Probe threads safely through Saturn's rings

A space craft the size of a school bus today began an historic four-year mission to explore Saturn and its moons.

A space craft the size of a school bus today began an historic four-year mission to explore Saturn and its moons.

Early today, a Nasa spokesman said the Cassini probe slipped through two of Saturn's rings and set itself in orbit around the giant planet.

The £1.6 billion joint US-European mission is by far the most complex and ambitious unmanned space expedition in history.

Not only does it include the first orbit of Saturn, but also the most distant landing on an interplanetary body ever attempted.

Huygens, a European Space Agency lander carried by Cassini, will parachute down to the surface of Saturn's cloud-covered moon Titan early next year.

The spacecraft was launched in the United States in 1997 and has taken seven years to complete its 2.2 billion mile journey through space.

When Cassini reached Saturn today was expected to be travelling at 70,700 mph. The brakes are then applied by firing the probe's main engine for 96 minutes during a crucial "orbit insertion manoeuvre".

This reduces Cassini's speed and allows it to enter orbit around Saturn.

Cassini passed through a gap between two of Saturn's rings, called the F and G rings, before swinging close to the planet and beginning the first of 74 orbits.

Over the next four years it will have 52 close encounters with seven of Saturn's 31 known moons.

Cassini has already visited one moon, an icy outpost of the Saturnian system called Phoebe. The spacecraft flew past the moon at a distance of 1,243 miles on June 11, sending back detailed pictures of its heavily scarred and cratered surface.

Saturn's most famous feature is its glorious rings, which Cassini will scan from a distance of just 15,000 kilometres. Scientists are anxious to know how the rings, which may only be about 100 million years old, came into being.

One theory is that they were created from an object that was captured and torn apart by Saturn's powerful gravity.

The spacecraft carries two cameras, one of which is so precise that if placed on St Paul's Cathedral it could resolve a 1p coin on top of the London Eye.

Other instruments will study the composition of Saturn's atmosphere, the planet's turbulent weather, and its magnetic field.

Britain is playing a key role in the mission, providing half the 12 instruments on board the Nasa orbiter, and two of the six instruments on the Huygens probe.

Landing on Titan will be the high-point of the mission. On Christmas Day, Huygens will detach itself from Cassini and head towards the moon, arriving three weeks later.

Titan is larger than Mercury or Pluto with a thick smoggy atmosphere whose surface pressure is 50% higher than the Earth's.

The robot probe will think for itself as it parachutes down onto Titan.

No-one knows what it will find, but scientists believe there is a good chance it will make a splash landing in a sea of liquid methane or ethane.

Titan has a surface temperature of minus 180C and is thought to be too cold to sustain life. But scientists think the conditions there mirror those that existed on early Earth billions of years ago.

Professor Ian Halliday, chief executive of the Particle Physics and Astronomy Research Council, said: "Saturn is hailed as the 'jewel' of the Solar System. It has fascinated us for centuries. Right now we stand on the threshold of a wealth of scientific discovery as we unveil Saturn's secrets."