Two derelicts, or just one?

I'm torn between whether the derelict craft we see in Prometheus is the same one on LV-426, or its own ship. I read arguments for it being the same that are plausible. Yet, the ships do appear different as has been pointed out. Here is the head scratcher. Its been said that the 'Alien' derelict could be thousands of years old, that the SJ is fossilized or mummified. If this is the case, then why does its 'warning' signal get picked up by the Nostromo at that particular time. Why not 10 or 20 years earlier (I would go further back, but I'm taking into account the technology of human intergalactic travel)? This must tie the LV-426 ship to Prometheus. The company knew what to look for and when, which is why Ash was put on the Nostromo just before it launched. Yet, 'Alien' is suppose to take place 30 years after Prometheus. Now, it could still be a different vessel. Who's to say that the 'Alien' derelict wasn't carrying a bioweapon to another planet, not Earth like the derelict in Prometheus is implied to do. Which could mean that the events that take place in Prometheus (the film) continue after the credits roll. Ridley did mention a sequel, but said it would steer further away from 'Alien.' Ugh, my head is spinning.