The Huygens probe has travelled 789 million miles to the Saturnian system tethered to the Cassini spacecraft. In less than a month's time, it will separate from the mothership and begin its final journey to the surface of the moon, Titan. The probe's journey has taken it more than seven years, but once it enters the atmosphere of the moon, its mission will last for just over four hours.

On 13 December, Cassini-Huygens will make another flyby of the moon, Titan, passing just 2,400km away from the surface. On 21 December, the separation unit will be readied, and two days later, the course of the craft will be adjusted to put the pair on a collision course with Titan.

On Christmas day, Cassini will jettison the Huygens probe, severing the electronic umbilical link. Three springs will push the probe, very gently, away from the mothership. Huygens will move away from Cassini at around 30cm per second, spinning gently at seven revolutions per minute. Both craft will continue to fly towards Titan, until Cassini makes a course correction that will take it away from an impact course.

The probe will the drift for three weeks. Mission scientists have no way to communicate with Huygens once the electronic umbilical cord is cut, so the craft will be on its own.

On 14 January, a series of three alarm clocks will wake the probe as it approaches the very edges of Titan's atmosphere. It will be travelling at 18,000km per hour at this point, but as it encounters the thick upper atmosphere it will be slowed to just 400km per hour in a mere three minutes. As the craft plunges into the atmosphere, it will generate a huge amount of heat. The heat shield will reach temperatures of up to 1800°C as it is exposed to energies 1000 times more intense than the solar constant.

The probe contains three parachutes. The first, pilot, parachute will slow the craft to less than 300km per hour. The shell of the module will then fall away, exposing the instruments to the atmosphere, and the second parachute, eight metres wide, will deploy. Once the craft reaches an altitude of around 160km, this large parachute will be cut away and a smaller one deployed. This is to ensure that the craft reaches the ground in time to carry out its surface experiments.

What it will encounter when it lands is still a mystery. It could land on a solid surface, in an alien sea, or in a big puddle of extraterrestrial slime.

John Zarnecki, lead scientist on the surface science package, says that the mission will be considered successful even if the craft doesn't manage to land. "This is not a lander," he told journalists on Wednesday morning. "Our main objectives will be fulfilled if we get measurements of the atmosphere."

But he confessed that his ideal scenario would be a splash down. "The waves on a Titan sea could be ten times the size of those on Earth. It would be great to have the camera pointed at huge wave gradually approaching..." he said, with a slightly dreamy look on his face.

Once on the ground, Huygens will have at most two and a half hours of radio contact with the Cassini mothership. Once Cassini passes over the horizon, Huygens will be totally out of contact with Earth, and its part in the mission will be over. Cassini will then turn its antenna towards Earth, and will transmit Huygens' data back scientists on Earth, who will set about unraveling the mysteries of the murky moon. ®

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