One of the standouts of the NASA Centennial Challenge to building structures on Mars is Solar Crafting, which developed a 3D printing system supported by a balloon. The process involves several robots who clear the local site, collect raw materials and feed it into a large-scale 3D printer that melts the sand using a solar power energy beam. The balloon not only acts as a support system for the printing gantry but also collects the sunlight that powers the melting beam. You can find out more about this awesome concept over on 3DPrint.com: http://3dprint.com/96812/solar-crafting-3d-print-mars/