WSCAOS is a simple game that is a delight to play. It cuts through the glut of rules that most sci-fi TTRPGs claim you need and gets right to answering the question in its title. (The answer is: what you do in outer space is what's so cool about it!) And if you need more rules, the game is up-front about giving you enthusiastic permission to make them.
It is also eminently hackable as What Is So Cool About Jam demonstrates. Anyone with a cool idea can make a game about it.