Space probe ready for critical stage of mission to Saturn

Space scientists are bracing themselves for the most critical moment in an ambitious seven-year journey that aims to put the first spacecraft into orbit around the planet Saturn.

After travelling two billion miles since its launch in 1997, the spacecraft Cassini-Huygens is being prepared for the vital hour and a half that will make or break the £2bn mission, which is funded jointly by the European Space Agency and Nasa. In the early hours of 1 July the craft's rocket motors will be ordered to burn for precisely 96 minutes to slow it down enough for it to captured by Saturn's gravity and end up safely in orbit.

Once the craft is in place it will provide the best opportunity yet to investigate Saturn's vivid rings as well as Titan, the most mysterious of the planet's 31 known moons.

Andrew Coates, of the Mullard Space Science Laboratory at University College London, said: "The Saturn orbit insertion is a particularly critical point in the mission. It's absolutely key that this rocket burn works correctly."

The spacecraft, the biggest ever launched to explore the planets, is the size of a bus and weighed 5.5 tons at launch, three tons of which was fuel shed as the probe flew twice past Venus, once past the Earth and once past Jupiter.

If all goes to plan, the Cassini part of the craft will release the smaller Huygens probe on Christmas Day. Huygens will then fly off to on a 22-day journey to Titan, ploughing through the moon's dense atmosphere to land on its surface.

While Cassini - named after the French-Italian astronomer Jean Dominque Cassini - will continue to orbit Saturn 70 more times over the next four years, analysing its rings, atmosphere and planetary core, Huygens - named after the Dutch scientist Christiaan Huygens - will make a historic exploration of one of the strangest moons in the solar system.

Titan is largely obscured by a thick photochemical smog but scientists believe its dense atmosphere of methane and nitrogen could be very similar to that of the Earth four billion years ago. Professor John Zarnecki of the Open University said: "Examining Titan is our chance to go back in time and see some aspects of the Earth's origins. In one sense going to Titan is like studying our own early history here on Earth."

Huygens' heat shield is designed to slow it down enough for it to deploy a set of parachutes that will soften its descent while onboard instruments take readings. Scientists are unsure about the surface of Titan - it might be solid, liquid, or anything in between. If the probe lands in a sea of liquid methane it will probably not float for longer than half an hour, and even less if it is buffeted by large waves.

Although Titan is rich in hydrocarbons such as methane - the building blocks of life - scientists do not expect to find anything living there as temperatures never rise above minus 180C (minus 290F).