In a land far, far away in a galaxy similar to our own, a probe called Huygen will float down upon a world called Titan. Leaving the mother craft after a seven year journey it will parachute down in a 22 day free- fall onto the unknown surface of Saturn's moon. It may be destroyed on impact or swallowed up in a methane lake. It may also provide scientists with unique evidence of the atmosphere of the young Earth and the origins of life. It is the year 2004.

Design for an Alien World, broadcast the day after the eclipse of Earth's moon, examines the science behind the journey to Saturn. Successfully launched in 1997, the Cassini spacecraft is now carrying the Huygen probe on the first stage of its thousand million kilometre journey through the night sky. Using `slingshot' acceleration from Jupiter it will hurtle towards Titan at 13,000 mph.

Titan remains one of the final frontiers in the solar system. The largest of Saturn's 20 moons, the Voyager missions took spectacular images of many planetary satellites but no camera has ever penetrated Titan's clouds of thick chemical smog. Even Hubble's telescope failed to make the breakthrough. "Titan is the only body of any size whose surface is still a mystery,"' remarks Tobias Owen of the University of Hawaii. "It may be lakes of liquid hydrocarbons or maybe volcanoes spewing methane into the atmosphere."

Although the atmosphere prevented the Voyagers from recording clear images of the surface, the missions did provide evidence of a dense atmosphere rich in nitrogen and other chemicals. The detection of molecules of hydrogen cyanide in Titan's atmosphere has led some scientists to speculate that it may one day be able to support life as this is one of the key organic chemicals involved in the evolution of life on Earth. Yet the surface temperature of about minus 180 degrees centigrade makes the presence of life as we know it unlikely.

This programme shows the work of research scientists and project managers and the collaboration of laboratories across the world - a kind of global cottage industry funded by NASA and the European Space Agency. John Zarnecki of the University of Kent takes us through the scientists' horse-trading over the cost, power, and design of the probe.

The researchers develop two pieces of equipment knowing that one of them will fail with no second attempts. When the probe reaches the surface, the key question will be whether it is liquid or solid.

If the surface is solid ice, the force of the impact on landing will be measured by the force of the impact on a titanium sphere recording the duration of impact and the angle of impact from the vibration of the mounting rod. In case of a liquid landing, Huygens is designed to bob to the surface with the buoyancy gauged accurately by delicate instruments recording the density of the liquid. A bad landing could smash Huygens apart or it could sink forever in a methane rich lake.

Nervous scientists can only wait five years to discover the truth. Unless an asteroid hits - but that's a story for another time.

Simon Newton

Design for an Alien World: Thursday 12 August 01.30 BBC2.