Why Did Mars Planners Put Venus on the Route?
Venus appeared in these plans for two related but distinct reasons: it could change a spacecraft’s trajectory, and a Venus encounter could add science to a proposed program of planetary flybys.
In Gaetano Crocco’s 1956 proposal, a crew would fly past Mars and return to Earth without firing a rocket after departure. But Mars’s gravity would make a close flyby miss Earth on the return leg. Crocco proposed flying past Venus afterward so its gravity would bend the spacecraft back toward Earth; the crew would also gain a chance to observe a planet hidden beneath thick clouds[1][2].
Later NASA concepts treated crewed flybys as steps toward a broader planetary-exploration program. A 1966 planning report proposed Mars flybys during 1975–80, including possible Venus/Mars and Venus/Mars/Venus itineraries, with crews releasing automated probes and gaining experience for later missions[3][4]. Venus could still serve a trajectory purpose in particular studies: a TRW proposal used a Venus swingby to lower Earth-reentry speed after Mars[5][6]. These were proposals, not flown crewed missions; the cited itineraries should not be read as using Venus for one identical reason[7].
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