5. Colliding Worlds
Probability of death from above
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 (Fig. 14.15). And if it is large enough, the impact will severely disrupt terrestrial life upon impact. It’s only a matter of time.
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 (Fig. 14.16). 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 3 of 4)
Copyright 2010, Professor Kenneth R. Lang, Tufts University