Descriptions of the international Cassini-Huygens mission to Saturn tend toward superlatives. The Cassini orbiter and the Huygens probe, which slid into orbit around this famously ringed planet in July 2004, represent the most ambitious effort in the history of interplanetary space exploration. Three probes launched by the United States National Aeronautics and Space Administration (NASA) had already flown by Saturn in the 1970's and 1980's. But the Cassini-Huygens spacecraft, the first to orbit Saturn, dwarfs all three in size and weight and in the complexity and power of its instruments. Scientists from NASA, the European Space Agency (ESA), and the Italian space agency, Agenzia Spaziale Italiana (ASI), spent more than 20 years and billions of dollars to complete the mammoth spacecraft, which stands 7 meters (22 feet) tall and weighs 5,700 kilograms (12,600 pounds). In fact, the spacecraft is so huge that it needed to swing by three planets—Earth, Venus, and Jupiter—to get the energy boost it needed for its seven-year journey to Saturn.

Instruments on the Cassini spacecraft have given us the most spectacular images as well as the most detailed information ever collected about Saturn. For example, Cassini's instruments recorded hurricanelike storms a million times stronger than those on Earth as well as the sudden appearance of a strange oxygen cloud that surrounded the planet. Cassini's cameras caught Saturn's moon Prometheus stealing particles from one of the planet's rings. A camera also spotted a previously unknown moon hidden within the rings. Cassini scientists were delighted to confirm that Titan, Saturn's planet-sized moon, has methane rain and an upper atmosphere with complex compounds similar to the chemical building blocks of life on Earth.

In January 2005, the ESA's Huygens probe—carried piggyback on Cassini during the journey from Earth—became the first human-made object to land on Titan. The probe, which collected and transmitted data for twice as long as anyone had expected, revealed its share of amazing discoveries, including detailed images of Titan's surface. As the probe floated through Titan's haze-choked atmosphere, its cameras revealed a hauntingly Earthlike landscape, complete with streamlike drainage channels, evidence of erosion, features that look like shorelines, and rocks (though the rocks are made of ice).

The Cassini mission is scheduled to last until at least 2008, by which time it will have orbited Saturn 76 times. The data the orbiter collects will keep scientists busy for years. Perhaps these data will answer some of the questions intriguing scientists. Where, for example, did the material in Saturn's rings originate? Why are the rings subtly colored? How many more moons does Saturn have? (By mid-2005, the count had reached 47.) How does the moon Enceladus produce particles for one of Saturn's rings? Why does the moon Iapetus have a huge mountain range around its equator? And where are the lakes and oceans of liquid methane and hydrocarbons that scientists expected to see on Titan?