Asteroid estimates 'too low'

Nine objects have come close to the Earth since 1991

Current predictions for the number of potentially dangerous asteroids have been underestimated by at least 20%, say astronomers.

According to recent calculations, there are between 750 and 900 asteroids circling the Earth with the potential to cause devastation on impact.

But a researcher at the US-based Massachusetts Institute of Technology (MIT) says these predictions should be revised upwards, on the basis of new data.

The late astrogeologist Eugene Shoemaker

Scott Stuart believes there are more than 1,100 objects within our Solar System that could come close to Earth.

The new number is consistent with earlier estimates made by the late Eugene Shoemaker, who based his analysis on the number of impact craters on the Moon.

Astronomers find and track asteroids by imaging the sky with telescopes and searching for objects that move against the background of fixed stars.

Improved detection

By observing an asteroid's path over several months, they can calculate its orbit and whether it might pose a threat to the Earth.

Asteroids bigger than one kilometre (0.6 miles) in diameter are thought to be capable of causing extensive damage on a global scale.

But no-one knows exactly how many undiscovered asteroids are out there.

Scott Stuart of MIT told a meeting of the American Astronomical Society in Pasadena, California, that there were likely to be more undetected near-Earth asteroids than previously thought.

His estimate is based on new calculations of the inclinations of asteroids, their angles of orbit in relation to the plane of the Earth's orbit around the Sun.

Asteroid defences

The raw data come from the Linear (Lincoln Near-Earth Asteroid Research) project which has detected more than 400 different near-Earth asteroids over the past two years.

This 10-fold increase in detection rate has allowed astronomers to predict more accurately the distribution of asteroids at both high and low inclinations.

The number of asteroids estimated recently - between 750 and 900 - is likely to be too low, said Scott Stuart.

He believes such an estimate could have been skewed by the fact that asteroids with low inclinations are the easiest to find.

Experts in the UK recently called on the British Government to lead the way in creating an early warning system to defend the planet against potentially dangerous asteroids and comets.