A quartet of Saturn's moons is seen here with the planet's F and A rings, but something special is happening to the moon in the middle of this Cassini image.
As Saturn approaches its August 2009 equinox, the planet's moons have been casting shadows onto the rings (see Moon Shadow in Motion). Now the rings take a turn casting a shadow on a moon. Tethys is the moon second from the left. The northern part of this moon is darkened by a shadow cast by Saturn's A ring.
From left to right, the moons shown are Mimas (396 kilometers, or 246 miles across), Tethys (1062 kilometers, or 660 miles across), Rhea (1528 kilometers, or 949 miles across), and Pandora (81 kilometers, or 50 miles across). Pandora is a tiny speck inside the rings.
This view looks toward the unilluminated side of the rings from about 1 degree above the ringplane. The rings and Pandora have been brightened relative to the other moons to enhance visibility. The image was taken in green light with the Cassini spacecraft wide-angle camera on April 11, 2009. The view was acquired at a distance of approximately 1.4 million kilometers (870,000 miles) from Tethys and at a Sun-Tethys-spacecraft, or phase, angle of 21 degrees. Image scale on Tethys is about 82 kilometers pixels 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