Before the Apollo program had even started manned flights, NASA was looking forward and planning its post-lunar missions. A new goal using existing hardware developed for the lunar program was ideal, so NASA commissioned a study on possible application of Apollo hardware to a manned mission to Venus. (Left, Venus traces an S as seen in the Earth’s sky.)
The study was published in 1967 when NASA knew fairly little about our cosmic neighbour – the Mariner 2 unmanned probe flew by the planet in 1962. Nevertheless, the agency determined that a manned version of the mission was feasible with late 1960s technology.