The Cassini-Huygens spacecraft hurtled through the rings of Saturn and settled into planetary orbit late Wednesday, putting a pinpoint finish to a bold 2 billion-mile journey through space to embark on a four-year exploration of the solar system's sixth planet, its rings and its moons in one of the most ambitious space science projects ever undertaken.

Locked in the grip of Saturn's gravity and traveling at a speed of 50,331 mph, Cassini-Huygens breached the 15,000-mile-wide gap between the planet's F and G rings at 10:11 p.m. Eastern time on Wednesday, turned its back end forward and fired its main rocket in a 96-minute "burn" to brake its progress.

At 10:25 p.m. scientists at NASA's Jet Propulsion Laboratory applauded as the spacecraft, apparently unscathed, sent a radio transmission confirming that it was safely through the rings.

Eleven minutes later, louder applause broke out as the burn began steering the spacecraft on an arc that brought it within a scant 12,800 miles of Saturn's cloud tops. At 12:12 a.m. Thursday, the burn ended within a second of its predicted finish and the entire roomful of scientists and engineers stood and cheered.

Cassini-Huygens then turned its cameras for close-up photographs as it sped back through the rings on the first of 76 planned orbits of Saturn. Its first images were expected before noon Thursday.

The spacecraft's mission, unmatched in the history of space travel, is to tour Saturn and its environs for four years -- and perhaps decades longer -- using 18 instruments provided by 17 nations. Saturn and its entourage in many respects operate like a smaller version of the solar system, and astronomers hope that the spacecraft will yield crucial insights into how the sun and its planets, and perhaps life itself, came to be.

For mission controllers, Wednesday was a day of nail-biting anticipation. The maneuvers associated with orbit insertion, including contingency plans to cope with emergencies, had been programmed into the spacecraft more than a year ago. After a final uplinked software "tweak" last weekend, there was nothing for them to do but listen for radio transmissions from their distant traveler as each maneuver unfolded.

The final approach began at noon Wednesday when the spacecraft calibrated its accelerometer, which used the rate of speed to signal when to switch the main rocket engine on and off, said spacecraft team chief Julie Webster. Next, at 3:30 p.m., the spacecraft began warming up parts of its engine to aim its thrust properly before beginning the burn.

Navigational team chief Jeremy Jones watched Cassini-Huygens gain speed as Saturn's gravity embraced it in an ever-tighter grasp. At noon, it was traveling at 26,843 mph, Jones said, but velocity had almost doubled by the time it passed through the rings, and crested at 68,763 mph when it reached its closest point of approach to the planet.

Had the 96-minute burn gone awry, the spacecraft was programmed to diagnose the problem and take action -- including firing an auxiliary engine after a short interval. "My team has run this sequence 200 or 300 times in simulation and we have already had 17 engine burns, and nothing has ever gone wrong," Webster said.

Although her prediction apparently proved true, planners made no secret that the dash through Saturn's outer rings was the most exacting and potentially dangerous maneuver of the entire mission. "We're not flying where we are for science," program manager Robert Mitchell said. Passing through the rings was the only way to get close enough for Saturn's gravity to pull the spacecraft into the proper orbit, he said.

Mitchell described "two classes of risk -- environmental and spacecraft" -- during the insertion into orbit. To assess the environmental risk, planners used cameras from Cassini-Huygens and the Hubble telescope to scan the gap in the rings for debris that could hit the spacecraft.

"Something the size of a marble could do a lot of damage" in a high-speed collision, Mitchell said. "But we don't anticipate anything larger than a dust grain."

The spacecraft risks -- all of them anticipated and modeled on computers -- involved a series of complex maneuvers beginning with a 10-minute turn to point Cassini-Huygens's rugged graphite-and-epoxy high-gain antenna toward the ring gap to absorb dust impacts. Once through the rings, Cassini-Huygens turned again to point its stern in the direction of travel so the exhaust from its rocket engine would act as a brake.

And once in orbit, Cassini-Huygens had 75 minutes to take pictures of Saturn and its rings before it turned again to point the antenna forward for its second transit through the ring gap.

In its explorations, Cassini-Huygens -- a $3.3 billion joint venture involving NASA, the European Space Agency and the Italian Space Agency -- will treat Saturn as a surrogate "sun," with its rings modeling the "dust disc" of a solar system and its 31 moons acting as planets. The spacecraft will study the dynamics of the Saturnian system and do close flybys of seven moons.

In its most spectacular maneuver, Cassini on Christmas Eve will detach the piggybacking Huygens probe on a three-week trip that will end with a parachute fall through the methane atmosphere of Titan, Saturn's largest moon.

Scientists hope that Titan, with its hydrocarbon-rich atmosphere and the possibility of ethane oceans below, may offer insights into how the primordial Earth looked 4.5 billion years ago, when carbon molecules began to form the complex compounds that eventually served as the building blocks of life.

Cassini-Huygens was launched from Florida's Cape Canaveral Air Force Station on Oct. 15, 1997, after a planning and construction period that began in the early 1980s. The spacecraft is 22 feet tall and 13 feet wide, and, with a full load of fuel, weighed 12,593 pounds at liftoff.

In all, Cassini-Huygens flew 2.2 billion miles to reach Saturn, using flybys of Venus (twice), Earth and Jupiter to accelerate into ever larger orbits of the sun. Webster said software for the spacecraft's on-board computers was upgraded and changed twice during the journey, most recently in January and February of last year, when the orbit insertion programs were uploaded.

For much of the trip, Cassini-Huygens flew about 12,000 mph. Cruising speed began to increase about a week ago when Saturn's gravity took hold, and by midday Wednesday, Jones said, "we're really feeling it, and we're dropping really fast."