The purpose of Saturday's flyby was to probe the internal structure of Rhea by measuring its gravitational pull via the spacecraft's radio link to NASA's Deep Space Network here on Earth. The results will help scientists understand whether the 950-mile-wide Rhea is homogeneous all the way through or whether it has differentiated into separate internal layers of core, mantle and crust.
PHOTOS: The Moons of Saturn
During the flyby Cassini came as close as 620 miles (997 kilometers) from Rhea's rugged surface, capturing images of the moon's terrain as it went.
The second-largest of Saturn's 62 moons (Titan is 3 1/2 times bigger) Rhea is a very reflective world, indicating that its surface is made up of a lot of ice. It is also extremely cratered. In fact, Rhea is one of the most heavily-cratered worlds in the entire solar system.