Yesterday, Asteroid 2007 TU24 zipped past Earth. It's a chunk of real estate about 250 meters (750 ft) across. TU24 actually came WITHIN lunar orbit which is kind of scary when you sit down and think about it. That's like if somebody shot a gun at me and the bullet broke the lens of my glasses but missed me all together. The mean distance from Earth to the Moon is about 384,000 km (239,000 miles) or just a hair over a light second. That may sound like a pretty large number but remember in the solar system the average unit of measurement in terms of distance is an AU which is equal to 149,598,000 km (92,956,000 miles) or about 8.3 light MINUTES. Anything that passes within lunar orbit should be considered a near hit.
It really is scary how much junk is out there flying around and how little money goes into the search for it. This brings me to a fun little thought experiment proposed by Arthur C. Clarke (I assume he originated the idea, I certainly heard it first from him) you take a nuclear bomb and detonate it in space. The resulting shock wave would spread out in all directions and bounce off any object it encountered. Essentially it is like sonar or radar only with the shock wave of a nuclear bomb. We could then see all the "echoes" and plot the vast majority of the junk in the solar system, especially if we set another one off a few weeks/months later and compared the different positions of objects, thus providing some sense of trajectory.