(at around 2 mins) At the start of the film, a probe finds parts of the wreckage of the Nostromo floating in space near Zeta² Reticuli following its destruction a couple of decades earlier at the end of
Alien (1979). Despite it being dubious there would be any wreckage left after what was such a monumental explosion, any parts of the ship that were not vaporised would not be simply floating in space around the area the explosion happened decades later. The force of the Nostromo's self destruct would have projected remnants of it in all directions at considerable speed, and since this is the vacuum of space, the remnants wouldn't slow down, they would simply keep flying through space at speed.
