The Mars micromission (MM) spacecraft (MMSC) is a multi-purpose platform capable of orbiting Mars for communications/navigation and science payloads or for precision Mars flybys to drop up to six probes. An MMSC is launched as a secondary payload on an Ariane 5 rocket from Kourou, French Guiana, to GTO using the Ariane 5 structure for auxiliary payloads (ASAP5). The maximum launch wet mass is 222kg and can include up to 45kg of payload depending on ΔV needs. The on-board propulsion system is used for maneuvering in the Earth-Moon system and injecting the spacecraft into Mars transfer orbit and, if needed, Mars orbit. The first MMSC to be launched in early 2003 will be a Communications/Navigation (Comm/Nav) Orbiter. JPL is responsible for the Comm/Nav payload. Ball Aerospace, working with JPL, provides the multi-purpose spacecraft bus, system integration and test, and assists in operations over the 3 year mission duration. This paper summarizes the baseline spacecraft concept for the first Mars Com/Nav mission as defined at the start of the program in February 2000 and describes the spacecraft bus implementation in more detail.