After much anticipation, Cassini has finally spotted the elusive spokes in
Saturn's rings.
Spokes are the ghostly radial markings discovered in the rings by NASA's
Voyager spacecraft 25 years ago. Since that time, spokes had been seen in
images taken by NASA's Hubble Space Telescope but had not, until now, been
seen by Cassini.
These three images, taken over a span of 27 minutes, show a few faint,
narrow spokes in the outer B ring. The spokes are about 3,500 kilometers
(2,200 miles) long and about 100 kilometers-wide (60 miles). The motion
of the spokes here is from left to right. They are seen just prior to
disappearing into the planet's shadow on the rings.
At the bottom left corner of the left and center images, the bright inner
edge of the A ring is visible. Continuing radially inward (or toward
Saturn) are several bands that lie within the Cassini Division, bounded
by the bright outer edge of the B ring. The rounded shadow of Saturn cuts
across the rings in the image at right.
Cassini's first sighting of spokes occurs on the unilluminated side of
the rings, in the same region in which they were seen during the Voyager
flybys. Although the most familiar Voyager images of spokes showed them on
the sunlit side of the rings, spokes also were seen on the unilluminated
side.
In Voyager images, when spokes were seen at low phase angles, they
appeared dark; when seen at high phase angles, they appeared bright. The
spokes seen here are viewed by Cassini at a very high phase angle, which
is about 145 degrees at the center of each image.
Imaging team members will be studying the new spoke images and will
maintain their vigil for additional spoke sightings.
These images were taken using the clear filters on Cassini's wide-angle
camera on Sept. 5, 2005, at a mean distance of 318,000 kilometers (198,000
miles) from Saturn. The radial scale on the rings (the image scale at the
center of each image) is about 17 kilometers (11 miles) per pixel.
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 mission for NASA's Science Mission Directorate, 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.
For additional images visit the Cassini imaging team homepage http://ciclops.org.
