Human-robot systems will play a critical role in space exploration, should NASA embark on missions to the Moon and Mars. A unified framework to optimally leverage the capabilities of humans and robots in space exploration will be an invaluable tool for mission planning. Although there is a growing body of literature on human-robot interactions, there is not yet a framework that lends itself both to a formal representation of heterogeneous teams of humans and robots, and to an evaluation of such teams across a series of common, task-based metrics. In this paper, we review the literature, and synthesize multiple considerations for architecting heterogeneous teams of humans and robots. We discuss considerations related to formally specifying tasks and representing human--robot systems, enumerating task allocations, and evaluating human--robot systems against common, task-based metrics. Our objective is to lay the foundations of a unified framework for architecting human--robot systems for optimal task performance given a set metrics.