Chelyabinsk meteor explosion a "wake-up call," scientists warn

According to Dr. Duffy, these findings are a wake-up call — that a large object need not be on a direct collision course with Earth to be a threat to us.

For example, the asteroid 86039, although not likely to hit us, could still pose a danger by sending its fragments on a path towards Earth.

Power of the airburst
In a second study led by Peter Brown, a professor of physics and astronomy at the University of Western Ontario, the authors observed recordings from 400 amateur videos and multiple instruments (such as infrasound, seismic and U.S. government sensors) all over Earth.

According to Dr. Duffy, such listening posts are usually used to monitor nuclear explosions.

But, because the energy from the Chelyabinsk airburst was so large, it caused a “ringing” in the Earth’s surface that got picked up by seismic stations as far as 4,000 km away.

“Never before have we been able to study a meteor event of this magnitude at this level of detail. As far as I know, we have never studied an asteroid impact before, during and after the event,” said Simon O’Toole, a research astronomer with the Australian Astronomical Observatory.

“This is what I call ‘forensic astronomy’ – to take in all the evidence about an event and infer what actually happened and why.”

Collecting evidence of the ground damage caused by the airburst, which shattered thousands of windows, the authors tested their models of the resultant shockwave from the explosion.

They estimated that the airburst was as powerful as 530 kilotons of TNT exploding, roughly the strength of thirty Hiroshima bombs, and about thirty times brighter than the sun at its peak brightness.

Lastly, the authors used worldwide datasets from the past twenty years to show that existing models used to estimate airburst damage did not match the observations from the Chelyabinsk asteroid.

They predicted that the number of global impacts from other Chelyabinsk-sized objects per year was underestimatedby a factor of ten.

Lucyna Kedziora-Chudczer, an astrophysics researcher with the University of New South Wales, explained that the underestimation was a result of using methods like telescopic observations of asteroids, models based on the number of lunar craters and dedicated searches for Near-Earth Objectsthat could be potentially dangerous.

According to Margaret Campbell-Brown, associate professor at University of Western Ontario and co-author of the study: “The Chelyabinsk airburst didn’t cause as much damage from the energy released as we had expected. This is because the energy was laid down in a long trail and so the damage was spread over a longer path.”

Dr. Duffy believes that these findings present both good and bad news.

“The good news is that the damage from ‘killer asteroids’ is significantly less than what the models estimate. The bad news is that the number of collisions of these Chelyabinsk-sized objects per year have been greatly underestimated,” he said.

“So even though each asteroid may do less damage individually, they are much more common.

“It is therefore vital that we invest in newer, more powerful telescopes so that the sky can be surveyed more carefully.”

Damage assessment
The authors of a third study, led by Olga Popova from the Russian Academy of Science, visited fifty villages in outlying Chelyabinsk in the weeks following the airburst.

They counted the homes damaged by meteor fragments and the number of people injured by the shockwave and radiation, which included UV and thermal burns as well as retinal damage.

“This data is significant because it shows that, even though the asteroid fell to Earth already severely weakened and its smaller fragments limited the damage on the ground, people still suffered burns from the UV glow of the very hot fireball,” said Dr. Duffy.

The findings also included an exhaustive analysis of the mineralogy of the recovered fragments from the Chelyabinsk meteor.

The mineralogy reports suggest that the Chelyabinsk object was a relatively rare LL5-type meteorite, which is a low-iron, low-metal, stony chrondrite material made of silicates and other minerals compressed together, said Dr. Kedziora-Chudczer.

Using robust Uranium-Lead, the authors determined the asteroid was about 4.4 billion years old.

But what is unusual is that the surface of the Chelyabinsk meteor appears to have been exposed to cosmic rays for only one million years, which is insufficient time for the object to have split from its larger parent.

Based on this, the authors believe that the culprit is likely Earth itself.

It is possible that the parent asteroid, thought to be billions of years old and originally from the Asteroid Belt, came close to Earth and got pulled apart by Earth’s gravity, or from a collision with another asteroid.

The fragments then fell towards Earth over the next million years.

Dr. Duffy warns that with the larger remains of the parent object still out there, we are at risk of possibly being hit even though there is no evidence to suggest that another fragment is on its way to us.

If, like in Chelyabinsk’s case, the asteroid is small and faint and coming at us from the direction of the sun, it would be next to impossible for a ground-based survey to see such an asteroid before the impact, explained Dr. Campbell-Brown.

Dr Kedziora-Chudczer agreed, saying: “That is why it is important to learn how often such bolides can enter the area that may send them on the collision course with Earth.”

Shireen Chan is the editor, The Conversation.  This story is published courtesy of The Conversation (under Creative Commons-Attribution/No derivatives).