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How many spacecraft have landed on neighboring Venus, and is there anything left of them by now?

JIM CHICKERING, GEORGETOWN, MASSACHUSETTS
Twelve probes have survived the trip to the broiling surface of Venus. Ten were Soviet landers, eight of which were part of the Venera series. The cores of the Venera probes consisted of 8-foot-diameter (2.4 meter) pressure vessels. The bases were doughnut-shape cushions. Although parachutes slowed each craft's initial descent, the last moments relied on metallic plates above the landers that caused enough drag in the dense atmosphere to slow the craft.
In 1970, Venera 7 became the first spacecraft to land on Venus' surface. It returned signals for 35 minutes as it fell through the atmosphere, and another 23 minutes from the surface. Two years later, Venera 8 also made it down, surviving the blistering heat for nearly an hour. Both Venera 7 and 8 measured surface temperature and pressure.

The first surface images came to us from Venera 9 and, shortly thereafter, Venera 10 in 1975. The Venera 9 landing-site panorama shows a sloping field of eroded volcanic rocks. Venera 10 landed 3 days later, about 1,370 miles (2,200 kilometers) away, and returned images of a more eroded landscape with rounded rocks on dark soil.

In 1978, NASA's Pioneer Venus Multiprobe mission sent four probes into the venusian atmosphere. Two of them made it to the surface, and one survived for 70 minutes after it landed. Venera 11 and 12 made landfall the same year. Several instruments failed, and no images were transmitted due to defective lens caps. (Both Venera 9 and 10 had twin cameras, and each lander had one lens cap that did not jettison. Apparently, the "fix" made the problem worse for Venera 11 and 12).

The landers discovered electrical discharges in the atmosphere similar to lightning. Studies since have indicated lightning may crackle through the clouds 35 miles (56 km) above the surface.

More surface panoramas came back from Venera 13 and 14 in 1982. Both cameras on each lander worked, allowing for dramatic 360° panoramas. Both also carried an arm to drill into the soil.

The final Soviet Venus missions were the most spectacular. The Vega 1 and 2 missions (with international collaboration from the French) dropped off landers and balloons on their way to Comet Halley in 1986. Vega 1's instruments were inadvertently turned on at 12.4 miles (20 km) above the surface — due to buffeting by a strong wind gust — and did not return data from the surface. Vega 2 returned atmospheric and surface data. The balloons lasted nearly 2 Earth days until they drifted into the morning Sun. Then, as expected, they popped.

What's left of these explorers? The sulfuric acid rain that falls from venusian skies probably does not make it to the ground, but circulates as a gas. The surface pressure would have little visible effect on the landers. Winds are fairly slow at the surface, although there may be some erosion from windblown grains of dust and sand.

Temperature has the greatest effect. While most of the Venera landers were metallic, thermal blanketing covered the tubes that carried refrigeration liquid into the craft. Other blankets wrapped portions of the main pressure vessel itself. These may well be peeling away.

Additionally, a color strip designed to calibrate color photos taken by the cameras would have faded almost immediately in the heat. Heat-resistant ceramic was switched, at the last minute, to a cheaper material that would have turned brown in the oven-like temperatures on Venus. But overall, the Veneras may still be in good shape — especially considering their environment. — MICHAEL CARROLL, SPACE ARTIST AND AUTHOR, LITTLETON, COLORADO


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