On Christmas Day, the Huygens satellite will attempt to lift off for Saturn's moon, Titan, which has an atmosphere like early Earth and could harbour life, writes Dick Ahlstrom.
By the time we start opening presents on Christmas morning, a satellite orbiting Saturn 1,300 million kilometres away will have set off on a momentous journey to visit the nearby moon Titan.
The mission marks a key juncture in a 20-year effort to drop a satellite onto the surface of a Saturnian moon, a body that could harbour life. It will also be - if things go to plan - the furthest man-made object to land on a remote celestial body, a remarkable achievement.
The mission involves a pair of satellites: the Cassini orbiter now circling Saturn; and its piggyback partner, the Huygens lander. The partners in the endeavour - the US National Aeronautics and Space Administration; the European Space Agency; and the Italian Space Agency - launched the two seven years ago.
In two days, Huygens will break away and begin its final adventure: a hoped-for landing on Titan's surface.
Titan is Saturn's largest moon, a strange environment with a nitrogen-rich atmosphere and the possibility of lakes or even oceans of liquid ethane, methane and nitrogen.
Mission scientists in the UK have had to plan for a range of landing eventualities given that we know so little about Titan's surface, explains Prof John Zarnecki of the Open University, the principal scientist for the science surface package on board Huygens.
"It is a distinct possibility that I could be the very first scientist to carry out oceanography on an outer planet of the solar system," he says. "But equally, the probe could land with a thud on hard ground or squelch into a morass of extra-terrestrial slime. No one knows for sure."
Following the separation early on Christmas morning, Huygens will coast along unpowered towards Titan for 20 days. It is due to arrive and begin its rapid descent on January 14th. Huygens will rush into Titan's upper atmosphere 1,270km above the surface at Mach 2 or 1,500 mph and will be slowed by friction and then parachute.
It will reach an eventual impact speed of just five metres per second when it touches down a few minutes later. The jolt should be similar to that felt when jumping from a chair onto the ground on Earth.
Throughout its descent, Huygens will relay data from six on-board experiments to the Cassini orbiter above. It should also be able to get a last message back if it does sink without trace into a sea of ethane and might also be able to send data for at least two more hours if it bumps down on solid ground.
Hopefully Huygens will fare better than last year's Christmas Day satellite delivery, the ill-fated Beagle 2 lander. Twinned with Europe's Mars Express it should have touched down softly last December 25th, but it disappeared without trace.
Titan is planet-sized, and larger than Mercury and Pluto. Scientists believe it has an atmosphere much like that of early planet Earth (3.8 billion years ago).
One of the big questions is where the atmospheric methane comes from, says Prof Ian Halliday, chief executive of the Particle Physics and Astronomy Research Council that funds UK involvement in the mission.
Titan is a strange place with a strange atmosphere, he explains. "Its thick atmosphere is mostly nitrogen, but there are also methane and many other organic compounds. Some of them would be signs of life if they were on our planet," he says.
"Organic compounds form when sunlight destroys methane. If sunlight is continuously destroying methane on Titan, how is methane getting into the atmosphere?" he asks.
For more information about the mission, visit www.esa.int/SPECIALS/Cassini-Huygens/index.html