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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. |
It took almost 30 years of hunting
to find giant convection cells that may play a major role in
how the Sun rotates and how sunspots move across its face and
even influence space weather. This composite image contrasts
the relative sizes of Jupiter with giant convective cells (red
and blue blotches) on the face of the sun. The discovery was
made with the MDI aboard the now-reviving SOHO spacecraft. (links
to 252x249-pixel image.) |
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 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. |
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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. |
SOHO instrument status
and
planned activation dates
- CDS (Coronal Diagnostics Spectrometer)
Oct. 18
- CELIAS (Charge, Element, and Isotope
Analysis System) Oct. 23
- COSTEP (Comprehensive Suprathermal
and Energetic Particle Analyzer) TBD
- EIT (Extreme ultraviolet Imaging
Telescope) Oct. 13
- ERNE (Energetic and Relativistic
Nuclei and Electron experiment) TBD
- GOLF (Global Oscillations at Low
Frequencies) Oct. 8
- LASCO (Large Angle and Spectrometric
Coronagraph) Oct. 13
- MDI/SOI (Michelson Doppler Imager/Solar
Oscillations Investigation) Oct. 12
- SUMER (Solar Ultraviolet Measurements
of Emitted Radiation) working well
- SWAN (Solar Wind Anisotropies)
Oct. 18
- UVCS (Ultraviolet Coronagraph Spectrometer)
Oct. 10
- VIRGO (Variability of Solar Irradiance
and Gravity Oscillations) working well
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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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