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November 5, 1998: Shuttle Discovery is carrying two telescopes, SOLCON and SPARTAN, designed to unravel two of the most perplexing mysteries in science. One mystery concerns the temperature of the Earth: Did the Sun cause a "Little Ice Age" two hundred years ago? A second conundrum involves the temperature of the Sun: Why is the solar corona, the outermost layer of the sun's atmosphere, 2,000 times hotter than the sun's surface? The STS-95 mission may not solve these longstanding puzzles, but scientists hope to uncover some important clues. |
Mystery the First: the Solar ConstantIn the late 17th century there was a 70 year period called
the Maunder Minimum when no sunspots were observed on the Sun.
The normal 11-yr sunspot cycle essentially stopped, and solar
activity was abnormally low. At the same time Northern Europe
experienced the "Little Ice Age", a series of bitter
winters lasting 50 years. There was another decrease in sunspot
activity between 1800 and 1830. It wasn't as severe as the Maunder
Minimum, but temperatures in Europe and America took another
dip. The year 1816 is sometimes referred to as "The Year
without a Summer" because of unusually cold weather. Many
of the novels of Charles Dickens, which depict harsh winters
in London, were set in this period. |
Our Sun has spots!
These spots appear dark in photographs like the one above, but
in fact sunspots are quite bright - they are just dark compared
to the rest of the Sun. Although sunspots are cooler than the
rest of the sun, the sun is generally hotter when there are many
sunspots. The Solar Maximum Mission found that solar radiance
increased by 0.1% at the maximum of the sunspot cycle. Scientists
are predicting that the next
solar maximum will occur in the year 2000. |
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An instrument called SOLCON has been sent aloft on STS-95 to help answer this question. SOLCON is a radiometer, a device used to measure the total power radiated by the sun at all wavelengths. Its measurements will be used to calibrate instruments on satellites that are continuously monitoring the sun's output. "One problem with instruments in orbit is that they tend to be good at measuring small changes in the solar radiance, but not so good at measuring its absolute value," continued Hathaway. "Calibration is very important." |
Below: SOLCON in the cargo bay of
Shuttle Discovery prior to launch last Thursday. SOLCON is mounted
on the International Extreme Ultraviolet Hitchhiker platform.
Since SOLCON returns to Earth in the shuttle for recalibration, its measurements are not subjected to possible degradation from solar radiation like other orbiting instruments. It provides a "standard candle" for comparison with all other measurements. Measuring the variability of the sun on 100-yr time scales takes time, and SOLCON is a crucial component of the process. With enough measurements by SOLCON and other satellites scientists are hopeful that they can answer some of the outstanding questions about climate change here on Earth. |
Mystery the Second: the Solar Corona The SPARTAN
solar observatory was captured and returned to its berth yesterday
after successfully completing a two-day solar science mission.
About 30 percent of the science data has already been transmitted
to the ground and the remainder will be off-loaded at landing.
SPARTAN Scientist Dr. Richard Fisher noted that investigators
were pleased to have the satellite in orbit near a solar maximum
cycle and that its instruments had captured sought-after data
on a solar mass ejection event. |
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The main target of SPARTAN's observations was the solar corona. Astronomers believe something odd is happening there. Here on Earth as you move from sea level to higher altitude the air generally becomes colder, but the Sun works in reverse. Its outermost layer, the corona, is hotter than 1,000,000 degrees C while the visible surface, or photosphere, has a temperature of only about 6,000 degrees C. How the corona is heated is one of the great mysteries of solar physics. |
"It should be possible to heat the corona with waves,"
says Dr. John Davis at the NASA/MSFC Space Sciences Lab. "All
kinds of waves are generated in the photosphere - such as acoustic
waves, from mechanical motions, or Alfven waves, from shaking
magnetic fields. These waves spread upward into the corona which
absorbs energy from the waves. Left: A
composite of images captured by SPARTAN from Nov. 1 to Nov. 2,
1998. It shows coronal streamers in white polarized light. |
SPARTAN is equipped with two telescopes that can measure both
white light and UV emissions from the sun's corona. By comparing the data collected by the two telescopes and
combining the observations of the SPARTAN 201 missions and Ulysses
and observations made by ground-based instruments, scientists
expect to gain a much more complete picture of the solar corona
and some insight into what might be heating it. |
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Sunspots and the Solar Cycle from Science@NASA SpaceWeather.com Current conditions and forecasts of solar storms, plus news about space weather. NASA Space Shuttle home page ShuttlePressKit.com
-- background information about STS-95 |
The
physics of Orange Juice -- STS-95 experiments target the
behavior of fluid mixtures in low gravity. 03 Nov. 98 NASA Space
Science News Living Better and Breathing Easier -- protein crystal growth experiments on STS-95. 03 Nov. 98 NASA NASA Science News Astronomy Blasts off with John Glenn -- 29 October 98 NASA Science News One small switch for a man... John Glenn activates a landmark materials science experiment while aboard STS-95. 30 October 98 NASA Science News |
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