we will never find out before it hits, even if we could see it comming, there is not much we could practically do, it will be very quick to evacuate areas, announcement could just create local, global, social and economical chaos.

imagine if an approaching asteroid was announced with rate chances 39/70 was announced and then skipped orbit by a tiny millimeter, avoiding earth by chance.
big mess

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<span style='color:purple'><span style='font-family:Impact'>i dont know much </span></span>