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Re: Hell Creek (long)



On Wed, May 29, 2002 at 10:55:47PM -0700, James R. Cunningham sent:
> MariusRomanus@aol.com wrote:
> >> , for massive impactors the approach angle doesn't have too
> >> much of an effect on crater shape or primary direction of blast
> >> wave  (jrc)
> >
> >  Actually, there is much to the angle of attack when it comes to
> > impacting and the resulting blast wave..... no matter what the size of
> > the impacting body.
> 
> Then why is it that most all very large impact craters throughout the
> solar system are essentially (nearly) circular?

Because off-centre collisions are relatively rare, for reasons involving
the way orbital dynamics typically work out.  Those very low angle
impacts are (near) misses without gravitational affects, and the folks
who model this sort of thing find that it's a rare case; it's more
common to get a miss or a centre-of-mass impact.

This has to do with the real, scary, repeated-interacting-pass,
semi-chaotic gravitational attraction math, and I can't follow it.

> > This brings me to the angle of attack issue and the resulting blast
> > field. If you look at certain craters on the moon..... and on other
> > planets such as Mercury, Venus, and Mars, (we see this on Europa and
> > Calisto too) not all of the craters are circular..... Some are
> > elliptical with ejected blast material and such blown downwind from
> > the crater...
> 
> Quite right.  Some of them are elliptical.  But the big ones are pretty
> much circular.

The standard explanation for that is that the fireball dynamics dominate
the kinetic energy distribution from the trajectory; once one gets above
a certain impactor energy, the fireball swamps the contributions of
angle.

This is one of those subjects where it's hard to test people's
simulations, though.

-- 
graydon@dsl.ca   |  Hige sceal þe heardra, heorte þe cenre,
                 |  mod sceal þe mare þe ure maegen lytlað.
                 |   -- Beorhtwold, "The Battle of Maldon"