Space Watch 
A hundred-thousand ton space rock is barreling toward a close encounter with Earth on Friday. It’ll miss us by about 15,000 miles. But it demonstrates that our planet is like a cosmic target, with projectiles zipping past us all the time. A hit by a big one could zap life for miles around. And a hit by a really big one could destroy much of the life on our entire planet.
Astronomers have been hunting for these possible dangers for a couple of decades. In the early 1990s, Congress instructed NASA to find 90 percent of all the asteroids that could potentially hit Earth that are at least one kilometer across — big enough to cause a planet-wide catastrophe. And a few years ago, that mandate was expanded to cover asteroids that are even smaller, which could cause major regional damage.
Several automated searches have discovered about 10,000 asteroids with orbits that come fairly close to Earth’s orbit around the Sun. Of those, almost 1400 are classified as potentially hazardous — they have some risk of eventually hitting us. And a recent study concluded that another 3,000 or more are still awaiting discovery.
Despite all the big numbers, though, the threat of a major collision is small — Earth is a pretty small target in this cosmic shooting gallery. Still, the search continues — for a bullet that might have Earth’s name on it. And we’ll have more about how astronomers conduct that search tomorrow.
Script by Damond Benningfield, Copyright 2012