World watches the skies in hope of something 'cosmic'

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The Independent Online
THE WORLD should see today the first pictures of what has been billed as the biggest cosmic event in the solar system witnessed by humans.

At about 9.15pm last night, the first of 21 fragments of the comet Shoemaker-Levy 9 collided with Jupiter, releasing the explosive energy of up to a million Hiroshimas. The last of the 'string of pearls' - their appearance through a telescope - will hit the planet this Friday.

Nasa said the total energy released by all 21 fragments could be as great at 100 million megatons of TNT, 'roughly 10,000 times the total destructive power of the world's nuclear arsenal at the height of the Cold War.

'The impacts will be as energetic as the collison of a large asteroid with Earth 65 million years ago. This cosmic catastrophe is believed to have led to the extinction of the dinosaurs and hundreds of other species.'

Astronomers warned, however, that despite the enormous energy of the collisions, only the most powerful telescopes have any chance of detecting the fireballs they create. Jupiter is 540 million miles away and the collisions are occurring on the far side of the planet. Only their indirect effects will be visible as the planet rotates to face Earth.

Early this morning Nasa scientists should nevertheless have received the first pictures of the initial collision from the best seat in the house, the repaired dollars 1.5bn ( pounds 1bn) Hubble Space Telescope, which is parked in a high orbit above Earth. Dozens of terrestial observatories, from Hawaii and Chile to South Africa and the Canaries, will be training their telescopes on the event. The first images from them are also expected to be processed today.

In addition to this armoury of telescopic power, data from a handful of other spacecraft monitoring Jupiter will be gathered over the next few days for transmission to Earth.

The Galileo spaceprobe, which is 143 million miles from the far side of Jupiter, will have a direct view, but we will have to wait weeks for that.

Three American astronomers - Carolyn and Gene Shoemaker and David Levy - discovered the comet on 24 March 1993 on a photograph taken with the Schmidt telescope on Mount Palomar in California.

'I did not know what I had but to me it looked like a squashed comet,' said Carolyn Shoemaker. 'It was the strangest thing that I or anyone else had ever seen,' said her husband Gene. 'In the zoo of the sky, this comet is a unicorn,' said David Levy.

A more powerful telescope found the elongated comet was in fact a series of 21 fragments. Astronomers believe the original comet - which has probably existed since the birth of the solar system some 4.5 billion years ago - fragmented in 1992 when it came close enough to Jupiter for the planet's gravity to pull apart its soft ice and dust.

Astonomers believe that comets are 'building rubble' left over from the birth of planets. They consist of a nucleus of ice and dust - a dirty snowball. Their tails are trails of vaporised gases created by the heat from the sun, around which they usually orbit.

Because comets lose material permanently, they eventually become defunct unless, like Comet Shoemaker-Levy 9, they are captured by a passing planet.

Jupiter is the giant of the solar system and it is no accident that it has attracted a comet. It is almost 11 times the diameter of the Earth and 300 times the mass. Jupiter is, in fact, about 2.5 times heavier than all the other planets combined and so exerts an enormous gravitational attraction.

Scientists estimate the fragments of Comet Shoemaker- Levy 9 could be up to 2.5 miles wide and be travelling at 135,000mph. As they begin to hurtle into the Jovian atmosphere, the ice and dust starts to boil, completely vaporising within seven seconds when it reaches a temperature of 2,700C.

Whether the resulting fireball is intense enough to see from Earth depends largely on whether the fragments of the comet split into smaller pieces before reaching Jupiter. One pessimistic forecast is that the fragments will divide into hundreds of objects before impact, producing nothing more spectacular than a shower of meteorites.

Iwan Williams, professor of astronomy at Queen Mary and Westfield College in London, said that some of the fragments are nothing more than loose agglomerations of other fragments. 'They are evolving all the time.' Two of the original 21 have already disappeared, but two more have spilt into two, keeping the total number the same as when they were first counted, he said.

Another complication is the Jovian atmosphere, which is a maelstrom where winds can whip up to 270mph. Scientists are unsure what lies below clouds of ammonia crystals. One suspicion is that the core of the planet is composed of metallic hydrogen, which only exists under intense pressures.

They hope that by 'listening' to the reverberations of the impacts, they will be able to resolve whether this strange substance exists or not.

Computer simulations of the impacts show waves travelling outwards from the impact sites and propagating around the planet in the days following. Such 'inertia waves' are analogous to the ripples from dropping a pebble in a pond. Other theorists, however, predict that the impacts from the comet will produce waves which will be like the seismic shocks from an earthquake. If so, the atmosphere of Jupiter will 'ring like a bell', they claim.