NASA’s Manned Grand Tour of the Inner Planets

The inner planets – Mercury, Venus, Earth, and Mars. Credit: NASA

In the 1960s, planetary flybys were all the rage at NASA. In 1966, graduate student Gary Flandro discovered that the planets were about to align for a planetary grand tour, a discovery that became the Voyager missions. The same year, NASA contractor Bellcomm started researching possible missions that, using flybys and Apollo hardware, could send a crew to Venus and Mars in one shot.  

Planetary flybys are ambitious missions that use gravity assists to send spacecraft further with less fuel. As a spacecraft swings around the trailing side of a planet it gains speed, and if the geometry of planets is just right the spacecraft will accelerate right to its next destination with little need for fuel to guide or control its flight. The proposals came from Bellcomm, a division of AT&T established in 1963 to assist the space agency with research, development, and overall documentation of systems integration. Using upgraded Apollo hardware, these flybys were thought to be a natural stepping stone between Apollo and loftier goals like Earth-orbiting space station missions and manned Mars landings and Venus orbiters.

The Earth as seen from a high orbit on Gemini 11 in 1966. This could be the view of a crew right before firing their main engine for a triple flyby mission. Credit: NASA

At a spaceflight mechanics conference in 1966, Bellcomm mathematician A. A. VanderVeen presented a study on possible manned Mars flyby opportunities between 1978 and 1986. This time frame was ideal, he said, because manned Mars missions would probably be of great interest at this point. Apollo would be over, and the next logical step in manned exploration after the Moon was our less hostile planetary neighbour.

According to his study, very little propulsion was needed to for this mission. After launch, the crew would make minor course corrections to adjust their path, but physics would be in the driver’s seat. The mission would look like a Mars version of Apollo 13 but without the exploding oxygen tank; one big burn would send the crew to Mars where they would whip around its far side and be flung towards Earth. Probes would do the hard work on a mission like this. At Mars, the crew would release a variety of automated probes, at least one of which would land and ideally bring a sample back to the crew.

The Earth and Moon as seen by Mariner 10 en route to Venus in 1973. Credit: NASA

Between 1978 and 1986, VanderVeen identified 5 oppositions – the point where Mars and Earth are at the closest points in their orbits. This was the best time for a Mars flyby since the geometry between planets allowed for the shortest transit time. But transit time wasn’t the real limiting factor for a mission, weight at launch was. The initial burn to send a crew to Mars demanded a lot of fuel, and some planetary alignments were just unfavourable and would need more fuel than was possible to launch into Earth orbit. But VanderVeen identified two favourable launch opportunities in 1979 and 1983. Both were equally conducive to the mission given the planets’ positions and the available technology for propulsion.

Manned Mars flybys got easier in 1967 when VanderVeen wrote another report on this mission type but added Venus to the roster. If launch weight from Earth was the biggest problem, using Venus’ gravity to slingshot a spacecraft to Mars could help solve the problem.

Mars, Earth, and Venus align with the Sun five times every 32 years, but Venus and Mars align more often making dual-planet flyby opportunities more frequent. The report found that adding a swing by Venus on both the outbound and inbound leg of a Mars flyby mission was feasible, turning a flyby mission into a triple-planet flyby.

Venus, adjusted for human eyes, a triple flyby crew’s first target. Credit: NASA

The first triple flyby opportunity identified in the report was in February 1977; the next chance afterwards wouldn’t come until 1983. These dates made a triple flyby mission look doubtful – manned Mars landings and Venus orbital missions were expected to be well underway by the early 1980s.

But the possibility of a triple flyby started looking up when VanderVeen and another Bellcomm engineer, J. Bankovskis, found another triple flyby opportunity with a launch window in 1981. They described the mission in a report from September 1967. The ideal launch on May 26, 1981 would send the crew on a 790 day mission. They would swing by Venus on December 28, past Mars on October 5, 1982, past Venus again on March 1, 1983 before splashing down on Earth on July 25. But even a non-optimal launch was a good option. There was a 30 day launch window associated with this mission, and even the worst opportunity in that window only lengthened the mission to 850 days.

Mars. The triple flyby crew’s second destination. Credit: NASA

Finding a previously unknown launch opportunity inspired Vanderveen to look for other triple flyby launch windows. And he found some. In an October 1967 report he announced a November 1978 dual-planet flyby, an Earth-Venus-Mars-Earth mission. With a slight modification, it could become a triple flyby. Adding a swing by Venus on the inbound leg of the mission wasn’t out of the question. Launching on November 28, 1978, the crew would pass Venus on May 11, 1979, Mars on November 25, Venus again on January 29, 1980 and return to Earth on January 31, 1981. In all, the mission would be 800 days but a different launch date within the 35-day window could shorten the mission to 760 days.

What’s really interesting about these triple flyby missions isn’t just the chance for a crew to go on a miniature grand tour of the inner solar system, it’s that every flyby would be a unique scientific opportunity. Because of the geometry of the planets’ orbits, each mission, and even each leg of one mission, would take the crew by a very different part of the planets. Some trajectories would take the crew to a planet’s day side or near the equator while others would take them around the dark side or near a pole. Neither was, scientifically speaking, a bad path. Infrared sensors and mapping radar could make observations the crew couldn’t manage visually.

The Earth and the Moon as seen by the HiRISE camera on NASA’s Mars Reconnaissance Orbiter. October 3, 2007. Credit: NASA

Whatever the final shape of the mission, triple flybys promised great scientific return on a very interesting mission. But, like a lot of the exciting plans for the 1970s and 1980s, this never went any further than a proof of concept mission. Maybe triple flybys will come back in vogue if NASA or some private company moves ahead with mission to Mars. If you’re going to do planetary recon for a landing mission, why not throw in some more science and a great, up-close shot of Venus at the same time.

Suggested Reading

VanderVeen and Bankovskis. “The Existence of a Triple-Planet Ballistic Flyby.” Bellcomm. September 19, 1967. Washington.

VanderVeen and London. “Existence of a Favorable 1976 Dual-Planet Ballistic Flyby.” Bellcomm. February 14, 1967. Washington.

VanderVeen. “The 1975 Mars-Venus Ballistic Dual-Planet Flyby.” Bellcomm. December 19, 1967. Washington.

VanderVeen. “Venu Swingbys for Manned Mars Missions During the 1978-1986 Period.” Bellcomm. August 9, 1966. Washington.


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