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.

December 23, 1995

Prometheus, Pandora and Saturn's F Ring
Credit: NASA, JPL, Voyager

Explanation: The third and fourth innermost moons of Saturn were unexpectedly discovered to be gravitational "shepards." The inner moon Prometheus and the outer moon Pandora use their gravitational attraction to define Saturn's outermost ring. Were any of the smaller chunks of ice and rock that compose Saturn's F Ring to stray, the ring particle would be gravitationally pulled back into place by one of these passing moons. This complex interaction creates a ring structure with two narrow braids and several unusual knots.

Tomorrow's picture: Uranus' Moon Miranda

Authors & editors: Robert Nemiroff (GMU) & Jerry Bonnell (USRA).
NASA Technical Rep.: Sherri Calvo. Specific rights apply.
A service of: LHEA at NASA/ GSFC