After a nearly seven-year journey covering 3.5 billion kilometers, the Cassini probe is poised to spend the next four years exploring Saturn and its many moons.

But before the multibillion-dollar spacecraft gets down to business this summer, it must survive a close encounter with Saturn's rings, passing through the bands of ice and dust to settle into orbit around the second largest planet in the solar system. Scientists and engineers have chosen a relatively hazard free path between two of Saturn's rings, but a collision with even the smallest particles could damage the craft and end the mission.

"Our confidence in our spacecraft is very high," Cassini Program Manager Robert Mitchell told reporters at a recent news conference at NASA headquarters here. "But having said that, I'll sleep better once we have this thing in orbit."

The thing that has been keeping Mitchell up at night is a critical 96-minute burn of Cassini's main engine. That burn, scheduled for June 30 at 10:36 p.m. EDT, is designed to slow Cassini down so gravity pulls it into orbit around Saturn.

NASA has only one shot at a Saturn orbit insertion. "If the burn doesn't work," Mitchell said, "then we would have a Saturn flyby and that's not what we are about."

NASA, the European Space Agency and the Italian Space Agency already have spent about $3 billion building, launching and operating the Cassini-Huygens Saturn mission. The three partners are hoping for at least four years of operations yielding 76 orbits around the Saturn system, including 52 close encounters with seven of the planet's 31 known moons. Cassini has enough propellant on board, however, to last 10 years or longer, Mitchell said, depending on what kind of objectives are set for an extended mission.

Charles Elachi, director of NASA's Jet Propulsion Laboratory, Pasadena, Calif. and a 15-year veteran of the Cassini science team, said the objectives of the primary mission are threefold: to study Saturn itself, to study the planet's brilliant rings, and to study its many moons. To accomplish these science goals, Cassini is equipped with 12 sophisticated instruments and a 320-kilogram stowaway, the European-built Huygens Probe.

Cassini is expected to release the Huygens probe Dec. 24 during a flyby of Titan, Saturn's largest moon. About three weeks later it will plunge into Titan's atmosphere for a leisurely two-hour parachute drop down to the moon's surface, its six instruments sending back pictures and other data all along the way.

Although Saturn has at least 31 moons, Elachi said none appears more interesting than Titan. Titan is bigger than Mercury and Pluto and has a nitrogen-rich atmosphere one-and-a-half times thicker than Earth's. Photos of Titan's atmosphere and surface formations -- which could include methane lakes and oceans -- are expected to be among the most dramatic images from this ambitious mission.

Cassini has already sent back the closest images ever of Phoebe, a Saturn-orbiting object that may or may not be a moon. The June 14 flyby captured detailed images of the heavily cratered body that is the largest of Saturn's outer satellites but only one-fifth the size of the Earth's moon. Cassini came within about 2,000 kilometers of Phoebe. Voyager 2, the last spacecraft to visit Phoebe, passed by in 1981. Its closest encounter, however, was about a thousand times further out than Cassini's.

Cassini was designed, developed and assembled at the Jet Propulsion Laboratory. Although many contractors had a hand in building the spacecraft, the largest contracts went to Bethesda, Md.-based Lockheed Martin, which in addition to supplying Cassini's main engine, also supplied the spacecraft's nuclear-powered batteries and launched the 5,700 kilogram payload aboard a Titan 4 rocket in October 1997. NASA's Glenn Research Center in Cleveland worked with the U.S. Air Force to maintain close oversight over the Cassini launch, which was considered far from routine given the spacecraft's mass and onboard nuclear power source.

More than 5,000 people have worked on some portion of Cassini, according to NASA. By the time Cassini completes its four-year mission, NASA and its partners will have spent about $3.3 billion, about $1.8 billion of which is the cost of the spacecraft and launch.

At one point in Cassini's 15-year history, NASA had planned to build and launch a virtual twin dubbed the Comet Rendezvous Asteroid Flyby (CRAF). But in 1992 at the dawn of then NASA Administrator Daniel Goldin's faster, better, cheaper revolution, the space agency cancelled CRAF as a cost-saving measure.

Under Goldin's rule, Cassini was widely seen as the last of NASA's so-called Battle Star Galactica class spacecraft, as the agency shifted its focus to flying more interplanetary probes built at a fraction of Cassini's cost.

But now that NASA has set its sights on a new generation of nuclear power and propulsion systems, the price tag for planetary exploration is creeping back up. The Jupiter Icy Moons Orbiter, a science behemoth NASA proposes to launch in 2015, is expected to cost in excess of $1 billion and require a launch vehicle powerful enough to lift 80 metric tons. Industry proposals for building the spacecraft are due in July with an award expected by the end of the year.