Optimized Spacecraft Fault Protection for the WISE Mission

The WISE project is a NASA-funded medium- class Explorer mission to map the entire sky in four infrared bands during the course of a 6-month survey. Because of the mission´s limited financial resources, a traditional robustness strategy of full block-redundancy was not feasible. By leveraging aspects of the mission design that tend to reduce the risk associated with certain failures, the project has been able to adopt a robustness strategy of mitigating high-risk failures, while accepting the risk of low-impact faults or unlikely faults in heritage equipment with proven reliability. The resulting WISE flight system design is primarily single-string with some select functional- and block-redundancy and includes fault tolerance measures targeted at achieving the most cost- effective risk reduction possible for the system design. The fault protection team has been challenged with balancing the risk of faults, cost, and down-time with Ground Segment capabilities, heritage, and effectively designed fault mitigations. Faults were identified via a collection of analyses, and a criticality rating was applied to each fault to assess its impact to the mission. Taking into consideration each fault´s impact and time criticality, mitigations to each possible fault were considered in areas such as on-board autonomy, the addition or use of functional and block redundancy, and ground system detection. Through this exercise, the project has realized a robust and reliable system design in line with the project´s risk posture and cost constraints.