Probe Set to Enter Titan's Atmosphere

John Roach
for National Geographic News
Updated January 13, 2005
If all goes according to plan, Huygens will parachute through Titan's hazy atmosphere tomorrow. The 8.9-foot-wide (2.7-meter-wide), 703- pound (319-kilogram) probe should land somewhere just south of the moon's equator.

Scientists from around the world have been waiting for years for the moment when Huygens sends back to Earth images and readings from Saturn's mysterious moon's atmosphere.

The Cassini spacecraft jettisoned Huygens on Christmas Day, sending the probe on its merry way to Titan.

Huygens spent the last seven years bolted to the Cassini spacecraft, which arrived at Saturn July 1, 2004, for a four-year mission to study the planet, its rings, and its icy moons.

At 3:24 GMT on December 25 (10:24 p.m. ET on December 24) confirmation was received on Earth that all had gone according to plan: Cassini had fired bolts and sent Huygens on a collision course with Titan, the second largest moon in the solar system. Twelve hours later the orbiter sent one last image of Huygens to Earth—a tiny receding point of light which gives scientists confidence that the probe is indeed on the right trajectory for its landing on January 14.

"We've always known these dates, but they've always seemed infinitely far away in the distance. It really is quite unreal they're now actually here," said John Zarnecki, a space scientist at The Open University in Milton Keynes, England. Zarnecki is the principal investigator for Huygens's surface science package.

Mysterious Titan

The probe's two-and-a-half-hour descent through Titan's atmosphere is anticipated to be a highlight of the international Cassini-Huygens Mission, which is managed jointly by the ESA, the Italian Space Agency, and NASA.

Titan's atmosphere is composed of nitrogen, methane, and other organic compounds. Scientists believe the chemistry is similar to that of Earth's about 4 billion years ago, before life evolved on our planet.

Data gleaned from the suite of robotic instruments on Huygens may shed light on how chemistry that predates life works in a "cold, waterless environment and tell us clues on how life started on Earth," Lebreton, the Huygens project scientist, said.

Hidden beneath a hazy atmosphere, the surface of Titan remains a great mystery. Scientists have long speculated that the moon may be covered in icy mountains and seas of liquid methane, but no one really knows.

Zarnecki, the Open University space scientist, noted that recent images of the moon taken by cameras aboard Cassini that can see through the lunar haze have only added to the mystery.

"I've sat in a couple of meetings that had the majority of the world's experts on Titan sitting together, and for the first time they've been suspiciously quiet," he said.

Showing light and dark patches, the images lack any sign of impact craters. In Zarnecki's opinion, the missing craters stand out as the most telling feature. He believes geologic activity or weathering erased them.

"Every surface [in] the solar system is pockmarked with [impact] features except for those that are active in some way," he said.

Information gathered during Cassini's recent flybys of Titan has confirmed scientific models of the moon's atmosphere, according to Lebreton. The finding has bolstered the confidence of the space mission's project scientists that Huygens will enter safely, he said.

Titan Encounter

During the descent from Cassini all systems were shut down, except for three timers designed to wake up the probe four hours before its landing on January 14.

Upon arrival, the probe will be traveling at about four miles a second (six kilometers a second). It will deploy a series of parachutes and open a communications link with Cassini to relay images and scientific data to Earth.

If all goes according to plan, the probe will drift through the atmosphere for about two and a half hours before it reaches the surface, sending scientists more than a thousand images and details about the lunar atmosphere's structure, composition, and winds.

Huygens, which is designed primarily as an atmospheric probe, is expected to land about 10 degrees south of Titan's equator and several hundred kilometers west of a bright landmark known as Xanadu. Some scientists speculate the feature is a giant icy mountain.

But precisely what the probe will land in or on—rock-hard ice, a sea of gungy methane, or something else—remains an open question. The probe's atmospheric entry point was selected to capture the best images during descent. The landing surface was irrelevant to mission planners.

"The probe may drift by several hundred kilometers in the winds during descent, so there was no way, and no reason, to target a specific patch on the surface," Lebreton said.

If the probe survives touchdown, it will have no more than two hours to relay images and data about Titan's surface before Cassini drifts over the horizon, severing the communications link.

"If we got two hours worth of data on the surface that would be beyond my wildest dreams, that would be truly wonderful," Zarnecki said.

Cassini will continue to make observations of Titan on more than 40 subsequent flybys during the spacecraft's mission.

"Titan is really a fascinating body to explore and its surface a big puzzle," Lebreton said. "It will take all the synergy between the repeated Cassini orbiter observations and the detailed [direct] observation of Huygens to put all the pieces of the puzzle together."

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