This paper questions the widely held assumption that a single crew habitat can serve equally well as both an interplanetary vehicle and as a Mars surface habitat. This paper argues that these two uses and the designs to support them are so fundamentally different that it is not possible to serve both optimally with the same habitat element. This distinction leads to a reassessment of the Mars Direct approach and similar mission architectures. As an alternative, this paper presents the Being There versus Getting There (BTvGT) approach, so-named because it comprehends the distinction between the two habitats and the mission scenarios that they support.
The first distinction is the emphasis upon placing optimal facilities upon the Mars surface and in the interplanetary vehicle, even at the cost of greater total mission launch mass. This shift signifies a focus upon the quality and content of the masses involved, rather than just total undifferentiated tonnage delivered to the surface.
The second major difference is the recognition of the need for substantial isotropic radiation shielding on the interplanetary habitat, which is not necessary on the Mars surface, and which poses a major mass penalty to any Mars lander. Besides radiation shielding there are significant differences in the functional outfitting of the habitats. Designing for six months to a year of zero-gravity with specialized exercise equipment to counteract the debilitating effects of weightlessness are essential in a trans-Mars vehicle, but may not be suited to the Mars environment. The Mars surface habitat -- that the crew will occupy for up to 600 days -- requires a substantial suite of science laboratories to prepare and analyze Martian samples, which a trans-Mars vehicle does not require.
Combined interplanetary vehicle-planetary habitat schemes depend upon an assumed commonality between interplanetary and surface habitats. This paper shows that each type of habitat requires specialized design responses to the environments in which they will provide shelter for human crews that will make commonality difficult to achieve between the Interplanetary and Planetary surface functions.