As an astronomer I can tell you that the threat is very real. How humanity has made it this far is amazing. Everyday we face global cataclysms on a massive scale, most of these threats come from space. We have only scratched the surface on asteroids and comets, nearly everyday there is some undocumented object found. NASA has been funding massive amounts of money on the NEO (Near Earth Object) project and has built many many RRRT's (Rapid Response Remote Telescopes) to help in finding objects, and responding to gamma ray burst, and close to earth super nova's. There is a planet killer that has been named 99942 Apophis (A-pof-is) That may be a major threat in the next 20 years. Although the trajectory calculations are looking better, any gravitation force can change the orbital path and throw our trajectory charts out of the window. Something to keep in mind when preparing is that when the SHTF it may not always be the human element that starts the downward spiral. But you better believe that the people that have not prepared and others will be close to capitalize on the situation.