Near Miss

This kind of thing is always a little disconcerting. That said, I'm not sure how productive it's going to be to put lots of research and effort into deflecting asteroids with current technology. Current launch technology is very expensive, it's slow to set up, and all the research needed to know the various methods of deflecting hurtling rocks or cometary fragments would take ages to perform... so if we end up in that situation in the next twenty years, we're a) probably not going to have enough time to respond, and b) won't know what do to about it anyhow. I mean, it's been argued many times before -- nukes might just break it into a thousand smaller but equally lethal pieces, and any vehicle we might send up to push it into a new trajectory... well, we don't know what that would take, energy-wise. And finding out would take enormous amounts of time and money.

I'm pretty confident that the problem will solve itself eventually, however. If and when the coming space tourism boom happens, it will make all kinds of new things possible. Big business in low Earth orbit will lower costs dramatically, leading to the mastery of all kinds of basic space technology -- habitats, construction, propulsion, etc. Lower costs in themselves create all kind of new possibilities -- for example, if you told a senior politician today that you could set up a solar-system covering detection system that could track every rock in Earth-crossing orbit for multiple billions of dollars, he'll probably show you the door. But if it's just a hundred million, and most of the technology has already been created as spin-offs from other civilian ventures, it all becomes more feasible.

Next thing -- the private sector will start tracking and studying asteroids anyhow, for mining purposes. Even cheap transport from Earth will still likely be more expensive than having your own raw mineral sources already in orbit, climbing out of a gravity well is always going to be expensive, especially in the kinds of volumes large construction projects might require. Plus asteroids have lots of rare metals that might even become economically feasible to sell back to Earth. So if the mining companies start going after asteroids, they'll have to find a) where they are, and b) what they're made of. That information will give any future asteroid-shield agency most of what they need to know in order to operate. Telescope arrays in orbit or on the moon ought to then be able to find anything within a few years of hitting the Earth (unless it's one of those rogues from the Ort Cloud that bullseyes Earth on its first pass -- but that's astronomically unlikely, usually they whizz around a few dozen (or million) times before hitting anything) and it'll then be dealt with by the agency in question (some branch of the yet-to-be-invented International Space Administration, I guess) using technology largely pioneered by asteroid miners.

All this raises some interesting complications, however. Shifting asteroid orbits, even if purely for mining purposes, is going to be very sensitive. These things can kill planets, after all. Imagine the paperwork required to move a two-kilometer diameter chunk of rock into near-Earth-orbit (if you needed to... maybe you wouldn't).