Once it gets within a few tens of megameters, pulsed radar systems could easily distinguish an incoming projectile thanks to the 2.4 second round-trip-time to t

Once it gets within a few tens of megameters, pulsed radar systems could easily distinguish an incoming projectile thanks to the 2.4 second round-trip-time to the moon. Metallic objects have ideal visibility. If you are concerned about solar thermal radio emissions bouncing off the moon and reducing SNR, that could be an issue -- a phase-of-the-moon bug in the system, if you will. But based on what I’ve overheard from friends working on 21-cm radar teaching telescopes, I doubt the luminosity of the continuous background is high enough to pose a big problem, assuming you have a decently large antenna. I’m more concerned that there’s nothing much you can do to redirect such an object in the few hours it takes to fall on your head. By that time it will have reached 4km/s.