The G-Ring and "Star-Trails"

Caption NASA originale:"Cassini's ability to remain sharply pointed at its targets allowed this image of Saturn's faint, dusty G-Ring to be made. The thin streaks visible here are "Star-Trails", which are created during long exposures, when the spacecraft remains locked onto a single target. The camera shutter was open for 3 and 1/2 minutes during this particular exposure. A long exposure was required to see details of this quite tenuous Ring.
The feature inside the G-Ring, at upper right, is also a star trail.
The image was taken in polarized visible light with the Cassini spacecraft narrow-angle camera on March 7, 2005, at a distance of approx. 1,2 MKM (such as approx. 750.000 miles) from Saturn. The image scale is approx. 10 Km per pixel".