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Killer asteroids more common than was thought, Sandia scientists say

By Sue Vorenberg
Saturday, December 22, 2007

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A crisp winter morning in Albuquerque, the scent of breakfast burritos wafting from local restaurants, unwary drivers zipping through red lights, and suddenly - whoosh.

A White House-sized object descends from the skies, creating a fireball with hurricane-force winds that knock over buildings and blow out windows for miles around.

Statistically, the scenario isn't all that likely - at least not at a specific location like Albuquerque - but new estimates from Sandia National Laboratories indicate that sort of devastating event is more common than was previously thought.

That's because Sandia researchers used a supercomputer to study a place where it happened before.

Not in a city, but near the Tunguska River, in the Siberian wilderness on June 30, 1908. During that event a White House-sized asteroid created a fireball that burned and knocked over trees for hundreds of square miles.

The event, which was previously thought to be caused by a solid rock the size of about three White Houses mushed together, now appears to have come from a much smaller object.

And, because there are more smaller objects drifting through space than larger ones, that means the possibility of another fireball or impact - which is created when a meteor breaks up in the sky or a meteor hits the ground - has increased, said Mark Boslough, a Sandia researcher studying the problem.

"Asteroids constitute a geologic hazard, so there is a national security issue associated with characterizing this hazard," Boslough said. "We call this a low-probability, but a high-consequence event, so it's important to study it."

Scientists prefer to describe impacts or fireball events like Tunguska in terms of explosive energy. And in that vein, the White House-sized object created an explosion equivalent to three to five megatons of TNT.

Past estimates of the larger object would have created the equivalent of 10-20 megatons of TNT.

"If it had been a 15 megaton event the damage at Tunguska would have been a lot worse," Boslough said. "If something like that hit Albuquerque, it would actually incinerate several square miles around ground zero. It would turn sand to glass, and there would be nothing left."

Prior estimates suggested that Tunguska-like events happened every 1,000 years or so. New estimates put that closer to the 300-400 year range, Boslough said.

"The smaller ones at most could hit perhaps every 100 years or so," Boslough said. "And the last one was 99.5 years ago, but any estimates are based on a lot of uncertainty."

Knowing the odds is one thing - but doing something about them will provide another, bigger challenge, said Rusty Schweikart, chairman of the B612 Foundation and a former Apollo 9 astronaut.

The B612 Foundation is focused on developing and testing technology to deflect asteroids that threaten Earth.

"These smaller objects are in many ways the most challenging to identify and track," Schweikart said. "It's harder to get precise orbits, and the warning times will be shorter."

The National Aeronautics and Space Administration is working on a mandate to catalogue 90 percent of near-Earth objects bigger than 140 meters in diameter by 2020.

Realistically, that probably won't happen until 2025, Schweikart said.

But the size of the Tunguska object was perhaps 30-40 meters in diameter - or loosely the size of the White House - and the process to track most of those will take longer, he said.

"In the process of finding the bigger objects, you find some of the smaller ones," Schweikart said. "But there are perhaps 400,000 objects that are 45 meters in diameter out there, and we've found less than 1 percent of them so far."

By 2025, NASA should have found about 40 percent of them, leaving a lot that could still smash into Earth unannounced, he said.

"This is a process that's going to take considerable time before we know of all the rocks out there that can do serious damage," Schweikart said.

If we know in advance that an object is on a path to Earth, NASA or some other agency can launch a missile to smash it, then use something called a gravity tractor to alter the trajectories of the remaining fragments so they don't strike Earth, he said.

That science is already fairly well tested, he said, adding that the device, which would cost about $300 million, hasn't yet been funded.

But the Sandia work emphasizes the importance of moving that sort of project along - since it turns out smaller objects are more dangerous than previously thought, he said.

"Mark's work changed fairly substantially what we understand to be the most frequent and smallest objects that can do significant damage," Schweikart said.