Given these kinds of considerations, a choice was made to rely primarily on nuclear power for systems operating on the martian surface. Power provided by the solar arrays used during the transit to Mars will be available for backup and emergency situations. However, the solar arrays will not be sufficient to power the propellant manufacturing plants that are also a key feature of this mission architecture. Abort to the Surface Mars missions differ from Space Shuttle and lunar missions in that once the crew is committed to launch, orbit mechanics force the crew to remain away from Earth for approximately 2 to 3 years. There has been a tendency to view the martian surface as the most hostile location for a crew during a Mars mission.
Practicality dictates a relatively small habitable space for the crew during transit.
To do otherwise causes a corresponding increase in the size and cost of the systems, primarily launch vehicles and transfer stages, associated with the transportation system. But to confine the crew to a small habitable space for an extended duration can lead to cabin fever. Zero g has known debilitating effects on the human body that must be addressed. Radiation from a constant background and the threat of solar flares require that protection be adequate for background sources and that a safe haven be provided for extreme events. All of these threats have engineering solutions that can make the extended stay in interplanetary space a viable prospect for the crew.