Here is a neat concept for colonizing asteroids I found today:
http://sei.aero/eng/papers/uploads/archive/ISTS_2008-k-13_DysonAsteroidS...
"The Dyson Asteroid Shell (DAS) involves completely enclosing a small body such as an asteroid in a rigid or semi-rigid shell" - first it is covered with roofing, and then the interior can be mined and converted at leisure. This can produce some pretty interesting habitats, either leaving the asteroid in place and with a lush surface, or fully converted into a 3D environment inside the shell.
See http://sei.aero/gallery/index.shtml#unmanned for some pictures (and plenty of other space sketches usable as EP illustrations).
http://sei.aero/gallery/fullres/unmanned-space-systems/Asteroid-Shell-1.jpg
http://sei.aero/gallery/fullres/unmanned-space-systems/Asteroid-Shell-2.jpg
http://sei.aero/gallery/fullres/unmanned-space-systems/MADMEN.jpg
http://sei.aero/gallery/fullres/unmanned-space-systems/PETSAT-Sketch.jpg
http://sei.aero/gallery/fullres/miscellaneous/Robotic-Space-Platform.jpg
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