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

[MEM]

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"