sentences:  The plan describes the first human missions to Mars with concept of operations and technologies to be used as a first cut at an architecture.The architecture for the Mars Reference Mission builds on previous work, principally on the work of the Synthesis Group (1991) and Robert Zubrin's (1991) concepts for the use of propellants derived from the Martian atmosphere.The primary purpose of the Reference Mission was to stimulate further thought and development of alternative approaches which can improve effectiveness, reduce risks, and reduce cost.