Hi !
I have some additional details about scientifc problems ^^
For the gravity of LV-426, as it was said, it's really impossible (if it's 1200 km).
We have a body which is 10 times smaller than the Earth. With a diameter 10 times smaller than the Earth, LV-426 would have a mass 1000 times less than the one of the Earth. Or gravity is proportional to mass and inversely proportional to the square of the distance from the center. Therefore, the gravity on the surface of such a planetoid would be 10 times weaker. Eventually, for about 0.86 G, the density of LV-426 would have to be 8 times greater than that of the Earth. Or, the density of the Earth is already high enough (5.5) and there is no material that predicts such a difference. If the planetoid were made of platinum (21.4), we would have a density 4 times greater but not 10 times.
So it's really impossible what we see in the movie, again, if it's 1200 km ^^. Personaly I prefer 1200 km because I like the idea that it's a tiny object. Imagine that LV-426 is almost the size of the Earth ... It bothers me ^^
After, we have the FTL, so that of course is impossible.
And another thing that hasn't been mentioned yet : The system of propulsion of the Nostromo (with the refinery) which is not compatible with the center of gravity of the vessel. Such a vessel would turn on itself at the slightest impulse because the system of propulsion of the ship is not aligned with the center of gravity.
The perspectives with the planet with the moon are exaggerated, when we know that Calpamos is 306 400 km in diameter.
For the rest I'm not competent enough, only astronomical data ^^ I think that there are a lot of other problems but those are the most significant I think
