Comet collision more spectacular than expected: Astronomers delighted with explosive impact as fragment makes Earth-sized hole in planet Jupiter's atmosphere

COMET Shoemaker-Levy 9 punched a hole the size of the Earth through Jupiter's atmosphere on Saturday evening, releasing far more explosive energy than astronomers were expecting.

Five of the 21 fragments in the 'string of pearls' that make up the comet had hit the giant gaseous planet by Sunday evening, in a display that had astronomers jumping for joy. Early images showed the cosmic collision, billed as a once-in-a-millennium event, was far more spectacular than most had dared hope.

Fragment A is thought to be one of the smallest of the string, and observers are anticipating a week of fireworks as the other pieces crash into Jupiter between now and Friday. Two more should have hit early this morning - one thought to be 25 times more massive than the first. One of the most spectacular collisions is expected on Wednesday, coinciding with the 25th anniversary of man's first step on the Moon.

The first fragment - about 1km across - hit Jupiter at 9.20pm on Saturday. It plunged down through the planet's outer atmosphere at a speed of more than 200,000km an hour (138,000mph). Once it had travelled beneath Jupiter's ammonia crystal clouds, the fragment collapsed under the force of its own shock waves, creating a gigantic explosion. Astronomers believe this reached the equivalent of at least 10 million megatons of TNT - or 25,000 Hiroshimas.

Dave Laney of the South African Astronomical Observatory (SAAO), in Sutherland, said he did not expect the explosion to be visible for so long. 'That is a big surprise. A 20-minute fireball is not what the astronomers were looking for. They were looking for a quick flash,' he said.

If the fragment had collided with Earth it would have left a crater about 20km across, with devastating effect for many hundreds of kilometres beyond. It would also have brought global climate changes.

The explosion sent a huge fireball outwards, rather like a nuclear explosion, creating a plume that rose 1,000km (the distance from London to Marseilles) above Jupiter. This lasted for about nine minutes before it collapsed back into the bubbling gases below. The explosion should have sent shock waves thundering out, but astronomers will have wait a day or so before they can analyse data collected by the Hubble Space Telescope for signs of these.

Astronomers are divided over what to expect from Wednesday's collision. Since the fragment is far larger it could travel deeper towards the centre of Jupiter, so any explosion may be less impressive.

The Hubble telescope, parked in space watching the far side of the planet where the comet hit, has sent back pictures of the impact itself. These show what look like waves around the crater, marking where debris is falling back into the hole. Ground-based telescopes had to wait for Jupiter to turn to face the Earth before they could pick up the 'hot spot' - reaching temperatures of around 30,000C - that marks the aftermath of the crash.

The patches on the images showing the collision sites are far brighter than astronomers expected, and have survived for longer - the first was still clearly visible after Jupiter had completed one full 10-hour rotation. Images have come in from telescopes around the world. The best include pictures from South Africa, Spain and Chile. Astronomers in Britain were disappointed because viewing conditions were so poor - Jupiter was very low in a hazy sky.

Astronomers are now predicting that the collisions will leave semi-permanent features on the planet's surface, and create long- lasting changes to its weather patterns. 'That whole band of latitude is going to be pock-marked,' one of the Hubble astronomers said yesterday.

In infra-red pictures from the SAAO, the collision sites show up as bright, white spots, because they are so hot. In reality, they are very dark, and include remains from the comet itself, mixed with chunks of material dredged up from deep inside the planet. These act as a sort of geological sample of Jupiter because astronomers can analyse the light they give out for clues to what lies beneath its cloudy surface.

The comet was first spotted on 24 March 1993 by a team of three astronomers, Carolyn and Gene Shoemaker and David Levy. Mrs Shoemaker was the first to spot it on astronomical plates. Yesterday, all three celebrated the spectacular finale to their discovery with champagne.

Mr Shoemaker said he was 'enormously relieved' that the event had not proved the damp squib some were predicting. 'These are fairly big objects that are giving us a good show,' he said. When asked how this compared with preparations for the Moon landing 25 years ago, he said. 'What's different with this is we didn't know how nature was going to perform. We are just elated because nature has outdone herself.'

Yesterday, Mr Levy said he was 'thrilled to pieces'. 'It's such a rare night when nature calls you on the phone and says, 'I'm going to drop 20 comets on Jupiter . . . all I want you to do is watch' . . . We are watching with everything we've got tonight, and nature winked at us.'

Mr Shoemaker described the comet pieces as made up of ice, rocks and organic material mixed into a fizzing, tarry gunk. Scientists at Nasa, the United States space agency, believe the first and third fragments were of roughly the same size. The second was probably a cluster, or swarm, of smaller pieces of rubble which had less effect on Jupiter. He hopes data from the collision will help settle an astronomical dispute over whether comets are solid, or 'porous and fluffy'.

Alan Fitzsimmons, a lecturer in astrophysics at Queen's University, Belfast, said the impact was far better than he had expected. 'We had this major uncertainty which was we didn't know the exact size of these fragments. Personally I was pessimistic, but now we have several images of the fireball and the hot spots . . . all its going to be a very interesting week.'

Dr Fitzsimmons is one of a team of British astronomers analysing images from a telescope on La Palma in the Canaries.

Two amateur astronomers in Israel, using only 4- and 10-inch telescopes, reported seeing flashes on Jupiter, although none of the professional centres have seen these.

Nevertheless, keen amateurs are expected to be out in force this week to see if they can spot bright flashes on the planet as the remaining fragments crash. (Photograph omitted)