In 1610 Galileo pointed his crude spyglass at Saturn and was dumbfounded by what he saw: “The planet Saturn is not alone, but is composed of three, which almost touch one another and never move nor change with respect to one another.” Worse, the two bulging planets on each side of the main planet had disappeared when he looked again a few months later. “What is to be said concerning such a strange metamorphosis?” he cried. Eventually, the frustrated Galileo decided never to look at Saturn again.
Now, of course, we have much better telescopes, and we know that Galileo was looking at the planet’s unique set of wide, thin rings. Seen broadside, they resembled companion planets through Galileo’s weak lenses; later, seen edge on, they shrank to nothingness—an invisible sliver. But nearly 400 years after Galileo’s observations, Saturn still teases astronomers, and the closer we look, the more oddities we see. Saturn’s magnificent rings, for example, consist of trillions of bits of ice, some no bigger than a speck of dust, making up a weird and complex system of satellites. The ring particles are so puny that you would expect them to quickly scatter and fall into the planet, yet they are still there. And the planet is so loosely built that it would float on water.
At another extreme, Saturn’s giant moon Titan seems more like a planet in its own right, larger than Mercury and cloaked in a dense atmosphere. Titan’s surface seems to be covered with ethane oceans and an organic goo that may resemble the Earth’s early surface chemistry, but nobody knows for sure, because astronomers can’t see through the moon’s maddeningly opaque orange fog. In between, Saturn has at least 30 other smaller moons, some smooth, some battered, some strangely mottled.
When NASA sized up Saturn’s expansive mysteries and decided to get a probe there, it chose the scientific equivalent of the big cannons on a battleship: a 22-foot-tall, 12,600-pound spacecraft called Cassini-Huygens. This behemoth is the largest object ever launched into deep space by the United States. And at a total cost of $3.3 billion, it is also among the most expensive planetary missions. Cassini is so hefty that NASA did not have a rocket powerful enough to send it on a direct course, so engineers devised a 2.2-billion-mile loop-the-loop trajectory that exploited the gravity of Venus, Earth, and Jupiter to hurl the spacecraft to its ringed destination.
At the end of June, Cassini reached Saturn and finally began erasing the question marks that have surrounded the planet ever since Galileo studied it. Aboard are a dozen instruments, including sensors for visible light, ultraviolet, and infrared as well as magnetometers, radar, and plasma detectors. All the equipment will be powered by three plutonium-driven generators cranking out 750 watts of electricity. The smaller Huygens probe will separate from Cassini on December 24 and set off for a rendezvous with Titan. Huygens has a suite of cameras and sensors all its own. “You want a complete set of instruments to make measurements simultaneously so you can correlate all the results,” says Robert Mitchell, the Cassini program manager at the Jet Propulsion Laboratory in Pasadena, California.
Many of the scientists on the Cassini team have waited a good fraction of their professional careers for this moment. Pioneer 11, the first craft to visit Saturn, returned a handful of provocative pictures in 1979. In the early 1980s, Voyagers 1 and 2 flew by, sending back vastly sharper images of Saturn’s rings and intriguing data about Titan’s atmosphere. “We knew very little before Voyager,” says planetary scientist Jeff Cuzzi of NASA’s Ames Research Center, a member of the Cassini mission’s scientific executive committee and a veteran of the Voyager imaging team. “It was a stunning and an exhilarating and a humbling experience.” But the Voyagers used 1960s-era technology, and they did not hang around long. Like typical American tourists, they swept in, took a few snapshots, and left. Planetary scientists then spent two decades sifting and resifting through a limited cache of data.