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New research shows an asteroid that exploded over Siberia a century ago wasn't nearly as large as previously thought.

A physicist studying the case says that may suggest a greater danger for Earth.

The asteroid — which exploded over a Siberian forest in 1908 — left about 2,000 square kilometres of scorched or blown down trees.

Simulations done with a super-computer show the asteroid had a blast force equivalent to one-quarter to one-third of the 10- to 20-megaton range scientists previously estimated.

Physicist Mark Boslough, who works at Sandia National Laboratories in Albuquerque, New Mexico, says better understanding of what happened back then will allow for better estimates of risk today.

And, he says, that would allow policy makers to decide whether to try to deflect an asteroid or evacuate people in its path.

“It's not clear whether a 10-megaton asteroid is more damaging than a hurricane Katrina,” Mr. Boslough said. “We can more accurately predict the location of an impact and its time better than we can a hurricane, so you really could get people out of there if it's below a certain threshold.”

On Tuesday, an asteroid at least 240 metres long was making a rare close pass by Earth, but scientists said there was no chance of an impact.

The closest approach of 2007 TU24 will be 537,500 kilometres — about 1.4 times the distance of Earth to the moon. An actual collision of a similar-sized object with Earth occurs on average every 37,000 years.

Smaller asteroids approach Earth about three times more frequently than large ones. So if large asteroids approach about every 1,000 years, a smaller one would be about every 300 years, Mr. Boslough said.

“Of course there's huge uncertainties,” he said.

The three-dimensional computer simulations of the asteroid exploding over Siberia's Tunguska forest were done last summer.

Mr. Boslough presented the findings at scientific meetings in September and December. A paper on the phenomenon, co-authored by Sandia researcher Dave Crawford, has been accepted for publication in the International Journal of Impact Engineering.

The simulation, which better matches what's known of Tunguska than earlier models did, shows that the centre of the asteroid's mass exploded above the ground, taking the form of a fireball blasting downward faster than the speed of sound.

But the fireball did not reach the ground, so while kilometres of trees outside the epicentre were flattened, those at the epicentre remained standing — scorched, with their branches stripped off.

Mr. Boslough said they were likened to telegraph poles by the first Russian expedition to Tunguska, an expedition that didn't arrive until 1927 because of the distance, primitive travel conditions and turbulent times in Russia.

If the asteroid had been as large as previously thought, “it would have had really different effects on the ground,” Mr. Boslough said.

“It wouldn't have just blown over trees. There would have been a zone of completely scorched earth for several miles,” he said. “That fireball would have come all the way down to the surface and everything it came in contact with would have basically just vaporized.”

Alan Harris, a planetary scientist at Space Science Institute in Boulder, Colo., said he's been following Mr. Boslough's work on Tunguska for several years “and I think the idea that he has there seemed very sound.”

“A meteorite or asteroid coming into Earth's atmosphere has a lot of momentum,” he said. “The idea that it would push down into the atmosphere seems very plausible.”

“The bottom line is it takes a lot less energy, a small explosion, to create ground damage” such as that at Tunguska, said Mr. Harris, who studies the frequency of such impacts to assess hazards.

In the future, he said, he'll take Mr. Boslough's work into account and revise estimates of damage from impacts by smaller objects.