NASA's spectacular feat of placing the Cassini spacecraft in orbit around Saturn ranks with the greatest achievements in human history and surpasses even the stunning successes of the twin Mars rovers.

The spacecraft - a cooperative project of NASA, the European Space Agency and the Italian Space Agency - entered orbit around Saturn in 2004 after traveling nearly 2.2 billion, circuitous miles (3.5 billion kilometers) since its launch from Kennedy Space Center in Florida on Oct. 15, 1997.

Over its scheduled mission lifetime, the 6-ton, bus-sized Cassini is scheduled to fly by Saturn a total of 76 times, and it will visit Titan, its largest moon, 45 times. It also will execute 52 close flybys of seven of the other moons.

In December 2004, the Huygens probe, which piggy-backed aboard Cassini during its odyssey to Saturn, successfully detached and parachuted to Titan's surface, giving it the record for landing on the most distant object from Earth.

Huygens, which descended through Titan's thick and murky atmosphere, showed Titan to be remarkably Earth-like - albeit at much colder temperatures. The probe found evidence for methane rain, erosion, drainage channels, dry lake beds, possible volcanoes and vast dune fields that run for miles.

From its orbital perspective, Cassini has acquired the highest resolution images ever taken of Saturn's rings, providing views of strange ring structures that became apparent even during the earliest days of the spacecraft's Saturnian encounter.

For example, Cassini found waves ripping through the rings, knots and banded structures shaping them, and clumps of ice several kilometers wide.

Scientists also have witnessed moons influencing the rings. Prometheus has been caught stealing particles from the F-ring. Enceladus, meanwhile - via ice geysers spraying into space from its south pole - seems to be contributing particles to Saturn's expansive E-ring, and scientists think the moon could harbor liquid water.

Cassini also detected evidence for a whole new class of small moonlets that could lie within Saturn's rings, and evidence for brand new rings, which could indicate the presence of more, as-yet-undetected moonlets.

One of the spacecraft's most bizarre discoveries is a giant mountain range that runs the full length around the equator of Iapetus. The mountains rival Olympus Mons on Mars, which is nearly three times the height of Mt. Everest.

So far, Cassini has discovered three new Saturnian moons - bringing the current total to 34.

The spacecraft's biggest bounty of data so far, however, has come from its 15 flybys of Titan.

"We especially focused on Titan because we thought it could tell us something about the early Earth," said Toby Owen, Cassini interdisciplinary scientist at the University of Hawaii at Manoa. "Examining this world frozen in time, we find evidence that Earth may have begun with the same methane-ammonia atmosphere that marked the birth of Titan."

Owen said so far, Titan seems to be "only a frozen echo of early Earth: methane, nitrogen, and a suite of small organic molecules. Our planet's carefully balanced, warm global climate is the underlying reason that we are investigating Titan, instead of Titanians investigating Earth."

Next, Cassini's tour of Saturn and its moons is about to take on an accelerated pace.

"This summer, we will begin our express-ticket ride," said Jerry Jones, Cassini chief navigator at Jet Propulsion Laboratory. "That's 11 months with 17 Titan encounters and 51 spacecraft maneuvers to adjust the flight path, more than one maneuver per week."

In all, during the second half of its prime mission, Cassini will swing by Titan 30 more times. "The past two years have been just like a warm-up," said Robert T. Mitchell, the spacecraft's program manager at JPL.

The first of these encounters will be a Titan flyby on July 2, followed by the closest Titan encounter yet on July 22, at 950 kilometers (590 miles) above the surface.

Perhaps even more dramatic, mission controllers in late July will order Cassini to begin to flip its orbital orientation with respect to the Sun by nearly 180 degrees.

This unprecedented maneuver - made possible by gravitational assists from multiple moons - will result in a bird's-eye view of Saturn's glorious rings. The entire execution will take about one year.

"One of the biggest mysteries confronting Cassini is the changes we've seen in Saturn's radio emissions" said Bill Kurth, Cassini scientist at the University of Iowa in Iowa City.

"We've seen the radio period, the frequency of emissions that tell scientists how fast or slow the planet is rotating, change by as much as 1 percent (or a few minutes) over just 10 years, and we don't know why," Kurth said. "Pinning down how long the day is on Saturn is key to understanding other things, such as wind speed."

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