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.
- By Craig Mellow
- Air & Space magazine, July 2004
(Page 2 of 4)
Huygens carries a transatlantic suite of instruments. An Aerosol Collector and Pyrolyzer, built in Paris, will grab samples of Titan’s atmosphere during descent, vaporize them, and feed the results for analysis to a spectrometer constructed at NASA’s Goddard Space Flight Center in Maryland. The Italian-made Huygens Atmospheric Instrument, which senses lightning and other electrical activity in the moon’s sky, works with a Descent Imager and Spectral Radiometer (that is to say, a camera and radiation sensor), contributed by the University of Arizona.
Cassini-Huygens scientists insist that their real purpose will be fulfilled above Titan’s surface, as the instruments log chemistry, electricity, winds, and the rest of it. “The main mission is going through the atmosphere,” Lebreton says. “The rest is bonus, if we get anything back.”
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.