We can find most smaller space rocks pretty easily, and we've yet to find one that is a danger.

But we haven't begun to look for non-periodic comets, a la Shoemaker-Levy variety. These buggers are on million-year orbits around the Sun. They come barrelling toward us out of deep space, from so far out that by the time they pass pluto they are already close to maximum velocity. At 70,000 mph + they can travel from Pluto to Earth, a distance of 30 AUs, in just 6 years. There is no warning. They don't circle around the Sun for another pass. It's a one-way trip. They practically follow a straight line all the way in.

If we needed 10 years to deflect such an object, we would have to design a spacecraft that could rendezvous with it at a distance of 50 AUs from the Sun. If the spacecraft could be launched from Earth from the moment of detection, and travel out to meet the rock at the same speed that it was travelling toward us, roughly 5 AU per year, then we would have to be able to detect the rock at a distance of 100 AUs from the Sun, or 20 years before impact.

The spacecraft would be gigantic. It would lauch with a velocity of 70,000 mph, and after 10 years in transit be capable of reversing direction and altering speed by 140,000 mph. Then, after "docking" with the rock, it would have to push it off course. This would be the most powerful and sophisticated spacecraft ever built.

But even before all this is done, we would need a space telescope that could detect these rocks. Notwithstanding the more remote existence of a 10-km diameter planet-killer asteroid like the one that killed the dinosaurs, dangerous rocks capable of destroying a city might be 1000 meters in diameter. The space telescope would therefore need a "conservative" resolution of 1000 meters at a distance of greater than 15 trillion meters (100 AU). And it would have to be able to estimate mass, trajectory, and scan the entire sky in both the northern and southern hemispheres. It would have to do this perhaps within weeks, so that a response could be put into action.