Planetary flybys are awesome.

As a spacecraft swings around the trailing side of a planet it gains speed and direction, momentum engineers can use to accelerate it to its next destination using little if any fuel for mid-course corrections. Its not a new idea. Gravity assists are how the Voyager probes visited the outer planets with one launch, its how NASA got Apollo 13 home and its how Denis Tito plans to whip a married couple around the far side of Mars within the decade.

And in the mid 1960s, its something NASA considered as a future application for its Apollo hardware.

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NASAs study of manned flybys came via Bellcomm, a division of AT&T established in 1963 to assist the space agency with research, development, and overall documentation of systems integration. In the mid 1960s, flybys with upgraded and modified Apollo hardware seemed like a natural stepping stone between the Apollo lunar missions and the agencys inevitable next steps of an Earth-orbiting space station, manned Mars landings, and manned missions in orbit around Venus.

It was Bellcomm mathematician A. A. VanderVeen who studied the manned flyby possibilities for NASA.

In 1967, he identified 5 favorable launch opportunities for a Mars flyby between 1978 and 1986. Two windows in 1979 and 1983 were ideal, feasible with then-existing launch technology and had the shortest transit time between planets. VanderVeen found that very little propulsion was needed with these launch windows.

After the initial burn towards Mars, physics would take over and guide the spacecraft to its rendezvous with Mars. Probes would do the hard work. Approaching Mars, the crew would release automated probes, one of which could even land on the surface, collect a sample, and launch to rendezvous with the spacecraft on its way back to Earth. VanderVeen also noted that these dates were perfect: Mars was bound to be NASAs next target after Apollo.

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But weight was a persistent issue in all the Mars flyby scenarios; the propulsion needed to launch a spacecraft into Earth orbit then fire it off to Mars was substantial. VanderVeen found an elegant, and scientifically exciting, solution: add a Venus flyby to the Mars trip. Mars, Earth, and Venus align with the sun five times every 32 years, but Venus and Mars alignments happen more frequently making double (Earth-Venus-Mars-Earth) or even triple (Earth-Venus-Mars-Venus-Earth) flybys a viable mission. Taking advantage of favorable launch windows to Venus also reduced overall launch weight.

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NASA's 'Inspirational' Mars Flyby