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Saturn's Deep, Dark Secret

Titan, the only major body in the solar system that we haven't gotten a good look at, is about to be outed.

But Louis Friedman, executive director of the Planetary Society, lets the cat out of the bag when he reveals what his California-based space advocacy group and the growing community of Internet space watchers want from Huygens. “If we could ever get an image from close to the surface,” he says, “that would be just awesome.”

Astronomers have argued about the surface of Titan since the 1970s when they found evidence of a thick cloud deck in its atmosphere. Data from the Voyager spacecraft in 1980 spurred the theory that Titan was all one great methane ocean, but near-infrared observations by the Hubble telescope in 1994 showed dark and light patches, indicating a mix, but not revealing what was producing the differences: Continents? Oceans? Craters? One prominent surface feature in the Hubble images is a bright area about the size of Australia. Huygen’s landing site is on the Titan equator west of the feature, in a dark area.

So if it survives its descent through the atmosphere, what might the little probe see? “Almost certainly craters,” says Jonathan I. Lunine, a Huygens scientist with the University of Arizona. “Maybe interesting and exotic landforms and erosional features.” Other scientists have suggested mountains covered in methane snow.

Lebreton says he would rather Huygens come down in a methane lake. Amid all the wizardry of sending robots to Saturn, his reason is comically simple. If the capsule hits solid ground, the impact might topple it, pointing its antenna away from Cassini and sending its transmissions into empty space. If it lands on liquid, Lebreton says, Huygens should float for five to 10 minutes, and for at least three of those precious minutes, its batteries will continue to power transmissions. In the ideal scenario—Huygens on land and upright after a fast descent—the transmission window could be as long as two hours before the mothership and its receivers disappear beneath Titan’s horizon. But Lebreton, who started working on what would become Huygens in 1984, would rather take his three minutes guaranteed.

As he approaches the spectacular climax of his career, Lebreton worries about more than Huygens tipping over. ESA is still coming to grips with the failure of another planetary lander, Beagle II, which last Christmas disappeared inexplicably during its descent to Mars. European cosmocrats are at pains now to distinguish Huygens from Beagle. The Mars craft was a shoestring adventure, they say, built for $80 million. It was built with partial funding from private U.K. companies, and its operational headquarters was at England’s Open University in Milton Keynes, north of London. Huygens, adequately funded at $400 million, was incubated and bred at ESA’s own scientific center, a sprawling campus-like complex tucked behind the North Sea dunes at Noordwijk, in the Netherlands.

Huygens ran into a serious problem all its own in February 2000, when a semi-annual systems checkup revealed that its communications with Cassini were garbled. Diagnosing the fault was relatively simple: The receiver that ESA had installed on the mothership had not been designed to cope with the extreme Doppler shift in radio frequencies that would occur after the probe separates from and transmits its data back to Cassini. Fixing it, on a spacecraft cruising around the solar system, was anything but simple.

With Cassini-Huygens well beyond repairman range, the only solution left was to attempt to reduce the Doppler shift so that the transmitted signals would fall within the receiver’s designed bandwidth. That required ESA and NASA’s Jet Propulsion Laboratory in southern California to remap much of the Saturn mission, so that Cassini would be moving past Titan on the right track to receive the data transmitted from Huygens.

The long-distance adjustments took three years, as the two mission controls modeled possible trajectories, then programmers furiously wrote software instructing the spacecraft in their new routines. Huygens will now disengage from Cassini on the mothership’s third pass by Titan after entering orbit around Saturn. That will happen this December 25, about six weeks later than originally planned. At the time of separation, both craft will be on a course to collide with the moon. The lander will coast another 2.5 million miles to do just that, but five days after the separation, Cassini will fire maneuvering thrusters—“slam on the brakes,” as deputy program manager Earl Maize puts it—to change course and orient itself so that when Huygens starts transmitting, Cassini’s high-gain antenna will be pointed toward Titan. The success of the Huygens mission depends on this maneuver.

In the meantime, Huygens will sleep until an onboard timer, set at separation, wakes up the craft’s batteries and computers as Huygens approaches Titan’s atmosphere. The batteries have enough power for the two-and-a-half-hour trip to the surface—plus an adequate reserve, in case the fall doesn’t pulverize the probe.

Huygens folk say they were compensated for the labor and stress of redesigning the mission by the Americans’ helpful attitude. “NASA’s cooperation has been magnificent,” says David Salt at Darmstadt. “JPL altered its whole four-year tour of Saturn to accommodate us. At this level it’s not just a design problem. It’s a political compromise too.”

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