Still Another Touch of Venus

Spacecraft will look again at our enigmatic neighbor

Despite 13 unmanned missions to Venus since 1961, ten by the Soviet Union and three by the U.S., Earth's nearest planetary neighbor has remained an enigma. Shrouded in a veil of fast-moving, pale yellow clouds, its surface temperature about 480DEG C. (900DEG F.)--hot enough to melt lead--Venus has tenaciously resisted attempts to probe its secrets. As Earth and Venus move closer together this year, two American and two Russian space probes will again test the formidable Venusian defenses. Last week, after a successful launch from Cape Canaveral, the first U.S. ship was speeding toward a Dec. 4 rendezvous with Venus and the most extensive atmospheric and topographical survey ever made of that planet.

Pioneer Venus 1 will travel more than halfway around the sun, flying outside Earth's orbit for the first three months, then crossing inside Earth's orbit for the last four months of a circuitous, 480 million-km (300 million mile) journey to Venus. This flight path will lessen the accelerating effect on Pioneer of the sun's gravity. As a result, the ship will make its approach to the planet at a lower speed than if it had taken a more direct route across space. Thus a smaller retrorocket will be needed to slow Pioneer down to the speed necessary for it to slip into its orbit around Venus.

The smaller rocket motor enabled scientists to cram a dozen instruments into Pioneer's drum-shaped "bus," 2.5 meters (8.2 ft.) in diameter. In addition to carrying devices that will map the Venusian atmosphere and cloud formations, analyze the composition of the clouds and measure the interaction between the "solar wind" and the atmosphere, Pioneer 1 will use radar to penetrate the clouds and produce a rough topographic map of much of the Venusian surface. Previous radar scans, made by the giant radio telescope in Arecibo, P.R., have already detected some craters, a large chasm, possible volcanoes, mountainous areas, and what seems to be a giant lava flow.

Pioneer 2's mission is considerably different. Scheduled for launching on Aug. 7, the probe will take a more direct route--only 354 million km (220 million miles) long--and will arrive at Venus only five days after Pioneer 1. While it is still several million miles away from Venus, the Pioneer 2 bus will release a large probe and then three small ones. The bus and separated probes will continue their journey until they plunge into the Venusian atmosphere. Packed with instruments, they will all begin to take readings of atmospheric temperatures, density, composition and distribution as they descend. None is expected to land intact.

Together, Pioneer 1 and 2 are expected to reveal much about the atmosphere and the weather patterns of Venus, and perhaps aid meteorologists in understanding Earth's weather. The data may also help explain why Earth and Venus, neighbors and nearly identical in size and density, should have developed so differently. Indeed, one important experiment will be to confirm earlier estimates that as much as 97% of the atmosphere is carbon dioxide. If that is correct, the planet's hellish temperature may well be caused by what scientists call the "greenhouse effect": sunlight penetrates the carbon dioxide and generates heat on the surface, but the gas blocks the escape of heat back into space and the surface temperature rises.

Thus the data that the Pioneers transmit could be a world-size object lesson to mankind, which is gradually increasing the percentage of carbon dioxide in Earth's atmosphere by burning oil, coal and other fossil fuels. Only a small increase could raise terrestrial temperatures enough to melt the ice caps and flood coastal cities. A significant rise could make it Venus on Earth.

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