A Date with Venus

Soaring through space nearly a million miles from the earth, Mariner 5 responded smartly last week to signals radioed from Pasadena's Jet Propulsion Laboratory, thereby ensuring that its Oct. 19 date with Venus would be as intimate as intended. The spacecraft pitched, rolled and fired its rocket engine for 17.66 seconds, giving the spacecraft a 36-m.p.h. boost and arcing into a trajectory that should carry it past Venus at a distance of only 1,250 miles.

Mariner had company on its 216 million-mile journey. Relatively near by was Russia's Venus 4, launched only two days before the U.S. craft and also scheduled to rendezvous with Venus in mid-October. Although the two vehicles seemed to be engaged in a space race, their launch dates were determined not by competitiveness but by the fact that Venus will be only 49 million miles from earth in October--closer than at any time in the next 19 months.

Tightlipped as usual, the Russians would say only that Venus 4 weighs 2,438 lbs. and will attempt to collect data on "super long-distance measurements," as well as to investigate surface conditions and other characteristics of cloud-covered Venus. Impressed by the weight of the Soviet craft--the heaviest ever sent to Venus--envious U.S. space experts speculated that it could be carrying a TV camera and a capsule capable of being ejected into the Venusian atmosphere and lowered to the surface--perhaps by parachute --in a soft-landing attempt.

Puny by comparison, Mariner 5 weighs only 539 lbs. and has no camera, ejection capsule or landing system. But it is crammed with the miniaturized instrumentation that has performed so effectively on earlier U.S. planetary probes. Unlike the last successful Venus shot--Mariner 2, which sailed within 21,600 miles of the planet in 1962 and sent back a surface temperature reading of 800DEG Fahrenheit, the newest Mariner may come close enough to detect a Venusian magnetic field--if it exists--and the equivalent of the earth's Van Allen radiation belts. It is also equipped to measure the density and temperature of the Venusian atmosphere, investigate the wake left by Venus as it plows through the solar wind, and provide new data on the mass and precise location of the planet.

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