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By Deborah Zabarenko
Cassini is nearing the end of it's seven-year voyage to Saturn, only to begin an intensive study of the second-largest planet, its rings and the stable of moons that orbit it. The spacecraft is on schedule to enter orbit around Saturn on June 30. (AP Photo/NASA, JPL, Space Science Institute)

WASHINGTON (Reuters) - Dark, rough and contrary, Phoebe has long been an object of fascination to astronomers, and on Friday, NASA (news - web sites)'s Cassini space probe will fly by this moon of Saturn for the closest look yet.

Cassini will capture data on Phoebe on its way to a four-year orbital mission around the ringed planet, scientists said in a statement. The craft's closest pass to Phoebe will take place at 4:56 p.m. EDT on Friday.

Phoebe is a bit of an eccentric among Saturn's 31 known moons, orbiting the planet in the opposite direction from most of the planet's other large satellites, at a 30-degree tilt when compared to Saturn's equatorial plane.

"That means it's really odd," said Bonnie Buratti, who has studied Phoebe for 20 years and is a lead scientist on the Cassini mission.
Last observed from space in images snapped by the Voyager probe in 1981, Phoebe is known to be rough and craggy, so uneven that there may be a huge mountain or crater, Buratti said in a telephone interview from the Jet Propulsion Laboratory in Pasadena, California.

Cassini will take pictures from a much closer vantage point -- 1,240 miles as opposed to 1.4 million miles with Voyager -- and could shed some light on what Phoebe's uneven surface is made of and where it originated.

Astronomers already know Phoebe's surface is extraordinarily dark, reflecting only about 6 percent of the light it gets from the sun. This darkness could be a sign that it was originally from the Kuiper belt, an area at the far reaches of the solar system that teems with such objects.

Earlier observations indicate that the darkness may indicate the presence of carbon on Phoebe, an important finding since carbon is one of the building blocks of life, Buratti said.


There is also frozen water on Phoebe, another finding of potential interest to astronomers looking for clues to how life came to Earth.
One theory is that materials essential to life -- not life itself -- were introduced into the planetary area of the solar system as objects carrying them were captured from the Kuiper belt by big outer planets like Saturn.

In fact, Saturn's distinctive rings, hundreds of them, are thought to be made up of shattered bits of comets, asteroids and moons that broke up before they reached the planet.

Cassini will fly by Phoebe on its way to an orbital path around Saturn. The spacecraft is expected to enter Saturn's orbit on June 30.

The Cassini mission is a joint project of NASA, the European Space Agency and the Italian Space Agency

PHOTO: In this artist's conception, NASA (news - web sites)'s $3.3 billion spacecraft, the Cassini Orbiter, nears the rings of Saturn.