As you've no doubt already heard, on Friday an asteroid 150 feet across will pass closer to the Earth than our geosynchronous communications satellites, moving more than 17,000 miles per hour. There's no risk of a collision this time, but similar objects have struck the Earth many times before, roughly on the order of once every 1200 years, with an impact comparable to exploding more than two million tons of TNT. If it hit somewhere the ocean -- which is likely, given that water covers the great majority of the Earth's surface -- it would probably cause a tsunami. If it hit on land, the amount of damage would vary greatly depending on exactly where it struck.
For any given person the risk is small but non-trivial, so it makes sense to consider what could be done to get an early warning of an asteroid collision and to prevent it given enough lead time. Here's a brief clip on the subject from Bill Nye: