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SOHO Wakes Up Mumbling

7 August 1998 4:30 pm
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After 6 weeks of silence, the Solar and Heliospheric Observatory (SOHO) has answered NASA controllers' pleas with brief bursts of a still-unintelligible radio signal. The sporadic reply to commands, which had been beamed up continuously for as much as 20 hours a day, means that the panels are now in a position to collect some sunlight. "This was a big, big step" toward recovery, says Bernhard Fleck, the SOHO project scientist for the European Space Agency, which operates the craft with NASA. "You can imagine the relief."

Ground controllers lost contact with the spacecraft early on 25 June, when a chain of errors threw it into a spin that left its solar panels unable to collect sunlight and generate power (ScienceNOW 26 June). When SOHO first responded on 3 August, controllers received only a so-called carrier signal, which was "analogous to lifting your phone and getting a dial tone," says Joseph Gurman, the SOHO project scientist for NASA. Later the 10- to 15-second bursts arrived with slight modulations, carrying information in the same way an FM station's signal carries music. Engineers have not yet been able to decode the short bursts, but they probably carry information from temperature and other sensors on SOHO.

The development was accompanied by a second bit of good news. A separate effort, in which radio waves were bounced off SOHO using the 305-meter dish at Arecibo, Puerto Rico, has determined that the craft is spinning at precisely one revolution every 53 seconds. "That's good from a structural point of view," says Gurman. "A bad number might have been 10 times that," or about one revolution every 5 seconds. The information also suggests why SOHO's signal is intermittent: Solar panels face the sun during only half a rotation, then the spacecraft loses power and falls silent again.

After another attempt today to read the patchy signal, controllers plan to shut down all systems on the craft and command it to charge its batteries over the weekend. That way, communications would not be at the mercy of the solar panels and the sensor readings might be sent continuously. And that, in turn, would provide crucial information such as whether fuel tanks could be safely thawed and thrusters fired to pull SOHO out of its spin.

JAMES GLANZ

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