I mean all the features, except main missions, tormented, and prospector. And none of them load up high scores.
Aside from Colonial Space and highscores, it should work perfectly fine without an internet connection.
Colonial space that is maps created by other players, can be accessed only by internet since you have to download them. After you download a map you can play it anytime.
Similarly highscores require internet connection to download the highscore list and upload your score.
So the game itself don't need internet but if you want to do anything that's not on your computer then yes you need the internet.
So without internet connection you can play:
Main Story
Prospector zone
Tormented space
Alpha sector
Credits
Editor
Previously downloaded colonial space maps.
The DMD (auto-generated maps) also work without Internet access.