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Jupiter's main ring, a narrow tenuous structure about 6000 km in width and encircling the planet at a planetocentric distance of approximately 126,000 km, was first imaged by the Cassini narrow angle camera on December 11, 2000 in a movie sequence that lasted 39.5 hours. (The orbital period of material in the ring is about 7 hours; Jupiter's magnetic field period is 10 hours.)
The distance of the spacecraft during this time ranged from 20.3 million km and 3.3 degrees above the ring plane to 19.0 million km and 2.98 degrees above the ring plane. The ring is about 100,000 times fainter than Jupiter, and during this movie sequence extended only about one sixth of a degree from Jupiter's edge. Accordingly, the pixels on the CCD detector were summed to enhance visibility of the faint ring against a severe scattered light background. The resulting resolution is ~230 km/pixel.
The 10 frames shown here are each a small section of a sum of 6 to 7 separate narrow angle images taken through the clear filter and spanning the entire 39.5 hour period. (The ring was also detected in the red region of the electromagnetic spectrum at ~ 757 nanometers, where the human eye is incapable of seeing.) The scattered light background has been removed, the images have been contrast stretched to enhance the ring, and the contours in the image, as well as the small variations in brightness of the ring from one frame to the next, are a result of the image processing and background removal. The ring clearly grows in angular extent, from upper left to lower right, as the spacecraft approaches the planet.
The Sun-ring-spacecraft, or `phase', angle reaches below 1 degree during this movie sequence. From this viewing perspective, the visible material tends to be grain size and larger. During the next month, the Cassini view of the Jovian ring system will grow in phase angle until the spacecraft is looking back from a phase angle of 120 degrees, at which point fine dust material should be more readily visible. Images of the ring system will be taken in a variety of spectral and polarimetric filters during this swingby. From this information, together with data collected by Galileo, will emerge the most complete picture of the Jovian ring system yet obtained.
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 consists of scientists from the US, England, France, and Germany. The imaging operations center and team lead (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colo.