Plus we would probably have a rather long warning of their approach due to how far out they would most likely begin adjusting their orbit, which would make an ever growing signal for us to detect. As we add more and more to our telescope capacity the odds of detecting them earlier go up, meaning we probably have a year, if not more, before they come within contact range. Assuming we can figure out WHAT it is, that gives us a lot of time to get defensive tools in place.
I feel like this is the point that we start to try to deflect/destroy these rocks before they get to the Earth. It might be a desperate attempt, but it's what we'd end up doing. Plus if someone started lobbing rocks at the Earth, we'd see it coming, and again, unless the aliens are incredibly good at stealth/cloaking, we'd know they are here, so we would have some degree of preparation in place. How one would prepare for rocks slamming into the Earth though....there wouldn't be very many places to hide, would there?
Would some alien race interested in taking (colonizing) the planet want to lob rocks at it in the first place? Wouldn't that possibly endanger the habitability of it? Or am I just inventing that?
I feel like their is a severe over-estimation in what we'd be able to actually see/detect. yes we are getting better at tracking NEO's, but an alien armada is likely to be extremely tiny in comparison to the size of most asteroids. the resolution and amount of data to track such tiny objects is massive, and we're not exactly pouring funding into this topic. hell if they approach from the sun, we'd never even see them coming. just think about the Russian meteor last year that no one even detected was heading for us.