Rob L. wrote:
"the fact that our sun is going to become a planetary nebula someday seriously enough.... "

No, NOOOOO! Say it isn't sooo! Quick, lets grab a tank of hydrogen and get it to the center of our sun STAT! Howabout a hydrogen bucket brigade?

Why won't anyone listen! Why won't anyone help!

EP:
While there is a chance that an impactor could destroy human civ/earth, I think there are much more real and immediate causes of our demise that CAN be avoided with our current technology. (Bio warfare agents, Nuke war, Environmental destruction... )

We *might* be wiped out ay any moment by any number of cosmic events, most of which we can't really do anything about because of the improbability of notice or the current state of technology.

We couldn't really stop much of anything in terms of asteroids/comets/etc , even with our best efforts. Maybe we could stop little stuff that we would survive anyway.

Welcome to the plight of the insignificant peach-fuzz-film clinging to a miraculously benign rock floating in space surrounded by energies and forces that could wipe it clean in an impersonal second. fecal matter happens....

In a few decades, if we survive our own idiocy, we might be able to deploy systems that would have a chance of saving our behinds in the situation you speak of... But, in all honesty, what are the chances of a KT/Dino Killer nailing us in the next few decades?

Nobody, no species, gets out alive in the end.
Davin