The asteroids of paramount importance to planetary defenders are near-Earth asteroids, those whose orbits around the Sun bring them at least as close as 28 million miles of Earth’s own orbit. Any asteroid that’s at least 460 feet long is capable of destroying a large city, but those smaller than that can also cause significant damage. Even an asteroid just a few dozen feet long can, with a direct hit, wreck a city with a mid-air blast akin to a non-radioactive nuclear explosion. Frustratingly, the smaller an asteroid is, the harder it is to see.