If all goes according to plan, the death dive of the European probe Huygens could push back the frontiers of knowledge about the Solar System.

For more than four hours, the scout will relay back to its mothership precious data which could help explain the chemical recipe that enabled life to appear on Earth several billion years ago.

"Titan has a very thick nitrogen atmosphere which also contains lots of methane, and where you see methane you have complex organic [carbon] chemistry," Huygens project manager Jean-Pierre Lebreton told AFP from mission control in Darmstadt, Germany.

"We suspect that Titan's atmosphere is undergoing the same type of chemical reactions that took place on Earth way before life appeared. These precursors are called prebiotic chemistry, in other words, the chemistry which took place on Earth before the emergence of life."

The descent is the high point of a 3.2-billion-dollar (2.5-billion-euro), 20-year cooperative venture between the United States and Europe, the two biggest powers in the scientific exploration of space.

Cassini, a powerful unmanned NASA orbiter studded with hi-tech scanners to map Saturn, was launched in October 1997, with Huygens taking a piggyback ride.

Put together, the tandem was a monster: 5.6 tonnes in weight, 6.7 metres (21.75 feet) long and four metres (14 feet) wide, making it one of the biggest interplanetary payloads ever launched.

Cassini-Huygens was so heavy that no rocket of sufficient size was available to give it a big enough push for reaching Saturn directly.

So the spacecraft was sent on an elaborate game of Solar System pinball.

On a 2.1-billion-kilometer (1.3-billion-mile) trek, it looped twice around the Sun, twice around Venus, once around Earth and once round Jupiter, picking up gravity "assists" that, like a slingshot, helped it build up enough speed to reach the outer Solar System.

In July last year, Cassini-Huygens tandem finally reached Saturn, the largest planet after Jupiter, and separated on December 25. Since then, Huygens has been drifting towards Titan's surface on a finely-calculated path.

At 0907 GMT Friday, it will be show time.

Huygens will enter the fringes of Titan's roiling atmosphere, its six sensors protected from friction-generated temperatures by a tough composite shell.

Over the next four and a half hours, during a long descent by parachute and then -- hopefully -- for some time after landing, the probe's cameras and gas analysers will transmit back their findings to the orbiting Cassini.

The mothership will then turn around so its antenna points to Earth and transmit the digital treasure back home via NASA's Deep Space Network, an array of sensitive ears tuned to the void.

"We hope to get the first information around 2pm, 3pm", said Lebreton.

Titan is the only moon in the Solar System to have clouds and a thick, planet-like atmosphere.

Even though it may have clues as to the chemical processes that unfolded on the infant Earth, Titan is quite unlikely to have life.

With a surface temperature of -180 C (- 292 F) and barely illuminated by the distant Sun, it has none of the ingredients -- warmth, light and liquid water -- deemed essential for organisms. If there is an ocean on Titan, it is likely to be of frozen methane, scientists believe.

Huygens is named after the Dutch astronomer Christiaan Huygens, who discovered Titan in 1655. Cassini's name comes from the Italian Jean-Dominique Cassini (1625-1712), who discovered the Saturnian satellites Iapetus, Rhea, Tethys and Dione. In 1675, he discovered what is called the "Cassini Division," the gap between Saturn's rings.

After Friday's drama, the orbiter will continue to circle Saturn for another three and a half years, pursuing a detailed survey of the ringed giant and its moons.

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