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A group of the world's leading scientists has urged the United Nations to establish an international network to search the skies for asteroids on a collision course with Earth.
The spaceguard system would also be responsible for deploying spacecraft that could destroy or deflect incoming objects.
The group - which includes the London-based Royal Society president Lord Rees and environmentalist Crispin Tickell - said that the UN needed to act as a matter of urgency. Although an asteroid collision with the planet is a relatively remote risk, the consequences of a strike would be devastating.
An asteroid that struck the Earth 65 million years ago wiped out the dinosaurs and 70 per cent of the species then living on the planet. The destruction of the Tunguska region of Siberia in 1908 is known to have been caused by the impact of a large extraterrestrial object.
"The international community must begin work now on forging three impact prevention elements - warning, deflection technology and a decision-making process - into an effective defence against a future collision," said the International Panel on Asteroid Threat Mitigation, which is chaired by former American astronaut Russell Schweickart. The panel made its presentation at the UN's building in Vienna.
The risk of a significantly-sized asteroid - defined by the panel as being more than 45m in diameter - striking the Earth has been calculated at two or three such events every 1000 years, a rare occurrence, though such a collision would dwarf all other natural disasters in recent history.
The panel added that developments in telescope design mean that by 2020 it should be possible to pinpoint about 500,000 asteroids in orbit round the Sun and study their movements. Of these, several dozen will be revealed to pose threats to Earth, the panel added.
However, the group warned it would be impossible to predict exactly which of these "at-risk" asteroids would actually strike until it was very close to our planet. By then, it would be too late to take action.
As a result, the panel said it would be necessary to launch missions to deflect or destroy asteroids that have only a one in 10, or even a one in 100, risk of hitting our planet.
"Over the next 10 to 15 years, the process of discovering asteroids will likely identify dozens of new objects threatening enough that they will require proactive decisions by the UN," the report added.
In addition, such missions will have to be launched well ahead of a predicted impact, so that slight deflections by spaceships can induce major changes in an asteroid's paths years later. The world will not be able to rely on Bruce Willis saving it from an asteroid at the last minute as he does in the movie Armageddon, in other words. Considerable planning and forethought will be needed.
Funding such missions will therefore require far greater investment than is currently being made by international authorities. At present, about US$4 million ($7.5 million) a year is spent by Nasa on asteroid detection, while the European Space Agency's planned mission to study the asteroid Apophis - which astronomers calculate has a 1 in 45,000 chance of striking the Earth this century - is likely to be a modest project costing only a few tens of millions of dollars.
By contrast, any effective protection system will require funding of about US$100 million ($187 million) a year to provide a full survey of the skies, combined with investment in spacecraft that can reach an asteroid and then deflect it.
This would be achieved either by crashing the spacecraft on to the asteroid or by triggering a nuclear explosion in space.
However, the cost of such missions should not be used as an excuse for failing to act, added the panel. "We are no longer passive victims of the impact process.
We cannot shirk the responsibility."
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