re: Density waves
[At least we have a space infrastructure that could allow us
to either stop or escape such a disaster, which the dinosaurs
and the trilobites sadly lacked.]
Warn, yes, stop... to be determined.
A good warning site: JPL's and NASA's NEO program. More details here. I know amateur astronomers have contributed greatly.
The makeup of these things (comets or asteroids) has a great deal to do with how to 'deal' with them. Yes, trilobites and dinosaurs had no idea.
They had no idea about this (part of the ride) either: http://www.astronomycafe.net/qadir/q1372.html">Bubbles, what bubbles?