It's mind-boggling and stupendous

Dr Laurence Soderblom, DS1's imaging team

Astronomers are already saying that the pictures of the 10-kilometre- (6-mile-) wide core of Comet Borrelly will revolutionise our understanding of these frozen wanderers.

DS1 passed within 2,200 kilometres (1,400 miles) of the comet's rocky, icy heart late on Saturday GMT.

It sent back black-and-white photos, as well as data on gases and infrared waves around the comet, and how the gases interact with the solar wind (the process that drives a comet's characteristic tail).

"Deep Space 1 plunged into the heart of Comet Borrelly and has lived to tell every detail of its spine-tingling adventure," said project manager Dr Marc Rayman. "The images are even better than the impressive images of Comet Halley taken by Europe's Giotto spacecraft in 1986."

Better understanding

"Up to Saturday night, we had only one example of a comet's nucleus. Now, we have another one, and with it a much better understanding of comets," said Dr Don Yeomans, of the American space agency's (Nasa) Jet Propulsion Laboratory, at a press conference to unveil the images.

Data will give scientists a better idea of how the solar wind plays with a comet

Dr Rayman added: "After years of nursing this aged and wounded bird along - a spacecraft not structured to explore comets, a probe that exceeded its objectives more than two years ago - to see it perform its remarkably complex and risky assignment so well was nothing short of incredible."

"It's mind-boggling and stupendous," said Dr Laurence Soderblom, the leader of DS1's imaging team. "These pictures have told us that comet nuclei are far more complex than we ever imagined. They have rugged terrain, smooth rolling plains, deep fractures and very, very dark material."

To see [DS1] perform its remarkably complex and risky assignment so well was nothing short of incredible

Dr Marc Rayman, project manager

The Comet Borrelly images have thrown up several surprises. As DS1 flew through the coma, the cloud of dust and gas surrounding the nucleus, scientists had expected that the solar wind would flow symmetrically around the cloud, with the nucleus in the centre.

But they found that although the solar wind was indeed flowing symmetrically around the cloud, the nucleus was off to one side, shooting out a great jet of material. "The shock wave is in the wrong place," said Dr Rayman. "We have to understand that."

Varied terrain

Dr David Young, of the University of Michigan, added: "The formation of the coma is not the simple process we once thought it was. Most of the charged particles are formed to one side, which is not what we expected at all."

It was a job DS1 was not designed to do

Holding up the first image at the media conference, Dr Rayman said: "Earth, meet Borrelly." Several scientists said that the new images were better than those taken of Comet Halley by Giotto, the only previous comet fly-by that produced detailed scientific data.

The highest-resolution image of the nucleus of Comet Borrelly shows a variety of terrain, including mountains and fault structures. Darkened material is visible over the surface.

Scientists speculate, on the basis of what they have now seen, that the comet may split apart in the future.

Only one other probe has obtained data to match that returned by DS1

There are cross-cutting fractures in the neck of the dumb-bell-shaped nucleus. Jets of material come from the neck, so it seems they are eroding away a saddle of material. Eventually the nucleus could be so weakened that it could calve in two, said one scientist.

DS1 finished its main technology-proving mission two years ago, and will end its usefulness in November when all its fuel is consumed.

The probe was not built specifically to encounter comets. That it has succeeded in gathering close-up information on Borrelly is therefore a tremendous fillip to Nasa and the project scientists and engineers at the Jet Propulsion Laboratory in California.

Comet Borrelly is currently 200 million kilometres from the Sun.