http://rockymountainnews.com/drmn/local/article/0,1299,DRMN_15_2481982,00.html
Saturn shot shines light on shadows Aging CU 'camera' runs rings around view from Earth By Jim Erickson, Rocky Mountain News For University of Colorado researcher Larry Esposito, the latest planetary portrait sent home by NASA's Saturn-bound Cassini spacecraft is a reassuring reminder that "the payoff is clearly near." Esposito has worked on the Cassini mission for 16 years. The $3.4 billion spacecraft was launched in October 1997 and is due to begin orbiting the giant ringed planet July 1. "It's a big boost to see Saturn so close up," Esposito said of the color picture released Friday. The natural-color image was captured Nov. 9, when the spacecraft was about 69 million miles from Saturn - that's roughly three-quarters of the distance between the Earth and the sun. "You can see the shadow of Saturn on the rings and of the rings on the planet, and it's a perspective we can't get from Earth," Esposito said. "So it's literally something that's only possible from space." Esposito heads the CU team that built Cassini's $12 million ultraviolet imaging spectrograph. The device will capture images of ultraviolet light reflected from Saturn's cloud- cloaked atmosphere, its majestic, multi-banded ring system, and its 31 known moons. The ultraviolet images will help scientists learn more about the structure and composition of those objects. Esposito's team will begin observing Saturn on Christmas Day and will monitor it periodically through the final approach to the planet. In mid-February, Cassini's main visible-light camera, which captured the image released Friday, will begin monitoring Jupiter on a daily basis, said Boulder planetary scientist Carolyn Porco, head of the Cassini imaging team. Thousands of pictures will be snapped between mid-February and mid-May to study ring structure and to look for previously undetected moons, said Porco, who works at the Space Science Institute. During the three-month monitoring period, researchers will measure wind speeds on Saturn by creating time-lapse imagery of clouds moving across the planet's surface. "Once we're in orbit around Saturn we are - surprisingly - too close to do a number of scientific objectives of interest," Porco said. "One of them is to make global movies of the planet and the rings." The color photo released Friday is a composite of images taken through blue, green and red filters. The hues approximate what the human eye would see. Five Saturnian satellites are visible as small bright dots beyond the rings. The moons are Rhea, Dione, Enceladus, Tethys and Mimas. Some details within the ring system can be seen in the latest Cassini image. The 2,980-mile-wide Cassini Division is the distinctive dark, central band that separates Saturn's outermost A ring from the brighter B ring. Saturn's rings are composed of countless chunks of ice and ice-coated rock that range in size from peas to houses. From end to end the Saturn ring system spans 171,000 miles - about three-quarters of the distance between Earth and its moon. Small satellites embedded within Saturn's rings sculpt the icy bands and provide a fresh supply of debris that perpetuates them, Esposito said Friday. The small moons are broken apart periodically by the impact of asteroids and comets. The resulting fragments are shattered to form the particles of new rings, Esposito said. Individual rings come and go, but the ring system is maintained, he said. "If you visited New York City, you'd see old people and young people. If you came back a century later, you would see a different bunch of people, but the streets would still be crowded. "In the same way, a separate, individual ring has a short lifetime, but the whole ring system may well have been around since the origin of the solar system," he said. Esposito and his colleagues will present new calculations concerning the lifetime of planetary rings next week at a meeting of the American Geophysical Union in San Francisco.
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