Sunday, January 3, 2016

Epimetheus Up Close and Personal

Credit: NASA/JPL-Caltech/Space Science Institute

NASA's Cassini spacecraft captured this view of Saturn's moon Epimetheus (116 kilometers, or 72 miles across) during a moderately close flyby on Dec. 6, 2015. This is one of Cassini's highest resolution views of the small moon, along with Epimetheus Revealed. This view looks toward the Saturn facing side of Epimetheus. North on Epimetheus is up. The image was taken in green polarized light with the Cassini spacecraft narrow-angle camera.

The view was acquired at a distance of approximately 22,000 miles (35,000 kilometers) from Epimetheus and at a Sun-Epimetheus-spacecraft, or phase, angle of 28 degrees. Image scale is 697 feet (212 meters) per pixel.

Epimetheus is a cold, icy object, average surface temperature of minus 195 Celsius (minus 319 Fahrenheit). The density is very low, only 0.64 grams per cubic centimeter or 64 percent that of regular water ice. Epimetheus appears to be an icy rubble pile held together by gravity and also frozen together.

One view shows a black and white view of Epimetheus against the cloud tops of Saturn. Because of the speed of the Cassini spacecraft in relation to Epimetheus, it was not possible to get a colour view as that requires three exposures through Red, Green and Blue filters, Epimetheus was literally moving straight in front of the Cassini spacecraft and timing was crucial to get that particular shot. Twelve were taken in quick succession using the Wide Angle Camera for insurance and only one shot caught Epimetheus in that sequence. The rings of Saturn appear as a narrow band to the upper left as they are pretty well edgewise on. Credit: NASA/JPL-Caltech/Space Science Institute
One view shows a black and white view of Epimetheus against the cloud tops of Saturn. Because of the speed of the Cassini spacecraft in relation to Epimetheus, it was not possible to get a colour view as that requires three exposures through Red, Green and Blue filters, Epimetheus was literally moving straight in front of the Cassini spacecraft and timing was crucial to get that particular shot. Twelve were taken in quick succession using the Wide Angle Camera for insurance and only one shot caught Epimetheus in that sequence. The rings of Saturn appear as a narrow band to the upper left as they are pretty well edgewise on. Credit: NASA/JPL-Caltech/Space Science Institute

Epimetheus more or less shares an orbit with the somewhat larger and four times more massive Janus (also a large  cratered, irregular icy lump) and every four years, the two swap places as Epimetheus catches up with a slightly slower Janus, Janus slows down Epimetheus and Epimetheus speeds ap Janus, Junus swings slightly outwards from Saturn, and Epimetheus falls slightly closer to Saturn and then four years later Janus catches up with Epimetheus and the two swap back, and so on. 

The Cassini mission is a cooperative project of NASA, ESA (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 mission for NASA's Science Mission Directorate, Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colorado.

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