Speculative: With no ability to trace Whisper appearances and new outbreaks on Venus, MarCo restricts travel in Sol. This dark period sees the end of the Stellarum megacorp. Ganymede saw Luna as its sister colony and is hit particularly hard by the end of Luna One. Ganymede and Europa hostility flares as suspicion of TTI increases, but the two colonies grow to rely on each other. “Ganymede grows up.”
This is an abrupt and catastrophic end to the golden age, and echoes through much of the 500s.
Reference: Suggested by events in history and conversations with game author.
Date Notes: Likely ending, 540-550, establishment of TTI as Megacorp