In the 1960s, NASA devoted nearly as much study effort to manned Mars and Venus flybys as it did to manned Mars landings. Manned flybys were viewed as the most ambitious planetary missions possible in the 1970s using upgraded Apollo moon program hardware and as a natural stepping stone between Earth-orbiting space station missions and manned Mars landings and Venus orbiters.

The NASA-wide Planetary Joint Action Group (JAG) proposed in its October 1966 internal report that the first manned planetary flyby mission leave Earth in September 1975. The four-man Apollo-derived flyby spacecraft would swing past Mars in late January-early February 1976, and return to Earth in July 1977. Except for modest course adjustments, no propulsion would be needed after the spacecraft left Earth orbit. As it flew past Mars, its crew would release a variety of automated probes, including at least one which would land, collect samples of Mars rocks, dirt, and air, and return them to a sealed laboratory on board the piloted flyby spacecraft.

In its report, the Planetary JAG described candidate follow-ons to the 1975 mission for the remainder of the 1970s. Of great scientific interest was a "triple-flyby" mission, in which the manned spacecraft would fly past Venus, then Mars, then Venus again, before returning to Earth. Unfortunately, the only known opportunity to begin a triple-flyby in the late 1970s was poorly timed. The spacecraft would need to leave Earth in February 1977, while the 1975 Mars flyby was still underway. No other opportunity to begin a triple-flyby mission was predicted before 1983. Planetary JAG planners assumed that by then manned Mars landings and Venus orbiters would have superseded manned flybys.

In September 1967, J. Bankovskis and A. Vanderveen, engineers with NASA planning contractor Bellcomm, identified a triple-flyby opportunity with an optimum Earth-departure date of May 26, 1981. A spacecraft launched on that date would fly past Venus on Dec. 28, 1981, past Mars on Oct. 5, 1982, and past Venus again on Mar. 1, 1983. It would return to Earth on July 25, 1983. Mission duration for the May 26 departure would total 790 days. Departures on other dates within the 30-day launch window would yield durations of 720 to 850 days.

Discovery of the 1981 triple-flyby opportunity led Vanderveen to look for other triple-planet opportunities previous researchers had missed. In October 1967, he announced that he had found that a previously identified November 1978 "dual-planet" (Venus-Mars) flyby opportunity could be modified slightly to create a triple flyby.

Vanderveen determined that, if one assumed a launch from Earth orbit on November 28, 1978, then the triple-flyby spacecraft would pass Venus on May 11, 1979, Mars on Nov. 25, 1979, and Venus again on Jan. 29, 1980. Return to Earth would occur on Jan. 31, 1981, yielding an 800-day mission. Earth departure on other dates within the mission's 35-day launch window could reduce duration to as little as 760 days.

Vanderveen explained that the two Venus flybys would have "very diverse. . .characteristics," so they would need different scientific programs. In both, the flyby spacecraft would pass about 1,200 miles from Venus. The May 1979 flyby would see it speed past the center of the dayside hemisphere, its ground track nearly paralleling the equator. This might favor visible-light mapping through breaks in the dense Venusian clouds, Vanderveen wrote.

The January 1980 flyby, on the other hand, would see the spacecraft slowly approach Venus' dayside southern hemisphere, its ground track passing near the south pole. It would reach closest approach 30° south of equator near the terminator (the line between day and night), then would recede from Venus' nightside hemisphere. Vanderveen recommended that the flyby crew turn infrared sensors and mapping radar toward the night side as they began their year-long return to Earth.

References:

"The Existence of a 1981 Triple-Planet Ballistic Flyby - Case 103-2, A. Bankovskis and A. Vanderveen, Bellcomm, Inc., Sept. 19, 1967.

"Verification of the Existence of the 1978 Triple-Planet Flyby Opportunity - Case 720," A. Vanderveen, Bellcomm, Inc., Oct. 19, 1967.