NASA's Cassini spacecraft will be swooping close to Saturn's moon Rhea on Saturday, March 9, the last close flyby of Rhea in Cassini's mission.

The primary purpose will be to probe the internal structure of the moon by measuring the gravitational pull of Rhea against the spacecraft's steady radio link to NASA's Deep Space Network here on Earth.

The results will help scientists understand whether the moon is homogeneous all the way through or whether it has differentiated into the layers of core, mantle and crust.

In addition, Cassini's imaging cameras will take ultraviolet, infrared and visible-light data from Rhea's surface. The cosmic dust analyzer will try to detect any dusty debris flying off the surface from tiny meteoroid bombardments to further scientists' understanding of the rate at which "foreign" objects are raining into the Saturn system.

Cassini will fly within about 600 miles (1,000 kilometers) of the surface. The time of closest approach is around 10:17 a.m. PST (1:17 p.m. EST). This is Cassini's fourth close flyby of Rhea.