Space Engineers is, in the words of its developer, "a sandbox game about engineering, construction, exploration and survival in space and on planets." You can build space stations, planetary outposts, and pilot ships.
"Space Engineers is inspired by reality and how things actually work," they say. "Think about modern-day NASA technology extrapolated 60 years into the future. Space Engineers strives to follow the laws of physics and doesn't use technologies that wouldn't be feasible in the near future."
As well as a 50% off sale on Steam this weekend, the game will also be free to play. Check your Steam library and it'll be sitting in there waiting for you. It's an Early Access game, but packed with features, including the recent addition of moddable, destructible planets. You can read all about that here.