Enceladus briefly passes behind the crescent of Rhea in these images, which are part of a "mutual event" sequence taken by Cassini. These sequences help scientists refine our understanding of the orbits of Saturn's moons.
The images were taken one minute apart as smaller Enceladus (505 kilometers, or 314 miles across) darted behind Rhea (1,528 kilometers, or 949 miles across) as seen from the Cassini spacecraft's point of view.
The images were taken in visible light with the Cassini spacecraft narrow-angle camera on April 14, 2006, at a distance of approximately 3.4 million kilometers (2.1 million miles) from Rhea and 4.1 million kilometers (2.5 million miles) from Enceladus. The image scale is 20 kilometers (13 miles) per pixel on Rhea and 24 kilometers (15 miles) per pixel on Enceladus. http://photojournal.jpl.nasa.gov/catalog/PIA08180