Astronomy Picture of the Day

Discover the cosmos! Each day we feature a different image or photograph of our fascinating universe, along with a brief explanation written by a professional astronomer.

October 14, 1995
See Explanation. Clicking on the picture will download the highest resolution version available.
Rhea: Saturn's Second Largest Moon
Credit: NASA, Voyager, Copyright Calvin J. Hamilton

Explanation: Rhea is the second largest moon of Saturn, behind Titan, and the largest without an atmosphere. It is composed mostly of water ice, but has a small rocky core. Rhea's rotation and orbit are locked together (just like Earth's Moon) so that one side always faces Saturn. A consequence of this is that one side always leads the other. Rhea's leading surface is much more heavily cratered than its trailing surface. The above photograph was taken with the Voyager 1 spacecraft in 1980.

Tomorrow's picture: Iapetus: Saturn's Disappearing Moon

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