A relevant SDC article:
Beware the Small Ones
Put aside the vision of Bruce Willis wrestling with huge space rocks threatening to doom Earth "Armageddon"-style. It turns out that people should be more worried about smaller space rocks that explode in our atmosphere.
While smaller than Earth-busting asteroids, these "airbursters" — like the space rock that exploded in 1908 high over Tunguska, Siberia — are more immediate threats, scientists say. They can cause localized destruction and may intrude in our airspace with little warning time...
Such objects are expected to impact the Earth on average every two to 12 years...
The risks of exploding asteroids and the need to keep watch for hazardous near-Earth objects took center stage at September's Space 2010 conference in California, sponsored by the American Institute of Aeronautics and Astronautics.
"We used to think that the only real threat was from impacts that hit the ground ... and that the atmosphere would protect us from the small ones," said physicist Mark Boslough of Sandia National Laboratories in Albuquerque, N.M. "We never really thought about the physics of airbursts. ... There hasn't been that much research."
The classic asteroid event occurred 102 years ago in Tunguska, Boslough said. It involved an object that broke up in a cascading way, leading to a rapidly expanding fireball and subsequent blast wave.
"That blast wave hit the ground, and the wind associated with it was high enough to actually blow over trees," he said.
The downed trees covered at least 2,000 square kilometers (more than 770 square miles) — with no crater associated with the explosion located.
Boslough said that, in his opinion, the Tunguska asteroid was probably a 40-meter (131-foot) object. "Tunguska wasn't the lower threshold. You could imagine something 30 meters (98 feet) across," he said, and in that case, it would explode with a little bit less energy and a little higher in the atmosphere.
"But if you just happened to be directly under it, yes, it could be fatal," Boslough added...
Similar in view was Don Yeomans, manager of NASA's Near-Earth Object Program Office at the Jet Propulsion Laboratory in Pasadena, Calif.
"We're doing very well with detecting the large ones. But we've got a long way to go for the small ones," Yeomans added.
His message regarding planetary defense:
"We need to find them before they find us."