A slingshot ride through Saturn's marvel of ice, dust

Any answers would hold an importance that stretches far beyond Saturn. Astronomers are finding more and more solar systems in their earliest stages - still compassed by a spinning disc of rock, ice, and gas. Understanding the mechanics of Saturn's rings would give scientists a deeper understanding of how disc systems work - and perhaps provide insight into the scenes at distant stars.

On many levels, it is an apt comparison. As a gas giant, Saturn is essentially a star that failed to gain enough mass to ignite. It has no "surface." Beneath icy clouds of hydrogen and helium that whip around the planet at more than 1,100 miles per hour, the atmosphere gradually thickens into a liquid and then a solid as heat and pressure increase deeper in the planet. It rotates so quickly - a day is less than 11 hours - that the centrifugal force flattens the poles and bulges the equator.

Then, there are the moons - 31 of them. Farthest out, Phoebe is a bundle of space rock that orbits the wrong way, apparently stolen from the most distant and ancient region of the solar system. Closest in, tiny shepherd moons embedded in Saturn's rings push dust and ice into sinuous paths, while larger moons nearby disturb rings to form waves.

In between, there is the peculiar half-bright, half-dark Iapetus, and a half-dozen other major moons that Cassini plans to visit. Among them, the white moon Enceladus is among the more intriguing. Its relatively featureless surface suggests that the moon could still be active, resurfacing itself as liquid water is somehow melted and then refrozen. Some scientists posit that eruptions of water on Enceladus could be venting water particles into space, where they freeze and form the vast but almost invisible sheet of fine particles of the adjacent "E" ring.

The focal point of the $3.3 billion mission, however, is clearly Saturn's largest moon, Titan. Cassini will pass Titan 45 times, and on Christmas Eve, it will launch the Huygens probe. If Huygens makes it to Titan's surface a month later, it would be the first craft to visit a moon other than our own.

The allure of Titan is obvious. Larger than both Pluto and Mercury, Titan is the only moon in the solar system with a thick atmosphere. To the average eye, that atmosphere is a bland ball of unbroken orange. To the chemist, however, it is a cosmic freezer for the elixir of life - nitrogen, methane, and hydrocarbons.

At minus 300 degrees F, Titan is too cold for life, scientists think. But it is the only place in the universe known to have these materials in such abundance. What Huygens will see when it parachutes beneath the cloud deck is a mystery, but scientists imagine a sky no brighter than a darkroom safe light illuminating a landscape of methane lakes.

"Methane plays the role on Titan that water plays on Earth," says Jonathan Lunine, a Cassini scientist.

The first fuzzy glimpse of Titan's surface could come within a week. Already, Cassini is seeing dark and light regions beneath the haze, and Friday it will pass within 220,000 miles of the moon.

"If we can see through the haze," Porco says. "Titan is ours."

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