...with all the support and power facilities that entails? It sounds like a very ambitious project, to say the least. I know that large paraboloid antennas can bounce radar pulses off the planets, but to maintain a full 4pi steradian radar coverage (in all directions with a fast repetition cycle, since comets can approach from anywhere and they move pretty quickly in cis-solar space!) and still give several months warning against relatively tiny objects which might be on the order of several AU away...this sounds like a Herculean task even from earth's surface.

Not only that, the system would have to be precise enough to provide highly accurate impact forecasts in a reasonable amount of time so target cities or coastlines could be evacuated. For the bigger, planet-busting "dinosaur killers", they're really doesn't seem to be much point in doing anything at all, unless we have years of warning and time to implement an orbital deflection program of some sort. Of course, for this type of object, an earth-based optical survey would probably be sufficient.

As for the LIDAR approach, that sounds intriguing. It looks like that could do double-duty as an active SETI beacon!

Perhaps you could direct me to that NASA Langley study, I'd like to crunch the numbers for myself.