Spacecraft Nears Climax of 2bn-mile Voyage

This morning, if all went to plan, Saturn gained a new moon. At the climax of a seven-year, 2.2bn-mile journey, a giant US-European spacecraft named Cassini-Huygens sailed between two of the outer rings of Saturn, turned, fired its rocket engine for 96 minutes, and slowed down to become a prisoner of the planet's gravitational field.

Cassini will now go into a series of 76 orbits around Saturn, and in the next four years it will explore six of Saturn's 31 other moons.

On Christmas Day, in a space spectacular in the planning for at least 15 years, the Huygens probe will separate from Cassini, and the two will make for Saturn's moon Titan. Three weeks later, Huygens will make a suicide dive into the murky, orange clouds of Titan - the only moon in the solar system to have an atmosphere - and spend its last minutes of existence relaying precious data back to Earth via the Cassini mothership.

The mission is one of the most ambitious ever. Cassini is a plutonium-powered carrier the size of a small bus, launched in 1997. To rendezvous with Saturn, it had to make a series of "gravity slingshot" fly-bys of Venus, Earth and Jupiter to gain speed for its epic journey. Two weeks ago, it flew by Phoebe, a small ball of ice and mud probably captured by Saturn as it wandered through the solar system. On Sunday Cassini burst through Saturn's plasma "bow shock" wave - the region where debris from the sun collides with Saturn's magnetic field. Early this morning Cassini reached the rim of Saturn's rings, to begin a 98,000-mile flight over the mysterious orbiting concentric planes of ice and rock.

The mission so far has cost more than £2bn. It was put together by three space agencies: those of the US, Europe and Italy. It takes its names from two 17th century astronomers: Jean-Dominique Cassini, the Italian who named four of Saturn's moons; and Christiaan Huygens, the Dutchman who discovered Saturn's rings and first saw the moon Titan. The mission is the first to get close to Saturn since Voyager 30 years ago.

Cassini's first pass over Saturn took it to 12,000 miles above the cloud tops - the closest it will get to the planet itself during the entire mission. It carries cameras, and could send back the first pictures of the "dark side" of Saturn's rings this afternoon.

Cassini will study the magnetic field and monitor the 12,000mph winds that race round Saturn's equator. In hugely elliptical orbits, it will make fly-bys of Titan, Enceladus, Hyperion, Dione, Rhea and Iapetus. Huygens, for the next six months a mute passenger on Cassini, will send back data as it parachutes into Titan's dense atmosphere of nitrogen, methane and ethane.