I wasted soooo many missiles until a suspected this.I find a really nice way to kill turrets on planets is to use beam lasers and drop to the deck and hover just out of their range, you can pop them off in no time. You can hear an audio indication of when you are hitting them. You can see a little puff of smoke when they blow up. The big turrets do seem bugged though, when they are destroyed they don't change model like the small ones, they still look like they are fully operational even though they are not.