Are NEOs dangerous?
In the Solar System, there is strong evidence that asteroids hit our and other planets from time to time: today, there are more than 160 impact craters recognized around the world and Earth is not the only place where this phenomenon takes place. In fact, all solid bodies such as the rocky planets, their moons and asteroids are covered with craters ranging from a few meters to huge structures.
Coming back to Earth, the first crater ever recognized as being generated by a NEO impact was Arizona's mile-wide Meteor Crater, also called Barringer Crater. A second famous impact site, located off the coast of the Yucatan Peninsula in Mexico, is the Chicxulub crater, buried under ocean sediments and believed to be a record of the event that led to the extinction of the dinosaurs 65 million years ago. Fortunately for us, these big asteroid impacts are extremely rare.
But how dangerous can an impact be? The answer to this question depends on the impactor’s characteristics, first of all on its dimension and its chemical composition: most of the objects that fall on Earth are not bigger than a meter and are therefore destroyed before they reach the ground. In fact, on our planet about 100 tons of interplanetary material rift down to Earth daily. Luckily, most of these objects are too small to reach the ground, and don't represent a danger, since our atmosphere acts as a real shield, burning out all small debris. Burning up, the debris can leave beautiful trails of light known as meteors or "shooting stars”.
…but just how often?
When the impactor is big enough, it can survive to this fall and reach the ground with some very evident consequences, such as the formation of impact craters or other possible destructive after effects.
The hazard represented by NEO impacts increases with the size of the projectile: the greatest risk is associated with objects larger than 1-2 kilometers, which are capable of causing major regional or global disasters, injecting large quantities of dust into the stratosphere and perturbing the Earth's climate on a global scale.
Luckily, these very dangerous asteroids are extremely rare: they are thought to impact Earth only a few times every 1,000,000 years, on average.
Last update: 30 March 2006