The Lure of Our Sister Planet

As the two major space powers began their programs of robotic lunar exploration, it was realized that with some modifications, space probes could survive the five-month journey to Venus, Earth's nearest planetary neighbor. It was necessary to increase rocket power slightly and to harden the probes to survive multi-month exposure to solar radiation. Also, due to the fact that the Earth and Venus separately orbit the Sun, and that poor alignment sometimes makes a journey between them impossible, optimal launch opportunities, or "launch windows'' are less frequent than those for missions to the Moon.

Two views of cloud-shrouded Venus were found in the scientific literature in the late 1950s (and in science fiction novels from that era). Might Venus be shrouded in water clouds—and be a benevolent, though warm, habitat for life? Or might the clouds consist instead of carbon dioxide, indicating blistering surface temperatures and an absence of biology? Unfortunately for astrobiologists, the first successful Venus flyby, NASA's Mariner 2, confirmed in 1962 that the high-temperature model was the correct one. Mariner 5 performed more extensive studies of Venus' atmosphere, during its 1967 flyby.

In spite of the far-from-clement conditions near the surface of Venus, Soviet planetary scientists began to concentrate on space probes that would attempt soft landings on Earth's sister world. They finally succeeded with Venera 7, which touched down in August 1970. Alas, contrary to how the Romans imagined her, the planet Venus is no gentle love goddess! No robotic visitor from Earth has survived more than a few hours on the planet's surface. Probes have found that the surface temperature on Venus exceeds 700 Kelvin. The planet's atmosphere is far more massive than that of Earth—and the atmospheric pressure at the surface is about 90 times that at Earth's surface. If that's not bad enough, acidic material constantly rains down on the surface from the planet's lower atmosphere. The conditions on Venus' surface approximate the medieval conception of Hell!

Sadly, Venus will be not become our second home in the solar system. But even though astronauts may never walk upon that world's surface, there is tantalizing science there. And because ofVenus' similarity to Earth in terms of size and surface gravity, planetary experts would like to know what strange course of events twisted the evolution of this planet, only 42 million kilometers from the Earth, onto such a divergent path. It's a sobering thought to realize that Earth might not be immune from a Venus-type environmental catastrophe.

Venus exploration has continued at a reasonable pace since the dawn of the Space Age. Many Russian Veneras have landed upon the surface, returned color photos and spectrographic analysis. Two Russian probes— Vega 1 and Vega 2—deployed balloons that survived for two days at an altitude of about 54 kilometers above Venus' surface.

En route to Mercury in 1974, America's Mariner 10 imaged the clouds of Venus in the ultraviolet through infrared spectral bands and surveyed particles and fields in the vicinity of the planet. In 1978, Pioneer Venus 1 and 2 became the first spacecraft to radar map the planet's surface. The radar mapping of Venus continued from several Venera probes and culminated in the magnificent Magellan orbiter, which began its studies in 1989 and demonstrated that Venus is volcanically active. Later, in 1989, NASA's Galileo probe performed imaging studies and spectroscopy of Venus' atmosphere, during its Venus gravity-assist maneuvers en route to Jupiter. The robotic exploration of this mystifying world will surely continue.

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