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It is Titan, however, that will be the main attraction. One of the largest moons in the solar system--larger than Mercury or Pluto--Titan would be a perfectly good planet if it were orbiting the sun under its own steam. NASA scientists were keenly disappointed when the Voyager 1 spacecraft flew by Titan in 1980. The moon's dense, orange atmosphere completely concealed its surface from view, revealing not a clue about what was happening on the ground.
Scientists speculate that there may be quite a bit happening. Rich in nitrogen as well as ethane, methane and other carbon-based gases, the Titanian air contains the raw chemical material believed to be needed to give rise to life--and just the kind that probably existed on the primordial Earth. Titan's frigid temperature--about --280F--would surely have prevented life from emerging. Nonetheless, over time the candlelike heat of the distant sun may have slow-cooked some of the organic materials, forming more complex molecules. What's more, if there is lightning in Titan's atmosphere, the random jolts could have shocked even bigger molecules into existence.
The Cassini-Huygens mission will investigate Titan from many angles. Of the 59 flybys of the nine selected moons, 45 will be devoted to Titan--most at a distance of just 590 miles. Preliminary images received last weekend revealed a bright cloud pattern about the size of Arizona near the south pole and what appeared to be a massive impact crater.
But there will be much more. Radar will pierce the Titanian cloud cover, mapping plains, mountains and perhaps even lakes of liquid ethane and methane--though early observations last weekend cast new doubt on the existence of the lakes. Spectrometers and other instruments will take the chemical measure of the moon's air, and cameras will again try to photograph Titan from outside in.
Cassini's best shot at the moon will come on Christmas Eve, when the Huygens probe is fired toward Titan, heading for a Jan. 14 rendezvous. On arrival, it will make a 2 1/2-hr. descent through the atmosphere by parachute. If it isn't destroyed by the landing, the probe could survive on the surface for an extra 30 minutes or so.
The brief three hours that Huygens lasts will be busy. The probe carries six instruments, including radar, an aerosol collector, a camera and wind instruments. The hardware will switch on by an altitude of 93 miles and will record data all the way down. When Huygens lands, sensors will continue to take readings--assuming it doesn't smack against a mountain or capsize in a methane lake. Even if it does, the landing--NASA's first splashdown since the return of the last Apollo spacecraft in 1975--would make space history.
Barring breakdowns or accidents, Cassini should send back data at least until 2008. If it exceeds its nominal life span it could survive for nearly a decade. When it finally does wink out, it could mark an end in more ways than one.