ALBUQUERQUE, New Mexico (AP) -- An asteroid that exploded over Siberia a century ago, leaving 800 square miles of scorched or blown down trees, wasn't nearly as large as previously thought, a researcher concludes, suggesting a greater danger for Earth.


Mark Boslough shows a computer simulated "fireball" that might be expected from an asteroid explosion.

According to supercomputer simulations by Sandia National Laboratories physicist Mark Boslough, the asteroid that destroyed the forest at Tunguska in Siberia in June 1908 had a blast force equivalent to one-quarter to one-third of the 10- to 20-megaton range scientists previously estimated.

Better understanding of what happened at Tunguska will allow for better estimates of risk that would allow policymakers to decide whether to try to deflect an asteroid or evacuate people in its path, he said.

"It's not clear whether a 10-megaton asteroid is more damaging than a Hurricane Katrina," 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 800 feet 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 334,000 miles -- 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.

Although the computer simulation shows the Tunguska asteroid was smaller, its physical size isn't known. That would depend on such factors as speed, shape, how dense or porous it was and what it was made of, Boslough said.

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, Boslough said.


"Of course there's huge uncertainties," he said.

The three-dimensional computer simulations were done last summer. 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 center 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 miles of trees outside the epicenter were flattened, those at the epicenter remained standing -- scorched, with their branches stripped off.

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Yes, I would not be concerned that there is some big ball of stuff this way, first because there is not and any rock that hits the outer atmospheric shell will burn up. But, look at what happens when there is a solar flare, a CME. Tunguskas was something like that but here is the thing, There has been discovered a 'finger' of Hydrogen coming our way from the neighboring galaxy. I guess it is the same thing that the sun puts out, but thousands of times larger.

Interesting, yes.

The number of asteroids <km in size has apparently been downrated. Around 1999, a Harvard physicist estimated the use of improved astro cameras would conclude a smaller number of <km size objects, than previously estimated.

The older average was 1500 smaller objects. Improved estimates are down to 750 objects of <km in size.

That is a reduction of 50%.