A slingshot ride through Saturn's marvel of ice, dust
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."Skip to next paragraph
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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.