If you want to play a Sci-fi rpg but you don't want to have to learn an entirely new system, you might want to check out Farsight. It's a sci-fi RPG based on 5th edition D&D. So, if you know that system, then you already know how it all works. The game's up on Kickstarter now.
From the campaign:
FARSIGHT is a pen & paper role-playing game designed to bring science fiction to your table, allowing you to explore pristine worlds, roam the cosmos, and confront with unknown species, alien civilizations and prodigious technologies. Well, provided you can survive the dangers lurking just beyond the edge of the known!
Produced by Lightfish Games during years of design and playtest, FARSIGHT is finally ready to project players into the future!
Its game system, evolved from the 5th edition of the most popular rpg of all time, is simple and easy to learn, but capable of giving you all the depth and details that science fiction demands.
With an extremely flexible character-building system, players will be able to create the heroes they want and forge their own destiny, while game masters will have at their disposal fine-tuned tools for creating endless worlds for their adventures and campaigns.
The campaign's just up and over its funding goal with 29 days left to go.