From a recent story on asteroids hitting earth as they recently hit Jupiter:
So far 784 near-Earth objects (NEOs) more than a half-mile wide (1 km) have been found.
"If an object of about the same size that just hit Jupiter also hit Earth — it was probably a typical cometary object of a kilometer or so in size (0.6 miles) — it would have been fairly catastrophic," explained astronomer Donald Yeomans, manager of NASA's Near-Earth Object program office at the Jet Propulsion Laboratory in Pasadena, Calif.
Scientists have ruled out the chances of an Earth impact for all of these 784 large NEOs. Still, lesser objects also pose a risk, and researchers estimate more than 100 large NEOS remain to be found.
Currently just one NEO of all the objects scientists are tracking poses any significant chance of hitting the Earth — 2007 VK184. If this roughly 425-foot-wide (130 meters) asteroid hit our planet, it would strike with an energy of roughly 150 million tons of TNT, or more than 10,000 times that of the atom bomb dropped on Hiroshima.
Roughly 100 telescopic observations made so far suggest that 2007 VK184 has a 1-in-2,940 chance of hitting Earth 40 to 50 years from now.
Great, so what they see poses no threat, but there is a lot we can't see. And if that doesn't make you nervous, try this on for size:
Of remaining concern are the NEOs that we do not see. Researchers suspect about 156 large NEOs 1 kilometer in diameter or larger remain to be found, and when it comes to dangerous NEOs in general, "when we get down to 140 meters (460 feet) or larger diameter objects, we think we've discovered about 15 percent of them, and with 50 meters (164 feet) or larger diameter, we've discovered less than 5 percent of them," Yeomans explained.
In other words, if one does head right for us, we may not get much warning. Let's hope our 500,000 years isn't up just yet.