System Theoretic Process Analysis (STPA) of the Mars oxygen ISRU experiment (MOXIE)

Throughout the history of space exploration, astronauts have been limited by the resources they need to bring with them. Now, technologies for In-situ Resource Utilization (ISRU), the art of “living off the land” in space, are paving the way for a sustainable human presence in space. Carbon dioxide is an example of an in-situ resource that comprises a majority of the Martian atmosphere and can be used as a source of oxygen for propellant and life support systems. The Mars Oxygen ISRU Experiment (MOXIE) will unlock this resource using a process called solid oxide electrolysis. As an experimental payload aboard NASA's upcoming Mars 2020 rover, MOXIE is a complex system nested inside in a larger system. For the success of the mission, it is important to understand not only component failure modes but also organizational and component interactions. System Theoretic Process Analysis (STPA) is a hazard analysis technique based on systems theory to identify scenarios leading to accidents and losses, and to find solutions and ways to prevent them. Where traditional hazard analysis technique are based on reliability theory, those methods often miss component-interaction effects in complex systems that could lead to hazards and losses. This paper applies STPA to the MOXIE system with an emphasis on the solid oxide electrolysis (SOXE) subsystem. In this analysis, we examined some hazardous control actions and propose controls for these hazards. Finally, we propose changes to these safety control loops, by adding missing feedback, actuators, and controls. Results of early analysis lead to the implementation of a critical flow sensor. Recommendations from later analysis include time based state transition checks, a soft fault recovery mode, and voltage control that actively tracks the CO-C Nernst potential. This paper is a framework for an expanded safety analysis of MOXIE. Further analysis will expand our knowledge of the system and enable the MOXIE team to identify and avoid additional hazards. With a focus on system safety, we will guide MOXIE on to achieve its greatest goal: to expand the reach of human exploration across the solar system.