An asteroid colliding with Earth could wipe out humanity. Alison Goddard reports on research to curb the threat

In 1908, an asteroid exploded above the Tunguska area of Siberia with the energy of about 1,000 Hiroshima bombs. If it had hit central London, everything within the M25 would have been destroyed. Now astronomers are warning that such an event is likely to happen once every century.

Governments are not noted for acting on long-term risks. But science minister Lord Sainsbury recently announced the creation of a task force to assess the threat of an asteroid hitting the Earth. "The government has realised there is a problem to be addressed," says Mark Bailey, director of the Armagh Observatory and a member of Spaceguard UK, which campaigns about the dangers posed by asteroids.

The Health and Safety Executive has developed what it calls the national scrutiny line, which measures risks in terms of fatalities. If a British nuclear power station lay above this line, it would be closed. By the same measure, asteroid impacts are also unacceptably dangerous.

ÖйúAƬ

Nuclear risk assessor Nigel Holloway of AWE Aldermaston explains: "The risk of asteroid impact is about as likely as the accident at the Chernobyl nuclear power station. We estimate that an accident like Chernobyl could happen every few hundred years, and that it would cause between 1,000 and 10,000 premature deaths. A large asteroid could hit the Earth once every 100,000 to a million years, and at least ten million people would die in the United Kingdom alone."

The risk posed by an asteroid crashing into the Earth is unique. "First, the risk to civilisation is mass extinction; secondly, the risk is predictable, years or decades in advance; and thirdly, it is avoidable. The means exist to mitigate the threat, provided we have enough warning. These features highlight the urgency of developing a global spaceguard programme," says Bailey.

ÖйúAƬ

In the United States, work has already begun on identifying large asteroids. The project, run by the Massachusetts Institute of Technology, is funded by the US Air Force. Using a telescope based at the White Sands Missile Range in Socorro, New Mexico, astronomers are searching for asteroids one kilometre across and bigger - the size that, according to asteroid theory, wiped out the dinosaurs.

A European research project should look for smaller asteroids hundreds of metres across, Bailey believes. A 1.5-metre telescope in the southern hemisphere, sweeping the entire sky, would cost about Pounds 1 million a year to build and run, he says. Perhaps in three or four years' time it could be upgraded to a four-metre wide field telescope costing about Pounds 20 million.

Benny Peiser, a social anthropologist at Liverpool John Moores University, has studied the consequences of past asteroid impacts. "For objects 100 metres across, the environmental effects would be regional but the social and political impact would be much worse: people would be very scared. The likelihood of such an object hitting the Earth in our lifetime is great. There is little we can do about it, except make the public aware that they might have to deal with such an event."

No one has ever destroyed an asteroid, but Bailey says there is a range of measures that could be taken. "For example, if an asteroid 300 metres across hit the Atlantic, it would create a huge tidal wave. If we removed the population from coastal areas, it would save millions of lives."

ÖйúAƬ

There are several proposals for how to avoid asteroid impact, ranging from firing nuclear weapons at asteroids to mining them. "We have evolved to the extent that mankind has the technology to take hold of its future; we are not entirely in the hands of nature," Peiser says.

Bailey comments: "There are proposals on paper but we would need time to develop them. It is a bit like putting a man on the Moon, which took ten years. And the earlier we start the survey, the more notice we will have of a potential disaster."

THE CHANCES OF AN ASTEROID ATTACK - AND THE HORRIFIC RESULTS

Asteroid diameter Frequency of impact Consequences

50 to 100 metres Once every 100 years Could devastate a city

ÖйúAƬ

200 to 300 metres Once every 5,000 years Destroy a small country

1,000 metres Once every 100,000 years Kill a quarter of the world's people

ÖйúAƬ

10,000 metres Once every 30 million years Mass extinction