There have been several movies made about this subject in the last few years. Giant asteroids crashing to Earth, snuffing out life as we know it. Yet, in true Hollywood style, a hero saves the day and the earth is spared.
There is growing evidence that the celestial body that helped form the Gulf of Mexico may have also wiped out the dinosaurs and given man his chance in the sun. There are a number of asteroids that NASA tracks because they are known to have "near-earth" orbits, some that actually pass between the earth and the orbit of the moon! (See chart, click here.)
The famous "Meteor Crater near Winslow Arizona is about a mile across and 600 feet deep. It is estimated that the meteor that caused it was no more that 150 feet in diameter. Now, imagine that crater in New York City.
(Click here to visit the source of this photo - http://www.barringercrater.com/science/ )
By comparison, the "Dinosaur Killer" that many scientists believe hit the Yucatan Peninsula is estimated to have been only 6 miles in diameter - and it changed Earth forever.
Professor Binzsel of MIT has created an informative chart to illustrate the chances of a large body coming close or actually hitting the earth, and the consequences. Click here to visit Professor Binzel's Torino scale (opens a new window, so close the new window when you are done to return to this page). In 1937, near-Earth-asteroid Hermes, which could impart much more destructive energy into the biosphere than the global nuclear arsenal is capable of releasing, was observed to have missed Earth by less than seven hours.
And an asteroid doesn't have to be all that big to pose a huge threat to our survival as a species. According to ABC Science News "A "doomsday" asteroid is defined as one with a diameter greater than 0.6 mile, which could cause global climatic catastrophe if it collided with Earth. Debris from such a collision would be predicted to cause worldwide clouding and cooling, with possibly disastrous effects on crops and animals." Imagine trying to spot something barely half a mile in diameter hurtling towards us from the vastness of outer space, and you begin to understand why we need to keep developing systems to watch the skies.
Although, even if we saw one coming, we don't really have any way to stop it... yet.