LOS ANGELES -- A hulking, 5,384-pound spacecraft is nearing the end of a seven-year voyage to Saturn, where it will begin an intensive study of the solar system's second-largest planet, its rings and the stable of moons that orbit it.
The $3.3 billion Cassini is on schedule to enter orbit around Saturn on June 30, shortly after it makes a dash through a gap in the shimmering rings that encircle a planet second to Jupiter in size.
It was to have its first encounter in the Saturn system Friday, hurtling within 1,240 miles of the outermost moon, Phoebe, at 4:56 p.m. EDT. The tiny moon is just 137 miles across. Saturn, in contrast, is nearly 75,000 miles in diameter.
The joint U.S.-European spacecraft, which also carries a probe to explore the moon Titan, was launched in October 1997. NASA built the plutonium-powered spacecraft; the European Space Agency contributed the Huygens (pronounced Hoy'-genz) probe.
Once at Saturn, Cassini should spend at least four years orbiting the planet, 76 times in all. Cassini's two cameras could take as many as 500,000 pictures.
Scientists hope study of the Saturn system will provide insight to the solar system's evolution.
"Saturn, its ring system, its moons, are a miniature model of a disk of gas and dust that surrounded the early sun as the planets formed in the solar system," said Orlando Figueroa, director of NASA's Solar System Exploration Division.
Mission members caution that getting into orbit won't be a cakewalk. Cassini must fire its engine on cue for 96 minutes to slow itself sufficiently and allow Saturn to pull it into orbit. If the maneuver fails, the spacecraft would sail past.
Cassini is set to release Huygens in December. A month later, scientists expect the probe to parachute through the murky atmosphere of Titan, Saturn's largest moon, and land on its surface.
Titan, larger than the planet Mercury, is the only moon in the solar system with an atmosphere - mostly nitrogen and about 6 percent methane. Titan is believed to resemble what the Earth was like several billion years ago.
"Titan may preserve in deep freeze many of the chemical compounds that preceded life on Earth," Figueroa said.
Huygens carries six instruments to gather visible and infrared images, data on the properties of Titan's atmosphere and winds, as well as surface composition if it survives the landing impact.
The wok-shaped probe will enter Titan's atmosphere and deploy parachutes to slow its descent. If it lands on a hard surface it could transmit data for about 30 minutes. A landing on liquid - liquid ethane rather than water - would only allow it to float and operate for a few minutes.
Friday's Phoebe flyby is a warmup for what's to come: Mission planners expect Cassini to conduct more than 50 similar flights past other Saturn moons, said Bob Mitchell, the mission's program manager.
Scientists believe Phoebe originated in the outer reaches of the solar system and that it was later flung toward Saturn, which captured it into orbit.
"If it is, this will be our first encounter with something from that far out in the solar system. People are more excited about this object than its size would lead you to believe," said Carolyn Porco, imaging team leader.
NASA this week released three fuzzy images of Phoebe taken by Cassini between June 4 and June 7 as it closed in on the moon. The images showed a great deal of contrast that scientists said likely indicated topography such as sunlit peaks and deep shadowy craters.
Cassini's best possible pictures of Phoebe could show features as small as 66 feet across.
"It's going to be like walking - like hiking across these objects," Porco said.
On the Net: http://saturn.jpl.nasa.gov/
Associated Press John Antczak contributed to this report.