After six-year odyssey, probe arrives at Saturn

After a journey of more than six-and-a-half years, the international probe Cassini will tomorrow start its investigation of Saturn, gliding between two of its rings and entering an orbit around the mysterious planet.

After a journey of more than six-and-a-half years, the international probe Cassini will tomorrow start its investigation of Saturn, gliding between two of its rings and entering an orbit around the mysterious planet.

Having been launched from Cape Canaveral in 1997 and travelled 2.2 billion miles across the solar system, scientists hope the craft's planned four-year investigation of the planet and its 31 known moons will provide new information on the origins of its rings and explanation of their colours.

In that time Cassini, a $3bn (£1.6bn) joint US-European venture, will orbit the Saturn system 76 times and have 52 close encounters with seven of its moons. One of the highlights of the project should be the landing of a mini-probe called Huygens on Saturn's largest moon, Titan, in January 2005. Titan is the second-largest moon in the solar system and the only one with a stable atmosphere. Scientists are unsure what the Huygens probe, constructed by the European Space Agency, will find. "Titan is like a time machine taking us to the past to see what Earth might have been like," said Dennis Matson, Cassini project member at Nasa's Jet Propulsion Laboratory.

Ed Weiler, at Nasa headquarters in Washington, said: "The system represents an unsurpassed laboratory, where we can look for answers to many fundamental questions about the physics, chemistry and evolution of the planets and the conditions that give rise to life."

Cassini is already sending back remarkable pictures of its approach to Saturn, whose orbit it will join at 00.47 GMT tomorrow by firing its main engine to create drag and drift towards the planet. As it passes a gap in the planet's rings it will use its large main antenna as a shield against possible debris.

At 1.12 GMT the engine should start a 96-minute burn to cut Cassini's speed by 1,400mph. That should put it in an elongated orbit.

If something goes wrong, however, Cassini will fly right past Saturn and into space. Mission control would be helpless to intervene because it takes more than an hour for signals from Earth to reach Saturn.

The engine burn should end at 2.48 GMT. At this point Cassini should have reached its closest point to the planet, just 12,1243 miles above Saturn's multi-coloured clouds.

Most of what is already known about the planet is due to the Voyager explorations in 1980-81 and the Pioneer II project. Cassini will be the first probe to orbit the planet and conduct extensive observations.

Its twin probes are named after the 17th-century astronomers Christiaan Huygens, who discovered Titan and Saturn's rings, and Jean- Dominique Cassini, who pinpointed several other of the planet's moons and a gap in its ring system.

Saturn is the sixth planet from the Sun and the secondlargest in the solar system, with an equatorial diameter of 74,130 miles.