Everyone knows that there's lots of trash flying in the Universe and it can happen that a big asteroid hits Earth one day and destroy our whole civilisation, juts like few hundred ( ? ) Millions years ago it killed the dinosaurs.
Even the asteroid that hit Russia in 1908 was comparable in energy to the bomb dropped on Hiroshima.
According to this article the risk of such an event is far greater than we thought before, since there are many invisible comets in our Solar System that can hit Earth literally at any time.
Have a nice reading here:
http://www.nature.com/news/2004/0410.../041018-3.html
PS I wonder how much $$$$ is spent on research on such issues compared f.e. to the Iraq war...