Single Event Functional Interrupts during the Cruise and EDL Phases of the Mars 2020 Mission

On July 31st 2020, only one day after the launch of the Mars 2020 mission, the flight system experienced a Single Event Functional Interrupt (SEFI), a type of memory upset leading to a temporary interruption of normal operation. While past NASA missions (e.g. Mars Science Laboratory, Juno) have also experienced SEFIs, the higher frequency of such events on the Mars 2020 vehicle during its journey to Mars complicated flight operations and presented unique risks that could have jeopardized mission success, especially during the pre-landing preparation activities as well as Entry Descent and Landing phase. First, this paper presents the issue of SEFIs and discusses how these events differ from more benign memory upsets. The Mars 2020 Avionics architecture as it relates to SEFIs is then briefly introduced. Second, the multiple SEFIs that impacted the Mars 2020 flight system during its cruise to Mars are examined. It identifies the specific signature that distinguished them from more benign memory upsets, and discusses the use of a model of likelihood of occurrence that was updated with each new event to support cruise operations planning. The recovery plan developed by the team to clear out SEFI conditions during cruise is then described, as these events introduced residual risk and could have left the system in an unstable state. Third, the paper focuses on the impact of a SEFI occurring in the last days preceding Entry, Descent and Landing (EDL). Due to the limited time available to interact with the vehicle from the ground during this critical phase, much effort was spent by the team to understand the risk to the EDL timeline in event of a SEFI, and to devise a swift response in order to mitigate its impact. The specific “EDL Approach” SEFI contingency plan that was developed is presented, including the possible configurations of the system after a particular anomaly, along with the resulting paths to recovery. These paths were pre-defined by balancing the residual risk remaining after performing recovery actions versus the time left until atmospheric entry. Additionally, various operational changes were decided to further increase the team's readiness to a SEFI recovery and are discussed herein. Finally, the paper examines the lessons learned from the reoccurrence of SEFIs during the Mars 2020 cruise phase, by discussing the potential changes to hardware, software and operations that could mitigate the impact of such events on vehicles designed to fly through deep space or perform planetary landings.