[meteorite-list] Russian Fireball Explosion Shows Meteor Risk Greater Than Thought

[Ron Baalke]

http://www.space.com/23423-russian-fireball-meteor-airburst-risk.html

Russian Fireball Explosion Shows Meteor Risk Greater Than Thought
By Leonard David
space.com
November 1, 2013

DENVER - As researchers recover more leftover pieces from the space 
rock that detonated earlier this year near the Russian city of Chelyabinsk, 
the event is helping to flag a worrisome finding: Scientists have misjudged 
the frequency of large airbursts.

Computer simulations also imply that such airbursts cause more damage 
than nuclear explosions of the same yield, which are typically used as 
an analogue to ballpark impact risk.

The meteor explosion over Chelyabinsk gives the bottom-line message that 
the risk from airbursts is greater than previously thought.

Meteor explosion data points

Mark Boslough, a physicist at Sandia National Laboratories in New Mexico, 
broached the implications of the Chelyabinsk airburst event on Oct. 7 
here at the American Astronomical Society's 2013 Division for Planetary 
Sciences meeting.

According to Boslough, when you add the Chelyabinsk incident to the 1908 
Tunguska explosion over Siberia - along with a 1963 bolide blast near 
the Prince Edward Islands off the coast of South Africa - the data suggest 
that the incoming rate of small space rocks is actually much higher than 
asteroid experts have assumed based on astronomical observations. 

That Prince Edward Islands event was a 1.1-megaton explosion picked up 
by a global network of infrasound sensors, but not apparently seen by 
any observers.

"These three data points together suggest that maybe we have underestimated 
the population," of smaller sized objects that can create air bursts, 
Boslough said. "We think the airburst hazard is greater than previously 
thought."

Chelyabinsk consortium

The Feb. 15, 2013, explosion of a previously undetected asteroid about 
25 miles (40 kilometers) from the Russian city of Chelyabinsk led to many 
injuries and widespread blast damage. But it has also spurred a wealth 
of data helping scientists to gauge the object's size, angle of entry 
and other specifics, Boslough said.

A "Chelyabinsk consortium" has been hard at work to better discern the 
varied characteristics of the fireball and the subsequent damage, he said.

The best estimate places the explosive yield of the space rock at 400 
to 500 kilotons, Boslough said, making Chelyabinsk the most powerful such 
event observed since Tunguska, which is pegged at 3 to 5 megatons. (There 
are 1,000 kilotons in a megaton.)

The Chelyabinsk space intruder came in mostly from the east at 9:20 a.m. 
local time in Russia, bursting apart close to 19 miles (30 km) above the 
ground.

Pre-entry, the object had a diameter of roughly 65 feet (20 meters), with 
a mass of approximately 12,000 tons, Boslough said. It came in at an 18-degree 
angle, "a glancing blow," spreading its energy sideways and at an angle, 
which generated less damage on the ground, he said. Still, the outburst 
broke windows over several thousand square kilometers.

Out of the sun

Thanks to the work of colleague Peter Brown, a physics professor at the 
University of Western Ontario in Canada, Boslough said over 500 videos 
of the Chelyabinsk fireball have been collected, some of those no longer 
available via the Internet.

Boslough traveled to Chelyabinsk, where he performed calibrations of dashcam 
videos to help pinpoint the altitude and coordinates of the explosion.

"It came pretty much straight out of the sun," Boslough said. 

Roughly 1,500 people were injured, "almost all by flying glass," due to 
a powerful, post-explosion shockwave, he said.

Duck and cover

On one hand, Boslough said that the old Cold War practice of "duck and 
cover" during a nuclear bomb drill is a bit quaint, "but it turns out 
that it would be the right thing to do" for the Chelyabinsk meteor.

"If you're not at ground zero, the way to keep yourself from getting hurt 
from a large explosion in the atmosphere is to stay away from windows," 
Boslough said.

But even armed with that fact, he admitted that scientific curiosity would 
assuredly have driven him for a look-see out the window.

"I probably know as much about airburst as anybody," Boslough said, "and 
I'd probably have my face pressed against the window, and as soon as the 
blast died down, I would have gone 'Oh, yeah.'"

Still, if scientists had discovered this asteroid before it struck, be 
it five months or even a week in advance, it would have been wise to send 
out a call for people to stay away from windows, Boslough said. 

In an October 1 online version of Acta Astronautica, a journal sponsored 
by the International Academy of Astronautics, Boslough has written about 
airburst warning and response, concluding that it is "virtually certain" 
that the next destructive Near Earth Object (NEO) event will be an airburst.

"Because early warning and civil defense will almost certainly be needed 
long before the first deflection is ever required, the credibility of 
the planetary defense community and its recommendations will be put to 
its first serious test by an airburst," Boslough said.

In his Acta Astronautica paper, Boslough has proposed an "airburst warning 
scale" to assist decision makers.

Misinterpreted attack

Also observing the Chelyabinsk episode, but from space, were U.S. government 
sensors. They, too, caught the affair, with that data released to the 
scientific community.

"If you compare some of these numbers to our estimates, they are different. 
It is worthwhile starting to consider these sorts of events as ground 
truth," Boslough told the DPS gathering.

Boslough underscored a concern voiced to Congress several years ago by 
Pete Worden, then deputy director for operations at the U.S. Space Command, 
now director of the NASA Ames Research Center near Silicon Valley, Calif. 
One of the largest potential threats associated with an airburst is the 
risk that it could be misunderstood as a preemptive attack launched by 
one country at another, Worden had said.

"I always thought that was a little exaggerated," Boslough said, until 
he watched a number of videos taken of the Chelyabinsk blast.

"Say that this had happened on an overcast day, where nobody actually 
saw the streak across the sky. Then you see smoke, hear a large explosion 
and a lot of things that sound like artillery fire," Boslough said. Then 
imagine it's over a place that's already politically unstable, he said.

"One of the biggest threats could be that this might lead to a counterattack 
of somebody because something was misconstrued," Boslough said.