This article presents a quantitative framework that can be used to determine whether a spacecraft is “Safe Enough” for crewed flight. Three major elements are established, namely: (1) An unequivocal definition of “Safe” (and its inverse, “Unsafe”), derived from and consistent with current NASA terminology and empirical practice, (2) a quantitative risk spectrum, which highlights spacecraft risk (measured using Probabilistic Risk Assessment) against a reference standard, and (3) a probabilistic threshold value, which delineates risk that is acceptable (e.g., “Safe Enough”) with risk that is unacceptable (e.g., “Not Safe Enough”). Each element of the framework is developed concomitantly, step-wise, and from the bottom-up, to ensure “Safe Enough” can be reliably and systematically determined.
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