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 operations center is based at the Space Science Institute in Boulder, Colo.
Credit: NASA/JPL/Space Science Institute