Saturn orbiter mission makes final approach

The international Cassini-Huygens spacecraft is on final approach to Saturn in a multi-agency mission that includes British involvement that will see the probe commence a four-year tour of Saturn, its rings and moons.

After a 2.2 billion-mile journey lasting seven years, the 5.6 tonne Cassini probe, the largest man-made object ever sent to another planet, is scheduled to enter orbit around Saturn on June 30 this year. But on June 11, Cassini - which has already returned the most detailed-ever pictures of Saturn - will have a close encounter with an object in the Saturn system; a 2000km fly-by of a small moon of Saturn called Phoebe.

One of Saturn's 31 known moons, Phoebe is thought to be a captured object, probably originating from a region known as the Kuiper Belt. In a retrograde orbit and of atypical appearance, the small moon with a diameter of 220km is about one-fifteenth the size of Earth's moon and scientists are keen to see the object close-up.

Launched on October 15 1997, the £2 billion mission has 12 instruments on the Cassini orbiter and six more on the Huygens probe, representing the best technical efforts of 260 scientists from the US and 17 European nations. The Cassini-Huygens mission will study Saturn's rings, magnetosphere and moons - in particular Titan, the planet's largest moon.

For the critical Saturn orbit insertion manoeuvre, the spacecraft will fire its main engine for 96 minutes to reduce Cassini's speed and allow it to be captured as a satellite of Saturn. If all proceeds according to plan Cassini will pass through a gap between two of Saturn's rings and swing close to the planet to begin the first of 76 orbits around the Saturn system.

The sixth planet from the sun, Saturn is the second largest planet in the solar system. A study of the dynamics and interactions among Saturn's elaborate rings and numerous moons is expected to provide valuable data to aide understanding of how the solar system's planets evolved.

The study of Titan, Saturn's largest moon, is one of the major goals of the mission. Cassini will execute 45 flybys of Titan, the closest at approximately 590 miles above the surface. This will permit high-resolution mapping of the moon's surface with an imaging radar, which can peer through the opaque haze of Titan's upper atmosphere.

Cassini will release the wok-shaped Huygens probe on its 20-day journey toward its encounter with Titan on December 25 2004.

At a press conference yesterday lead scientist for the surface science module of the Huygens' probe, John Zarnecki, said that the mission was "far and away the most distant controlled descent ever attempted".

(SP)