Astronomy Picture of the Day

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

December 5, 1999
See Explanation. Clicking on the picture will download the highest resolution version available.

Rhea: Saturn's Second Largest Moon
Credit: Voyager, NASA; 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: Pinwheel in Space

< Archive | Index | Search | Calendar | Glossary | Education | About APOD >

Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA)
NASA Technical Rep.: Jay Norris. Specific rights apply.
A service of: LHEA at NASA/ GSFC
& Michigan Tech. U.