The New York Times > After 7 Years, Craft Set for Descent Onto Mysterious Saturn Moon

By JOHN NOBLE WILFORD

DARMSTADT, Germany, Jan. 13 - At the European Space Operations Center here, scientists and engineers are tracking a spacecraft around Mars, one monitoring Earth's atmosphere and another on the way to orbit a comet. But the mission very much of the moment is the attempt Friday morning to plunge through the hazy atmosphere and land on Saturn's largest moon, Titan.

The European Space Agency's Huygens spacecraft is scheduled to begin its two-and-a-half-hour descent through Titan's dense and mysterious atmosphere shortly after 11 a.m. here (5 a.m. Eastern time). It will be at least five hours before the first relayed data are received; pictures will come still later.

'We have good hope that we will survive the touchdown,' said Dr. Jean-Pierre Lebreton, the mission manager. 'I'm quite confident we will have something special.'

At a news briefing Thursday, Dr. Martin G. Tomasko of the University of Arizona, a Huygens scientist, said the craft's camera and science instruments were expected to provide 'a spectacularly new view of Titan and an understanding of this mysterious world.'

If Huygens succeeds, it will give scientists their first peek inside one of the most intriguing atmospheres in the solar system. Its dense hydrocarbon smog suggests complex chemical processes like those that led to life on Earth. Dr. Tomasko said previous observations indicated that the atmosphere would appear green at higher altitudes and then turn orange closer to the surface.

This atmosphere has also been frustrating. The perpetual smoggy veil has limited any view of Titan's surface to little more than a vague patchwork of light and dark regions, with only some hints of rugged topography.

Mission officials acknowledge the risks that Huygens runs. All of the spacecraft's parachutes must deploy flawlessly. No one can predict the effects of Titan's stiff winds on the craft's stability. And if the craft makes it to the surface intact, its batteries will be running low - time enough for no more than 30 minutes to 2 hours of observations and picture taking.

It would be the first landing of a spacecraft on the moon of another planet. And Titan is no ordinary moon, but a body larger than the planets Mercury and Pluto, and almost the size of Mars.

The 700-pound Huygens, built and operated by the European Space Agency, had help reaching this climactic moment. The mission is part of a $3.2 billion undertaking by the National Aeronautics and Space Administration, ESA and the Italian Space Agency.

Huygens (named for the 17th-century Dutch astronomer) rode piggyback on NASA's six-ton Cassini spacecraft for the seven-year journey to the wide-ranging orbit of Saturn. On the night of Dec. 24-25, Cassini released its fellow traveler for its solo cruise on course for Titan and is now in a position to relay all the data sent from Huygens during the descent and landing.

The possibility remains that a design flaw in Cassini's radio receiver system will hopelessly scramble the data. Engineers anticipated that signals from the wind-tossed Huygens would vary widely in frequency and strength, and thus compensated for it in the receiver's design. But they had failed to take into account frequency shifts that would also throw off the timing of the encoded data, leaving it a garbled mess.

In early 2000, an ESA engineer recognized the problem. Finally, ESA and NASA engineers found a way to reduce the frequency shifts to acceptable levels by altering the trajectory and orientation of Cassini during the critical maneuvers."