Short answers, pulled from the story.
Rhea was discovered by Giovanni Domenico Cassini on the 23rd of December 1672, using a telescope made by Giuseppe Campani with a focal length of 10.4 meters. It was the second moon of Saturn that Cassini discovered, and the third found around Saturn overall.
Rhea is composed of roughly 75 percent water ice and 25 percent rock, giving it a low density of about 1.236 grams per cubic centimeter. Its interior is thought to be largely homogeneous, with little or no separation between a rocky core and an icy mantle, though this remains debated.
Rhea has a mean diameter of 1,528 kilometers, making it the second-largest moon of Saturn and the ninth-largest moon in the Solar System. Its surface area is approximately 7,330,000 square kilometers, roughly comparable to the land area of Australia.
Rhea has an extremely tenuous exosphere, announced by NASA on the 27th of November 2010. It consists of oxygen and carbon dioxide in a ratio of about 5 to 2, with a surface density of roughly 100,000 to 1,000,000 molecules per cubic centimeter.
NASA announced in March 2008 that Rhea might have a ring system, which would have been the first rings found around any moon. Subsequent targeted observations by the Cassini spacecraft found no evidence of ring material, leaving the original electron-flux anomalies without a confirmed explanation.
Rhea was formally named in 1847, when astronomer John Herschel proposed in his book Results of Astronomical Observations made at the Cape of Good Hope that Saturn's moons be named after the Titans of Greek mythology. Rhea is named for the Titan who was the wife of Cronus, the Greek equivalent of Saturn, and the mother of the first Olympian gods.