🚀 go-pugleaf

I would rather be shot at with a .45 than a 12-gauge, wouldn't you?  At 
least there would be only one bullet to duck.

It's been a while since I did the calculations, but a very small delta 
velocity can make the difference between a planet hit or not, if that 
velocity has decades or centuries to act.  Maybe someone would compute 
the necessary energy for representative cases, and compare that, for 
example, to the kinetic energy of a Shuttle in orbit.  The point is to 
identify the hazardous objects early enough. 

Along those lines, Tom Gehrels is hosting a symposium entitled "Danger 
of Near-Earth Asteroids" in Tucson the first week of January.  There is 
intense interest from scientists, engineers, and the press.  We have to 
watch out for the "giggle factor" from the press and possibly a few nuke-
happy engineers, but the latter will get no sympathy from Gehrels. 

As to the terrestial effect of mega-explosions:  I think a big difference 
is that the largest volcanic or nuclear explosions are still confined to 
the atmosphere and thus subject to ordinary aerodynamic forces, whereas 
something like the fireball from the K-T impact would break entirely out 
of the atmosphere.  A lot of the debris would go ballistic, and thus blanket 
the whole earth rather uniformly.  You would get total, worldwide darkness 
for months or years, with all the implications for biology.

As to radioactivity, breaking up the earth's crust into little pieces always
releases radioactivity, I think mainly trapped radon.  People have noted
that coal mining has put a lot more radioactivity into the atmosphere than
all of nuclear power.

Ben