With its handlers waiting nervously 934 million miles away, the Cassini-Huygens spacecraft headed toward a fateful rendezvous Tuesday, poised to pass between the rings of Saturn in a spectacular maneuver to open the most ambitious deep-space science mission ever undertaken.

On Wednesday at 10:03 p.m. Eastern time, Cassini-Huygens is scheduled to swing upward through a 15,000-mile gap between Saturn's F and G rings, fire its main engine for 96 minutes, and sweep into planetary orbit before it passes back through the rings. If all goes well, it will mark the beginning of a four-year voyage of discovery that could uncover secrets of the solar system's formation and perhaps the origins of life.

"It's been a long time coming," said the Jet Propulsion Laboratory's Robert Mitchell, NASA program manager for the mission that began 61/2 years ago with Cassini-Huygens's launch from Florida's Cape Canaveral Air Force Station. "We are all a little bit nervous, but we're all very excited."

On the day before "orbital insertion" began, scientists could do nothing but watch the mission's progress as pre-programmed commands sent the spacecraft on its way. Should its rocket "burn" go awry, Cassini-Huygens's second engine will ignite and, they hope, correct the mishap, but planners anticipated no problems on what up to now has been a flawless journey.

"I've never flown a mission this easy," spacecraft team chief Julie Webster said in an interview. "We're the biggest and most sophisticated of the large missions. When we built the spacecraft, I was working with people who had 40 years' experience. They retired when it was finished."

Cassini-Huygens -- 22 feet tall, about 13 feet wide, carrying 18 instruments and costing $3.3 billion -- is a joint project involving NASA, the European Space Agency and the Italian Space Agency. In the next four years, it is to orbit Saturn 76 times to study its atmosphere, its rings, its magnetic field and its 31 known moons.

Scientists describe Saturn as the "sun" in a mini-solar system, with the rings serving as surrogates for the solar "dust disk" that spun out from the young sun, and the Saturnian moons playing the role of "planets."

Of special interest is the largest moon, Titan, bigger than the planets Mercury or Pluto and cloaked in a nitrogen-and-methane atmosphere that may provide clues to what Earth was like 4.5 billion years ago, before carbon molecules combined in a "primordial soup" to form the building blocks of life.

"It's probably the most exotic object in the solar system," said the European Space Agency's Jean-Pierre Lebreton, mission manager and scientist for the Huygens probe, Cassini's flying-saucer-shaped passenger. Huygens is to be released on Christmas Eve for a three-week trip to Titan to transmit data until it touches down on the moon's surface and its batteries give out.

Titan's methane clouds obscure its surface, but Cassini's cameras have already penetrated this blanket somewhat, revealing a pale sphere with darker patches. "This is just a prelude to the treats we will see from Titan," said NASA's Dennis Matson, a Cassini project scientist.

Last week, Cassini did a flyby of Saturn's oddball moon Phoebe, a jagged piece of pitted stone and ice that is only 137 miles in diameter and may be an ancient bit of cosmic stuff captured by Saturn as the sun flung it out toward the solar system's nether regions billions of years ago.

But the main event, "the last hair-raising part of the mission," Mitchell said at a JPL news conference, is the orbital insertion. Planners have used Cassini and the Hubble Space Telescope to image the target canyon between the rings and seen no big pieces of debris lurking. Nevertheless, the spacecraft is to turn as it enters the gap so that its heavy-duty, dish-shaped high-gain antenna will take any blows from fast-traveling particles that cameras could not see.

Instructions for virtually all of the orbit insertion maneuvers have been locked in Cassini's memory since the beginning of 2003, when controllers uploaded the necessary commands as the spacecraft zoomed past Jupiter on its last leg to Saturn. "Since then, all we've had to do is tweak it," Webster said, "and we have tweaked it."

The last command -- a precautionary instruction so the orbit insertion sequence does not get triggered prematurely -- was uploaded Saturday night. At midday Tuesday, Jeremy Jones, the mission navigation chief, said Cassini will cross the ring plane within seven miles of its ordered position.

Saturn will be 934 million miles from Earth when Cassini-Huygens goes into orbit, but the spacecraft has traveled about 2.2 billion miles since its launch Oct. 15, 1997, looping around the sun to get gravity assists from Venus, Earth and Jupiter as its speed accelerated to 13,320 mph.

Slowing Cassini down so it will drop into orbit around Saturn is the object of Wednesday's burn. Thirty-three minutes after swooping up through the rings, the spacecraft is to point its back end in the direction it is traveling and fire its main engine for 96 minutes in a braking maneuver. Nine minutes before the burn ends, the spacecraft is to pass within 12,400 miles of Saturn's cloud tops, the closest approach of the entire mission.

Cameras are to be shut down during the burn, but they should be on and operating for 75 minutes as the spacecraft speeds back through the rings, producing the closest pictures of them ever taken. Even then, however, the images are not expected to show individual chunks of the stone and ice that compose them. Some of these reach the size of automobiles or even houses.

Should something go amiss during the initial burn, the spacecraft's second engine will fire up and readjust the orbit, "but it will cost us" in fuel consumption, Webster said, and probably eventually curtail the mission. If the mission proceeds without mishap, the spacecraft could last decades beyond its four-year design life.

And once through the rings on the return trip, Cassini will never again pass through them. Instead, said Jones, it will use gravity assists from 45 flybys of Titan to adjust its orbit outside the rings, using main propulsion only for fine-tuning.

"Gravity never puts you exactly where you want to go," Jones said. "And we don't always get it exactly right."

An artist's rendition of Cassini-Huygens, which will pass within 12,400 miles of Saturn's cloud tops, the closest approach of the mission.