OAKLAND, CALIF. — When Galileo first peered though his telescope at Saturn some 400 years ago, he was bewildered. "I do not know what to say in a case so surprising," he wrote, confounded by a planet that appeared to have "ears."
Since then, four centuries have revealed much about this pale wanderer through the night sky, including the rings that so perplexed Galileo. But when the Cassini spacecraft passes through those rings Wednesday night and fires its engines to settle into orbit around the great gas giant, it will arrive at a place where scientists still stand open-jawed with a sense of both wonder and incomprehension.
Of all the must-see objects in our solar system, Saturn is the only one that has yet to be fully explored - a mysterious miniature solar system with puzzling rings and a planet-size moon that holds the recipe for organic life. In the past, probes have buzzed by like bullets. But for the first time, Cassini's four-year tour will offer an intimate view of a planet instantly recognizable, yet only dimly perceived.
"This is like going into a new solar system for the first time," says Torrence Johnson, a scientist at the Jet Propulsion Laboratory in Pasadena, Calif., which is managing the joint mission for NASA and the European and Italian Space Agencies. "We're going to find lots of surprises - and find that we have only just scratched the surface."
The first of 76 orbits around the ringed planet begins Wednesday night with a white-knuckle ride. On its initial trajectory, Cassini skitters though a gap between Saturn's thinnest rings, then arcs over the rings like a needle poised over a record. Above the rings, Cassini fires its rocket for 96 minutes, slowing the spacecraft enough to be grabbed by Saturn's gravity, and then flung back down though the rings on the opposite side of the planet and into orbit.
For rocket science, the maneuver is not a Top Gun feat of death-defying difficulty. One of the Pioneer probes passed through the same gap decades ago. Yet the double dip through the rings with its accompanying burn is the crux of the mission. If it fails, there is no mission.
If all goes according to plan, however, the maneuver will do more than set up the mission. It could result in some of the most astounding and significant pictures ever taken of Saturn's rings.
In all its passes, Cassini will never be closer to the rings than it is in those first few hours. And that means Carolyn Porco will never get a closer look. Like her colleagues, she has puzzled over pictures from Voyager and terrestrial telescopes for more than a decade.
The rings, scientists believe, are ever-spiraling rivers of icy dust and debris, gathered by gravity into seven channels, known as rings "A" through "G." Yet they act in ways both curious and confusing, in some places forming waves nearly a mile high, in others braiding themselves together.
Some of these features are massive, such as the radial features called spokes. They can be studied throughout the mission. But deciphering the smallest structures, like wakes only 300 feet long, could hinge on Wednesday night's flyby.
"It's the only time we have a prayer of imaging them," says Dr. Porco, leader of the imaging team.
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."