After a 61/2-year journey spanning nearly 2.2 billion miles, the Cassini-Huygens spacecraft is speeding toward its final rendezvous with Saturn, opening the discovery phase of one of the most ambitious scientific space missions ever attempted.

Last week, Cassini-Huygens was about 10 million miles from Saturn's underbelly, traveling at 13,320 mph. On June 30 (10:36 p.m. Eastern time), NASA controllers will fire its main engine for 96 minutes, causing the spacecraft to swing upward between two of the planet's brilliant rings, and then sweep into orbit around the planet.

Scientists describe crossing the rings as the mission's riskiest moment. Cassini-Huygens will avoid the densest bands of ice and dust that make up the rings, but even particles 0.04 inches in diameter scattered in the spaces between them could damage the spacecraft.

"It's not a slam-dunk," warned Cassini program manager Robert Mitchell of NASA's Jet Propulsion Laboratory. Although images sent back by the spacecraft three weeks ago appeared to show a safe route, Mitchell said, "I'll sleep better when we're in orbit."

Scientists have planned a four-year mission, during which the spacecraft will circle Saturn 76 times. It will study the planet, its rings, its 31 known moons, its magnetic field and especially how its largest moon -- Titan -- harbors the building blocks of life. With judicious use of remaining propellant, planners suspect they could extend the mission for several additional years, perhaps decades.

What they will examine is a solar system in microcosm, with Saturn as the sun and the rings as the "dust disk" that surrounds young stars and can lead to planet formation. By understanding the dynamics of Saturn and its moons, scientists say they can learn more about the early evolution of the solar system.

But "the [current] focus of planetary exploration is also the solar system's ability to sustain life," said astronomer Michael J.S. Belton, who led the imaging team for the 14-year Galileo mission to Jupiter that concluded last year. "We're expecting great things."

In a rehearsal for "Saturn Orbit Insertion," engineers late last month ignited Cassini's main engine for the first time in four years. The dry run also set up a close encounter for Friday with Saturn's moon Phoebe, during the spacecraft's final approach to the rings.

Phoebe -- only 137 miles across -- is a dark object with a "retrograde" orbit, moving in the opposite direction from Saturn's rotation. Astronomers have long suspected that Phoebe, unlike Saturn's other moons, is an asteroid or a captured migrant from the remote Kuiper Belt on the solar system's outskirts.

But the Phoebe flyby is only a teaser for the expedition's virtuoso exploit. On Christmas Eve, Cassini is to detach and launch the bowl-shaped Huygens space probe, its 705-pound passenger, for a three-week journey that is expected to end with a controlled descent and landing on the surface of Titan.

"All indications to date are that the observatory is behaving well and exactly as designed," said Orlando Figueroa, director of NASA's Solar System Exploration Division, who joined Mitchell and others at a NASA headquarters news conference last Thursday. "I applaud [the] meticulous approach."

Cassini-Huygens, conceived in the 1980s, is a $3.3 billion partnership among NASA, which designed the Cassini orbiter; the European Space Agency, which developed the Huygens probe; and the Italian Space Agency, which provided the high-gain antenna, most of the other radio equipment and parts of several scientific instruments. The mission has involved 260 scientists from 17 European nations and the United States.

Fully fueled, the spacecraft weighed more than six tons. It stands about 22 feet high and is 13 feet wide. Cassini has 12 scientific instruments, and Huygens, another six. NASA has described the hardware as the most sophisticated set of analytical tools assembled for a planetary spaceflight.

The instruments include a variety of spectrometers and instruments for particle collection and analysis, as well as imaging systems covering everything from ultraviolet wavelengths to microwaves. For most of the journey, they and Cassini-Huygens' other components remained largely dormant.

The mission was also conceived in the 1980s. Saturn was an attractive target. Known since antiquity, it was first examined with a telescope by Galileo in 1609. Half a century later, Dutch astronomer Christiaan Huygens advanced the then-controversial theory that Saturn was surrounded by rings, a view later supported, among others, by Italian-French astronomer Jean-Dominique Cassini.

For modern scientists, Saturn has the additional lure of Titan, a moon bigger than the planets Pluto and Mercury and the only known moon in the solar system with an atmosphere -- composed largely of nitrogen and methane gas.

Even as the Saturn system as a whole behaves like a miniature solar system, Titan's hydrocarbon environment may emulate, at least in part, "the kind of chemistry that Earth had 4 billion years ago" in the "pre-biotic" years before life evolved, said Huygens project manager Jean-Pierre Lebreton.

"I don't expect to find any creatures moving around down there," Lebreton said, noting that Titan's surface temperature is 330 degrees below zero Fahrenheit. But with Huygens's cameras, "if they're there, we'll see them."

Cassini-Huygens was launched Oct. 15, 1997, from Cape Canaveral Air Force Station in Florida. Instead of flying directly to Saturn -- impossible, given its mass -- the spacecraft got four gravity assists, adding velocity during swingbys of Venus (twice), Earth and finally Jupiter, in 2000.

Mitchell said the spacecraft will make its final approach to Saturn by passing through the gap between the "F" and "G" rings -- about 110,000 miles from the planet's gaseous "surface."

As it enters this canyon, Cassini-Huygens will angle its shock-resistant, high-gain antenna forward so it can perform the same function as linemen on a football team -- knocking away any debris before it can reach more vulnerable instruments tucked in behind. Once the burn begins, it cannot be aborted, Mitchell said.

During its mission, Cassini will make 52 close passes by seven of Saturn's moons. Forty-five of the flybys will approach within about 590 miles of Titan, including the Christmas Eve encounter.

Two days after Huygens is released, Cassini will maneuver into position to monitor the probe's radio signals during its descent, scheduled to take place Jan. 14 and last a maximum of 21/2 hours.

Huygens will be traveling at 12,400 mph when it hits Titan's atmosphere, but a series of parachutes will brake its descent, so that when it finally touches down it should be traveling no faster than 15 mph.

By that time, all of Huygens's work will have been finished. Instruments transmitting to Cassini from the probe will measure chemical composition of the atmosphere, wind speed and even sound. Imagers will record everything from clouds to landscape.

Huygens will not live long after landing. If it falls in an ethane lake or ocean, the cold will finish it almost immediately. If it survives a hard-surface impact it will be able to transmit data, but only for about 30 minutes. Then, Lebreton said, the batteries will give out, even as Cassini disappears over Titan's horizon.

An artist's conception of the Cassini orbiter shows the Huygens probe, left, separating to enter the atmosphere of Titan, a moon of Saturn, in distance.