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A Parting Shot for Cassini

3 November 1997 8:00 pm
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Don't look back. The Cassini spacecraft (arrows) hurtles away from Earth, captured in three images taken 2.5 minutes apart. Field of view is about 3 arc minutes.

Planetary scientists thought they had seen the last of the Saturn probe Cassini when it roared skyward on 15 October. But thanks to astronomers who usually hunt for asteroids, they have caught a poignant glimpse of it more than 3 million kilometers into its journey. "I got goose bumps when I saw [the images]," says Carolyn Porco of the University of Arizona, Cassini imaging team leader. "I cried when it took off, never to be seen again, and there it was, among the stars."

Joseph Montani and his colleagues at the University of Arizona's Lunar and Planetary Lab in Tucson spend most of their time hunting for asteroids near Earth, but late last month they pointed their 36-inch Spacewatch telescope toward Cassini, which was 1 week into its mission to Saturn. Montani says he used Cassini as a test for the telescope: "Because Cassini was moving at approximately three times the speed of an asteroid in the main belt, this was a good test of whether we could detect a very fast-moving object."

George Lewis, a member of Cassini's navigation team at NASA's Jet Propulsion Lab in Pasadena, California, sent Montani and his group a detailed description of Cassini's predicted path through the sky. Montani's team searched for the receding craft using a computerized light sensor called a charge-coupled device, which searches for objects moving across the background stars, such as asteroids or in this case a spacecraft. He and his group spotted Cassini on 23 October, even though it was about a million times fainter than the dimmest object visible to the human eye. "It was right where it was supposed to be," Montani says.

By now the 5600-kilogram craft is nearly twice as far away as when Montani saw it, nearly 5 million kilometers from Earth. During the next 2 years of its journey, Cassini will steal some extra speed as it flies past Venus twice and Earth once. In December 2000, it will swing by Jupiter, gathering even more speed for the final leg of its trip to Saturn. When it arrives in 2004, Cassini will be the one taking pictures, giving scientists the most detailed images and data ever collected from Saturn and its moons and rings.

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