October 2, 2009
A pair of Saturn's moons accompany the planet and its rings in this image taken shortly after the planet's August 2009 equinox.
Dione (1,123 kilometers, or 698 miles across) is in top left of the image. Tethys (1,062 kilometers, or 660 miles across) is on left, below Dione in the image. The vertically thin rings cast a narrow shadow on the planet around the time of equinox. This view looks toward the northern sunlit side of the rings from about 12 degrees above the ringplane. The rings have been brightened relative to the planet to increase visibility.
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).
The image was taken in visible light with the Cassini spacecraft wide-angle camera on Aug. 18, 2009. The view was obtained at a distance of approximately 2.2 million kilometers (1.4 million miles) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 114 degrees. Image scale is 131 kilometers (81 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