Since most of the stages have a prototype, I wonder when the space stage prototype will be in the game?
Hello, @Coolberry! A space stage prototype really isn’t very far away - in the latest progress update there’s quite a few screenshots you can see of it! If I’m not wrong, it’s going to be added to the game in the next release, which isn’t far off now! I hope this reply helped.
Yep, that’s right. I just finished the space prototype this week. And if you are not our patron, you get the prototype in the next release (0.6.3). Of course as Thrive is open source anyone can compile the game from source code to get the latest version.
Here’s hoping for a full n-body orbital mechanics simulation in the prototype. 
That would have been extremely time consuming to implement. It is not shown in any of the progress screenshots, so don’t expect it. The screenshot literally has everything showing: a fake star, one planet, one fleet and building some space stuff. Nothing except the space fleet can even move around.
I’m aware of that. My comment was merely a joke. The Kerbal Space Program team never even considered n-body physics (calculating gravitational influences from multiple planetary bodies acting on an object), as it was far to complicated to program, and would generally make little difference to the game.