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Science & Space

Space probe fly-by of Saturn's moon

By Kate Tobin

The small dark moon of Phoebe is resolving quickly in the camera of Cassini as it approaches Saturn. These images were taken 13 hours apart on June 10.
The small dark moon of Phoebe is resolving quickly in the camera of Cassini as it approaches Saturn. These images were taken 13 hours apart on June 10.
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(CNN) -- The Cassini spacecraft will execute a close flyby of Saturn's moon Phoebe on Friday, snapping pictures and making science observations as it passes.

The closest approach to the moon -- about 1,240 miles -- will occur at 4:56 p.m. EDT. Mission scientists expect to make images available Saturday afternoon on the Jet Propulsion Laboratory website.

Cassini is on course to enter orbit around Saturn on June 30.

Astronomers regard the Cassini mission to Saturn as the most ambitious planetary exploration ever attempted. Launched nearly seven years ago, the craft has traveled over 2 billion miles through the solar system to rendezvous with the ringed giant. Once it enters orbit, it will begin a four-year scientific tour of the planet and its moons.

The high point of the mission will come in January 2005, when the orbiter deploys a probe called Huygens that will land on the surface of Titan, Saturn's largest and most interesting moon.

But first will come the Phoebe fly-by. Saturn has 31 known moons, and over the course of its mission Cassini will make up-close observations of seven of them. This will be Cassini's only chance to study Phoebe. The moon is in an off-plane, highly elliptical orbit around Saturn, and Cassini will not pass near it again.

A small moon just one-15th the size of Earth's moon, Phoebe has been studied before, but at a much greater distance. The Voyager 2 spacecraft passed within 1.4 million miles of it in 1981. Cassini's images are expected to be 1,000 times better.

Cassini will also use on-board radar and spectrographic instruments to study Phoebe's structure and composition, in hopes of gathering clues to the moon's origin. Mission scientists say it is an odd moon with a retrograde orbit, meaning it orbits Saturn in the opposite direction from the larger moons. It is also very dark, and reflects very little sunlight.

Astronomers suspect it is a "captured" object, meaning it formed elsewhere in the solar system and was passing by Saturn at some point in the distant past when it got caught in the planet's powerful gravitational field. There is a chance Phoebe could be from the Kuiper Belt, a mysterious area in the distant outer reaches of the solar system where icy, rocky debris such as that which formed the planets remains.

"We are already learning new things about the Saturn system," said Cassini science team member Candice Hansen. "We will fly by Phoebe on June 11th and that's a very interesting moon. We presume that it's a captured object. We don't know what population it represents. Is it an asteroid? Is it a Kuiper Belt Object?"

Cassini is a $3 billion joint project between NASA, the European Space Agency, and the Italian Space Agency.

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