Remember the episode were sg1 goes to a planet and screws up the sun i believe it was called "Red Sky". Well at the end they say I dont think we were the ones that fixed the problem. Meaning that it was the asgard who fixed it. They couldnt fix the problem in the first place because of the protected planets treaty. This was also the case when the asteroid was heading to earth the asgard could not interfear. Unless they did it but covered it up so the gou'ld could not call there bluff. What I am trying to say is the asgard could have caused the asteroid to go right through the planet with out causing any damage.