An interesting read here from Gregg Easterbrook regarding the risks from a major asteroid strike on the Earth. Once thought to be a terribly remote occurrence, it seems the more the astronomers look at the issue the less rare it appears to be.
The Sky Is Falling
A generation ago, the standard assumption was that a dangerous object would strike Earth perhaps once in a million years. By the mid-1990s, researchers began to say that the threat was greater: perhaps a strike every 300,000 years. This winter, I asked William Ailor, an asteroid specialist at The Aerospace Corporation, a think tank for the Air Force, what he thought the risk was. Ailor’s answer: a one-in-10 chance per century of a dangerous space-object strike.
Although from what I can glean from this table: Sentry Risk Table [NASA], there appears to be only one rock that is of any concern at this time. [this one - 2007 VK184] and that will happen June 3rd...2048.
The whole point of the first article seems to be that we may be more at risk than we had previously thought and spending some money on asteroid defense systems may be prudent.