联锁店博客

NASA won a major prestige victory over the Soviet Union on December 14, 1962, when Mariner II flew successfully past cloud-cloaked Venus at a distance of 22,000 miles. The 203.6-kilogram spacecraft, the first successful interplanetary probe, left Cape Canaveral on August 27, 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 July 22, 1962.

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° F over the entire planet. Cornell University astronomer Carl Sagan explained the intense heat; the planet’s thick carbon dioxide atmosphere, which bears down on its surface with about 90 times the pressure of Earth’s atmosphere, behaves like the 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 course, slow down, or speed up without expending rocket propellants. Some also began to view Venus as a proving ground for space technology development. In a 1967 Technical Memorandum, for example, NASA Lewis Research Center engineer Edward Willis proposed a manned Venus orbiter based on an “Apollo level of propulsion technology” for the 1975-1986 time period.

Willis rejected piloted Mars and Venus flyby missions, at the time under intense scrutiny within NASA, because they would provide insufficient time near their target planet. Though he supported a piloted Venus orbiter, he questioned the wisdom of launching an equivalent mission to Mars. “It is generally felt,” he wrote, “that the. . . objective of a manned Mars flight should be a manned landing and surface exploration,” not merely a stint in Mars orbit.

Willis calculated that, even in the most energetically demanding Earth-Venus transfer opportunity (1980), a manned Venus orbiter could have 70% less mass at Earth departure than a manned Mars orbiter in the least demanding Earth-Mars transfer opportunity (1986). A manned Mars orbiter would be so massive that it would push the limits of Apollo technology, Willis estimated.

A Mars landing mission would be “still heavier than the orbiting mission,” so probably would “best be done using nuclear propulsion.” Nuclear rockets would, however, require development work before they could propel men to Mars. “This suggests,” Willis wrote, “that 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 low-mass Venus orbiter, Willis explained, would be the selection of an appropriate Venus orbit. Entering and departing a highly elliptical orbit would need considerably less energy (hence, propellant) than would entering and departing a circular orbit. For the 1980 opportunity, he suggested 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 during a 40-day stay near Venus. Time within 26,300 kilometers (3 Venus radii) of Venus would total two days; that is, several times longer than the time a manned flyby could spend near the planet.

A 1980 Venus orbiter based on Apollo technology, with departure from a 400-mile-high circular Earth orbit, a 40-day stay in a highly elliptical Venus orbit, and a total mission duration of 565 days, would have a mass of only 1.5 million pounds, Willis calculated. A 1986 Mars orbiter, on the other hand, would have a mass of 2.4 million pounds.

During the two close approaches, the astronauts would use radar to explore landforms hidden beneath Venus’s clouds. Farther from the planet, they would deploy and monitor Venus atmosphere entry probes with a combined mass of 10,000 pounds. Their high apoapsis, combined with Venus’s slow rotation, would permit them to remain in direct radio contact with the probes for days (unlike probes deployed from a manned Venus flyby spacecraft, which would at best remain in contact with their mothership for a few hours). Throughout their stay in Venus orbit, the crew would explore the planet using 30,000 pounds of science instruments mounted on their spacecraft.