Surprise for Stardust Mission

A Nasa spacecraft sailed within 150 miles of a comet to take a closer look at one of the solar system's most ghostly visitors. Instead of a grubby, fluffy object of snow and dark gravel, Comet Wild-2 proved to be a miniature world marked by broad mesas, craters and canyons with flat floors and sheer walls.

The Stardust mission was launched in 1999 on a trajectory that would eventually lead it to an encounter with Wild-2 on the far side of the sun. On January 2 it flew through the comet's tail and caught particles of comet fabric streaming off at 13,000mph. It will parachute these to Earth in January 2006. But 72 photographs of Wild-2's nucleus have revealed an object like nothing else in the solar system

"We were expecting the surface to look more like it was covered with powdered charcoal," said Donald Brownlee of the University of Washington.

Instead, the Stardust team report in the journal Science today that they found a clump of brittle material three miles in diameter, that had preserved the stamp of any object that had hit it during its 4.5bn-year career through the solar system. The researchers identified one type of crater with a central pit and rough surrounding terrain, and another with a flat floor and vertical cliffs. They dubbed two craters Right Foot and Left Foot for their resemblance to footprints.

But what they could not detect was the characteristic layer of fine powder found around impact craters on the moon or other bodies in the solar system. The guess is that any debris dislodged by an impact would have flown off into space.

Comets are the rubble left over from the making of the solar system. They are believed to have delivered the water for the Earth's oceans, and they contain a mix of organic compounds that may have been important in the formation of life on Earth.

They normally have huge orbits far from the sun. Every now and then gravitational disturbance tips a comet towards the centre of the solar system, where it forms its distinctive "tail" in the heat from the sun's rays.