I dunno, I guess there could be hundreds of ships with SJ crews in hypersleep, but it's pretty amazing for one group of SJ's to meet a new race and decide to go off and invade their homeworld, without telling the others. It's pretty risky of the humans to sacrifice their only ship (that we know of) and hope the other SJ's don't notice the big explosion and crash.
i have a theory about this.
i think that, at the beginning of the movie, the "space jockeys" are a long extinct, ancient race of biological engineers. the ampules contain their genetic code, which is what infects the crew. note that the alien we see walking around noticably varies in size -- i think he's a former member of the prometheus crew.
Maybe there are more ships but only enough SJ's left to crew one.
well, i think, either way the one on LV-426 is way, way older than this one. and that in this movie, it's really only that one ship they have to worry for the time being -- until the end, which might end on a cliffhanger of "but wait there's more!"
Arachnophilia - Uranus's rings are pretty far from Saturn's rings.
proportionally, uranus's rings
look very much like saturn's. except that they're on sideways, because the planet's poles are roughly on the ecliptic, as opposed to roughly perpendicular to it.
The change in landscape is more of a stretch. I think it needs explaining, if it's the same place, but I've seen dumber things happen in movies. Like the flashback scene in Robin Hood. Ridley has shown us that there are times when he simply does_not_care.
and that's fine. maybe it's a f**k up. but considering the whole "not a prequel" schtick he's been on lately, i think the safer bet is on it being a different place. i mean, he'd have gotten the moon-of-a-ringed-gas-giant thing right. i think cameron even f**ked that one up. who remembers that kind of obscure detail?