On Fri, 15 Aug 2008 12:32:41 -0700, John Harshman
<jharshman.diespamdie@pacbell.net> wrote:

Hardly more asteroid strikes, since they all come from within the 
system.

Asteroids and comets come from within the system, but their dynamics may
be influenced by what is outside the system. There is some evidence in
the case of the Solar System that our regular passage through the
galactic plane causes an increase in comets entering the inner system
due to perturbations in the Oort cloud. It is entirely possible that the
behavior of planetary systems in denser regions of galaxies is much more
chaotic.
_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com