Death Knell for Surprise Spacecraft : Exploration: Venus orbiter launched in 1978 is pronounced technically defunct after transmitting 400 billion bits of information. It was designed for eight months of duty.
When Pioneer-Venus was launched in 1978, it was designed to orbit our closest planetary neighbor for eight months. The most optimistic predictions for its life span were two to three years. Now, 14 years and 400 billion bits of information later, the spacecraft has finally entered its death throes, providing--even in its final months--surprises and mysteries about Venus.
Researchers managing Pioneer from NASA’s Ames Research Center in Mountain View pronounced the craft technically dead Friday, one day after they lost radio contact with the orbiter that was about 80 miles above the surface, on the planet’s nightside. This development had been expected as the craft got down to mere ounces of fuel, started slowing by 6 m.p.h. on Thursday, and began to glow from the heat of friction.
The main antenna stopped transmitting at 12:22 p.m. Thursday. On Friday, Ames scientists kept checking in vain with the control room to see if auxiliary links could be used to reinstate communications, then gave up and went to lunch to hold an impromptu wake.
Though the actual burn-up is predicted for Oct. 20 or thereabouts, nobody will have any way of knowing the date, time and circumstances when Pioneer passes on. “It’s ‘Goodby, baby,’ ” said Ames spokesman Peter Waller.
The capsule used radar to pierce Venus’ opaque, yellowish clouds, making the first crude map of the planet--a map since improved upon greatly by the Magellan orbiter. Pioneer also studied Halley’s comet in 1986, found evidence of ancient oceans and of lightning, and recorded variations in the atmosphere during an entire 11-year solar-activity cycle.
At its height, Pioneer traveled 41,000 miles above the Venusian surface, then gradually dropped to an altitude below 1,500 miles. As it descended farther during this summer, scientists were amazed to find that the upper atmosphere is far colder than they had imagined--about 70 degrees Fahrenheit on the day side and about 300 degrees below zero Fahrenheit on the nightside. The same “runaway greenhouse” effect that makes the surface so hot that lead would melt is responsible, said Gerald Keating, a senior research scientist from the National Aeronautics and Space Administration’s Langley Research Center in Hampton, Va.
Oxygen atoms are bumping into the abundant carbon dioxide, losing energy into space. “It’s like perspiration,” Keating said. But at the surface, under the clouds, “the radiation is trapped and the atmosphere heats up,” he said.
The observations confirm other scientists’ conclusions that carbon dioxide added to the skies of Earth would cool the upper atmosphere before signs of a surface “greenhouse effect” could be detected. “It looks like the cooling mechanism is much stronger than we thought, though,” Keating said.
During the last several days, spacecraft insulation and other fragile components must have been melting or cracking, Waller said. The loss of communication must have meant that the antennae melted too. Only a pair of titanium fuel tanks are expected to survive the plunge to the Venusian surface.
The mission cost was $170 million, “60 cents per person in the U.S.,” said project manager Richard Fimmel.
The 800-pound spacecraft, built by Hughes Aircraft Co. in El Segundo, was launched from Cape Canaveral on May 20, 1978. In December of that year, it became the first U.S. spacecraft to orbit Venus. Pioneer was not built to be as durable as it turned out to be, and researchers do not know why it held up.
