This paper describes the vision for Space Exploration that would return humans to the moon by 2020. Creating architecture for returning humans to the moon requires the comprehension of the physics of spaceflight, knowledge of the hardware that can realize the physics, and an understanding of how these many parts interact and interconnect. The NASA team concluded early in its study that the direct–direct mode would be possible only if a single launch vehicle approaching twice the lift capacity of the Saturn V were available. The three mission modes were compared as higher levels of technology were engaged. The key was to find a workable architecture that involved the least amount of mass. The direct return mission that involved no operations in lunar orbit seems to be the least operationally complex, but it tends to be the least efficient because it moves the largest mass-including the Earth-entry heat shield- the entire velocity change of lunar landing and ascent.

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