This book covers the topic of Earth's Planetary Defense. This is not for the purposes of repelling an alien invasion, although the military has probably thought of that. Any civilization that could reach us is undeniably more technological advanced.

We are focused, however, on big chunks of rock that could cause massive damage, and end civilization as we know it. It has happened – ask the dinosaurs. Ocean impacts, and Earth is covered by 75% water, recall, could trigger massive Tsunamis.

The U. S. National Science and Technology Council is on record as saying we are unprepared as a nation for a large asteroid impact event. The last impact of a object some 10 km in size, 66 million years ago, caused an “extinction event.”