When I first heard about a plane hitting the WTC, it was on my car radio and the news reports were still vague. Not being able to actually see anything at first, I thought it might have been an accident like the plane that crashed into the Empire State Building in 1945:

http://history1900s.about.com/library/misc/blempirecrash.htm

After I started watching the news on television (and wondering if I should get my kids out of school; we weren't anywhere near NYC, but with the chaos and confusion going on, who knew what was about to be hit next), I remember the networks declaring that a truck bomb had rammed into the State Department, but that story quickly went away.

Anyone who grew up in the Cold War probably had similar thoughts as I did: That at any moment some day there would be a 30-minute head start warning that the USSR had launched nuclear missiles at the US and we were retaliating in kind. And not that it might do much good, but head to your basements.

While we certainly are not out of the woods yet, I'd like to think that if we made it through that era, then nothing of a cosmic-sized catastrophe will stop the human race.