Earth's surface undergoes many kinds of environmental changes that affect human life and the evolution of all living things. Some are caused by human beings, and others result from natural processes; some evolve slowly, whereas others are sudden: "accidents" (if caused by humans) and "natural disasters." Since life can adapt to slow changes, the most disruptive changes are sudden calamities. The worst calamity occurs when a large, errant asteroid or comet collides with Earth.

Sizes of Near-Earth Objects

Fragments of asteroids and comets pervade interplanetary space. Modest cosmic impacts occur all the time. On a dark, clear night one can see a flash of light (a meteor or "shooting star") every few minutes as an interplanetary grain of dust or sand strikes Earth's upper atmosphere. More rarely, larger space rocks cause brilliant "fireballs" when they crash to Earth, perhaps leaving meteorites in the ground. Every few years, Earth-orbiting...