The Huygens spacecraft is about to begin the final leg of its journey to Saturn's enigmatic moon Titan.
By Paul Rincon
BBC News science reporter
The US-European probe will separate from its "mothership" Cassini on 25 December and coast for three weeks toward the haze-shrouded world.
On 14 January, Huygens will parachute through Titan's atmosphere, collecting invaluable information about the moon during its two-hour descent.
But scientists are still uncertain what awaits the craft on Titan's surface.
Huygens has spent the past seven years piggybacked on the Cassini spacecraft, which arrived at the ringed planet in July.
The probe's target is the only known planetary satellite with an appreciable atmosphere.
Dominated by nitrogen, methane and other organic (carbon-based) molecules, conditions on Titan are thought to resemble those on Earth about 4.5 billion years ago.
As such, scientists hope to see many of the chemical processes that led to the conditions in which life developed on our planet. But with temperatures down to -179C, Titan itself is considered too cold to host biology.
Departure and arrival
Huygens' campaign will begin on Christmas Day, when Cassini is due to fire retaining bolts holding down its 319kg passenger.
Tension-loaded springs will then push Huygens just ahead of its mothership at a relative speed of 30-40cm/s and spinning at about seven revolutions per minute.
"Scientists don't believe in luck but we'll still be crossing our fingers on Christmas morning," said Mark Leese, Huygens project manager at the Open University in Milton Keynes, UK.
Ground controllers want to unleash Huygens at about 0200GMT (spacecraft time). But should any problems arise, separation could be delayed by up to a few days without affecting the mission.
After Huygens is set free, it will coast silently on a collision course with Titan. A full systems shutdown will preserve the battery power required for its scientific study of the Saturnian moon.
"Between separation on Christmas Day and entry into Titan's atmosphere lie 21 days of nail-biting," said Michael Kahn, mission analyst at the European Space Agency's operations centre in Darmstadt, Germany.
There are three ways ground controllers can tell whether separation was a success. First, electronic switches on Cassini will show whether Huygens has detached.
Second, as Huygens goes spinning into the void, a "kickback" will be felt as Cassini spins very slowly in the opposite direction.
Lastly, the temperature profile on Cassini will change when the surface covered by Huygens is exposed.
1. HASI - measures physical and electrical properties of Titan's atmosphere
2. GCMS - identifies and measures chemical species abundant in moon's 'air'
3. ACP - draws in and analyses atmospheric aerosol particles
4. DISR - images descent and investigates light levels
5. DWE - studies direction and strength of Titan's winds
6. SSP - determines physical properties of moon's surface
After a day or more, Cassini will be rotated to take a picture of Huygens to confirm it is on the right trajectory. By this time, Huygens will be no more than a bright speck in the distance.
On 14 January, Huygens will be switched on by a timer four hours before it is due to reach an "interface altitude" of 1,270km (789 miles) from Titan's surface.
On entering the moon's atmosphere at 0907GMT, the robotic lab's peak velocity will exceed Mach 20 - 20 times the speed of sound.
Once friction on the heatshield has slowed the probe's descent to about Mach 1.5, it will deploy the first of three parachutes.
Deployment of the first two parachutes must occur in a window of about 30 seconds, said Steve Lingard, who worked on the descent control subsystem.
"Parachutes don't work very well over Mach 2, and all probes the shape of Huygens become somewhat unstable around Mach 1," Mr Lingard, technical director at aerospace firm Vorticity, explained.
Huygens should get two to three hours to study its surroundings as it drifts gently to the surface.
Full of surprises
The probe's instruments should return a full "weather report" for Titan; detailing the atmosphere's composition, structure, temperature, pressure, winds and aerosols. The camera system will return more than 1,000 images.
All data will be relayed back to Cassini for onward transmission to Earth.
Despite several flybys of the moon, scientists are still uncertain about the type of surface Huygens will encounter on Titan.
The probe will experience massive heat flux on entry
"So far Titan has surprised us every time we've looked at it, so I suspect it will keep on surprising us," said Peter Challenor, a co-investigator on Huygens' surface science package (SSP) at the Southampton Oceanography Centre.
The SSP team envisage three possible scenarios: a hard landing on an icy-rocky surface; a squelch into tar-like gunge; and a splash-down in an oily sea.