True-color image of Rhea taken by Voyager 1
|Date of discovery||December 23, 1672|
|Name of discoverer||Giovanni Domenico Cassini|
|Name origin||Titaness and mother of Zeus by Kronos|
|Order from primary||19|
|Semi-major axis||527,040 km|
|Sidereal month||4.517500 da|
|Inclination||0.35° to Saturn's equator|
|Sidereal day||4.517500 da|
|Rotational speed||12.3 m/s|
|Mass||2.309 * 1021 kg|
|Mean radius||764.5 km|
|Surface gravity||0.2634 m/s²|
|Escape speed||0.6346 km/s|
|Surface area||7,344,544 km²|
|Minimum temperature||63 K|
|Mean temperature||76 K|
|Maximum temperature||99 K|
|Composition||Water ice and rock mixture|
Discovery and naming
Giovanni Domenico Cassini discovered Rhea, along with the moons Tethys, Dione, and Iapetus, in the latter half of the seventeenth century, during the reign of "Sun-King" Louis XIV of France. Cassini named these moons the "Sidera Lodoicea" in honor of the king. Later generations (especially after the French Revolution) would not retain such regal flattery.
Sir John Herschel, son of the astronomer William Herschel, suggested the current names of the seven largest satellites of Saturn, including Rhea. Titan received a generic name, and the other six received names of the Titans of mythology. Rhea was the wife of Kronos or Cronus and mother of Zeus, the eventual king of the gods.
Orbital and rotational characteristics
Rhea is in a relatively circular orbit around Saturn, at a mean distance of 527,040 km. It makes one orbit around Saturn in 4.52 days. Rhea is in tidal lock with Saturn and hence its rotation is synchronous with its orbit.
Rhea is a low-density body consisting mainly of water ice.
SurfaceCallisto, are almost flat, and their walls and central mountains are of relatively low altitude.
The trailing hemisphere of Rhea has multiple wisp-like formations that might be mountain ranges.
The density of Rhea at first led astronomers to believe that Rhea had a rocky core consisting of one-third the total mass of the body, and a mantle of water ice. But recent evidence suggests an undifferentiated interior. Specifically, the moment of inertia for Rhea is 0.3911 ± 0.0045 kg m², which suggests that Rhea's rock and ice are evenly mixed, with compression of the ice from Ice I to Ice II toward the core.
Ring systemVoyager 1 had found in 1980, strongly suggest that Rhea has rings.
Observation and Exploration
The first spacecraft to explore Rhea was Voyager 1. The Cassini orbiter has made two close rendezvous with Rhea during its first four years of operation, on November 26, 2005, and August 30, 2007. Mission planners have fixed a date for another rendezvous with Rhea on March 2, 2010, during the two-year extended mission.
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