Robot's Odyssey Across Space Reaches Saturn

Cassini, the most expensive robot space probe ever built, will this week make its final crucial approach to Saturn after a seven-year, two-billion-mile journey across the solar system.

On Thursday, the $3 billion spacecraft will slip between two of the planet's rings and ignite its main motors for a critical 96-minute burn. If successful, the rocket blast will send the 5.5 tonne probe spiralling into orbit round Saturn, where it will begin a four-year investigation of the mysterious planet and its family of moons.

Nasa-built Cassini, which is larger than a double-decker bus, is also carrying a car-sized mini-lander called Huygens. This probe, constructed by the European Space Agency (Esa) and containing many British-built instruments, will be released on Christmas Day and will be sent spinning towards Titan, Saturn's main moon, for a landing in January.

Scientists believe Titan may be lapped by seas of liquid ethane. 'Titan's atmosphere is similar to Earth's primitive atmosphere four billion years ago,' said Professor John Zarnecki, of the Open University. 'Examining Titan is our chance to see some aspects of Earth's origins.'

Cassini was launched from the Kennedy Space Centre in October 1997 and has spent the intervening years zigzagging across the solar system, making two fly-bys of Venus, one of Earth and one of Jupiter, in order to build up enough velocity to reach Saturn, the second-biggest of the Sun's family of planets. Three previous US space probes have flown close to Saturn, but Cassini will be the first to orbit the planet and make lengthy observations.

The probe has already swept over Phoebe, one of Saturn's moons, and sent back images that suggest this strange, crater-pitted world, only 125 miles in diameter, may have begun life as a passing comet that was captured by Saturn's gravitational field.

Yesterday Cassini beamed back its its first close-up images of Titan. This week the craft will continue plunging towards Saturn at 3.2 miles per second, until Thursday, when it will sweep over the planet at only 12,500 miles before its engines blast it into orbit.

The next day the probe will make the first of its 44 planned fly-bys of Titan.

During its mission, Cassini will orbit Saturn 75 times and send back more than 300,000 colour images of Saturn and its moons.

The main craft will also relay more than a thousand pictures that will be sent back by Huygens as it descends, on a parachute, through the dense, orange smog that forms Titan's atmosphere. (The probes are named after Jean-Dominique Cassini who discovered several of Saturn's moons and the main gap in its ring system, while Christiaan Huygens first realised that Titan was a moon of Saturn.)

However, researchers are still unsure whether the Huygens probe will land on ice or rock, or whether it will float on a sea of liquid gas. Some observations suggest Titan is battered by hurricanes and that it may have oceans swept by giant 10-yard waves.

'We have a great deal that we wish to learn, but only a very short time to collect our data,' said Jean-Pierre Lebreton, project scientist for Huygens.

'The batteries on Huygens will provide power for only three hours once they are turned on at the beginning of its descent. This will be our only opportunity to study Titan for many decades, so will have to make the very most of it.'

Huygens will be carrying more than scientific instruments, however. It will also transport the recordings of four pieces of music - 'Hot time', 'Bald James Deans', 'Lalala' and 'No love' - composed by French musicians Julien Civange and Louis Haéri.

This will be the furthest distance at which human-made sounds, which are intended to leave a trace of human culture in the unknown, will have landed on another celestial body.