The most ambitious interplanetary mission ever launched reaches a climax on Friday when a clam-shaped probe plunges towards Titan in a suicidal quest to unlock the mystery of Saturn's biggest moon.
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 mother ship 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 the agency 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.
{{/usCountry}}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.
{{/usCountry}}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.
Huygens will enter the fringes of Titan's roiling atmosphere, its six sensors protected from friction-generated temperatures by a tough composite shell.