Triple-Flyby: Venus-Mars-Venus Piloted Missions in the Late 1970s/Early 1980s (1967)

A NASA Manned Spacecraft Center-designed piloted flyby spacecraft departs Earth orbit. A series of three Saturn S-IVB stages would have ignited in turn to place the spacecraft on its interplanetary trajectory. The stylized image above shows two spent S-IVB stages, the third S-IVB with its single J-2 engine firing, and the piloted flyby spacecraft with portholes and a streamlined cover over its large telescope. In most piloted flyby plans, the Earth-departure burns together constituted the only significant propulsive velocity change of the mission. Image credit: NASA.

During its first decade (1958-1968), NASA devoted more advance planning study effort to piloted Mars and Venus flybys than it did to piloted Mars landings. Piloted flyby missions to the nearest planets were seen as the most ambitious voyages beyond the Moon potentially feasible in the 1970s. Such missions could, many believed, employ modified, upgraded Apollo spacecraft and Saturn rockets to serve as low-cost "bridge" missions linking planned temporary Earth-orbiting space stations of the early 1970s and piloted Mars landings and Venus orbiters of the early-to-mid 1980s. In this respect, they would be as Project Gemini was for Project Mercury and Project Apollo.

Despite the effort spent on them, NASA's 1960s piloted flyby plans are little remembered today. Proposals for piloted flybys in recent years almost never cite the mass of study documents NASA and its contractors generated half a century ago. Even careful historians confuse piloted flyby mission proposals and spacecraft designs with piloted landing proposals and spacecraft.

The NASA Headquarters-led Planetary Joint Action Group (JAG), which included representatives from Marshall Space Flight Center, Kennedy Space Center, and the Manned Spacecraft Center in Houston, proposed in a pivotal October 1966 internal report that the first piloted Mars flyby mission should depart Earth orbit in September 1975. The four-person Apollo-derived flyby spacecraft would swing past Mars in late January-early February 1976 and return to Earth in July 1977.

As it flew past Mars, its crew would release automated probes. At least one would soft-land, collect samples of Mars rocks, dirt, and air, and launch them to a hermetically sealed laboratory on board the piloted flyby spacecraft for initial study and transport to labs on Earth.

Except for modest course corrections, no propulsion would be needed after the piloted flyby spacecraft left Earth orbit. This was one of the mission's great attractions. As they neared Earth, the flyby astronauts would abandon the piloted flyby spacecraft and reenter the atmosphere in a beefed-up Apollo Command Module.

In its report, the Planetary JAG described several candidate follow-on piloted flyby missions for the remainder of the 1970s. Of great scientific interest was a "triple-flyby" mission, in which the piloted spacecraft would fly past Venus, then Mars, then Venus again, before returning to Earth. As with the simpler September 1975 Mars flyby mission, only minor course adjustments would be necessary after the triple-flyby spacecraft left Earth orbit.

Venus flyby: the piloted flyby spacecraft depicted in Earth-orbital launch configuration at the top of this post is shown here with its telescope, solar arrays, dish antennas, and Venus mapping radar antenna deployed. A robotic Venus atmosphere-entry probe — perhaps a lander — is shown departing the flyby spacecraft probe compartment. This NASA Manned Spacecraft Center concept, never described in a formal report, dates from near the end of the 1960s period of piloted flyby planning. Image credit: NASA.

Unfortunately, the only opportunity to begin a triple-flyby in the late 1970s known in 1966 was poorly timed. The spacecraft would need to depart Earth in February 1977, while the 1975 Mars flyby mission was still underway. This would create operational difficulties — NASA would need to operate two piloted planetary missions at once — and would deprive the space agency of the opportunity to apply lessons learned from the September 1975 flyby mission. No other opportunity to begin a triple-flyby mission was known before 1983. Planetary JAG planners assumed that by that date NASA would have moved on to piloted Mars landings and Venus orbiters.

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

Discovery of the 1981 triple-flyby opportunity led Vanderveen to look for other triple-planet flyby opportunities researchers had missed. In October 1967, a year after the Planetary JAG completed its report, he announced that he had determined that a previously known November 1978 "dual-planet" (Venus-Mars) flyby mission opportunity could be slightly modified to create a new triple-flyby opportunity.

Vanderveen wrote that, if one assumed a launch from Earth orbit on 28 November 1978, then the triple-flyby spacecraft would pass Venus on 11 May 1979, Mars on 25 November 1979, and Venus again on 29 January 1980. Return to Earth would take place on 31 January 1981. Mission duration would total 800 days. Earth departure on other dates within a 35-day launch window could reduce mission duration to 760 days.

