Mars landing studies in the USA from 1963 to 1969 focussed on use of NERVA nuclear rocket stages to take a manned expedition to the red planet.

MSC Director Robert R. Gilruth summarized Houston's position expressed during discussions with Associate Administrator for Manned Space Flight George E. Mueller two days earlier. Gilruth cited NASA s need for a manned space flight goal other than 'using Apollo hardware' (and suggested a Mars flyby or landing mission as an in-house focus for planning.) Also, he repeated his concern over the imbalance between AAP goals and resources, as well as the extent of engineering redesign and hardware modification that had been forced upon the project. Though expressing his and MSC's desire to contribute to and be a part of AAP, Gilruth voiced concern that 'the future of manned space flight . . . is in jeopardy because we do not have firm goals, and because the present approach appears to us to be technically unsound.'