Hi, I'm about to correct you.
Freighters don't carry weapons because some nation's laws would prohibit them from docking. They aren't even allowed to carry small arms.
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Can I suggest the station isn't invulnerable? If your goal is to sell the game, well, what could sell it more t a quick dose of adrenaline, and the fact that that beautiful station was bombed? It'd demonstrate the fluidity of the universe. There wouldn't be any set game mechanic where pirates show up at a certain time, it'd be subject to the same attack-defense rules as the rest of the galaxy. So you could be a newb, 30 minutes into the game and an innocent bystander. You'd see boarding action and space combat. You could defend the station or aid the pirates.