[meteorite-list] Cassini Spacecraft Near First Stop in Historic Saturn Tour

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Jun 9 18:51:30 2004
Message-ID: <200406092251.PAA10449_at_zagami.jpl.nasa.gov>

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

Dwayne Brown (202) 358-1726
NASA Headquarters, Washington

News Release: 2004-145 June 9, 2004

Cassini Spacecraft Near First Stop in Historic Saturn Tour

The most complex interplanetary mission ever launched is about to
meet one of the solar system's enigmatic moons. Cassini will fly
by Saturn's largest outer moon, Phoebe, on Friday, June 11. The
closest approach is at approximately 1:56 p.m. Pacific Time, just
19 days before Saturn arrival.

A final trajectory correction maneuver is scheduled for June 16.
On arrival date, June 30, Cassini will become the first spacecraft
to orbit Saturn. Once in orbit it will conduct an extensive, four-
year tour of the Saturn system, including its majestic rings and
many known moons.

"The arrival date and trajectory to Saturn were specifically
selected to accommodate this flyby, which will be the only
opportunity during the mission to study Phoebe at close range,"
said Dave Seal, mission planner for the Cassini-Huygens mission
at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "Phoebe's
orbit is simply too far from Saturn, at almost 13 million
kilometers (about 8 million miles), nearly four times as far as
the next closest major satellite, Iapetus. A later encounter is
not feasible."

"The last time we had observations of Phoebe was by Voyager in
1981," said Dr. Torrence Johnson, former Voyager imaging team
member, Galileo project scientist and current Cassini imaging
team member. "This time around, the pictures of the mysterious
moon will be about 1,000 times better, as Cassini will be
closer." Voyager 2 captured images of Phoebe from about 2.2
million kilometers (about 1.4 million miles) away. Cassini will
obtain images from a mere 2,000 kilometers (about 1,240 miles)
above the moon's surface.

Cassini will also collect spectroscopic and radar data that could
decipher the composition and origin of this distant moon.
Cassini's Phoebe images, already twice as good as any image
returned by Voyager 2, show large craters and variation in
surface brightness.

"We anticipate that Phoebe will be heavily cratered in the higher
resolution images we expect to see in the next few days," said
Dr. Peter Thomas, a member of the imaging team and a senior
research associate at Cornell University, Ithaca, N.Y., who
specializes in studies of small satellites. "The hints of
different brightnesses also suggest that the highest resolution
images, several hundred times better, will show a variety of
materials.

Discovered in 1898 by American astronomer William Henry
Pickering, Phoebe is of great interest to scientists. "With the
instruments Cassini carries, we might learn more about Phoebe's
internal structure and composition. What we have are many
unanswered questions: Did it ever melt? Does it have evidence of
past interior melting? Was it ever an icy body? Why is Phoebe in
such an odd orbit?" said Dr. Dennis Matson, project scientist for
the Cassini-Huygens mission at JPL.

Phoebe has a diameter of 220 kilometers (about 136.7 miles),
which is equal to about one-fifteenth the diameter of Earth's
moon. Phoebe rotates on its axis every nine hours and 16 minutes,
and it completes a full orbit around Saturn in about 18 months.
Its elliptical orbit is inclined approximately 30 degrees to
Saturn's equator. Phoebe's retrograde orbit means that it goes
around Saturn in the opposite direction of the larger interior
Saturnian moons. Previous ground-based observations have shown
water ice present on its surface.

Phoebe is also unusual in that it is very dark. It reflects only
six percent of the sunlight it receives. Phoebe's darkness and
retrograde orbit suggest that it is most likely a captured
object. A captured object is a celestial body that is caught by
the gravitational pull of a much bigger body, generally a planet.
Some scientists believe Phoebe might even be an object from the
outer solar system, similar to the objects found in the Kuiper
Belt. The Kuiper Belt is a collection of small icy bodies beyond
Pluto that were never drawn together by gravity to form a planet.

"The dark and odd-shaped Phoebe may be a piece of the building
blocks from which some of the planets formed," said Dr. Bonnie
Buratti, scientist on the Cassini-Huygens mission at JPL. "It
might hold clues about the early formation of our solar system."

The Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency and the Italian Space Agency. The Jet
Propulsion Laboratory, a division of the California Institute of
Technology in Pasadena, manages the Cassini-Huygens mission for
NASA's Office of Space Science, Washington, D.C. JPL designed,
developed and assembled the Cassini orbiter.

For the latest images and more information about the Cassini-
Huygens mission on the Internet, visit:

http://www.nasa.gov/cassini

For information about NASA and agency programs on the Internet, visit

http://www.nasa.gov .
                                                        
-end-
Received on Wed 09 Jun 2004 06:51:18 PM PDT