Safety assurance for crewed and safety-critical space missions demands a quantitative, risk-informed framework that integrates system performance, human reliability, and lifecycle processes. This paper proposes a structured methodology to evaluate system adequacy across five dimensions—requirements, design, test, process, and safety adequacy—within a unified Probabilistic Risk Assessment (PRA) framework supporting Risk-Informed Decision Making (RIDM).Requirements adequacy is evaluated using completeness and traceability metrics, ensuring that all safety-critical functions, human interactions, and hazard controls are fully captured and consistently linked. Design adequacy is assessed through fault-tolerant architectures, reliability modeling, and systematic analyses such as Fault Tree Analysis (FTA) and Human Reliability Analysis (HRA), explicitly incorporating human error probabilities into system risk models. Compliance is verified against mission-level safety targets, including stringent thresholds for Loss of Crew (LOC) and Loss of Mission (LOM).Test adequacy is addressed through scenario-based verification strategies with high coverage of credible mission conditions, supported by human-in-the-loop simulations, Hardware-in-the-Loop (HIL) testing, and statistical confidence measures. Bayesian updating is employed to reduce uncertainty in reliability and human performance estimates as test and operational data evolve. Process adequacy is characterized through maturity and capability indicators, ensuring robust configuration control, traceability, and integration of human factors into procedures, training, and operations.Safety adequacy is synthesized through integrated PRA combining Event Tree Analysis (ETA), FTA, and HRA to quantify the combined effects of system failures and human errors on mission risk. Uncertainty propagation and sensitivity analysis are used to identify dominant risk contributors and support prioritization of risk mitigation measures.
Application to human spaceflight programs within the Indian Space Research Organisation (ISRO) highlights the importance of human-system integration in achieving stringent safety goals. The proposed framework advances the integration of HRA within PRA, enabling defensible, metrics-driven safety assurance for crewed and safety-critical space missions.
Keywords: Human Reliability Analysis; Risk-Informed Decision Making; Probabilistic Risk Assessment; Loss of Crew; Uncertainty Quantification; Crewed Space Missions