The Asteroid IS the Spacecraft

Why would an advanced space-faring species create monstrously inefficient metal constructs for their space-going vessels? Sure, smaller shuttle craft or even “destroyer” class vehicles I can see… But the whole idea of Star Wars’ “That’s no moon - it’s a space station!” seems ridiculous on further thought. I mean, there’s most likely a host of asteroids in a star system (or even a whole belt of them in the event of a earlier planet’s demise.) With behemoths such as these, things like steel girders seem ludicrous, unless they're constructed out of thin air in some manner. Even if they're molecularly synthesized somehow, such construction still seems grossly inefficient and slow in comparison to simply locating an appropriately sized asteroid and forming it into whatever type of vehicle was desired.

At the presumed sizes where this would make sense (dimensions surpassing miles or kilometers), whatever propulsion system that might be used could be fitted onto the naturally occurring structure, and explosives used to shape the asteroid into something more appropriate. Considering the lack of need for any aerodynamics though, such efforts would probably need to be minimal, especially given the huge number of available asteroids to choose from as an appropriate subject for spacecraft-forming.

Once an asteroid was chosen, it would also be relatively simple (in comparsion to the infamous Death Star’s creation, for example) to tunnel through the thing, creating vast labyrinths. These would have the advantage of being deep underground, naturally protected from certain forms of attack. Screw the need for deflector shields!

These large vessels would not need to be fast moving, and might have slowly decaying orbits around planets themselves. Not only would they be naturally camouflaged, but using gravitationally assisted propulsion (perhaps occasionally complemented by technological means), such a vessel or base might be able to slowly navigate throughout a solar system, or even used as the ultimate doomsday weapon against a planetary based civilization if an interception course was plotted.

These are the things that enter my brain after watching a few too many episodes of Red Dwarf in a row. The notion of an asteroid spacecraft or space station brings to mind tunnels, caves, and the dungeon crawl experience. If you’re looking for a way to marry such a thing with a science fantasy setting, this seems like an excellent way to introduce the subject in some manner, complete with scientific rationale.

If that’s not enough food for thought, check out this theory that Phobos, the misshapen moon of Mars is exactly this type of ancient, asteroid/moon-sized and derived spacecraft. Agree with the premise there or not, it makes for some entertaining reading.