Vanderveen explained that the two Venus flybys would have different qualities, so they would require different scientific programs. In both, the flyby spacecraft would pass about 1200 miles from Venus. On 11 May 1979, the triple-planet spacecraft would race past the center of the dayside hemisphere, its ground track nearly paralleling the Venusian equator. This, Vanderveen wrote hopefully, might permit visible-light mapping through breaks in the dense Venusian clouds.

The southern hemisphere of Venus as imaged by the European Space Agency's Venus Express orbiter. Image credit: ESA.

The 29 January 1980 Venus flyby, on the other hand, would see the spacecraft slowly approach the planet's dayside southern hemisphere. It would pass closest to Venus 30° south of the equator near the terminator (the line between day and night), then would recede from Venus's nightside hemisphere. Vanderveen recommended that the flyby crew turn infrared sensors and a mapping radar toward the night side as they flew away from Venus and began their year-long return to Earth.

Piloted flybys did not become part of NASA's 1970s program for several reasons. NASA split over the efficacy of the piloted flyby mission concept, with the Manned Spacecraft Center in particular favoring as bridge missions piloted Mars and Venus orbiters over piloted flybys.

More important was a toxic political climate, which was partly of NASA's making. Increasing U.S. military involvement in Indochina drove up the Federal budget deficit, leading to cuts in many programs, including the space program.

The Apollo 1 fire (27 January 1967) damaged the relationship between NASA and Congress at this critical time, increasing the space agency's vulnerability to funding cuts. The fire broke out during a pre-flight test inside the first Apollo Command and Service Module (CSM) spacecraft scheduled to fly with a crew on board. Astronauts Gus Grissom, Ed White, and Roger Chaffee perished. It emerged that CSM contractor North American had delivered to NASA CSM spacecraft containing many manufacturing flaws, yet NASA had not shared this fact with Congress.

Efforts by NASA Headquarters under Administrator James Webb and the Lyndon Baines Johnson White House to secure substantial funding for post-Apollo piloted spaceflight, including piloted flybys, had switched into overdrive just before the Apollo 1 fire, so became a lightning-rod for Congressional displeasure. In August-September 1967, Congress slashed the Apollo Applications Program (AAP) budget request for Fiscal Year 1968 and heaped scorn on piloted and robotic Mars plans.

AAP, a series of Earth-orbital temporary space station and advanced moon missions based on Apollo hardware, shrank rapidly during the following year. The only U.S. automated probe program planned for the 1970s, the Voyager Mars/Venus program, was cancelled outright in part because the Planetary JAG had relied heavily on Voyager heritage for its piloted flyby automated probe designs.

NASA adapted to adversity, turning AAP into the Skylab Program (three three-man long-duration stays on board one Orbital Workshop space station) and the advanced Apollo J-class missions (Apollo 15, 16, and 17). The space agency also successfully negotiated with Congress for a new program of automated Mars spacecraft based on the low-cost Mariner design (Mariner 9 and Viking 1 and 2).

Piloted flybys would, however, never recover, in part because in early 1969, under the leadership of new NASA Administrator Thomas Paine, NASA advance planning became increasingly grandiose. Paine told NASA Center directors to "think big" in anticipation of riding the wave of spaceflight enthusiasm he expected would follow the first piloted Moon landing.

The result was an elaborate Integrated Program Plan (IPP) with a 12-man Space Station evolving into a 100-man Earth-orbital Space Base, reusable winged Space Shuttles, uprated Saturn V rockets, a lunar base, reusable Nuclear Shuttles for transport within cislunar space, and, by 1986 at the latest, a large piloted expedition to land on Mars. A forward step as small as an Apollo-derived piloted flyby mission had no place in the grand IPP.

The post-Apollo 11 wave was short-lived, however. Paine won over Vice-President Spiro Agnew to his plans for men on Mars, but it was a hollow victory, for Agnew had no power in the Administration of Richard Nixon. President Nixon, for his part, for a time considered ending piloted spaceflight.

Unlike the piloted flyby plans of NASA's first decade, the grand-scale plans of 1969-1970 would be long remembered. They would serve mainly to instill in the minds of many the expectation that initial piloted voyages to Mars must land and must be expensive.

Sources

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

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

"White House Stand Blocks NASA Budget Restoration," Aviation Week & Space Technology, 28 August 1967, p. 32.

After Apollo? Richard Nixon and the American Space Program, John M. Logsdon, Palgrave MacMillan, 2015.

More Information

EMPIRE Building: Ford Aeronutronic's 1962 Plan for Piloted Mars/Venus Flybys

After EMPIRE: Using Apollo Technology to Explore Mars and Venus (1965)

"Assuming That Everything Goes Perfectly Well in the Apollo Program. . ." (1967)

Think Big: A 1970 Flight Schedule for NASA's 1969 Integrated Program Plan