The risk of humankind being devastated by the impact of a meteor is so great that a world network of telescopes should be built immediately to provide an early warning system, an astronomer said last week. And Britain should finance one of the telescopes, says Dr Duncan Steel, research astronomer at the Anglo-Australian Observatory.

Dr Steel claims in a new book, Rogue Asteroids and Doomsday Comets, that a seriously damaging crash is likely to happen every 50-100 years. He also produces, in his book, a new theory about the building of the original Stonehenge, claiming that it was an early monitoring system built when there was a lot of meteoritic activity in the skies.

Dr Steel says that a person is twice as likely to die from asteroid or comet impact (probability of one in 10,000) than from an aeroplane disaster (one in 20,000 for United States residents). Although nobody has ever been killed by a comet, such a disaster, when it happened, would kill millions of people, pushing it to the top of accident statistics in an instant. But when crossing the road, he says, it's best to look at the traffic rather than up at the sky because probability of death is one in 100.

Dr Steel, who was in Europe last week to attend the International Astronomical Union working group on near-earth objects, has assessed the likely devastation from a crash. The impact of an asteroid, one kilometre in diameter, could cause the death of at least 25 per cent of humankind and be expected to happen once every 100,000 years.

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Possible effects of an impact could be a greenhouse effect, cold, winds, fires and acid rain. The impact itself would not necessarily be the worst feature of a disaster. "Most deaths would be from starvation worldwide," he says. The most vulnerable would be the societies that rely most on high technology. "The people who would most suffer would not be people in the third world because they know how to grow their own food and kill their own goats," he says.

He says that six telescopes should be built around the world, at a total cost of $50 million. These would note the orbit of every relevant asteroid of over one kilometre in size. The money should come from defence rather than science budgets.

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If astronomers discover that earth is in danger, the world would have time to build a nuclear rocket that could be sent into space to deflect the asteroid from its orbit: only a slight nudge would be required, he says. "It would be a piece of cake."

Iwan Williams, of the astronomy unit at Queen Mary and Westfield College, and a member of the working group, said that astronomers agree that the sky should be monitored for dangerous objects. The danger has become more apparent in recent years with the discovery and exploration of craters on earth. Also, the collision of Comet Shoemaker Levy with Jupiter last year heightened people's awareness of the possibility of something similar happening to the Earth.

"What astronomers disagree over is the speed at which we should do something about it," he said. "It could be done over the next few hundred years with no extra expenditure but are we prepared to wait?" Dr Steel is one of the most impatient advocates of action. "I want to get to the stage where I can mention this problem and the person on the street doesn't laugh," he said. Dr Steel believes that the probability of disaster is not evenly spread over the centuries but that there are clusters of activity, when times become particularly dangerous. He believes that Stonehenge was built during one of these times, which led to a major cultural change.

"Something was happening in the sky with a regularity that terrified people in diverse and disconnected cultures, leading to a lifestyle change induced by the perceived necessity to achieve certain things."

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This could have been the trigger for major cultural shifts that have led to today's civilisation, he thinks.

Rogue Asteroids and Doomsday Comets, Duncan Steel, Wiley, Pounds 16.99.