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The woes of Magellan.

This fall, federal budget cuts may prematurely end the Magellan spacecraft's highly successful effort to map the entire surface of Venus using radar. But Magellan also faces a more immediate threat. At least temporarily, it has lost its voice.

Although the craft, which has already mapped 97 percent of the Venusian surface, continues to bounce its radar off the planet's rugged terrain, problems with on-board transmitters have prevented Magellan from radioing any of these data to Earth since early July

In order to preserve the life of the more useful of its two faulty transmitters, the craft is likely to remain silent until Aug. 21, says Magellan project engineer David Okerson of Space Applications International Corp. in Washington, D.C. Since Magellan's tape recorder can store only a small amount of data, most of the mapping information gathered this summer will have been lost, he notes.

Magellan's communication woes began early this year, when the craft's main transmitter suddenly stopped adding the Venus mapping data to the standard carrier signal it beams to Earth. NASA engineers commanded the craft to switch to its backup transmitter, but that device has its own idiosyncrasy: Electronic noise, known as a whistle, prevents it from sending a clear signal to Earth (SN: 1/25/92, p.63).

By the end of January scientists had circumvented most of the problem by encoding the mapping data at a lower-than-normal electronic frequency Though the lower frequency relays information at less than half the normal rate, researchers found that this signal was not obscured by the whistle prevalent at higher frequencies.

Last month, however, transmission took a turn for the worse. After spending three weeks behind the sun, Magellan emerged with a backup transmitter that was slightly warmer and somewhat noisier. Researchers speculated that the temperature rise had triggered the increase in noise, since the backup device didn't whistle when it was first turned on and remained below 35 [degrees]C. But in June, they found that a slight cooling of the transmitter only made the noise worse.

Standard 34-meter antennas on Earth, part of a network designed to receive signals from space, could no longer decipher the mapping data. Although 70-meter antennas could still receive the mapping information, these instruments were often busy getting data from other spacecraft.

So on July 15, researchers took a drastic measure. They commanded Magellan to turn off the whistling transmitter - thereby substantially cooling it - and to switch back to the faulty main transmitter, which can relay the craft's engineering data but none of the mapping information. Scientists hope that giving the backup transmitter a rest now will enable it to operate normally during the first two weeks of September, when Magellan will fly over an unmapped portion of Venus' southern hemisphere.

NASA intends to use one of two strategies to keep the backup device working come September. Scientists may decide to turn the device on and off for short periods during each 3.1-hour orbit, so that the transmitter will remain below 35 [degrees]C. Alternatively they may choose to keep the device at about 58 [degrees]C, a relatively warm temperature at which it has sometimes performed well.

Tests scheduled to begin on Aug. 21 should determine the optimum strategy, Okerson says. He adds that the current difficulties will not affect Magellan's ability to probe Venus' interior by measuring surface gravity That part of the Magellan mission, which will begin in October but could be severely limited in scope by NASA:S tight budget, relies only on the craft's standard carrier signal and does not require transmission of mapping data.
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Title Annotation:spacecraft transmitter problems
Publication:Science News
Date:Jul 25, 1992
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