In Alien it was established that the Nostomo's journey time back to Earth was approximately 13 months. So it takes an FTL ship about 13 months to travel 40 light years, I am assuming a shuttle wouldn't have FTL capacity (or possibly limited FTL capacity? The shuttle did have a hypersleep capsule) Ripley would have burned all her fuel on course for Earth, did she reach light speed or a percentage of light speed? If so maybe she could have drifted 40 plus light years, but I agree it is a bit far fetched.
With 57 years of colonial expansion taken into consideration, the core systems could have encompassed more systems and expanded greatly since the time of the Nostromo's ill fated voyage, and possibly Burke was talking of the core systems of the outlying colonies, which the Narcissus drifted past? Doesn't really make sense though as you would assume the core systems would be our solar system and nearby colony systems.
The Alien franchise has never really acknowledged relativity either, on the Nostromo's journey (if they hadn't stopped off at LV426) travelling at or near to light speed, would mean Ripley's daughter would have been long dead anyway by the time the Nostromo made it's scheduled return to Earth. (Or does relativity not enter into the equation if you are travelling Faster Than Light?).
I suppose these are things that you just have to suspend disbelief over and it doesn't really affect your enjoyment of the films, but it's sort of got my mind working and I will see if I can come up with a theory to explain it logically.
I know this is a bit off topic but it's remimded me of another little lapse in logic regarding Aliens, Hicks says that once they are declared overdue they will have to wait seventeen days for a rescue team. So obviously there is a colony/space station/outpost 17 days travel from LV426, why then would the company send a rescue team 40 light years from Earth, instead of sending someone who could get there in seventeen days once contact was lost?
