The original idea was to fly within 600 miles of Enceladus, orbiting 148,000 miles from Saturn, but some researchers suspected the icy moon might have an atmosphere. Fly closer, they suggested.

"The rest is history," said planetary scientist Robert H. Brown of the University of Arizona's Lunar and Planetary Laboratory. From a height of 109 miles, the Cassini spacecraft trained instruments on a cloud of water vapor venting from fissures at the moon's south pole.

From that moment, tiny Enceladus, only 310 miles in diameter, joined Mars and Jupiter's moon Europa as the solar system's leading candidates for having liquid water beneath their chilly surfaces -- a likely precondition for harboring life. But why the south pole? And how does something so small have liquid water?

"We haven't a clue," Brown said.

Brown calls the discovery of water vapor at Enceladus "one of the most important things ever to come out of planetary science," but for NASA's Cassini, perhaps the most muscular planetary science mission ever launched, it was just another day's work.

Nearly 15 months after Cassini awed an international viewing audience by flying through the rings of Saturn and settling into orbit around the giant gas planet, the spacecraft is operating impeccably, producing data and discoveries that are occupying the professional lives of as many as 500 scientists and engineers.

"We feel like we're drinking from a fire hose," said Torrence V. Johnson of NASA's Jet Propulsion Laboratory. He is a member of Cassini's imaging science team.

Perhaps the mission's most memorable moment came in January, when the European Space Agency's Huygens probe, released by Cassini 11 days earlier, parachuted to the surface of Saturn's largest moon, Titan, penetrating its smoggy atmosphere to record the first clear images of a forbidding world.

Since then, Cassini has made five more passes, and scientists' perception of Titan has begun to expand. They still regard Titan as a "pre-biotic" world, lifeless but rife with the compounds that existed on Earth before life evolved, but they now find its meteorology just as interesting as its chemistry.

"Titan is probably more Earth-like than anyplace we've looked at anywhere," Johnson said, "and we're seeing it every few weeks." Titan quite likely has a weather system based not on water but on liquid methane, which has eroded Titan's ice cliffs to create a landscape reminiscent of Norwegian fjords.

The methane vents from Titan's interior; and although much of it blows away into space and some of it falls back to the surface as hydrocarbon dust particles, scientists are confident that a great deal of it must also fall as rain. No solid body in the solar system except Earth has such a complex weather system.

And even though Cassini has not yet seen methane rain nor found any evidence of the predicted methane oceans, project scientists say rain is the only explanation that fits the landscape. "We're probably talking about some seasonal differences," said J. Hunter Waite Jr. of the University of Michigan. "There is some evidence from the southern hemisphere of a methane lake, and we'll find more."

Cassini, a $3.3 billion behemoth launched in 1997 by NASA and the European and Italian space agencies, has 12 instruments on board.

Cassini has found several new rings surrounding Saturn, has discovered that Saturn's winds blow 170 mph faster at lower altitudes than at high altitudes, and has observed huge thunderstorm columns that produce lightning bolts 10,000 times as strong as any on Earth.

The spacecraft has also provided new evidence that Saturn's moon Phoebe is an interloper that wandered in from deep space and was captured into orbit by the planet, and that Phoebe's surface is more diverse than that of any known body in the solar system except Earth.

The spacecraft discovered Polydeuces, a 3-mile-wide midget moon, orbiting in concert with the larger moon Dione. Saturn is the only planet known to have these companion moons, called "Trojan moons."

During a summer encounter with Saturn's rings, Cassini's infrared spectrometer found sharp temperature variances on the surfaces of both large and small particles in Saturn's three main rings, an indication that the particles are rotating slowly.

And in the A ring, 76,000 miles from Saturn's center, Cassini found that particle clusters, ranging in size from "sedans to moving vans," are constantly combining and being torn asunder by tidal forces caused by Saturn's gravity.

"You have to have enough big particles for gravity to stick them together, and enough distance from Saturn so the tidal forces rip them apart immediately," said University of Colorado planetary scientist Joshua E. Colwell. "Only the A ring has both."

On the heavily cratered moon Iapetus, scientists found an equatorial mountain range twice the height of Mount Everest, but they still do not know how half of Iapetus came to be coated with dark material while the other half shines white.

Iapetus vies with Enceladus among project scientists for the title of "favorite icy moon," Johnson said. "Iapetus is strange on the old end, and Enceladus is strange on the young end."

Known before the Cassini mission as the brightest object in the solar system aside from the sun because of its shroud of crystalline ice, Enceladus is now also known as the smallest body to have active volcanism.

"What we think is happening is that jets of gas are escaping at substantial velocity from fissures to form a large column of gas above the south pole," Brown said. Mass spectrographic analysis showed that the gas is composed of water vapor, carbon dioxide, methane and maybe nitrogen, he added, and though temperatures at the vents are well below zero, liquid water will flash into gas when it is flushed into the vacuum of space.

Scientists were also able to confirm from these observations that Saturn's E ring, as suspected, is made of microscopic icy "smoke" from Enceladus's vents. What they have not been able to figure out is how there could be enough heat to make liquid water.

"Deep down, you have a reservoir of stuff -- liquid water mixed with carbon dioxide and light organics that is hot in a relative sense," Brown said. "Why, and why only at the south pole? Those are the big questions, and none of the explanations advanced so far is satisfactory."