Space Watch II 
On an October morning in 2008, computers sifting through a night’s observations by a telescope in Arizona sounded the alarm: A space rock discovered that night would hit Earth in less than 24 hours. Fortunately, it was only about the size of a pickup truck. It exploded high above Sudan, producing a bright light show but no damage.
But future encounters might not be so harmless. An asteroid the size of a small office building could flatten hundreds of square miles. And bigger asteroids could inflict damage across the entire planet.
To prevent such a catastrophe, astronomers are searching for asteroids that could potentially hit Earth. With enough warning, it should be possible to deflect an asteroid from a collision course; more about that tomorrow.
The searches use fairly small telescopes to scan large regions of sky. Asteroids move quickly against the background of stars, so they show up as streaks of light in pictures with exposures of just a few minutes. Follow-up observations of newly found asteroids allow astronomers to calculate precise orbits — telling us if the discovery is headed our way.
The searches have discovered hundreds of thousands of asteroids. But most of them are in harmless orbits far from Earth. Only about 1400 asteroids follow paths that come dangerously close to us — some within just a few thousand miles. So far, though, not a single one is on a collision course. But the search continues — just to be safe.
Script by Damond Benningfield, Copyright 2012