What I am seriously worried about are asteroids - those big chunks of rock/metal/other stuff (like water...funny...? Ever been hit by a 100 ton snowball? Thought so.).
An impact by a 10 km asteroid on the Earth is widely viewed as an extinction-level event. The dinosaurs did't like it, I bet you we wont like it neither.
It is amazing how little has been done to prevent this from happening. Some talk and theoretical ideas, but action? Next to none.
Obviously, who wants to waste money on making sure our species has better chances of survival? It is not as popular as unemployement, job offshoring or gay marriage...
Just one more reason for politicans to go and Earth to unite under world wide single government, I say.