Cassini's Countdown

After a seven-year journey that included four planetary flybys, the Cassini spacecraft is finally poised to begin orbiting the giant, mysterious, ringed planet Saturn. The date of this "orbit insertion," June 30, promises to be a momentous one for the international team of scientists and engineers who've spent more than a decade constructing, launching, and nurturing the craft during its trek.

Cassini is the largest, most expensive planetary exploration mission in the history of the National Aeronautics & Space Administration. It's the last of a breed, a $3 billion extravaganza featuring an instrument-packed orbiter and a probe that will descend to the surface of Titan, the largest of Saturn's 31 moons.

A project of this scope would be inconceivable in today's "faster, better, cheaper" mind-set, where robotic space missions run perhaps a few hundred million dollars.

If all goes according to plan, Cassini, which is managed by the Jet Propulsion Laboratory (JPL) in Pasadena, Calif., will embark on a four-year orbit around Saturn, studying its atmosphere, rings, moons, and magnetic field. The spacecraft also will dispatch the Huygens probe to Titan to study the moon's hazy atmosphere and patchy surface.

As Cassini arrives at its destination, "there's a tremendous amount of anticipation," says Trina Ray, a member of the Cassini team at JPL. "We're very excited."

And nervous. "Until [orbit insertion] happens, we don't have a Saturn mission. I hope it all works," says Cassini team member Jonathan I. Lunine, chair of the theoretical astrophysics program at the University of Arizona, Tucson. "I'll have to make sure I grow my fingernails so I can chew them."

So far, though, Cassini appears to be in good health, as evidenced by its recent close-up images of Saturn's small moon Phoebe. The moon's crater-pocked surface shows bright speckling--evidence that it contains ice-rich material covered by a thin layer of darker material. This bolsters the theory that Phoebe is more akin to a comet or other icy body than an asteroid, hailing from the far reaches of the solar system.

Saturn is a giant ball of mostly hydrogen and helium, second only to Jupiter in size. Cassini's 12 instruments, including a dust analyzer and numerous spectrometers, will study the planet's roiling storms, ammonia clouds, and mysterious heat source that causes Saturn to emit more energy than it absorbs from sunlight. Cassini also will examine Saturn's complex rock-and-ice rings as it sails close to and through them, the craft's high-gain antenna serving as a protective shield.

FEW MOONS in the solar system have generated as much interest as Titan, which is larger than the planet Mercury and the only moon with an atmosphere. Titan's surface has long been obscured from telescopes by a methane haze. Scientists have speculated that the surface is covered with lakes of liquid hydrocarbons such as ethane. Recent observations from the Arecibo radio telescope showed large reflective patches that could be either smooth ice or liquid.

Whether Titan's chemical brew could produce the molecular precursors to life has been a long-standing question.

On Dec. 24, the Cassini craft will set loose the Huygens probe, which is managed by the European Space Agency. The probe will plunge through Titan's atmosphere and hopefully land on the surface. Huygens carries a gas chromatograph and mass spectrometer designed to identify organic molecules in the low- to mid-mass range. There's also a pyrolyzer that will break apart and analyze aerosols in the atmosphere.

The probe isn't expected to survive long after its descent, though the instruments on board the orbiter will continue to monitor Titan.

During the years that Cassini has been en route, technologies have evolved. What would team members put on Cassini now if they had the chance? Lunine would equip Huygens with a state-of-the-art chiral gas or liquid chromatographer capable of studying much higher molecular weights.

Ray's upgrade of choice would be a scan platform, which would allow the instruments to operate and point independently of the spacecraft. Right now, she says, they're all fixed. "If you want to point an instrument at Titan, you have to move the entire craft," she says. "But then the high-gain antenna is not pointed toward Earth, and so you have to share. One of the big tasks for the science planning team is deciding how we're going to share resources."