5. Colliding Worlds

    • Most of the impact craters on the Earth disappeared long ago, but a few of the relatively recent ones have been discovered from airplanes or spacecraft.

    • An asteroid wiped out the dinosaurs when it hit the Earth 65 million years ago, and similar cosmic impacts might be responsible for other mass extinctions.

    • If an asteroid or comet of about 10 thousand meters in size hit the Earth, the horrific blast could generate towering ocean waves, block out the Sunís light and heat, ignite wildfires that might set the world ablaze, drench the land and sea with acid rain, and ignite deadly volcanoes on the other side of the Earth.

    • The Earth is immersed within a cosmic shooting gallery of potentially lethal, Earth-approaching asteroids and comets that could collide with our planet and end civilization as we know it.

    • The lifetime risk of your dying as the result of an asteroid or comet striking the Earth is about the same as death from an airplane crash, but a lot more people would die with you during the cosmic impact.

    • It is estimated that the Earth receives a direct hit by an asteroid or comet about two kilometers in size every million years or so, resulting in a global catastrophe. It could happen tomorrow or it might not occur for hundreds of thousand of years.

    • Astronomers are now taking a census of most of the Near-Earth Objects that are big enough and close enough to threaten us. When the survey is complete, they will have found 90 percent of those larger than one kilometers in size and determined their future trajectories during at least much of this century.

    • With enough warning time, we could redirect the course of an asteroid or comet that is headed for collision with the Earth.

Copyright 2010, Professor Kenneth R. Lang, Tufts University