Comet heads for fireball collision: Steve Connor on the cosmic impact of 21 'giant snowballs', each with the explosive energy of a million Hiroshima atom bombs, on the planet Jupiter

HUGE dirty snowballs, some the size of mountains, will begin smashing into the planet Jupiter this Saturday evening with the explosive energy equivalent to a million Hiroshimas, astronomers said yesterday.

At about 9pm British Summer Time, the first of 21 fragments of the Comet Shoemaker-Levy 9 will crash into the Jovian atmosphere creating an intense fireball out of the comet's core material of dust and ice. Over the next six days the remaining fragments will hurtle into the planet at 135,000 mph, creating the largest set of explosions in the Solar System witnessed by scientists.

Although it will be the first cosmic collision astronomers have been able to predict well in advance, only those with the biggest telescopes are likely to see anything. Jupiter is 540 million miles from Earth and a million Hiroshimas will be difficult to see even with a good telescope. In addition, the cometary fragments will collide with the side of Jupiter furthest from Earth.

Professor Iwan Williams, an astronomer at Queen Mary and Westfield College in London, said: 'We will not see any of the impact at all and almost none of the effects. What we can hope for is to see the impact sites about 20 minutes after the event as Jupiter rotates to face us.'

Comet Shoemaker-Levy 9, named after the American astronomers who discovered it in March 1993, is believed to have been knocked off its orbit around the Sun by the huge gravitational pull of Jupiter, which is 11 times the Earth's diameter and 300 times its mass. As the comet flew past the planet in 1992, Jupiter's gravity split it into the fragments that astronomers can now see. Its new orbital trajectory also brought it onto a collison course with the giant planet itself.

Astronomers are not sure what will happen as the comet hits Jupiter, but one question they hope to resolve is whether the planet's core is made of 'metallic hydrogen', a solid that can only exist under intense pressures.

What they are certain of, however, is that there will be absolutely no effects from the impacts beyond the immediate vicinity of Jupiter. Patrick Moore, the broadcaster and amateur astronomer, said that people who suggested the collision would be felt on Earth were 'a few sandwiches short of a picnic'.

The Royal Astronomical Society said it was unlikely the public would see anything. 'Professional telescopes or the larger instruments used by experienced amateur astronomers will be needed to have a hope of seeing any change to Jupiter.'

Although most astronomers believe impacts between comets and planets are almost inevitable, they say the chances of it happening with Earth are so remote that it is not worth worrying about. 'If you go out onto the street you face a far greater risk, so don't worry,' David Hughes, lecturer in physics at the University of Sheffield, said.

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