SOHO opens its eyes
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SOHO opens its eyes Scientists await word on key instrument
October 9, 1998: Two of the 12 telescopes aboard the Solar and Heliospheric Observatory (SOHO) spacecraft have been successfully turned on raising hopes for normal operations. Scientists are eagerly awaiting word, though, on a key telescope whose fragile optics may have been damaged by long exposure to the intense cold of deep space.
"It's like putting a bottle of water in the freezer," said Dr. David Hathaway of NASA's Marshall Space Flight Center. "In one of these telescopes we have optical elements with metal mountings that contract differently than the optical elements, so we're worried that they may be cracked."
The instrument is the Michelson Doppler Interferometer - MDI - that is a key instrument for measuring magnetic fields and vibrations on the surface of the sun. Hathaway is an associate investigator on the MDI and used it to discover giant convective cells (right) that may play a major role in the sun's 22-year-long cycle.
Â SOHO has been high drama - literally - for the solar science community since June 24 when a communications error pointed it away from the sun. It lost electrical power and the thermal control that keeps the electronics, telescopes, and propellant at just the right temperatures.
Engineers scrambled first to locate the craft , and then to send a signal loud enough to be heard by the antenna which now was pointed away from Earth. Between Aug. 3 and Sept. 16 they received tentative signs that SOHO was alive, and then slowly brought it under ground control, pointed it in the right direction, and then thawed it out.
An artist's concept, above, of SOHO in flight configuration. Click for a spin animation of the spacecraft (87KB gif). Image and animation courtesy of the European Space Agency/ISD Visulab.
Over the last two weeks, scientists have been gradually reactivating the telescopes to see which ones work and whether SOHO can continue its phenomenal series of discoveries about the sun.
"It has been nice to see these other instruments come back on line," Hathaway said. "Some aspects of our helioseismology can be done by GOLF and VIRGO [see instruments listed at left], but they have more limited capabilities, and they have very narrow fields of view. The MDI shows the whole sun at high resolution, and gives us velocities and magnetic fields. It will be sorely missed if its lost."
The MDI instrument played a key role in showing that solar flares produce seismic waves, and gigantic seismic quakes, in the Sun's interior.
planned activation dates
One of the keys, Hathaway explained, is a calcite crystal within the MDI. The crystal is softer than glass and absolutely crucial to extracting from the sun's intense light a narrow set of measurements.
"If it's been cracked by the mounting contracting too tightly around the crystal, then it will just deliver scattered light and the MDI won't work," he said.
The MDI is scheduled to be powered up on Monday. The electronics and some moving parts will be exercised first, he said, followed by the camera itself. Scientists will know right away if the crystal survived the deep freeze. They will either get a clear image, or one that looks like a broken jumble.
Stay tuned to the SOHO recovery operations web site for further developments.
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