Defense Advanced Research Projects Agency

The complexity of a spacecraft, as with any other engineering system, is driven by the twin objectives of delivering a particular capability and of doing so robustly in the face of uncertainty. Whereas the intrinsic difficulty of the underlying mission minimally bounds the complexity of the engineering system needed to effect it, in reality such a minimalist system would be of little practical utility; it is not enough to deliver a given capability – it must be delivered with some degree of robustness in the face of various sources of risk or uncertainty.1