The role of NASA safety thresholds and goals in achieving adequate safety

NASA has recently instituted requirements for establishing Agency-level safety thresholds and goals that define long-term targeted and maximum tolerable levels of risk to the crew as guidance to developers in evaluating "how safe is safe enough" for a given type of mission. This paper discusses some key concepts regarding the role of the Agency's safety thresholds and goals in achieving adequate safety, where adequate safety entails not only meeting a minimum tolerable level of safety (e.g., as determined from safety thresholds and goals), but being as safe as reasonably practicable (ASARP), regardless of how safe the system is in absolute terms. Safety thresholds and goals are discussed in the context of the Risk-Informed Safety Case (RISC): A structured argument, supported by a body of evidence, that provides a compelling, comprehensible and valid case that a system is or will be adequately safe for a given application in a given environment. In this context, meeting of safety thresholds and goals is one of a number of distinct safety objectives, and the system safety analysis provides evidence to substantiate claims about the system with respect to satisfaction of the thresholds and goals.