Mongoose Traveller is a great game, with TONS of support.
I am currently playing Stars Without Number. It's an OSR rule set, with a more open setting than Traveller. However it is VERY influenced by the classic space RPG. SWN also has options for making it more science fantasy like Starfinder. If that's what you're going for.
I've also heard good things about Mothership and Alien, if you are going for a more space horror vibe.