PASADENA, Calif., July 1— The first spacecraft to orbit Saturn, arriving late Wednesday, swiftly turned its cameras on the planet's rings of ice and rock, and transmitted striking pictures of the encircling luminous strands, some with scalloped edges, strawlike textures and rippling waves that spread across the shimmering disk.

Scientists could not have been more delighted.

As the ring pictures from the Cassini spacecraft were received here Thursday morning, Dr. Carolyn C. Porco, the mission's chief photographic interpreter, said their beauty and clarity were ''just mind-blowing.'' They were far superior in number and close-up detail to any previous images of Saturn's most distinctive feature.

The black-and-white pictures -- taking color ones would have meant settling for fewer images -- were so good, Dr. Porco said at a news conference, ''I thought they were simulations of the rings and not the rings themselves.''

The photographs and other scientific data were the first payoffs of Cassini's success in orbiting the giant planet after a 2.2-billion-mile journey of nearly seven years. With precise navigation through the rings and a flawless 96-minute rocket firing, the spacecraft began orbiting Saturn at 9:12 p.m. Wednesday, Pacific time.

The $3.3 billion mission is a partnership of the National Aeronautics and Space Administration, the European Space Agency and the Italian Space Agency. The hope is that investigations of Saturn's rings will provide insights into how stars and planetary systems formed from disks of interstellar matter. And Saturn's greatest moon, Titan, may hold clues to how life emerged on the primitive Earth.

Robert T. Mitchell, the Cassini program manager at NASA's Jet Propulsion Laboratory here, said engineering and tracking data indicated that the spacecraft had reached its destination on time, on target and in excellent condition.

During the rocket firing, to slow itself into orbit, Cassini made what will be its closest approach to Saturn, passing within 12,800 miles of the planet's cloud tops. The spacecraft survived, without damage, two passages into and out of the rings through wide gaps.

''Every subsystem reports that their status was completely flawless,'' Mr. Mitchell said.

Te navigation team said that the American spacecraft, with Europe's smaller Huygens craft attached, was cruising on a wide orbital course that would take 116 days to complete a circuit of Saturn; that is almost exactly what mission planners wanted. Over the course of the mission, planned to last four more years, the orbit is to be reduced to a 32-day period.

The shorter orbital duration will give scientists more opportunities -- 44 are planned -- to make close approaches to Titan. Cassini's first close observations of that moon are to take place on Friday. In January, the Huygens craft, separated from Cassini, is to plunge through clouds and haze for an examination of the planet-size moon's surface.

The rings commanded the initial attention of Cassini -- named for the early European astronomer Jean-Dominique Cassini -- because never again in the mission will the spacecraft be so close to them. At least seven discrete bands of ring material extend more than 370,000 miles from the center of Saturn. Two Voyager missions, in 1981 and 1982, detected ring particles ranging in size from nearly invisible dust to chunks of water ice the size of a house.

The Voyagers also made the discovery that tiny moons, in the gaps between ring bands, appeared with their gravitational tugs to act like shepherds herding the flocks of ice and rock. They seemed to maintain the structure of at least some of the bands and keep the gaps open.

Cassini's photography captured in even greater detail the shepherding courses of the tiny moons Pan, Prometheus and Pandora, and detected hints of the existence of much smaller moonlets. Some of the moons already found are no more than 12 miles in diameter. Dr. Porco, a planetary scientist at the Space Science Institute in Boulder, Colo., said that in the best of Cassini's initial pictures, each pixel represented a detail as small as 500 feet wide; that is fine resolution by astronomical standards. And the spacecraft, she said, appeared remarkably stable, ''like a tripod in space.''

''We are seeing structure never seen before,'' Dr. Porco said. ''I can't describe how exciting this is for us.''

Although she and other scientists have had no time for careful analysis, they are already fascinated by new evidence for the existence of shepherd moonlets and the dynamics of the ring structure.

Dr. Porco described a region in the A ring, one of the widest, as having a wavy pattern, resembling corrugated cardboard. Other, similar features reminded her of ripples on a pond. They are assumed to be what is known as density waves, suggesting dynamic variations in particle distribution through the disk, possibly over time.

Many edges of the ring bands were fuzzy and some sharp. But the most fanciful, scientists said, appeared to be the scalloped outer fringe of the A ring, where it is bound by the wide Encke gap. They speculated that repeated passages of unseen moonlets in the Encke region were at least partly responsible for the scalloping.

Although Jupiter, Uranus and Neptune also have faint rings, only recently discovered, the rings of Saturn are so bold and dynamic that scientists consider them a natural laboratory for studying the processes that are at work in the creation and evolution of all disks, like those around stars out of which the stars themselves and their planets evolved.

''Ring scientists are going to have a field day'' with the Cassini pictures, Dr. Porco said.

The pictures alone are not expected to yield much new information on the composition of the ring material, mission scientists said. That will have to await spectrographic analysis of the light in the images, which can detect signatures of the ring chemistry.

While the spacecraft was entering orbit, especially as it made its closest approach to Saturn itself, other scientific instruments were alert to the magnetic forces of the Saturnian system. Indeed, some observations of Saturn's magnetic field had been made since Sunday, when the spacecraft crossed from interplanetary space into the sphere of the planet's magnetic forces.

Dr. William Kurth, a physicist from the University of Iowa, said the pressure of the supersonic particle winds of interplanetary space with the magnetic fields generated by Saturn created powerful disturbances, not unlike a sonic boom. The spacecraft recorded seven such crossing shocks in a single day, indicating that the boundary surrounding the planet's magnetosphere pulsates widely.

Perhaps it was Saturn's way of announcing the arrival of the Cassini spacecraft, the first ever to orbit the solar system's second-largest planet.

Photos: The Cassini spacecraft sent photographs of a gap in Saturn's rings, showing a scalloped edge that may have been formed by unseen moons. (Photo by NASA/Jet Propulsion Laboratory)(pg. A1); The rings of Saturn as captured by the spacecraft when it was almost four million miles away, on June 20.; The images of Saturn and its rings at left were taken June 20. Those on the right were taken yesterday. (Photographs by NASA/J.P.L.)(pg. A15) Chart/Diagram: ''Encircling a Giant'' Saturn's rings were named in the same order as their discovery. Diagram compares Saturn to the size of Earth and highlights: G-RING F-RING A-RING B-RING C-RING D-RING ENCKE GAP CASSINI DIVISION (Source by NASA)(pg. A15)