How far along is this game?

Its past tech demo now, I'd call it alpha stage. They're still building features and systems and extending what is in the engine into actual gameplay.

Similar to very early pre-steam KSP, but 100x as beautiful and 100x as performant. They are consciously avoiding several of the engine pitfalls KSP was known for and are using more modern techniques.

Overall I'm really happy and excited. It's moving fast but also keeping quality fairly high.

They’ve officially labeled it pre-alpha.

Right now, it’s a basic simulator set in our solar system. They haven’t really added many gameplay elements yet: for example, they just added the ability of high g-forces/impact to destroy ships/cats (just disappear, no explosions, no incremental damage.) Fictional solar system, resources (money, xp/science, etc.), missions/objectives, etc. are still a ways off.

They’re currently testing a Steam-like game client that will allow game updates without manually reinstalling every time a new version is released, so I plan to at least wait for that.

Interstellar travel is so damn cool. I'm so excited for this game.