Now showing in outer space: NASA's version of Lord of the Rings.
After nearly seven years ricocheting around the inner solar system, the Cassini spacecraft this week begins mankind's most ambitious survey of Saturn, its dazzling rings and quirky collection of moons -- including one that may harbor primitive precursors of life.
The $3.3 billion unmanned probe, crammed with instruments and billed as the biggest and most complex ever built, will streak past the moon Phoebe on Saturn's distant outskirts tomorrow, capturing the first close-ups of the oddball orb.
Then, in a dicey maneuver that's giving scientists slick palms, Cassini will punch up through the rubble-strewn ring plane, kick in its engine, and dive back down through the rings on Saturn's opposite side.
If it survives, on June 30 the spacecraft will become the first interplanetary probe to swing into orbit around the ringed world.
Cassini is scheduled to spend four years making 76 orbits around a planet whose profile has made it a pop culture icon -- appearing in everything from cartoons to car commercials -- and an irresistible draw for astronomers.
Near the top of most scientific curiosity lists are those arresting rings. First described in 1659 by Dutch astronomer Christiaan Huygens, the rings appear to be solid symmetrical hoops from afar. But observations by the Hubble Space Telescope and three previous NASA flybys have revealed that they're far more complex.
Saturn's rings comprise hundreds of discrete bands of dust, rock and ice, including hunks as big as a house. Scientists labeled the major rings in order of their discovery. From the inside out they are: D, C, B, A, F, G and E. If they encircled the Earth, Saturn's rings would barely squeeze into the space between us and the Moon.
Still, there's less than meets the eye. Packed into a dirty snowball, the debris would form a sphere no more than 62 miles in diameter, according to some estimates.
One enduring puzzle Cassini could help solve is how all this stuff got there and how it's changing over time. The answer, says University of Maryland astronomer Derek Richardson, could hold the key to more questions about the origin of planets and the solar system.
Rings up close
A theorist who specializes in planetary rings, Richardson notes that Saturn is often used as a model for the discs of debris around young stars that eventually coalesce into planets. "We look at the rings and we think: Maybe that's how our solar system looked early on," he says.
Scientists say Cassini's 18 scientific instruments may also help them understand why Saturn looks so different from other ringed planets.
For example, in the 1980s the twin Voyager spacecraft revealed that Jupiter, Uranus and Neptune are also encircled by rings. But it has never been clear why those rings are so faint and lackluster while Saturn's are so dazzling.
Cassini could also help pin down the age of Saturn's rings, which now appear to be far younger than the planet itself. According to Carolyn Porco of the Space Science Institute in Boulder, Colo., the best estimate is that they're a few hundred million years old.
"Which is to say that during the time that dinosaurs roamed the Earth, Saturn did not have rings," says Porco, who heads Cassini's imaging team.
The spacecraft is expected to have its best shot at studying Saturn's rings during its descent into orbit this month. As the craft streaks through a gap between the F and G rings, it will capture high-resolution close-ups, says Porco. These photographs can be compared with images taken by the Voyagers to see how the rings have changed.
Shooting through the gap will also be the most dangerous part of the mission. To help prevent Cassini's sensitive instruments from being pummeled by Saturnian space junk, the probe will pass through with its high-gain antenna pointed forward like a shield.
Still, "it doesn't rule out the possibility of running into a chunk of ice that's moving at the speed of a bullet," warns mission scientist Larry Esposito of the University of Colorado, Boulder. As eager as scientists are to solve ring-related puzzles, many are even more excited about another major objective: to learn more about Saturn's moons.
The sixth planet from the Sun is practically a mini-solar system unto itself: All told, 31 moons twirl around it. Thirteen of these were discovered in the past five years. Scientists suspect others are waiting, possibly nestled in gaps between the rings.
Phoebe is one of seven moons Cassini is scheduled to study up close. Scientists suspect that unlike Saturn's other satellites, Phoebe is not native to the planet. Just 137 miles across, the tiny moon is too dark and distant, and follows an orbital path too different from the other satellites to be home grown.
