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Saturn's
Rings To Shine As Never Before
09.18.06
Ring scientists have been waiting for this. Finally,
after more than two years orbiting Saturn, the Cassini spacecraft
reaches one of the ultimate vantage points. The rings should
shine with majesty worthy of the "Jewel of the Solar
System."
The
imaging team will take this opportunity to stitch together a
mosaic of the entire ring system.
Image
credit: NASA/JPL
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The event is a solar
occultation -- when the sun passes directly behind the planet as
Cassini looks on. And this is not just any solar occultation;
it's a very long one.
The Cassini spacecraft will be
right where scientists studying the rings want it: far enough
from Saturn to be able to image it all and, more importantly,
with the Sun blocked by the planet for 12 hours, long enough to
properly map the elusive microscopic particles moving within the
extended ring system.
Data collected during this
observation might also uncover clues about Enceladus' past
behavior and aid mission planners in refining ring hazard models
for future ring crossings. Thanks to the slow occultation, images
taken during this carefully designed orbit may also uncover new
ring structures and, at the very least, capture truly spectacular
views of the D, F, G and E rings.
"We are all sort
of on pins and needles waiting for the results," says Brad
Wallis, Cassini Rings Discipline Scientist. "When you get
these kinds of high phase angles, very small particles almost
focus the light right at the observer. So these faint rings that
are so hard to see are going to be considerably brighter and show
us details that are just not possible to see in other viewing
conditions. All the space between Enceladus and the G ring is
probably going to be pretty well lit up. It's really a unique
event."
While solar occultations in the mission
typically last only an hour, this time it will last almost 12
hours, truly a dream come true considering it takes about 30
minutes to turn the spacecraft to the proper position. During
"regular" occultations, scientists can only take a few
images before the Sun reappears and the spacecraft has to be
turned away to avoid damaging its sensitive instruments.
"So
far we have only had some tiny snapshots of the E ring at high
phase angles," Wallis says. "In this case we can
basically image the entire ring. During that period of time, we
can image all of the rings from the outer E ring inward at very
high phase angles, which means that we are almost looking
straight into the sun, but the sun is behind Saturn."
Besides mapping the E ring,
the observation will allow scientists to learn if there are
structures within the ring. Structure within the E ring would be
a good indication that Enceladus is not spewing material out at a
regular pace.
"We know that the E ring is a large,
diffuse cloud going out from a little beyond the G ring to far
beyond Enceladus," Wallis says. "Enceladus seems to be
the source of it, given the ice jets Cassini has recently
discovered, so if the whole ring looks uniform, that means
Enceladus has being spewing stuff at the same rate for quite a
long time. But if there are clumps and/or ringlets in it that we
can not explain, it means the spew rate out of Enceladus changes.
This is the one chance we have to see a snapshot of the history
of Enceladus."
The E ring is a very large cloud of
dust made up of extremely fine particles -- 1 or 2 microns --
more or less the size of smoke particles. It stretches from
150,000 to 240,000 kilometers (93,000 to 149,000 miles) from the
center of Saturn and is about 5,000 to 10,000 kilometers (3,100
to 6,200 miles) thick. Crossing the E ring, however, poses no
hazard to the spacecraft.
"We pass through the E
ring all the time, and we get samples of particles every time we
cross it, but we have only a rough idea of what the structure
looks like in detail because the difference between detecting
three particles or four particles at any ring plane crossing is
insignificant," Wallis explains. "This is the only
chance we get to point the cameras right at the Sun and see these
things in this very forward scattered mode."
The
same technique worked in the past, and it allowed mission
planners to avoid cross the rings safely.
"NASA's
Voyager 2 discovered a vast and complex new system of ringlets
around Uranus this way, using only one single image taken in
several minutes. We'll have 10 straight hours," Wallis says.
Voyager
2 looks at Uranus' rings. The view on the right, which was
taken a much higher phase angle, shows details not visible
from any other angle. Image credit: NASA/JPL
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Cassini is slated to send
home the images starting on Sept. 17.
To better exploit
the rare opportunity, Cassini will take images in multiple
exposures and colors. The Imaging Science Subsystem and Visible
and Infrared Mapping Spectrometer will be the prime instruments
in this set of unique observations.
"The imaging
team will use the wide-angle camera to do a mosaic of the entire
area in multiple colors and multiple exposures because the
brightness of the particles changes very steeply when you move
into these high-phase angle areas. The team will also use the
narrow-angle camera to get some high resolution looks at selected
areas." Wallis says. "We have looked at these rings at
140 degrees, and at 150 degrees. During this opportunity we will
see some parts of the rings at phase angles up to 179.4 degrees."
"Because this is somewhat uncharted territory
(viewing these faint rings at these very high phase angles), and
because we only have one opportunity like this in the entire
4-year mission, the teams planning this science want to be sure
they get it and get it right," adds Wallis.
Source / Credit: NASA
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