Rings of Rhea
On November 26, 2005, Cassini made the one targeted Rhea flyby of its primary mission. It passed within 500 km of Rhea's surface, downstream of Saturn's magnetic field, and saw the resulting plasma wake as it had with other moons, such as Dione and Tethys. In those cases, there was a cutoff of energetic electrons as Cassini crossed into the moons' plasma shadows (the regions where the moons themselves blocked the magnetospheric plasma from reaching Cassini). However, in the case of Rhea, the electron plasma started to drop off at eight times that distance, and decreased gradually until the expected sharp drop off as Cassini entered Rhea's plasma shadow. The extended distance corresponds to Rhea's Hill sphere, the distance of 7.7 times Rhea's radius inside of which orbits are dominated by Rhea's rather than Saturn's gravity. When Cassini emerged from Rhea's plasma shadow, the reverse pattern occurred: A sharp surge in energetic electrons, then a gradual increase out to Rhea's Hill-sphere radius.