The most complex interplanetary mission ever launched
is about to meet one of the solar system’s enigmatic moons.
Cassini will fly by Phoebe, Saturn’s largest outer moon, on
Friday. The closest approach is at approximately 4:56 p.m.
EDT.

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 (JPL),
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 Cassini imaging
team member. “This time around, the pictures of the
mysterious moon will 1,000 times better, as Cassini will be
closer,” he said. 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
returned by Voyager 2, show large craters and variations 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 brightness also suggest the highest resolution
images, several hundred times better, will show a variety of
materials,” he said.

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 of 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 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 as it is very dark. It reflects only
six percent of the sunlight it receives. Phoebe’s darkness
and retrograde orbit suggest it is most likely a captured
object. A captured object is a celestial body 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 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. JPL, a division of the California Institute of
Technology, Pasadena, manages the Cassini-Huygens mission
for NASA’s Office of Space Science, Washington. JPL
designed, developed and assembled the Cassini orbiter. For
the latest images and 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