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Chilling fails to open Galileo antenna.

First they tried heating it. Now they've tried cooling it. But the latest attempt to fully open Galileo's main antenna has failed.

The craft, launched in 1989, is scheduled to begin orbiting Jupiter in 1995. NASA scientists reported last month that they still hope to unfurl the jammed, umbrellalike instrument in time for the Jovian rendezvous. And they have a strong impetus to do so. Without its key antenna, the spacecraft would send back only a small fraction of the data it will collect during its two-year visit to the solar system's largest planet, and the $1.3 billion mission would prove largely fruitless, says NASA's Robert Murray, former program manager for the mission.

If functioning correctly, the high-gain radio antenna will transmit 134,000 bits of data per second. Each picture of Jupiter and its satellites, taken with Galileo's near-infrared camera, will be radioed to Earth in a mere 40 seconds. The 4.8-meter antenna will rapidly relay information from the spacecraft's 10 other instruments as well. But if it stays closed, scientists will have to rely on the craft's two smaller antennas. Near Jupiter,] these antennas would radio only about 10 bits of data per second and would take 10 days to transmit a single image, Murray says. And during those lengthy transmissions, he adds, researchers would lose other precious data too vast to be stored on the craft's tape recorder.

Scientists will face a more immediate, though less serious, problem if they can't fix the main antenna by October, when Galileo passes by the asteroid Gaspra. Because the craft cannot easily transmit data from the asteroid encounter without its main antenna, the onboard tape recorder will likely have to store all Gaspra information until Galileo passes near Earth again in December 1992.

NASA discovered the antenna problem on April 11, when mission scientists first signalled the spacecraft to unfurl the instrument. Special motors on the craft switched on but then stalled, as if they had met with a force too great to overcome. Ground tests indicated that two of the antenna's 18 "ribs" - graphite arms analogous to the spokess of an umbrella-did not open. The malfunction surprised researchers because six similar antenna on three communications satellites had unfurled without a hitch.

At NASA's Jet Propulsion Laboratory in Pasadena, Calif., recent ground simulations with an identical device suggested that the alignment pins, which help keep the ribs properly positioned around the main axis when the antenna is closed, somehow got stuck. Loosening the pins through thermal expansion or contraction appears the only practical means of solving the problem, says program manager Donald Ketterer.

On May 20, mission scientists rotated the craft so that its antenna basked in the sun's warming rays for two days. But tests with Galileo instruments, including two that help measure changes in the wobble of the spacecraft, revealed that the antenna remained tuck. Then, on July 10, NASA rotated the craft so that the antenna faced directly away from the sun for 32 hours. The same tests showed that nothing had changed. Murray says.

But NASA hasn't cooled off on the thermal strategy. On Augut 12, says Murray, scientists will try chilling the antenna to a lower temperature, this time for about 50 hours.
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Title Annotation:spacecraft
Publication:Science News
Date:Aug 3, 1991
Words:540
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