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If all goes as planned, the Huygens probe will plunge into Titan's methane-shrouded atmosphere at 5:13 a.m. Eastern time and parachute 789 miles to the moon's frigid surface, gathering images and data for as long as four hours before its batteries die.

On to a Moon of Saturn -- and the Unknown
'Surprise' Expected In Probe of Titan

By Guy Gugliotta
Washington Post Staff Writer
Friday, January 14, 2005; Page A03

DARMSTADT, Germany, Jan. 13 -- A 705-pound European spacecraft, spinning slowly like a movie flying saucer, will dive into the smog above Saturn's moon Titan on Friday in a spectacular attempt to force one of the solar system's most mysterious worlds to give up its secrets.

If all goes as planned, the Huygens probe will plunge into Titan's methane-shrouded atmosphere at 5:13 a.m. Eastern time and parachute 789 miles to the moon's frigid surface, gathering images and data for as long as four hours before its batteries die.

Scientists call Titan a "pre-biotic" laboratory, rife with the compounds that existed on Earth before life evolved but frozen in time by the surface temperatures that prevail about 900 million miles from the sun -- 290 degrees below zero Fahrenheit. But beyond these basics, Huygens's handlers do not know what to expect. Titan could have mountains of water ice, lakes filled with liquid ethane, hydrocarbon rainstorms, and glaciers made of pebbles and hydrocarbon slush. Or it may not.

"I think we are in for a big surprise," said Claudio Sollazzo of the European Space Agency (ESA), who leads Huygens's operations team. "We've seen a lot of variation in the surface between bright and dark patches, and [the surface] looks to be very active. But we don't know why or how."

The craft began its journey in 1997 as a passenger aboard Cassini, a $3.3 billion joint mission by NASA, ESA and the Italian Space Agency to explore Saturn, the sixth planet in the solar system. It is one of the most ambitious space ventures ever undertaken.

Huygens's first tour de force came last summer, when Cassini ended its seven-year transit by orbiting around Saturn after a nail-biting rocket burn to pass through the planet's rings.

Huygens carries six instruments, primed to photograph Titan and to record weather and chemical data throughout its 2 1/2-hour descent and for as long as 90 minutes after it reaches the surface, beaming the information to the Cassini mother ship orbiting Saturn. One instrument is listening for the sounds of electric storms.

Earth-based receiving stations may be able to detect Huygens's descent minutes after it begins, but it will take hours before mission control here begins receiving data from NASA's Cassini.

What happens at the end of Huygens's descent depends on whether the probe lands on solid ground, in liquid or slush, or behind a mountain, and whether it bounces or rolls. With winds expected to reach 380 mph, Huygens could drift as much as 370 miles over the ground before it comes to rest.

"Anything on land is extra," Sollazzo said. "If everything is perfect, we will have up to 30 minutes without any problems while the instruments are still warm. After that, everything will degrade." Even with optimum conditions, contact will end when Cassini disappears over Titan's horizon about four hours after the descent begins.

Engineers designed Cassini's mission to last four years. It could go on much longer while it collects data on such matters as the rings, composition and magnetic field of the giant host planet and the shape and characteristics of seven of Saturn's 31 known moons.

Chief among these is Titan, Saturn's largest moon and the second- largest in the solar system, after Jupiter's Ganymede. Titan is also the only known moon with an atmosphere, made up mostly of nitrogen, like Earth's, but impregnated with methane.

Titan was discovered in 1659 by Dutchman Christiaan Huygens, but it has frustrated astronomers because the enveloping haze conceals the surface, even from the most sophisticated ground- and space-based telescopes.

The methane gives Titan a greenish cast from above. The University of Arizona's Martin G. Tomasko, who leads the Huygens imaging team, said the sky should look orange from the surface.

Cassini has conducted two close Titan flybys and, after Friday, will pay 42 more visits over the next 3 1/2 years. Its instruments have penetrated the mist to provide the first images of Titan's surface, which have perplexed astronomers, but Huygens should be able to fill in many more details.

Tomasko said at a Thursday news conference that Huygens's camera can provide images 20 times better than Cassini's high-altitude shots, a "spectacularly new view of Titan." Friday's pictures will be black-and-white, he said, but spectrographic data will allow colorizing later.

Cassini released the gold-colored, saucer-shaped Huygens probe on Christmas Eve, imparting a spin of seven revolutions per minute to hold it stable on its three-week journey to Titan.

With no propulsion of its own, Huygens will coast until Titan's approach triggers a series of "alarm clocks" to begin the descent sequence. "There is no way we can talk to Huygens" at any point during the mission, Sollazzo said.

Huygens will smash into Titan's atmosphere traveling at 13,500 mph. Temperatures on the probe's composite heat shield will reach 2,700 degrees Fahrenheit during the first three minutes of the descent.

If all goes well, it will almost immediately release a pilot parachute, designed to brake the descent to 870 mph. The chute will then pull off the back cover of the spacecraft, deploying a larger, 27-foot main parachute.

That will be jettisoned 15 minutes later for a smaller drogue chute that will take the probe to Titan's surface. Huygens will land at 15 mph -- like jumping from a table to the ground. Surface atmospheric pressure is about 1 1/2 times that of Earth.

During the descent and beyond, Huygens's instruments will gather images, measure the physical and chemical properties of the atmosphere at different altitudes, gauge wind speed, and collect and analyze aerosol particles. A separate package of sensors will measure surface characteristics.