Carolyn Porco, eyes still rapt with wonder, watched the Cassini-Huygens spacecraft's first images of Saturn's rings blink across NASA's screens Thursday in a dazzling montage: a stringlike band in space, trailing a wispy tail of icy dust; a bright ribbon of white, with scalloped edges like sand ridges on a Carolina beach; a grainy hodgepodge of unknown stuff, matted like straw in a manger.

"I don't think you have to be a ring scientist to understand what this was for us," said Porco, the imaging team leader for the Cassini-Huygens mission to Saturn. She blinked and said: "It's just beyond description."

Benumbed from lack of sleep and the tensions of shepherding a $3.3 billion spacecraft into orbit around the solar system's sixth planet, scientists at California's Jet Propulsion Laboratory studied ring photographs, listened to sonic booms from the solar wind and marveled at how the most ambitious space science mission in history was unfolding -- just about perfectly.

"I feel humbled, because nothing ever happens this way," said David Southwood, director of scientific programs for the European Space Agency, which teamed with NASA and the Italian Space Agency to send Cassini-Huygens aloft in 1997. "You do the best you can, but the thing about being in space is you can't go up and fix it."

In a spectacular finale to a seven-year voyage from Earth, Cassini-Huygens soared up through Saturn's rings late Wednesday, settled into orbit and, by dawn Thursday, had swooped back down through the rings, outward bound on the first of 76 circuits of Saturn, transmitting data and photographs along the way from the closest encounter that any human-made object had ever had with the planet.

The spacecraft will tour Saturn and its environs for four years, and perhaps much longer, depending on when its fuel runs out. Cassini-Huygens will study a planetary neighborhood that mimics the solar system in many respects and that should provide clues to its formation more than 4 billion years ago, and, perhaps, the origins of life.

"It's like having a library in the solar system," said the University of Iowa's William Kurth, who is studying Saturn's magnetic field as part of the project. "We can explore all kinds of things that we can't explore on Earth."

Following the perfectly executed 96-minute rocket "burn" that put the spacecraft into orbit, ground controllers ran checks on all its operating systems as well as the 18 instruments it is carrying: "Every subsystem is completely flawless," project manager Robert Mitchell said at a news conference Thursday. "The spacecraft status is just about perfect."

Mitchell said Wednesday's burn had transpired so successfully that navigators predicted the first orbit of Saturn would finish "within a day" of the planned 116 days, and controllers were trying to decide whether to bother with a "trajectory clean-up maneuver" -- a brief engine firing to eliminate the discrepancy.

On Friday, Cassini-Huygens will have the first of 45 close encounters with Titan, Saturn's largest moon and the object that the spacecraft will use for the gravity boosts it needs to change the length and direction of its orbits as it travels the Saturnian system.

On Christmas Eve, Cassini will detach the piggybacking Huygens probe on a three-week journey that will end with a parachute landing on Titan's surface. Scientists hope the moon's hydrocarbon-rich atmosphere and ethane landscape can provide clues to how Earth began to form the complex carbon molecules that eventually led to the creation of life.

But for the scientists who had begun gathering data, the spacecraft was already a source of wonderment. Johns Hopkins University's Stamatios Krimigis described how his Magnetospheric Imaging Instrument was measuring Saturn's powerful magnetic field. Kurth also played a tape of the solar wind setting off sonic booms in space as it rammed into the magnetic field at speeds ranging from 2 million to 10 million mph. By the end of the mission, Krimigis said, "we will be able to characterize Saturn's magnetosphere so well that we will be able to do a movie."

Still, Thursday belonged mostly to Porco and her ring scientists, who stayed up all night to analyze 61 close-ups of Saturn's most distinctive features -- images so sharp and "so shocking," Porco said, "that I thought my team was playing a trick on me" by substituting animations.

Saturn has seven known rings extending from 41,000 miles to more than 110,000 miles from its center. They are composed of ice and stone and, despite their hooplike appearance, they are pulled to and fro by the gravity of Saturn's 31 moons, especially the five tiny "shepherd" moons that orbit within the rings. Perturbations by the moons cause the ripples in the rings described as either "density waves" or "bending waves."

"This is unprecedented," Porco said, pointing to a photograph that showed these "classic features" in never-before-seen detail.

Another image showed that the 30-mile-wide F ring has a gossamer tail that Porco said could be composed of dust-size ice particles that may have come adrift. The scalloped ridges bordering the Encke Gap that divides Saturn's ribbonlike A ring probably were caused by the shepherd moon Pan, she said.

But the gritty bed of "straw" in the A ring evoked nothing from Porco but amazement: "This is something new here, and I don't know what it is," she said. "We literally don't have a clue."

Carolyn Porco, the imaging team leader for the Cassini-Huygens mission to Saturn, said the first images the spacecraft sent back of the planet's rings were "beyond description."