The first to be recognised as an impact crater was Arizona's 1600 metre wide Barringer crater. The NEO that caused it is thought to have been just 40-50 metres wide. Another impact site off the coast off the Yucatan Peninsula in Mexico (image right), now buried by the sea floor, is thought by some to be the event that contributed to the extinction of the dinosaurs some 65 million years ago.
In more recent times, a small rocky meteor or comet of less than 50 metres in size is believed to have broken up in the atmosphere above the Tunguska region of Siberia in 1908. The resulting shockwave flattened over a hundred square miles of trees in all directions. Thankfully, these big asteroid impacts are rare.
But what are the chances of a large NEO hitting Earth?
The danger from NEO impacts increases with the size of the projectile. The greatest risk comes from objects larger than 1 kilometre. Such impacts could have devastating consequences for the region or even the whole world.
Luckily, the very dangerous asteroids are extremely rare and on average are thought to hit Earth only a few times every million years. Of course these statistics cannot tell us when the serious impacts will be. It could be next year or many millions of years from now.
That said, existing technology can protect us from future impacts, but it is important that we monitor the orbits of NEOs to determine if they may become a risk. If we can predict possible impacts well in advance, say by 10 or more years, then conventional rockets and explosives should be enough to nudge the NEO out of harms way.