Astronomy Picture of the Day 07-21-04

Explanation: This picture of Saturn could not have been taken from Earth. No Earth based picture could possibly view the night side of Saturn and the corresponding shadow cast across Saturn's rings. Since Earth is much closer to the Sun than Saturn, only the day side of the planet is visible from the Earth. Rather, this picture was taken by the robot Cassini spacecraft that began orbiting Saturn earlier this month. The dark western limb of Saturn looms large on the image right, while complex concentrations of small ring particles reflect sunlight on the image left. Saturn's enigmatic F ring is visible around the outside, showing mysterious knots. The small moon Epimetheus, only about 100 kilometers across, can also been seen on the far left. Cassini is scheduled to drop a probe toward the largest moon Titan in December.

Cassini captures crescent Rhea
CASSINI PHOTO RELEASE
Posted: July 20, 2004


Image credit: NASA/JPL/Space Science Institute.

As the first artificial satellite in the Saturn system, Cassini returned images of its native siblings following a successful insertion into orbit, including this unmagnified view of Rhea (1528 kilometres, 950 miles across). Rhea is Saturn's second largest moon, and like Dione, the Voyager spacecraft found one of its hemispheres to covered with bright, wispy streaks which may be water frost.

This view shows a heavily cratered surface, and thus it is most likely an ancient one. Many of the craters visible here have central peaks. Cassini soon will look for clues to help unlock the moon's geologic history. The spacecraft is slated to fly by Rhea at a distance of only 500 kilometres (311 miles) on November 26th, 2005.

The image was taken in visible light with the narrow angle camera on July 2nd, 2004, from a distance of about 990,000 kilometres (615,000 miles) from Rhea and at a Sun-Rhea-spacecraft, or phase, angle of about 109°. The image scale is 6 kilometres (4 miles) per pixel.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Office of Space Science, Washington, D.C. The imaging team is based at the Space Science Institute, Boulder, Colorado.

For more information about the Cassini-Huygens mission, visit http://saturn.jpl.nasa.gov and the Cassini imaging team home page, http://ciclops.org.

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