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Big asteroid population doubles in new census

Eros, a Near Earth Asteroid roughly the size of Manhattan
Eros, a Near Earth Asteroid roughly the size of Manhattan  

By Richard Stenger

(CNN) -- A new census in the solar system doubles the number of large asteroids thought to lurk between Mars and Jupiter.

Astronomers estimate that roughly 1.5 million space boulders 0.6 mile (1 km) or larger in diameter orbit in the main asteroid belt.

Rocks of such size could cause catastrophic results if they slammed into our planet, but European scientists caution that the new survey could have little bearing on risks to Earth.

The search was the first systematic one in infrared light rather than visible light. In using the European Space Agency Infrared Space Observatory, researchers looked for absorbed sunlight rather than reflected sunlight from asteroids.

Asteroids often possess dark, dusty surfaces, making them difficult to spot in visible light. But such dirty rocks soak in sun and emit infrared heat.

The infrared sampler, which ESA announced on Friday, suggests that between 1.1 million and 1.9 million large asteroids reside in the main asteroid belt, a ring of rocks orbiting the sun and thought to be the remnants of a failed planet.

"The result is about twice as high as that estimated by two other recent studies in the visible light," said Edward Tedesco of ESA.

The earlier surveys, concluded in 1998 and 2001, tallied 860,000 and 750,000 asteroids 0.6 mile (1 km) or larger in the belt.

Tedesco acknowledged that infrared and visible light surveys each have strengths and weaknesses in hunting elusive asteroids, which streak quickly across the field of vision of observatories and can vary considerably in brightness within a matter of minutes.

Taking into account the findings of the different surveys, the best population estimate would be "1.2 million asteroids larger than 1 kilometer in the main belt, give or take 500,000," Tedesco said in a statement.

Astronomers speculate that so-called Near Earth Asteroids originated in the main belt, but the gravitational pull of Jupiter disrupted their orbits eons ago and pushed them closer to our planetary neighborhood.

About 500 NEAs have been identified so far. None pose much of a risk to our planet for a century or more, but astronomers suspect that hundreds more remain undiscovered.

The new infrared survey did not include NEAs and preliminary calculations suggest that the survey will not affect the NEA risk assessment, ESA scientists said.




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