[meteorite-list] Peter Brown's Initial Accessment of Chelaybinsk Event

From: MEM <mstreman53_at_meteoritecentral.com>
Date: Sat, 16 Feb 2013 04:14:15 -0800 (PST)
Message-ID: <1361016855.97087.YahooMailNeo_at_web142401.mail.bf1.yahoo.com>

Update from Peter Brown (via Richard's FB post)
Fri Feb?15,?2013 2:11?pm (PST) . Posted by:
"Tomas" scibuffcom? via MPML

This report is written by Peter Brown, probably the foremost expert in the world on this type of impact event.

"This is what I know as of 17 h UT today...

Feb 15, 2013 Fireball Event Summary

What follows are *initial* information gleaned for multiple instrumental
 sources recording various aspects of the Feb 15, 2013 airburst over
Chelaybinsk, Russia (55.2N, 61.4E)

1. Time: The time of the main flare/airburst was 03:20:26 UT on Feb 15,
2013; the fireball began ablation about 30 secs before this time.

2. Based on the long duration of the event and videos, it is clear this
was a very shallow entry (certainly less than 20 degrees, maybe more
shallow).

3. It is *not* related to 2012 DA14

4. Energy: This is perhaps the hardest value to pin down so early in
this investigation. From multiple sensors using multiple technologies a
best initial estimate of the total energy of the event is about 300
kilotons of TNT equivalent = ~10^15 J). This could easily be in error by
 a factor of two. I am confident, however that it is in excess of 100
kTons, making it the largest recorded event since the 1908 Tunguska
explosion.

5. Speed: The fireball entered the atmosphere at 18 km/s

6. Damage: The airblast clearly caused window breakage and light
structural damage in downtown Chelaybinsk. The exact overpressure at
which window failure occurs tends to be probabilistic and varies by
construction design (ANSI S2.20, 1983). Normally some damage begins to
occur around 500 Pa of overpressure, widespread window damage is
expected to occur up to around ten-20 times this value. As the fireball
had a shallow trajectory, the cylindrical blast wave would have
propagated directly to the ground and would be expected to be intense.
This could be further compounded by any fragmentation, quasi-spherical
blasts. My impression is that the key here is that the terminal part of
the fireball (probably between 15-20 km altitude) occurred almost
directly over Chelaybinsk; this was perhaps the single greatest
contributor to the blast damage (short range to the main part of the
terminal detonation).

7. Comparators: The Sikhote-Alin fall (Feb 12, 1947) in the former
Soviet Union was the equivalent of about 10 kilotons TNT, BUT as an iron
 impactor much of this energy was deposited at the ground rather than at
 altitude. The Oct 8, 2009 Indonesia event is the most recent similar
event at about 50 kTons, but over the ocean (paper attached for quick
reference).

8. Size: The pre-impacting asteroid was about 15 meters in diameter and had a mass of ~7000 tonnes.

I fully expect revision of some of the numbers above, particularly the
estimate of the yield which could *easily* change by a factor of two
upon more complete analysis and will likely change as the day progresses

/end
------------ --------
Dr. Peter Brown
Director - Centre for Planetary Science and Exploration
Canada Research Chair in Meteor Astronomy
Professor
Department of Physics and Astronomy
University of Western Ontario
London, Ontario
N6A 3K7
CANADA"
Received on Sat 16 Feb 2013 07:14:15 AM PST