Asteroid: Earth faces 'small risk'

2002-11-21 11:04

Paris - US military satellites have helped scientists to conclude that the Earth faces a far smaller risk than previously estimated of being struck by a much-feared type of asteroid, a study says.

The research focuses on a medium-sized asteroid of the kind that devastated a swathe of Siberia last century with the power of a hydrogen bomb.

That object, believed to be around 50 metres across, exploded over Tanguska in 1908, unleashing a shock wave equivalent to that of 10 megatonnes of TNT, devasting the pine forest over 2 000 square kilometres.

Scientists have long fretted about asteroids in this category.

The largest of these rocks, which are between one metre and up to 100 metres across, have the ability to wipe out large cities or cause tidal waves should they cross our path.

They lie somewhere between small, harmless rubble that burn up prettily as meteors when they meet the Earth's atmosphere and large asteroids, which include brutes a kilometre across or more.

Large asteroids could extinguish all significant life on this planet, as the ending of the reign of the dinosaurs, 65 million years ago, will testify.

They are being detected and tracked by a co-ordinated international programme - but medium-sized ones, while still a risk, are a relatively unknown quantity because they are too small to be seen by telescope.

But a remarkable insight has been thrown up by a new study, published in the British journal Nature on Thursday, led by Peter Brown, an astronomy professor at Canada's University of Western Ontario.

His team used data provided by the Pentagon's constellation of military satellites, whose cameras constantly monitor the Earth for evidence of a nuclear explosion.

Over the past eight years, the satellites have detected nearly 300 optical flashes from medium-sized asteroids as they exploded in the upper atmosphere.

Asteroids of this size are too small to impact the land or sea, but can still deliver a punchy shockwave when they detonate in the air from friction with atmospheric molecules.

The biggest flash detected occurred over the middle of the eastern Mediterranean, at a point roughly equidistant from Libya, Greece and Italy, on June 6 this year.

The explosion was equivalent to more than 25 kilotonnes of TNT - more than twice that of the Hiroshima bomb.

Looking at the frequency of these asteroids and their effect, the researchers believe that a Tunguska-type event is likely to occur once only ever 1 000 years on statistical average.

That compares with previous estimates, based on observations from the ground, that the occurrence could take place once every 200 to 300 years.

"We estimate that the Earth is on average struck annually by an object of around five kilotonnes in energy (with a possible range of two-10 kilotonnes), and struck each month by an objection with 0.3 kilotonnes of energy," the authors say.

"Every 10 years, an object of around 50 kilotonnes in energy strikes Earth." - Sapa-AFP

- SAPA