NASA's Voyager 2 probe approached Saturn 20 years ago and spent the next eight months beaming back images of a planetary system far more complicated and beautiful than anyone had imagined.

It was a data bonanza for the solar system's most elegant planet. The level of detail revealed in the rings and the mysterious surfaces of so many of Saturn's moons were so fascinating that planetary scientists instantly began requesting funding and research for a dedicated mission to Saturn.

No one anticipated that the return would not come for more than two decades.

Tobias Owen, chairman of the Outer Planet's Working Group at the time, found the Voyager findings absolutely fascinating.

Owen and others finally should get their wish in 2004 when the Cassini spacecraft and its piggy-backing Huygen's probe reach Saturn to begin a four-year exploration of Saturn, its moons, and ring system. The mission, although far and away more advanced than Voyager, owes its creation to Voyager's early discoveries.

Owen and other Voyager scientists were most intrigued with findings on Saturn's largest moon Titan, so Cassini is designed to drop a probe to the surface of this mysterious orb.

Titan was once thought to be the largest moon in the Solar System, but Voyager found the moon's outer shell was actually an atmosphere 60 percent more dense than Earth's. This was surprising. Moons with atmospheres are very uncommon, said Cassini project scientist Dennis Matson, and those that do exist are as dense as a laboratory-made vacuum.

And aside from the atmosphere being extraordinarily dense, it is steeped in organic molecules -- methane, ethane, and even hydrogen cyanide, all important building blocks for amino acids, the precursors to life. This is the discovery that got the Cassini project off the ground.

"Part of the excitement, is that it's thought Titan in many ways may have started out like early Earth," Matson says. "But because it's so far from the Sun, [the closest Titan gets is 864 million miles (1.39 billion kilometers)] things chilled early on and got frozen out. Perhaps Titan is an in situ example of early Earth's development."

To get a closer look than the Voyager mission which spied this "Earth frozen in time" from a distance of 2.7 million miles (4.5 million kilometers), Cassini will release the 8.9-foot (2.7-meter) diameter Huygens Probe toward Titan on December 25, 2004. It will take three weeks to arrive at Titan's atmosphere, then it will descend for two and a half hours, through a rust colored haze layer, then the dense cloud tops of Titan, all the way down to the moon's surface.

On this descent, Huygens will measure temperature, pressure, density and energy balance of the atmosphere. A camera will take more than 1,000 images of Titan's landscape. Built by the European Space Agency, the probe will drift down at 15 miles per hour (25 kilometers per hour) with the aid of two stabilizing parachutes. If the probe survives the landing, it will continue collecting images for up to a half -hour before its batteries are scheduled to run out.

"Huygens will verify whether there is really liquid on the surface of Titan," said JPL's Ellis Miner, a science advisor for the mission. "We can only conjecture now, but there has to be a source of all that ethane and methane in the air, which would probably be lakes or oceans of it."

Titan is clearly unique in the whole solar system, Matson said. "Its atmosphere is one way it's unique, but there may be other ways it is unique, and that's what we'd like to find out," he said.