Death from the Skies?
Earth has been hit by asteroids before--a giant impact 65 million years ago likely caused the extinction of large dinosaurs. The larger the asteroid, the worse the devastation, not only from the initial impact and explosion, but also from tsunamis, global wildfires caused by super-heated asteroid fragments and ashy skies that lead to catastrophic climate change.
Any near-Earth asteroid or comet whose orbit comes within five million miles (8 million kilometers) of Earth's is potentially hazardous. So far, researchers have identified more than 1,200.
Giant waves, firestorms and widespread devastation to plants and animals. What's to like about the effects of a potential impact? Luckily, impacts are rare.
Every 100 years
Asteroids wider than 150 feet (50 meters) might crash into Earth, causing local damage in the area of the impact.
Every 2,000 years
Asteroids wider than 500 feet (150 meters) might impact Earth, causing damage equivalent to Earth's natural disasters--such as a volcano or earthquake.
Every 3,000 years
An asteroid more than 1 mile (1.6 kilometers) wide might impact Earth, causing widespread devastation.
Every 5 to 10 Million years
A potentially hazardous asteroid as big as the one that probably wiped out large dinosaurs 65 million years ago--wider than five miles (10 kilometers)--might impact Earth.