13. Colliding worlds

Probability of death from above

Fig. .. 

The Earth does not occupy a secure niche in space. Our planet is instead immersed in a cosmic shooting gallery, subject to a steady bombardment by lethal, Earth-approaching asteroids and comets. Somewhere in space, one of them is hurtling toward a future collision with Earth. And if it is large enough, the impact will severely disrupt terrestrial life upon impact. Its only a matter of time.

Fig. .. 

Thus, to estimate the risk of being hit in a way that matters, the potential impacting projectiles first have to be sorted according to size. Fragments smaller than a few tens of meters across burn up in the atmosphere and rarely reach the ground except for the exceedingly tiny particles of cosmic dust that drift down into your hair. Asteroids or comets a hundred meters in diameter are expected to strike Earth every thousand years on average. They could take out a city and cause severe local damage, but pose no threat to the Earth as a whole.

(page 5 of 6)

Copyright 2010, Professor Kenneth R. Lang, Tufts University