For scientists exploring the possibility of human settlements beyond Earth, Mars has long been the planet of choice with its CO2-rich atmosphere and 24-hour cycle. 

But it's not the only candidate. Finnish astrophysicist Pekka Janhunen has proposed Ceres, a dwarf planet in the asteroid belt between Mars and Jupiter as the site for an orbiting megasatellite.  Ceres is the same distance from Earth as Mars and has an abundance of nitrogen which is vital for creating an atmosphere. An orbiting base could be configured to create artificial gravity and with a modular structure could easily grow or shrink as required.

Space elevators could be used to bring material up from the planet's surface, but any settlement would need to import key minerals such as phosphorous to be sustainable. The colony would also rely on a number of unproven technologies required for space mining.