Asteroid hitting Earth. Artist's impression of an asteroid - a nearly planet-sized rock - colliding with Earth. The impact has thrown up a colossal circular wall of earth and water around a central explosion of liquid rock. Collisions like this are a statistical certainty in the Earth's future, and would be catastrophic for mankind. The nuclear winter" that could follow the impact would profoundly disturb global ecology, leading to mass extinction. Collision of an asteroid with Earth is thought by some to account for the mass extinction of dinosaurs 65 million years ago, although evi- dence suggests that the dinosaurs disappeared over millions of years."