CICLOPS: Cassini Imaging Central Laboratory for OPerationS
Captain's Log
February 11, 2000

Cassini Asteroid flyby

Since its swing-by of Earth on August 18, 1999 (GMT), Cassini has been making its way towards Jupiter at a breezy 21.4 km/sec. It entered the asteroid belt in mid-November 1999 and is presently more than half way through.

The belt is a collection of millions of small bodies larger than 1 km orbiting the Sun between Mars and Jupiter. Their origins are not well understood: some are made of very fresh and pristine material, while others seem certainly to have resided at one time deep in the hot interior of a minor planet. It is this ambiguity, and what it implies about the events that occurred in early solar system history, that drives scientists to want to learn as much about them as possible.

On January 23, 2000, the Cassini Project took advantage of a serendipitous close encounter with 2685 Masursky, a main-belt asteroid too small to be sized from Earth, but suspected (from its location in the solar system) to be akin in material properties to other asteroids previously studied up close by spacecraft. Gaspra and Ida were both imaged by the Galileo spacecraft, and Eros was imaged, and soon will be orbited, by the Near Earth Asteroid Rendezvous spacecraft. These bodies all belong to the most populous class of asteroids, composed of rock and metal and typically reflecting ~20% to 25% of the light falling on them.

Cassini's close approach to Masursky of 1.5 million km, close enough to come within reach of the cameras but far enough not to cause alarm, was first noted and brought to the attention of the Cassini Project by Tolis Christou, a graduate student of Imaging Team member Carl Murray at Queen Mary and Westfield College in London. Eager for new information on this alien body, the Cassini Imaging Team planned a series of exposures during the brief 1.5 hour flyby. The Masursky images represent the first time that Cassini has gathered information on a body not extensively studied from the Earth, and the first time that Cassini's autonomous object-targeting capabilities, which functioned as expected, have been used.

Though the images are not visually spectacular (Imaging Diary: Masursky), they suggest that Masursky may not be the type of asteroid it was previously believed to be, an intriguing result if confirmed by further analysis. They also are teaching us how to extract information from star-like images of small planetary objects, a lesson which will prove invaluable when the cameras are used to search the Jupiter and Saturn systems for small previously undetected satellites.

Cassini's next stop is Jupiter on December 30, 2000, when it will receive gravity assistance to ensure a timely appointment with Saturn in July 2004. Starting in October, the Imaging Team will begin making a 3 month long, planetary blockbuster movie of Jupiter's swirling atmosphere, its rings and orbiting satellites. Be sure to visit us again around October when the action begins.

Carolyn Porco
Cassini Imaging Team Leader
University of Arizona
Tucson, AZ

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