Hey hey, just wanted to add some details lol
Interea is what we call a spin-off: It's when another story is made to focus on a specific character of event (in this case, to focus on the Moon Tour). Therefore, it takes place DURING Adastra, hence why you should play it before Khemia.
Khemia however is a sequel, the events take place after Adastra. Though I must say, I'll call it a "half-sequel" cuz you don't get to play the human, but I won't spoil ;)