|Mariner II during its final days on Earth, July-August 1962. Image credit: NASA|
Astronomers knew that Venus was nearly as large as Earth, but little else was known of it, for its surface is cloaked in dense white clouds. Many supposed that, because it is a near neighbor and similar in size to our planet, Venus would be Earth's twin. As late as 1962, some still hoped that astronauts might one day walk on Venus under overcast skies and perhaps find water and life.
Data from Mariner II effectively crossed Venus off the list of worlds where astronauts might one day land. As had been suspected since 1956, when radio astronomers first detected a surprising abundance of three-centimeter microwave radiation coming from the planet, Venus's surface temperature was well above the boiling point of water. Mariner II data indicated a temperature of at least 800° Fahrenheit over the entire planet. Cornell University astronomer Carl Sagan explained the intense heat: Venus has a dense carbon dioxide atmosphere that behaves like glass in a greenhouse.
Venus's role in piloted spaceflight thus shifted from a destination in its own right to a kind of "coaling station" for spacecraft traveling to and from Mars. Mission planners proposed ways that a piloted Mars spacecraft might use Venus's gravity to alter its course, slow down, or speed up without expending rocket propellants.
Some also began to view Venus as a proving ground for incremental space technology development. In 1967, NASA Lewis Research Center (LeRC) engineer Edward Willis proposed a manned Venus orbiter based on an "Apollo level of propulsion technology" for the period immediately after the Apollo moon missions.
Willis rejected piloted Mars and Venus flyby missions, which were under consideration as a post-Apollo NASA goal at the time he wrote his paper, in large part because he believed that they would not provide enough exploration time near the target planet. Though he sought a piloted Venus orbiter, Willis questioned the wisdom of launching an equivalent mission to Mars. "It is generally felt," he explained, "that the. . .objective of a manned Mars flight should be a manned landing and surface exploration," not merely a stint in Mars orbit.
The NASA LeRC engineer calculated that the mass of propellants needed for a piloted Venus orbiter would be considerably less than for a piloted Mars orbiter even in the most energetically demanding Earth-Venus minimum-energy transfer opportunity. This meant that a piloted Mars orbiter would always need more costly heavy-lift rocket launches to boost its propellants and components into low-Earth orbit than would a piloted Venus orbiter.
A piloted Mars landing mission, for its part, would be "still heavier than the [Mars] orbiting mission," so probably would "best be done using nuclear propulsion." Whereas chemical rockets generally need two propellants - fuel plus oxidizer to "burn" the fuel - nuclear-thermal rockets need only one working fluid. Liquid hydrogen is most often cited, though liquid methane is also mentioned.
Because they need to lug around the Solar System only one propellant, nuclear-thermal rockets are inherently more efficient than chemical rockets. Nuclear-thermal propulsion would, however, need more development and testing before it could propel humans to Mars. Nuclear-thermal propulsion was unlikely to be ready by the time Apollo ended; therefore, Willis wrote, "in terms of [technological] difficulty and timing, the Venus orbiting mission has a place ahead of the Mars orbiting and landing missions."
The key to a Venus orbiter with the lowest possible propellant mass, Willis explained, was selection of an appropriate Venus orbit. Entering and departing a highly elliptical orbit about Venus would need considerably less energy (hence, propellants) than would entering and departing a close circular Venus orbit. He thus proposed a Venus orbit with a periapsis (low point) of 13,310 kilometers (1.1 Venus radii) and a apoapsis (high point) of 252,890 kilometers (20.9 Venus radii).
The spacecraft would complete two orbits of Venus during its 40-day stay. Time within 26,300 kilometers (three Venus radii) of the planet would total two days; that is, several times longer than a piloted Venus flyby could spend near the planet (Willis's Venus orbiter would, however, not pass as close to Venus as would a Venus flyby spacecraft). Throughout their stay in orbit, the crew would turn remote sensors toward Venus. During the two periapsis passes, the astronauts would use radar to explore the mysterious terrain hidden beneath the Venusian clouds.
Farther out from the planet, near apoapsis, they would deploy the Venus atmosphere entry probes. Their spacecraft's distant apoapsis, combined with Venus's slow rotation rate (once per 243 Earth days), would enable them to remain in direct radio contact with their probes for days - unlike a piloted Venus flyby spacecraft, which could at best remain in contact with its probes for a few hours.
At the end of their stay in Venus orbit, the crew would cast off the Venus scientific payload and ignite the Venus departure stage at periapsis, expending 86,970 pounds of propellants and adding 1.14 miles per second to their speed. During the trip home, which would take them beyond Earth's orbit, they would discard the Venus departure stage and perform a course correction, if one were needed, using the small course correction stage attached to the Command Module.
Near Earth, the crew would separate from the Command Module in the Earth atmosphere entry lifting-body and enter the atmosphere at a speed of 48,000 feet per second. After banking and turning to shed speed, they would glide to a land landing, bringing to a triumphant conclusion humankind's historic first piloted voyage beyond the moon.
Manned Venus Orbiting Mission, NASA TM X-52311, E. Willis, 1967
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Triple-Flyby: Venus-Mars-Venus Piloted Missions in the Late 1970s/Early 1980s (1967)
Floaters, Armored Landers, Radar Orbiters, and Drop Sondes: Automated Probes for Piloted Venus Flybys (1967)
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