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Voyages to the Planets: Venus

Editor's Note: This is the 22nd in a series of essays on exploration by Steven J. Dick.

Less than a century ago, the planet Venus was most often referred to as "Earth's sister planet." In the 1920s, Charles Greeley Abbot, the distinguished director of the Smithsonian Astrophysical Observatory, called that nearby Sunward world "A twin planet to the earth in size and mass," and suggested that "its high reflecting power seems to show that Venus is largely covered by clouds indicative of abundant moisture, probably at almost identical temperatures to ours." He concluded that "our sister planet appears lacking in no essential to habitability." By the 1930s the lack of spectroscopic detection of oxygen began to take its toll on the 'sister planet' status. Moreover, the detection of carbon dioxide in the thick Venusian atmosphere in the 1930s led astronomer Rupert Wildt to postulate a "greenhouse effect" with temperatures above the terrestrial boiling point of water. By 1958 ground-based radio observations gave first indications of temperatures around 600 F. The discovery in the early 1960s of a 243-day rotation rate - in a retrograde direction compared to the other planets! - left Venus a totally alien and mysterious planet.

It is not surprising, then, that Venus was one of the earliest targets for spacecraft. Mariner 2, the very first successful spacecraft in the famous Mariner series, and in fact humanity's first successful interplanetary mission, was dispatched not to Mercury or Mars, but to Venus. Mariner 2 had no imaging capability, but its flyby of the planet on December 14, 1962 at a distance less than 35,000 kilometers indicated a surface temperature of at least 800 F. Its 42-minute scan of the planet also found a dense cloud layer, but the nature of the clouds had to await study by future spacecraft. Five years later Mariner 5 indicated an even higher temperature of more than 900 degrees Fahrenheit, and crushing atmospheric pressures of 75 to 100 Earth atmospheres.

This global view of the surface of Venus is centered at 0 degrees east longitude. Magellan synthetic aperture radar mosaics from the first cycle of Magellan mapping are mapped onto a computer- simulated globe to create this image. Data gaps are filled with Pioneer-Venus Orbiter data. The simulated hues are based on color images recorded by the Soviet Venera 13 and 14 spacecraft. The image was produced by the Solar System Visualization project and the Magellan Science team at the JPL Multimission Image Processing Laboratory, and is a single frame from a video released at the October 29, 1991, JPL news conference.

Meanwhile, the Russians also had their sights set on Venus. After a series of failures, on October 18, 1967, the day before Mariner 5 began transmitting data, the Venera 4 spacecraft released a lander that parachuted to the surface. As it descended it transmitted data for 93 minutes before being crushed by high pressures at an altitude of 27 kilometers. Venera 4 returned temperature and pressure results similar to Mariner 5, and added another bit of data: the atmosphere was 95 % carbon dioxide. In 1973 ground-based observations by A. T. Young added yet another bizarre twist: the most probable constituent of the Venus clouds was sulfuric acid, a view later confirmed by spacecraft.

"As different as Venus is, data returned from this bizarre planet - as well as from other planets - may well help us understand our own atmosphere."

The Russians sent numerous other Venera spacecraft to the planet, ending with Venera 14 in 1982. Re-engineered to withstand the terrific pressures, some of these spacecraft soft landed on the surface, beginning with Venera 7 on December 15, 1970. In 1982 Veneras 13 and 14 proved especially lucky, returning the first color pictures of the surface of Venus immediately surrounding the landers, showing orange-brown rocks and soil. Veneras 15 and 16 actually mapped parts of the surface via radar. In yet another Venusian coup, in 1985 the Russians and their partners succeeded in landing the Vega 1 and Vega 2 descent modules on the surface of Venus. Moreover, each of those missions deployed a helium balloon that drifted through the atmosphere, returning data for 46 hours.

By this time the Americans had pioneered Venus radar mapping on a global scale. On its way to Mercury, in 1974 Mariner 10 had passed within 6,000 kilometers of Venus, returning 4,165 photos, and becoming the first spacecraft to obtain a gravity assist to its final destination. Then came the remarkable Pioneer Venus missions, consisting of Pioneer Venus Orbiter and Pioneer Venus Multiprobe. Pioneer Venus 1, which entered orbit in December, 1978, peered through the clouds and produced the first radar topographic map of most of the surface, at a resolution of 75 kilometers. It found a relatively smooth planet, with the highest point, Maxwell Montes, at about 11 kilometers above the surface. The orbiter's cameras also detected continuous lightning. A few days after Pioneer Venus 1 entered orbit, Pioneer Venus 2 arrived, consisting of a main carrier and four probes, all five of which descended into the Venusian atmosphere. Two of the probes survived impact, and one of them transmitted data for 67 minutes before being crushed. These probes confirmed clouds composed mainly of sulfuric acid droplets, and made possible a global picture of the structure of the Venusian atmosphere. Pioneer Venus 1 continued in orbit until 1992, when it burned up in the atmosphere.

The Cadillac of Venus spacecraft arrived when Magellan entered Venus orbit in August, 1990, a few months after the Galileo spacecraft had swung by the planet on its way to Jupiter. Living up to its explorer's namesake, Magellan carried synthetic aperture radar capable of mapping 70% of the planet's surface at a resolution of 120 to 300 meters, and 98% of the surface at lesser resolution. Peering through the thick clouds, Magellan's sophisticated radar found evidence of volcanism, tectonics, turbulent winds, lava channels and pancake-shaped domes. It operated until 1994.

It became clear during the course of the Space Age that Venus was about as far from Earth's sister planet as could be imagined. In the true spirit of exploration, what was once thought to be a lush planet ripe for life was instead revealed to be an alien environment. It is no less fascinating for that, and spacecraft will continue to explore the planet. Just a few months ago, in April 2006, the European Space Agency's Venus Express arrived in orbit. Within days, it had discovered strange atmospheric features at the Venusian South Pole, and a variety of instruments began to return data on temperature and chemistry of the atmosphere. It has found clouds extending up to 100 kilometers above the surface.

As different as Venus is, data returned from this bizarre planet - as well as from other planets - may well help us understand our own atmosphere. An understanding of the runaway greenhouse effect, and of the role of aerosols like sulfuric acid, may help us prevent the Earth from eventually becoming another Venus. As experts ranging from NASA's Homer Newell to historian Spencer Weart have pointed out, comparative planetology is not a purely academic subject, but may bear strongly on the future of our home planet. Such practical considerations are another reason why we must continue to explore.