All this has got me to speculating (rather wildly) about what it would mean if we really were under observation or study by a seriously advanced culture who for whatever reason don't want to reveal themselves.
If I were commanding such an outfit and saw that the planet we were studying was about to be hit by a rock big enough to destroy a city and kill millions, but I don't want to make my presence known to them, what do I do. It is immoral to sit back and do nothing when you have the power to prevent the catastrophe.
Well I shoot the damn thing down high up in the atmosphere so our explosion seems to be just normal meteoric atmospheric destruction.
So maybe we are being protected, we just don't know it.