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

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Sun, 3 Nov 2013 19:29:00 -0800 (PST)
Message-ID: <201311040329.rA43T0fd028187_at_zagami.jpl.nasa.gov>

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.
Received on Sun 03 Nov 2013 10:29:00 PM PST