At this very moment, celestial forces prowl the Universe and threaten man's very existence. They're asteroids and comets--and they've left their imprint on planet Earth, literally. Initially, they helped build planets through violent collisions. During this fiery bombardment period, they may have even seeded Earth with water and the building blocks for life. Since the turbulent formation of the solar system, these space rocks have continued to impact earth. Some have been so violent that they've led to mass extinctions events, including one that wiped out the dinosaur. What's more, radical new theories suggest that asteroid and comet dust harbor deadly viruses that may have triggered some of our worst pandemics. The possibility of future cosmic collisions remains a legitimate threat. Yet, despite their dangers, asteroids and comets may hold vital natural resources, which could actually preserve mankind.