Close-up photographs of a comet have shattered the belief that these traditional portents of doom are so-called "dirty snowballs" composed of dust and ice.

Close-up photographs of a comet have shattered the belief that these traditional portents of doom are so-called "dirty snowballs" composed of dust and ice.

Pictures taken by a spacecraft that flew within 150 miles of the comet Wild 2 reveal that the subject is a solid chunk of rock with a spectacularly sculpted landscape.

The images taken by the Stardust spacecraft as the comet made its nearest approach last February clearly show that the comet's 20 square miles is covered in broad mesas, craters, pinnacles and canyons with flat floors and sheer walls.

Scientists led by Professor Donald Brownlee, the Stardust's principal investigator at the University of Washington, were prepared for featureless images of an icy surface coated in dust. "It's completely unexpected. We were expecting the surface to look more like it was covered with pulverised charcoal," Professor Brownlee said.

Instead the Stardust photographs - published in the journal Science - depict a mini world scarred by a series of collisions with other space objects over many millions of years. The scientists involved in the mission have identified two kinds of crater on the comet, one with a central rounded pit and a surrounding rough terrain, the other with a flat floor and steep sides.

Two craters look like footprints, and have been named Right Foot and Left Foot. Unlike craters seen on Earth or the Moon, the craters on Wild 2 are virtually devoid of the powdery debris seen scattered around typical impact craters. Professor Brownlee said that was because there is hardly any gravity on the surface of Wild 2.