Specification of spacecraft subsystem interactions is typically carried out using informal diagrams and descriptions that can obscure subtle ambiguities and inconsistencies. As a result, problems in the way subsystems are designed to interact may remain undetected until the integration and test phase, when the cost of change is high. Our Behavioral Analysis of Spacecraft Systems (BASS) modeling tool provides a structured way to define spacecraft subsystem interfaces and interactions, and access to an underlying formal model of interaction that allows the specified interactions to be rigorously analyzed. The enforced consistency of the diagrams produced by our tool and the analytical power of the underlying formal model increases a developer’s ability to discover and correct system design errors early in the development process.