If all goes according to plan, 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 data which could help explain the chemical reactions 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 said from mission control in 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, 20-year cooperative venture between the US and Europe.
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 long and 4 metres 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 to reach Saturn directly.
So the spacecraft was sent on an elaborate game of solar system pinball.
On a 2.1-billion-kilometre trek, it looped twice around the sun, twice around Venus, once around Earth and once around Jupiter, picking up gravity "assists" that, like a slingshot, helped it build up enough speed to reach the outer solar system.