This article is from the Alien Movies FAQ, by Darryll Hobsonand Eelko de Vos E.W.C.deVos@TWI.TUDelft.NL with numerous contributions by others.
- They knew about the derelict ship from the beacon signal that was picked
up by another space craft, maybe off course and with some technical
problems so that they could not investigate it themselves, or maybe it was
picked up by some automatic exploration vessel. Back on earth they had
enough computer power to unscramble the beacon. (remember that "Mother"
couldn't unscramble it completely). Some department of Weyland Yutani
decided to bring the next ship that came around that area close to LV-426.
It would then have to check out what was going on on the surface (this was
in the contract they signed). After the Nostromo was destroyed and didn't
return, the people who made the Nostromo alter its route got scared -in
the end they were responsible for the destruction of the Nostromo- and
deleted all files concerning the Nostromo's new route and LV-426. Ash was
planted on board for that reason: to find out what was on LV-426 and bring
it to them. They knew about a hostile creature from the beacon, but they
didn't expect something like "this". [this theory is supported by the
novelisation]
- The same reasoning as above, except for the fact that Weyland Yutani knew
all about the aliens from the beacon. Some people claim that Ash knows
everything about the alien lifeform. This seems unlikely because of the way
Ash tries to get rid of the facehugger on Kane.
- They did not know everything about the aliens and just wanted to see what
happens ("crew expendable"), and Ash was supposed to store all information
in the computer (Ash gives us a few details about the aliens, but he does
not necessarily know everything from the start, he might have gathered
some things from what he has already seen on the Nostromo). Later, when
the Nostromo had returned to earth with it's autopilot, they could first
remote-access the computer and then, with all the information, decide
how to get out the alien eggs (or whatever was found to be there). In
that case they would have only lost 6 employees and perhaps an expensive
android.
- The ship diverting to investigate the beacon (and assuming that the "crew
is expendable") is part of a standard procedure. All data collected
would be returned to the company when the ship returns to Earth. Since
the Nostromo did not return to Earth, the company did not know about
the aliens. (this theory assumes that no communication occurred between
the Nostromo and The Company AND that Ash was not necessarily "planted" on
the Nostromo for sinister reasons) [contradicting the novelisation
and the movie: there was a specific order to bring back the lifeform]
 
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