For the first time in 30 years, the planet Saturn will be at its closest point to Earth -- about 745 million miles (1.2 billion kilometers). At the same time, Saturn's rings will reflect the sun's rays perfectly and be tipped toward Earth for the best possible viewing angle. According to astronomers, the sight, even through an ordinary store-bought telescope, will be stunning.
First-time observers often gasp when they see Saturn through a telescope, said astronomer Mitzi Adams, with the National Aeronautics and Space Administration's Marshall Space Flight Center in Huntsville, Ala.
Adams recalls her own first encounter with the ringed gas giant with a sense of awe.
"It was November 1969, and I was a volunteer at the Fernbank Science Center in Atlanta. We were tracking the Apollo 12 spacecraft for an NBC news crew using the center's 36-inch telescope. It was a very cold night -- about 9 degrees Fahrenheit. For fun we rolled out a 10-inch telescope, too, and pointed it at Saturn. The sight of the planet and its rings took my breath away. Or maybe that was the cold!"
Adams said she never will forget the planet's image. "Now, whenever I show Saturn to 'first-timers,' I really enjoy watching their reactions, remembering my own experience."
Although oppositions of Saturn -- when the planet and the sun are at opposite positions from Earth -- occur about every 13 months, the Dec. 17 event is special, Adams explained, because the ringed planet is also at perihelion -- its closest approach to the sun. "Saturn's 30-year orbit is not a perfect circle," she said. "It has the shape of an ellipse with one side 6 percent closer to the sun than the other. When Saturn is closer to the sun it's also closer to Earth ... and we get a great view."
Saturn's most famous feature, its rings, are enormous -- about 170,000 miles (274,000 kilometers) wide, or twice as wide as the planet Jupiter. They also are incredibly thin -- only about 100 feet thick. That is why they seem to vanish when seen edge-on. About seven years ago, the rings were practically invisible to Earth-based observers. Because Saturn is tilted 27 degrees with respect to its orbit, the rings appear to wobble as the planet goes around the sun.
When Galileo Galilei began observing the sixth planet in 1610, he found Saturn's appearance curious and its behavior frustrating. Looking through his relatively primitive telescope, at first he thought he saw three planets grouped together -- one large planet flanked by two smaller ones. As he wrote in his journal in Latin, "Altissimum planetam tergeminum observavi" -- "I have observed the highest planet triple: o0o."
Then, two years later, the blobs seemed to vanish because, although Galileo did not know it, the plane of the rings had become parallel to Earth's orbit. Later, the blobs returned. This coming and going of Saturn's companions puzzled scientists for nearly half a century, until 1656 when Dutch astronomer Christiaan Huygens determined the mysterious objects were rings.
Astronomers have been observing and marveling at Saturn's rings ever since -- although there still is much that remains unknown about them. They know the rings are made of icy chunks ranging in size from dust to large houses, and they obviously are composed of debris, but the debris could have originated from several sources, from a shattered moon to a passing asteroid torn apart by Saturn's gravity.
There is mounting evidence the rings are much younger than Saturn itself. Some astronomers think the rings appeared only a few hundred million years ago -- about the time dinosaurs were roaming on Earth.
"After all this time we're still not sure about the origin of Saturn's rings," said Jeff Cuzzi, a planetary scientist at the NASA Ames Research Center at Moffett Field, Calif. Although current theory is all the planets formed about 4.8 billion years ago from a swirling cloud of interstellar gas, "there's a growing awareness that Saturn's rings can't be so old," he said.
Several hundred millions years ago, Cuzzi said, something unlikely happened. "A moon-sized object from the outer solar system might have flown nearby Saturn where tidal forces ripped it apart. Or maybe an asteroid smashed one of Saturn's existing moons." The debris encircled the planet and formed the rings we see today.
Saturn's ring particles range in size from microscopic dust to barn-sized boulders. If they were assembled all in one place, Cuzzi said, there would only be enough material to make an icy satellite about 125 miles (200 kilometers) wide, about the size of Saturn's moon Mimas.
The debris layer is extraordinarily thin, he said. "A sheet of paper the size of San Francisco would have about the same ratio of width to depth." A 1-meter-wide scale model of Saturn would require the rings to be 10,000 times thinner than a razor blade.
There are two reasons to believe the rings are young, Cuzzi said.
First, they are bright and shiny like something new. The wide-spanning rings sweep up bits of debris from comets and asteroids as Saturn orbits the sun. Rings much older than a few hundred million years would have been darkened by now by the accumulated dust. "The fact that they're bright suggests they're young," he said.
Second, small moons that orbit through the outermost regions of the ring system are gaining angular momentum at the expense of the rings. "During the next few hundred million years," Cuzzi explained, "the outer half of the rings will fall toward the planet, and the little moons -- called shepherd satellites -- will be flung away. This is a young dynamical system."
Some long-awaited answers about the rings may be forthcoming in July 2004, when the Cassini spacecraft reaches Saturn. Cassini -- a joint mission of NASA, the European Space Agency and the Italian Space Agency -- will become the first spacecraft to orbit the planet. Cassini also carries the Huygens probe, which it will release in early 2005 for descent through the thick atmosphere of Saturn's moon Titan.
Cuzzi said he hopes Cassini will solve other ring mysteries as well.
"In the early 80's," he said, "the Voyager spacecraft visited Saturn and took close-up pictures that revealed many strange things in the rings, including spokes, braids and waves. "Some of the waves have a spiral shape, like the spiral arms of galaxies," he said. To an astronaut floating among the rings, such waves would appear to be gentle swells, a few miles high and hundreds of miles wide. They move around the rings every few days or weeks.
"We understand these spiral waves," Cuzzi added. They are triggered by gravitational tugs from Saturn's moons -- the same ones that are sapping the rings' angular momentum.
Other structures, such as spokes and irregular ripples, are puzzling. Some of them might be signs of space rocks plunging through the ring system. Others might be spawned by tiny moonlets, as yet undiscovered, plowing through Saturn's rings. "Cassini, which will orbit Saturn for years, should provide some answers," he said.
Perhaps the most beguiling aspect of Saturn's rings is the possibility they are a temporary phenomenon. "If they are as short-lived as we think, we're lucky to be here at just the right time to see them," Cuzzi said.
If current ring theory is correct, in another few hundred million years Saturn's crowning glory will sag inward and our solar system will become a little more ordinary.
To find Saturn, Adams said, just look east after sunset. Saturn will be there, rising, among the bright stars of the constellation Taurus. Saturn is yellow in hue and is easy to spot because it does not twinkle like a star. It also is one of the brightest objects in the winter evening sky. At midnight on Dec. 17, Saturn will be almost directly overhead.
The following night, Adams said, Saturn will be close to the full moon. Although the moon's glare will impair night vision, it will not be enough to wipe out Saturn. The planet is so bright, she noted, because its vast rings reflect sunlight so well.
The only instruments needed to enjoy Saturn are "your eyes and a small telescope," Adams said. "Saturn and Earth will be close together for many weeks." If you miss it on Dec. 17, she said, not to worry -- the planet will be there to take your breath away for some time to come.