Why did Weyland not send a rescue mission?

Hi, I have been following this fantastic site for ages now and have just registered as a user, so apologies if what I am going to talk about has already been raised as a discussion. What I would like to know is that in the first film, 'Alien', the crew of the Nostromo were instructed that if they receive a distress message or something similar it was the companies policy to investigate, but surely if the comapany sent the Prometheus on a mission to investigate the possibility of life on another planet, 30 years earlier, I would have thought that as soon as they lost contact or a certain time had elasped when Weyland relealised that something had gone wrong, that some sort of rescue mission would have been made. The other theory would be that the Nostromo was sent into this region of space on purpose as the company knew that a failed mission had taken place, but if this was the case why leave it for so long with not knowing what had happened to the crew of the Prometheus.