There is a one-in-10,000 chance that an asteroid or comet, more than two kilometres in diameter, will collide with Earth in the next century, killing a large proportion of the population, according to space scientists.

An article in the British journal 'Nature' says the risk is great enough to justify a space surveillance system that would warn scientists of approaching objects and allow them to deflect them with nuclear explosions.

Dr Clark Chapman, from the Planetary Science Institute in Arizona, and Dr David Morrison, from the NASA Ames Research Centre in California, say a person living in the United States has a greater chance of dying from a comet impact (one in 20,000) than being killed in a flood (one in 30,000) or a one-in-100,000 risk of death from a venomous bite or sting.

Earth experiences a constant barrage of cosmic debris. Small particles burn up as visible meteors high in the atmosphere and reach the ground as individual meteorites. The researchers say there have been no recorded deaths from meteorite falls, although cars have been hit a few times, the most recently by the Peekskill meteorite on 9October 1992.

Earth's atmosphere represents a significant barrier to comets and asteroids. Most cosmic projectiles break up and are consumed before they reach the lower atmosphere. The US scientists say that one of these fireballs, with the energy of the Hiroshima atomic bomb, or .015 megatons, occurs annually. A few events of megaton energy occur each century.

"We are generally unaware of these events," the 'Nature' report said, "for most occur at very high altitudes and their shock waves do not reach the ground, although they have been observed from surveillance satellites."

Sixty-five million years ago a projectile, more than 10 kilometres in diameter, hit Earth. The crater, called Chicxulub, in Mexico, is 180 kilometres in diameter. The 'Nature' authors say the impact caused the extinction of more than half the species on Earth.

"It apparently resulted in devastating wildfires and changes in atmospheric and oceanic chemistry as well as dramatic short-term perturbations in climate" produced by dust in the air, the authors said.

They acknowledge that many hundreds of thousands of years pass between fatalities caused by comets and asteroids similar in size to the one that caused Chicxulub.

But they estimate that a 200,000-megaton event, capable of killing 1.5 billion people, occurs every half a million years.

They say that in the future there is a one per cent chance of an impact of more than 1000 megatons, or the equivalent of one-tenth of the world's combined nuclear arsenal. This could have happened in 1989 had asteroid 1989FC struck Earth rather than passing by at less than twice the distance to the moon.

The researchers say potential impactors can be identified and tracked.

In 1992, NASA held a near-Earth object detection workshop, which proposed a spaceguard survey. Over 20 years this survey would list all potentially threatening asteroids large enough to precipitate a global catastrophe.

The survey would cost $67million for capital construction and $14 million a year for operations. The researchers say the warning time offered by the survey would be several decades for asteroids down to a few months for some comets.

"Should such an object be found ... action could be taken to mitigate the hazard," the 'Nature' article said.

The researchers propose nuclear explosions aimed at deflecting rather than destroying the meteoroids. "It would be important to avoid fragmenting it into even more damaging swarms of pieces ... Even if the warning is too short for deflection ... people near the anticipated point of impact could take shelter or be evacuated."