By Jonathan Amos
BBC News science reporter
The Huygens probe is on target and all set for its encounter with Titan, the mysterious large moon of Saturn.
The 2.7m-wide robot lab has passed its final systems check-out and scientists have confirmed the rendezvous can go ahead on 14 January as planned.
Huygens has spent the past seven years riding on the Cassini spacecraft, which arrived at the ringed planet in July.
The probe's ejection and plunge into Titan's thick atmosphere should be one of the mission's major highlights.
"Huygens is in great shape," said Dr Gerhard Schwehm, from the European Space Agency (Esa). "We've got the green light from the review board to proceed with the mission."
The campaign will begin in earnest on Christmas Day when Cassini will fire retaining bolts holding down its 319kg passenger.
A spring mechanism will then give Huygens a gentle nudge, taking it ahead of the mothership at a steady, relative speed of 30cm/s and spinning at seven revolutions per minute.
At that point, Huygens will be on a collision course with Titan, although it does not actually arrive at the moon for another 20 days. It will be a silent coast with all systems shutdown to preserve the battery power needed for the scientific reconnaissance.
Huygens is timed to hit the top of Titan's atmosphere at 0907GMT on the 14th January; it will be travelling at some 6km/s.
Friction on the heatshield and three parachutes will slow the robotic lab's descent.
If all goes to plan, Huygens should get about two and a half hours to study its surroundings on the gentle drift down to the surface.
Titan is unique in the Solar System. It is the only planetary satellite with a substantial atmosphere. It is rich in nitrogen, methane, other organic (or carbon-based) molecules and nitriles (in which carbon atoms are triple-bonded to nitrogen atoms) such as hydrogen cyanide.
"The atmosphere is of really great interest because it is similar in some respects to the primitive atmosphere on Earth - except stuck in deep-freeze," explained Professor John Zarnecki, of the UK's Open University and a principal investigator on Huygens.
"Our atmosphere has evolved enormously, of course; but if we go to Titan we hope to see many of the chemical processes that happened here four billion years ago and which led, ultimately, to the conditions in which life was able to develop."
Finding life on Titan, however, is not thought likely. With temperatures down to -180C, there is no opportunity for aqueous chemistry at Earth-like temperatures, conditions considered crucial for the origin of life as we know it.
Nonetheless, Huygens should return a full "weather report"; detailing the atmosphere's composition, structure, temperature, pressure, winds and aerosols. The camera system will return more than 1,000 images.
All the information will be relayed up to Cassini for onward transmission to Earth.
Huygens is first and foremost an atmospheric probe, but scientists will be hoping it keeps operating all the way down to the surface to study the conditions there, too. What it will find no-one is really quite sure.
Surface features are obscured from direct visual study by a thick, organic haze. And even the close flyby (at a distance of 1,200km) by Cassini in October seemed to add to the mystery.
There are no major impact craters, for sure; which means the surface is being reworked, perhaps by icy volcanism or erosion. There are bright and dark features but their true nature has become a tantalising enigma. Are the bright regions "land forms" and the dark features seas of liquid ethane?
"We're really not sure," said Professor Carl Murray, of Queen Mary, University of London, and Cassini imaging team member.
"To get the whole picture, we will need repeated encounters with Titan which we will get with [45 flybys]. And to understand the surface in detail we need Huygens to give us the 'ground truth' for one particular region, which we can then compare with all the other Cassini instrument data."
No-one is keener to get to the surface intact than John Zarnecki.
He leads the team behind the surface science package. He envisages three possible scenarios: a relatively hard landing on a icy-rocky surface; a squelch into a tar-like gunge; and, his favourite outcome, a splash-down in an oily sea.
"Huygens would then take the first measurements ever in extraterrestrial oceanography," he said. "We've done some calculations for what wind-driven waves might look like on Titan and they are actually quite scary.
"Under the same conditions, waves on Titan could be 10 times larger than on Earth."
So, although Huygens is sealed and should float, if it lands on liquid there is the prospect it could be overwhelmed by a massive surge of liquid hydrocarbons.
"We've also calculated the waves will be very slow; so I guess my dream scenario is that the camera picks up this massive wave on the horizon trundling towards us and we take measurements all the time before the screens suddenly go blank," Professor Zanecki joked.
Cassini-Huygens is a cooperative project between the US space agency, Esa and the Italian Space Agency.