Saturn to Pull Celestial Houdini on August 11

http://www.jpl.nasa.gov/news/features.cfm?feature=2263

Saturn to Pull Celestial Houdini on August 11
Jet Propulsion Laboratory
August 07, 2009

In 1918, magician extraordinaire Harry Houdini created a sensation
when
he made a 10,000 pound elephant disappear before a mystified audience
of
over 5,200 at New York's famed Hippodrome theatre. But a vanishing
pachyderm is nothing compared to the magnificent illusion to be
performed by our solar system's own sixth rock from the sun on Aug.
11.
On that day, ladies and gentlemen, boys and girls, children of all
ages,
the planet Saturn, with no help from either Jupiter or Uranus, will
make
its 170,000-mile-wide ring system disappear.

How does a mere gas giant planet, without the benefit of a magic wand,
smoke and mirrors, or even sleeves for that matter, manage to hide an
estimated 35 trillion-trillion tons of ice, dust and rock fragments?
Saturn itself, perhaps adhering to the magician's code never to reveal
how a trick is performed, is not talking. But fortunately for us, dear
friends, Linda Spilker, deputy project scientist for the Cassini
Saturn
mission at NASA's Jet Propulsion Laboratory in Pasadena, Calif., is
not
in the magician's guild.

"Saturn has been performing the "ring plane crossing" illusion about
every 15 years since the rings formed, perhaps as long as 4.5 billion
years ago, so by now it is pretty good at it," said Spilker. "The
magician's tools required to perform this trick are pure sunlight, a
planet that wobbles, and a main ring system that may be almost
200-thousand miles wide, but only 30 feet thick."

All planets in our solar system wobble on their axes to some extent.
This change of attitude eventually places a planet's equator directly
in
line with the photons of light streaming in from the sun. This is
called
"equinox," and on Earth it occurs every year about March 21 (spring
equinox) and September 22 (autumnal equinox). On Saturn, it occurs
twice
during each 29 Earth-year-long orbit around the sun (about every 15
years).

"Whenever equinox occurs on Saturn, sunlight will hit Saturn's thin
rings, the ring plane, edge-on," said Spilker."The light reflecting
off
this extremely narrow band is so small that for all intents and
purposes
the rings simply vanish."

While the second largest planet in our solar system has been conjuring
its ring plane phenomenon for millennia, the audience for it only
began
showing up about 400 years ago. By December 1612, Galileo Galilei had
been studying Saturn and its "two large moons" (through his primitive
telescope he mistook the ring system for moons on either side of the
planet) for over two years. He had been noticing these "two moons"
getting thinner and thinner. After the rings disappeared from his
eyepiece entirely, Galileo shared his surprise in a letter in which he
wrote, "I do not know what to say in a case so surprising, so unlooked
for and so novel."

"Galileo had every right to be mystified by the rings," said Spilker.
"While we know how Saturn pulls off its ring-plane crossing illusion,
we
are still fascinated and mystified by Saturn's rings, and equinox is a
great time for us to learn more." Far from being a loss, a ring plane
crossing provides a unique opportunity for scientists. The sunlight
hitting the rings at 90-degree angles can illuminate, or throw
shadows,
revealing ring structures and oddities previously unseen.

But fair warning for those miserly types armed with their own
telescopes
and determined to get a free celestial magic show. This particular
conjuring of the ring-plane crossing illusion will have an audience of
one.

"Saturn's orbit has brought it so close to the sun that it is
extremely
difficult to see even with the best of telescopes," said Spilker.
"Fortunately, we have Cassini in the front row."

The Cassini spacecraft has been observing Saturn, its moons and its
rings from orbit around the planet for the past five years. The
spacecraft's instruments have discovered new rings, moons, as well as
changed the way we look at Saturn's ring system. Around equinox,
Cassini's thermal instrument is tasked with measuring the temperature
of
both sides of the rings as the sun sets to look at how the rings cool
as
they go through this seasonal change. The spacecraft's cameras are
looking for topographic features in the rings, like tiny moons and
possible ring warps, which are only visible at equinox, while the
near-infrared and ultraviolet instruments will be on the hunt for
signs
of seasonal change on the planet.

"The great thing is we are not sure what we will find," said Spilker.
"Like any great magician, Saturn never fails to impress."

The Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency and the Italian Space Agency. The Cassini
orbiter
was designed, developed and assembled at JPL. JPL manages the mission
for the Science Mission Directorate at NASA Headquarters in
Washington.

More information about the Cassini mission is available at
http://www.nasa.gov/cassini or http://saturn.jpl.nasa.gov.