For starters, the job of planetary defense is NOT to take out earth-killing space rocks, but to identify them--preferably as soon as possible. We can worry about how to avoid the collision later, after we know for sure one is coming and when.

Finding out which asteroids are a threat means studying and cataloging their orbits years, even centuries, ahead of time. There's really not much we can do about small rocks that sneak up on us like this latest near-miss, but getting accurate and updated orbital data on all potential hazards will give us years to get ready, and years to do something about it. In fact, a small nudge to an orbit decades ahead of time is much more effective than diverting or destroying a collider while its on final approach. It will not only be more effective, it will be faster and cheaper.

Even identifying future collisions that cannot be avoided will give us time to evacuate threatened areas, stockpile emergency supplies and take other steps to mitigate the damage. What we really need is time, time to find these objects, time to calculate their orbits precisely, and time to do something about it, early on while its still easy.

As for the very rare, but still possible, planet-buster, well...there's not a damn thing we can, or even should, do about that. But all the others can either be deflected, destroyed, or their ill effects mitigated. We already have organizations set up to do that, but they are not properly supported or funded by governments and they can do little about any threat they do identify except publicize it.

Fortunately, we already have the technology, procedures and talent to do all this with existing telescopes and computers, and it really isn't all that expensive; we can even enlist citizen-volunteer skywatchers from the amateur astronomy community, like we do now with SETI-At-Home, or the American Association of Variable Star Observers. They could collect data which would be sent to central clearing houses to process, prioritize and schedule professional telescope time. This would involve searches for new, undiscovered space rocks and the calculation of their orbits, as well as scrutiny of known "safe" asteroids to make sure their orbits will not be perturbed by some future encounter with another body that might send them to a rendezvous with Earth, perhaps decades in the future.

Setting up and funding an organizing agency to coordinate all this could very easily be done. In fact, some universities and observatories already specialize in this, although not at a sufficiently large or thorough scale. It would certainly cost much less than The Wall, and it would work a lot better at keeping out unwanted visitors.