Stereo Eclipse
- From: baalke@xxxxxxxxxxxxx
- Date: 12 Mar 2007 16:15:05 -0700
http://science.nasa.gov/headlines/y2007/12mar_stereoeclipse.htm
Stereo Eclipse
NASA Science News
March 12, 2007
March 12, 2007: When scientists announce they're about to calibrate
their instruments, science writers normally put away their pens. It's
hard to write a good story about calibration. This may be the
exception:
On Feb. 25, 2007, NASA scientists were calibrating some cameras aboard
the STEREO-B spacecraft and they pointed the instruments at the sun.
Here is what they saw:
STEREO eclipse movie
<http://stereo.gsfc.nasa.gov/gallery/stereoimages/preview/
preview_transit.shtml>
See the movie: small
<http://stereo.gsfc.nasa.gov/img/stereoimages/movies/
transit_label_sm.mov>,
medium
<http://stereo.gsfc.nasa.gov/img/stereoimages/movies/
transit_label_med4.mov>
or large
<http://stereo.gsfc.nasa.gov/img/stereoimages/movies/
transit_label_large.mov>.
"What an extraordinary view," says Lika Guhathakurta, STEREO Program
Scientist at NASA headquarters. The fantastically-colored star is our
own sun as STEREO sees it in four wavelengths of extreme ultraviolet
light. The black disk is the Moon. "We caught a lunar transit of the
sun," she explains.
The purpose of the experiment was to measure the 'dark current' of
STEREO-B's CCD detectors. The idea is familiar to amateur astronomers:
Point your telescope at something black and see how much 'dark
current'
trickles out of the CCD. Later, when real astrophotography is taking
place, the dark current is subtracted to improve the image.
In this case, the Moon served as a black calibration disk backlit by
the
sun. "The observation was no accident," she says. Mission controllers
arranged the alignment with a small tweak to STEREO-B's orbit last
December and engineers have been waiting for the dark current data
ever
since.
"The images have an alien quality," notes Guhathakurta. "It's not just
the strange colors of the sun. Look at the size of the Moon; it's very
odd." When we observe a lunar transit from Earth, the Moon appears to
be
the same size as the sun - a coincidence that produces intoxicatingly
beautiful solar eclipses. The silhouette STEREO-B saw, on the other
hand, was only a fraction of the sun's diameter. "It's like being in
the
wrong solar system."
The Moon seems small because of STEREO-B's location. The spacecraft
circles the sun in an Earth-like orbit, but it lags behind Earth by
one
million miles. This means STEREO-B is 4.4 times further from the Moon
than we are, and so the Moon looks 4.4 times smaller.
STEREO-B has a sister ship named STEREO-A. Both are on a mission to
study
the sun. While STEREO-B lags behind Earth, STEREO-A orbits one
million
miles ahead ("B" for behind, "A" for ahead). The gap is deliberate:
it
allows the two spacecraft to capture offset views of the sun.
Researchers
can then combine the images to produce 3D stereo movies of solar
storms.
Of particular interest are coronal mass ejections (CMEs), billion ton
clouds of electrified gas hurled into space by explosions on the sun.
"STEREO's ability to see these clouds in 3-dimensions will
revolutionize
our understanding of CMEs and improve our ability to predict when they
will hit Earth," she says.
The STEREO mission is still in its early stages. The two spacecraft
were
launched in Oct. 2006 and reached their stations on either side of
Earth
in January 2007. Now it's time for check-out and calibration. The
first
3D views of solar storms are expected in April.
So science writers, ready your pens. If the calibration runs are any
indication, the actual data will be something to write about.
.
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