NASA won a major prestige victory over the Soviet Union on 14 December 1962, when Mariner II flew past Venus at a distance of 22,000 miles. The 203.6-kilogram spacecraft, the first successful interplanetary probe in history, left Cape Canaveral, Florida, on 27 August 1962. Controllers and scientists breathed a sigh of relief as it separated from its Atlas-Agena B launch vehicle; failure of an identical rocket had doomed its predecessor, Mariner I, on 22 July 1962.
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, many hoped that astronauts might one day walk on Venus under overcast skies and perhaps find water and life.
Data from Mariner II doomed plans for piloted Venus landings. As had been suspected since 1956, when radio astronomers first detected a surprising abundance of 3-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's has a dense carbon dioxide atmosphere that behaves like glass in a greenhouse.
By 1967, Venus's role in manned spaceflight had 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 manned 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, because they would provide insufficient exploration time near the target planet. Though he supported 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 engineer calculated that the mass of propellants needed for a manned Venus orbiter would, even in the most energetically demanding Earth-Venus transfer opportunity, be considerably less than for a manned Mars orbiter. This meant that a manned Mars orbiter would always need more costly rocket launches to boost its propellants and components into low-Earth orbit than would a manned Venus orbiter.
A manned Mars landing mission, for its part, would be "still heavier than the orbiting mission," so probably would "best be done using nuclear propulsion." Whereas chemical rockets generally need two propellants -- fuel and oxidizer to burn the fuel -- nuclear-thermal rockets need only one working fluid -- liquid hydrogen, in most cases -- so are inherently more efficient. Nuclear-thermal propulsion would, however, need more development and testing before it could propel men to Mars. "[I]n terms of [technological] difficulty and timing, the Venus orbiting mission has a place ahead of the Mars orbiting and landing missions," Willis wrote.
The key to a Venus orbiter of the lowest possible 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).
Willis calculated that a Venus orbiter based on Apollo-level technology, departing from a 400-mile-high circular Earth orbit, staying for 40 days in his proposed Venus orbit, and with a total mission duration of 565 days, would have a mass of 1.412 million pounds just prior to Earth-orbit departure in the energetically demanding 1980 Earth-Venus transfer opportunity. An equivalent Mars orbiter launched in 1986, the least demanding Earth-Mars transfer opportunity of any Willis considered, would have a mass in Earth orbit 70 percent greater -- about 2.4 million pounds.
The 1.048-million-pound Earth-departure stage (*A *in the image above), was the largest single hardware element in Willis's Venus orbiter design. It would expend 930,000 pounds of chemical propellants to boost the spacecraft's speed by 2.8 miles per second and send it on its way toward Venus; after that, it would remain attached to the spacecraft to perform a course-correction burn roughly halfway to the planet, expending an additional 12,500 pounds of propellants.
After the Earth-departure stage was cast off, the Venus orbiter spacecraft would have a total mass of about 332,000 pounds. It would comprise, from aft to fore, 10,000 pounds of Venus atmosphere entry probes (B), the 103,000-pound Venus arrival rocket stage (C), a 30,000-pound Venus scientific payload (D) made up of remote sensors, the 95,120-pound Venus departure rocket stage (E), the 4,000-pound Venus-Earth course-correction stage (F), the Command Module (G) for housing the crew, and the Earth-atmosphere entry system (H), a 15,250-pound lifting-body with twin winglets for returning the crew to Earth's surface at the end of the mission. Of the Command Module's 66,000-pound mass, food, water, and other expendable supplies would account for 27,000 pounds.
As the spacecraft approached Venus, its crew would turn it so that the Venus arrival stage faced forward, then would ignite the stage as it passed closest to Venus to slow the spacecraft by 0.64 miles per second. This would enable Venus's gravity to capture it into its elliptical operational orbit. The maneuver would expend 91,950 pounds of propellants. The spent arrival stage would remain attached to the spacecraft at least until the Venus atmosphere entry probes were released.
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 manned Venus flyby could spend so near the planet. 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 manned 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 manned voyage beyond the moon.
Reference:
Manned Venus Orbiting Mission, NASA TM X-52311, E. A. Willis, Jr., 1967.
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