Saturn's moon Prometheus casts a shadow on the narrow F ring in this image captured weeks after the planet's August 2009 equinox.
The gravity of potato-shaped Prometheus (86 kilometers, or 53 miles across) periodically creates streamer-channels in the F ring, and the moon's handiwork can be seen to the right of the shadow. To learn more and to watch a movie of this process, see Soft Collision.
The novel illumination geometry that accompanies equinox lowers the sun's angle to the ringplane, significantly darkens the rings, and causes out-of-plane structures to look anomalously bright and cast shadows across the rings. These scenes are possible only during the few months before and after Saturn's equinox, which occurs only once in about 15 Earth years. Before and after equinox, Cassini's cameras have spotted not only the predictable shadows of some of Saturn's moons (see Across Resplendent Rings), but also the shadows of newly revealed vertical structures in the rings themselves (see A Small Find Near Equinox).
This view looks toward the sunlit, northern side of the rings from about 11 degrees above the ringplane. Prometheus was overexposed in this image and has been dimmed by a factor of three.
The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Sept. 23, 2009. The view was acquired at a distance of approximately 2 million kilometers (1.2 million miles) from Prometheus and at a Sun-Prometheus-spacecraft, or phase, angle of 74 degrees. Image scale is 12 kilometers (7 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