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Posted by baalke on September 15, 2006, 8:03 pm
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http://saturn.jpl.nasa.gov/news/features/feature20060915.cfm
Saturn's Rings To Shine As Never Before
Jet Propulsion Laboratory
September 15, 2006
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 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 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.
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.
"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.
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