATP per 25 cm(2) could be adopted as a benchmark for assessing spacecraft surface cleanliness. This study clearly demonstrates the value of using alternative methods to rapidly assess spacecraft cleanliness, and provides useful information regarding the process.