This paper presents the design of Alcyone, a modular, cyber-resilient flight software architecture developed for real-time spacecraft systems. The architecture applies secure-by-component principles to enforce fault isolation, privilege separation, and threat containment at the subsystem level. Cyber requirements are derived from threat models and instantiated as runtime enforcement points across key interfaces.

Alcyone will be implemented in Rust to leverage memory safety, concurrency guarantees, and compile-time enforcement of architectural constraints. The system design supports software-in-the-loop (SWIL) testing, runtime observability, and gradual extension to certified hardware platforms. Threat-informed decomposition, testable enforcement surfaces, and mission-grounded validation strategies are used to demonstrate practical cyber resilience in embedded space software. This work establishes a foundation for verifiable, high-assurance spacecraft software with built-in fault tolerance and attack resilience.