Cassini Caps off 2004 With Flyby of Icy Moon Iapetus

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JET PROPULSION LABORATORY
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Carolina Martinez (818) 354-9382
Jet Propulsion Laboratory, Pasadena, Calif.

News Release: 2004-300 December 30, 2004

Cassini Caps off 2004 With Flyby of Icy Moon Iapetus

NASA's Cassini spacecraft is set to cap off 2004 with an
encounter of Saturn's ying-yang moon Iapetus (eye-APP-eh-tuss) on
New Year's Eve.

This is Cassini's closest pass yet by one of Saturn's smaller icy
satellites since its arrival around the ringed giant on June 30
of this year. The next close flyby of Iapetus is not until 2007.

Iapetus is a world of sharp contrasts. The leading hemisphere is
as dark as a freshly-tarred street, and the white, trailing
hemisphere resembles freshly-fallen snow.

Cassini will fly by the two-toned moon at a distance of
approximately 123,400 kilometers (76,700 miles) on Friday, Dec.
31. This flyby brings to an end a year of major accomplishments
and rings in what promises to be a year filled with new
discoveries about Saturn and its moons.

"I can think of no better way than this to wrap up what has been
a whirlwind year," said Robert T. Mitchell, program manager for
the Cassini mission at NASA's Jet Propulsion Laboratory,
Pasadena, Calif. "The new year offers new opportunities, and
2005 will be the year of the icy satellites."

In 2005 Cassini will have 13 targeted encounters with five of
Saturn's moons. "We have 43 close flybys of Titan still ahead of
us during the four-year tour. Next year, eight of our 13 close
flybys will be of Titan. We will also have a number of more
distant flybys of the icy satellites, and let's not forget Saturn
and the rings each time we come around," said Mitchell.

With a diameter of about 1,400 kilometers (890 miles), Iapetus is
Saturn's third largest moon. It was discovered by Jean-Dominique
Cassini in 1672. It was Cassini, for whom the Cassini-Huygens
mission is named, who correctly deduced that one side of Iapetus
was dark, while the other was white.

Scientists still do not agree on whether the dark material
originated from an outside source or was created from Iapetus'
own interior. One scenario for the outside deposit of material
would involve dark particles being ejected from Saturn's little
moon Phoebe and drifting inward to coat Iapetus. The major
problem with this model is that the dark material on Iapetus is
redder than Phoebe, although the material could have undergone
chemical changes that made it redder after its expulsion from
Phoebe. One observation lending credence to the theory of an
internal origin is the concentration of material on crater
floors, which implies that something is filling in the craters.
In one model proposed by scientists, methane could erupt from the
interior and then become darkened by ultraviolet radiation.

Iapetus is odd in other respects. It is the only large Saturn
moon in a highly inclined orbit, one that takes it far above and
below the plane in which the rings and most of the moons orbit.
It is less dense than objects of similar brightness, which
implies it has a higher fraction of ice or possibly methane or
ammonia in its interior.

The last look at Iapetus was by NASA's Voyager 1 and 2 spacecraft
in 1980 and 1981. The Cassini images will be the highest
resolution images yet of this mysterious moon.
The Iapetus flyby by Cassini follows the successful release of
the Huygens probe on December 24.

More information on the Cassini-Huygens mission is available at:
http://saturn.jpl.nasa.gov and http://www.nasa.gov/cassini .

The Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency and the Italian Space Agency. JPL, a
division of the California Institute of Technology in Pasadena,
manages the Cassini mission for NASA's Science Mission
Directorate, Washington, D.C. JPL designed, developed and
assembled the Cassini orbiter. The European Space Agency built
and managed the development of the Huygens probe and is in charge
of the probe operations. The Italian Space Agency provided the
high-gain antenna, much of the radio system and elements of
several of Cassini's science instruments.

Cassini spacecraft targeted satellite encounters for 2005:

Titan: January 14, 2005
Titan: February 15, 2005
Enceladus: March 9, 2005
Titan: March 31, 2005
Titan: April 16, 2005
Enceladus: July 14, 2005
Titan: August 22, 2005
Titan: September 7, 2005
Hyperion: September 26, 2005
Dione: October 11, 2005
Titan: October 28, 2005
Rhea: November 26, 2005
Titan: December 26, 2005
-end-