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Surprising Saturn
Color image of glowing Saturn illuminated from behind by the Sun.
Enchanting Saturn: NASA's Cassini orbiter has revealed Saturn in stunning detail. Here the planet is backlit by the Sun.

Our solar system's ringed giant has been a consistent source of surprises for 400 years

Two drawings of Saturn by Galileo.
Galileo's Saturn: Galileo wasn't sure what he was seeing when he first spotted the rings of Saturn in 1610. He made the lower sketch of what he called Saturn's arms in 1616.

Galileo Galilei stumbled across one of the great beauties of our solar system when he turned his telescope to Saturn in 1610.

Unfortunately, his telescope wasn't powerful enough to allow him to really appreciate it.

Thought for centuries to be a perfect sphere like the other planets, Galileo found Saturn had a squashed look when compared to the other planets.

"I discovered another very strange wonder," Galileo wrote in 1610. "The star of Saturn is not a single star, but is a composite of three, which almost touch each other, never change or move relative to each other, and are arranged in a row along the zodiac, the middle one being three times larger than the lateral ones, and they are situated in this form: oOo."

We know now that Galileo was seeing Saturn's magnificent rings, which are not visible to the unaided eye.

The mysterious shape changes that puzzled ancient astronomers were caused by a ring plane crossing - a phenomenon that occurs every 14 to 15 years when Saturn turns its rings edge on to Earth, making them nearly invisible. (Note: The latest cycle will make the rings their thinnest on Sept. 4, 2009).

The vast, thin expanse of debris - ranging in size from a speck of dust to house-sized boulders - is unique in our solar system. All four gas giant planets in our solar system have rings, but none are as complex and vivid as Saturn's.

In the centuries since Galileo's discovery, Saturn has routinely delivered surprises.

Pioneer 11, the first robotic spacecraft to visit Saturn, and Voyagers 1 and 2 revealed the complexity of the rings, added to Saturn's growing moon and ring count and sent back the first detailed data about the planet's hydrogen and helium atmosphere.

Color image showing Saturns rings from below.
Ringscape: The Cassini spacecraft captured this view of Saturn's rings from below before it slipped into orbit in 2004.

Scientists later used the Hubble Space Telescope to peer into the hazy atmosphere of Saturn's massive moon, Titan, and resolve vague features on its surface. The powerful orbiting telescope also revealed Saturn's auroras and the tilt of its rings as it orbits the Sun.

But it wasn't until 2004 that Saturn and its rings and moons finally got the close-up they deserved. Launched in 1997, the Cassini orbiter arrived on July 1, 2004, slipping gracefully across the ring plane and becoming the first spacecraft to orbit the giant planet.

Cassini also delivered the European Space Agency's Huygens probe, which made the first parachuted descent and landing on Titan. Combined studies from the orbiter and probe found Titan to be a world that might be what Earth was like before life evolved.

Titan offers another vantage point to study another world with an actively evolving and complex climate.

One of the unexpected surprises of Cassini's tour was Enceladus.

The orbiter found the moon has a hot spot at its southern pole where geysers spew ice crystals. The geysers may be evidence of liquid water below the moon's surface - a sign that makes Enceladus a prime target for future mission seeking evidence of life beyond Earth.

The geysers are believed to feed particles into Saturn's most expansive ring.

Cassini is now on an extended mission that will allow scientists to continue to study Saturn, its rings and intriguing moons, especially Titan and Enceladus.

Color image showing brownish red rocks on the surface of Titan.
Titan Touchdown: The Huygens probe snapped a photo of these pebble-sized rocks shortly after landing on Titan in 2005.

"We've had a wonderful mission and a very eventful one in terms of the scientific discoveries we've made," said Bob Mitchell, Cassini program manager at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "We are incredibly proud to have completed all of the objectives we set out to accomplish when we launched. We answered old questions and raised quite a few new ones and so our journey continues."

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Last Updated: 2 February 2011

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Last Updated: 2 Feb 2011