It's not a surprise stuff like this is detected when it's already close to Earth. Brightness decreases exponentially with distance, so it's much
easier to see stuff the closer it is.
If these were aliens trying to attack us I would think they would:
1) Have better aim
2) Throw a lot more rocks and more closely together
But then why attack us? Anything the Earth has to offer they can get far more easily from the rest of the Solar System. Unless of course they want
us. Then why throw rocks that could kill us off?