One thing that I’ve used well is to set up a conflict between two or three big players in the area of space I know the players are going to be in. Nothing on the level of outright open warfare (at least, not at first), but some sort of conflict all three sides will be willing to kill over. Then I describe for my own use their methods and the resources they have. Finally, I’ll describe some low-level NPCs the heroes might run into, and the level of lieutenants just above them.
Then I craft the first adventure based on this info, having the PCs land smack-dab in the middle of some little plot by one of the factions. I make no reference to the larger conflict, and probably don’t directly link the people the PCs will be dealing with to their larger faction.
Then, after adventure one, I cut the players loose to do their trading or exploring or whatever it is they want to do with their starship. But every time they seem to get excited about or invest emotionally in something, I look for a way to tie it into the big fight. And I’m always updating what the different sides are doing. Did a plan succeed? How will that affect the PCs’ lives? Have the PCs done anything to affect any of the competing factions?
If I do my job right, about the time the PCs arrive at their mid-levels or have their feet under them or whatever defines mid-level play in your game, they should be actively seeking out confrontations with at least one of the factions. They shouldn’t understand what the fight is really about yet, that’s for the rising action and climax. But they should at least be getting an inkling of an idea that there’s something bigger going on behind the scenes.
4 hours ago