* Astronomy

The Nature of Airbursts and their Contribution to the Impact Threat
Mark Boslough (Sandia National Laboratory, United States)

First part describe airbursts, in the past when we model airburst we made oversimplifications, still simplifying but making fewer simplifying assumptions, more complicated phenomena, have not done a complete quantitative analysis - greater then we thought, relative threat from LAA (Low Altitude Airbursts) is increasing, most of the integrated threat is from the large objects, 100 m / 100 Mt has 1/100 chance in century (100-100-100 event, every 10K years) - will dominate threat once 90% of object larger than 140 m, the next destructive NEO is virtually certain (greater than 99% chance), to be LAA, IPP on Climate Change defining verbally descriptive terms, less than 200 m in diameter dominated by air burst, tech development similar to threat reduction time, other reasons to deflect and fragment small asteroids (for resources and possibility for GEO engineering), put one of these in L1 point and dust (could reduce number of photons), mitigation should focus on small ones (~100 meters ) - could just be civil defense, Al Harris iconic plot - plot of the long zoom, LAAs are not explosions (unlike bombs - point source), generate upward ballistic plumes (saw in 1994 from SL9), generate downward vortex rings (smoke rings that carry a lot of energy), enhance heat transport to surface - even as KE is lost - momentum still continues - unlike bomb, carries mechanical energy downward, anisotropic radiation patterns, two types of LAA, Tunguska type - fireball descends rapidly but does not reach the ground - expands - enough surface - shockwave to surface, area to start losing energy, 1-10Mt (1908), Libyan Desert Glass (29 M years ago), fireball is much larger and descends all the way to the surface, threshold yield is about 10 Mt, its own inertia tends to expand on ground - trees would vaporize, was at 100th anniversary, no evidence of glass in Tunguska, most of the Tunguska damage is mechanical, slope angle of 35% (like Tunguska), treefall at 50 m/s, threshold is less if terrain is not flat and trees not healthy (5 mt explosions at 12 km above a surface at 35 degrees angle), for type 2, consequences, get stationary wind at bottom at epicentre (fireball), cannot make this not happen for 15 MT explosion, Pancake model (Chyba, et al, in 1993) - revisited - vortex at surface (2-d potential effect, maybe at 3d would break up), previous Tunguska yield estimates were too high, momentum coupling to atmosphere t, solid, earth and tsunami may be higher, ,Type 2 airburst could generate fireballs in co=nat with surface over hundred of square kilometres for tens of seconds (possibility to find more evidence in the geological record).

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