US-European Mission Explores Saturn's Moon Mysteries

Mimas, one of the 33 moons of Saturn, suspended against the blue-streaked backdrop of the giant planet's northern hemisphere, is snapped by a departing spacecraft.

Cassini-Huygens is a joint Nasa-European mission to explore Saturn, its rings and satellites, and it has just made a close approach to the planet before swinging away for another loop around Saturn's mysterious moon Titan.

Mimas is about 250 miles in diameter. The bright blue swath closest to it is created by sunlight as it passes through a 3,000 mile gap in Saturn's rings known as the Cassini division. Shadows of thin ringlets within the division are also visible, and the dark band across the centre of the image is the shadow of Saturn's B ring, the densest of all.

The study, combined from images taken through red, blue and green filters, is one of thousands that will soon begin to unravel the riddle of Saturn and its orbiting hoops of ice, dust and rubble, less than a mile thick but flaring into space for more than 180,000 miles.

But while Cassini will spend the next four years looping around the planet, its robot passenger, Huygens, faces a short, brilliant career lasting a few hours. On Christmas Day, Cassini will release its passenger and send it gently on a collision course with the only moon in the solar system with an atmosphere.

On January 14, as it nears the thick veil of methane, ethane, ammonia and nitrogen around its target, the European part of the mission will wake up, to prepare for a 2 hour descent through the clouds, ultimately to perish on the unforgiving surface of a moon with an average temperature of around minus 170C. It will signal its adventures back to its mothership, which will relay them to Earth across 934m miles of space. By the time scientists get the first data, Huygens will be silent.