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 Cassini orbiter and its two onboard cameras, were
designed, developed and assembled at JPL. 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.
