[quote=@Durandal]
That could work. Maybe the Imperium did it as a way to consolidate power away from some earlier Earth-based nation.
[/quote]One of my ideas include the Martians messing up Mars so much it exploded. Maybe it also messed up Earth a bit and mankind chose to abandon the solar system?
Then they started over and built their galactic empire we know.
Depending on the size of the Arcturan Empire and its age I can even change the travel method of the ship from STL to slow FTL. Whichever fits.