Relying on spare parts from Earth for anything important is impossible, since it takes months to years for a one-way trip depending on where in their orbits the

Relying on spare parts from Earth for anything important is impossible, since it takes months to years for a one-way trip depending on where in their orbits the two planets are. I'm just speculating now, but I think this might be one reason Musk is so interested in robotic manufacturing (e.g. the failed "alien dreadnought" vision for the Model 3 production line). If you have mostly automated, flexible manufacturing systems that can be computer controlled and require minimal human expertise to operate then all you need is the computer files for whatever spare part you need to build. Set up 3-4 of these manufacturing centers at different spots for redundancy and you don't have to worry so much about spare part availability. The first effort (Model 3 assembly) was a disaster though.