A good what if
In that case, a 100-foot-wide (30 meters) asteroid exploded after it entered Earth's atmosphere, leveling 825 square miles (2,137 square km) of trees in the region. If asteroid 2012 DA14 were to strike the planet, it would probably behave in much the same way as the Tunguska object, NASA scientists have said.

The asteroid's impact wouldn't cause a worldwide catastrophe, Yeomans said, but it would be a regional disaster

[link to news.yahoo.com]