Seven years ago, the largest and most expensive interplanetary probe ever built blasted off from Cape Canaveral. It was loaded with 12 advanced scientific instruments, 72 pounds of plutonium to power them, and a capsule destined to be jettisoned toward the only other object in our solar system protected by a nitrogen-based atmosphere. After launch, the spacecraft began its voyage through the void of space and was promptly forgotten by all but a few scientists and space enthusiasts.
Since then NASA has launched the International Space Station, a Mars orbiter and lander (both of which failed), the doomed shuttle Columbia and the successful Mars rovers Spirit and Opportunity. President Bush even announced plans to send humans back to the Moon. All the while, the forgotten spacecraft continued to rack up miles, about 2.2 billion of them, as it quietly made its way across the solar system.
This month, the Cassini spacecraft will finally arrive at its destination: the ringed planet Saturn. And when it does, it will steal our attention back from the Moon, Mars and even Iraq–at least for a few precious moments. On July 1, Cassini's main engine will fire to slow down the spacecraft so that Saturn can capture it as the planet's first artificial satellite. Cassini will fly through a gap between two of Saturn's rings, then aim its cameras and remote sensing instruments at the rings as it passes above them. "These pictures are probably going to knock people's socks off," says Candice Hansen-Koharcheck, a scientist at NASA's Jet Propulsion Laboratory, which manages the Cassini mission.
With a mission price tag of $3.3 billion, Cassini is more than four times as expensive as the Mars rovers Spirit and Opportunity combined. Cassini is the size of a school bus, while the Mars rovers are only the size of golf carts. And those rovers existed only in the imaginations of NASA scientists when Cassini began its long journey. Cassini is the biggest, baddest mission ever flown, not just because it has gone out so far–about twice as far from Earth as Jupiter–but also because of what it will do when it arrives.
When Pioneer 11 and Voyagers 1 and 2 visited Saturn more than 20 years ago, each only whipped by on its way out of the solar system. Cassini will orbit Saturn 74 times over a four-year period, studying this mysterious planet from every angle. The spacecraft will train a dozen instruments on the Saturn system–snapping pictures of the planet and its stormy atmosphere, mapping the surface of its icy moons, studying the composition and rotation of the rings to find out why strange "spokes" form and dissipate, and measuring the powerful magnetic field surrounding Saturn to learn more about its inner layer of metallic liquid hydrogen. Cassini will also drop the Huygens probe built by the European Space Agency onto Saturn's largest moon, Titan, which may have lakes of gasoline-like liquid hidden beneath its smoggy atmosphere.