Rings, as you've never seen 'em

Cassini's quest to Saturn may show that high-budget probes still play a key role in solar-system exploration.

Battlestar Galactica rides again.

The international Cassini-Huygens spacecraft has begun its historic four-year tour of Saturn, its spectacular rings, and its moons, following a flawless arrival late Wednesday evening.

Yet as the grand tour begins, the craft has undergone a symbolic metamorphosis. A decade ago, NASA administrator Daniel Goldin turned the craft into a poster child for what was wrong with solar-system exploration efforts. Today many researchers praise Cassini's robust capabilities - which some dub the "Battlestar Galactica" approach to space science - as vital to humanity's efforts to unravel the mysteries of the solar system. They say Cassini's mission, including this week's ring-belt encounter, will showcase the value of long, billion-dollar missions.

Among the initial results to be announced friday: measurements of Saturn's rings and magnetic field, and its moon Titan. The images represent the closest look humanity will get at some of these features during the four-year mission.

The craft's dozen instruments allow researchers to ask questions of the planet and respond to new questions as they arise - a level of flexibility that a "faster, better, cheaper" approach wouldn't allow. To travel so far and then find that instruments on the craft are seeking answers to the wrong questions would be a disaster, notes David Southwood, science director for the European Space Agency. "That inevitably means you have to build big."

In the past decade, NASA's pendulum has swung from infrequent, large, expensive missions like Galileo and Cassini-Huygens to the other extreme of frequent, small, cheap missions. Both approaches have yielded stunning successes and spectacular failures. Now, the pendulum appears to be settling in the middle.

"You can't do the science with just small missions," says Ed Weiler, NASA's associate administrator for space science. "There's a future for big, small, and medium missions." Indeed, in a report to NASA last year, a National Research Council panel recommended a Cassini-class "flagship" mission roughly once a decade. One top candidate: a mission to Jupiter's moons.

The focus now is on what Cassini reveals. Researchers are poring over images of Saturn's rings and Titan and are probing data on the planet's magnetic field to help resolve puzzles about the nature of the planet's interior and the dynamo that generates the field.

One key question involves the age of the rings. Before the Pioneer and Voyager flybys in the 1970s and '80s, "we thought the rings quiescent and were billions of years old," notes Larry Esposito, a University of Colorado scientist at the and discoverer of Saturn's "F" ring.

Those fly-bys testified to an active ring system full of motion. If it had formed 4.6 billion years ago, collisions would have pulverized the material and it would have fallen into the planet by now, say some researchers who see the rings as only a few hundred million years old.

"When dinosaurs roamed the Earth, Saturn had no rings. Think about that," says Carolyn Porco, who heads the Cassini imaging team.

But Dr. Esposito and others aren't so sure. He dubs it "philosophically unsatisfying" to think humans are living at just the right time to observe Saturn's spectacular rings. It also raises questions about where the material came from - some sort of collision involving moons or a comet - so late in the solar system's history.

He says the rings may represent a grand example of cosmic recycling. The rings we see today could be young, even while the system itself is as old as the planet. Saturn's gravity and that of the moons shepherding the rings would have trapped material in the system. There, moonlets of accumulated rubble could build up, then collide and return debris to the rings.

Cassini images could help answer the riddle.

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