Cassini Image: Prometheus and Pandora

http://saturn.jpl.nasa.gov/cgibin/gs...jpg&type=image

Cassini
Prometheus and Pandora
April 15, 2004

Full-Res: PIA05387
http://photojournal.jpl.nasa.gov/catalog/PIA05387

Cassini has sighted Prometheus and Pandora, the two F-ring-shepherding
moons whose unpredictable orbits both fascinate scientists and wreak
havoc on the F ring.

Prometheus (102 kilometers, or 63 miles across) is visible left of
center in the image, inside the F ring. Pandora (84 kilometers, or
52 miles across) appears above center, outside the ring. The dark
shadow cast by the planet stretches more than halfway across the A
ring, the outermost main ring. The mottled pattern appearing in the
dark regions of the image is 'noise' in the signal recorded by the
camera system, which has subsequently been magnified by the image
processing.

The F ring is a narrow, ribbon-like structure, with a width seen in
this geometry equivalent to a few kilometers. The two small,
irregularly shaped moons exert a gravitational influence on particles
that make up the F ring, confining it and possibly leading to the
formation of clumps, strands and other structures observed there.
Pandora prevents the F ring from spreading outward and Prometheus
prevents it from spreading inward. However, their interaction with
the ring is complex and not fully understood. The shepherds are also
known to be responsible for many of the observed structures in
Saturn's A ring.

The moons, which were discovered in images returned by the Voyager 1
spacecraft in 1980, are in chaotic orbits--their orbits can change
unpredictably when the moons get very close to each other. This
strange behavior was first noticed in ground-based and Hubble Space
Telescope observations in 1995, when the rings were seen nearly
edge-on from Earth and the usual glare of the rings was reduced,
making the satellites more readily visible than usual. The
positions of both satellites at that time were different than
expected based on Voyager data.

One of the goals for the Cassini-Huygens mission is to derive more
precise orbits for Prometheus and Pandora. Seeing how their orbits
change over the duration of the mission will help to determine
their masses, which in turn will help constrain models of their
interiors and provide a more complete understanding of their
effect on the rings.

This narrow angle camera image was snapped through the broadband
green spectral filter, centered at 568 nanometers, on March 10,
2004, when the spacecraft was 55.5 million kilometers (34.5 million
miles) from the planet. Image scale is approximately 333 kilometers
(207 miles) per pixel. Contrast has been greatly enhanced, and the
image has been magnified to aid visibility of the moons as well as
structure in the rings.

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. The imaging team
is based at the Space Science Institute, Boulder, Colo.

For more information about the Cassini-Huygens mission visit,

http://saturn.jpl.nasa.gov

and the Cassini imaging team home page,

http://ciclops.org .

Image Credit:
NASA/JPL/Space Science Institute