Raw materials could be brought from asteroid

Garrett P. Serviss : writer, astronomer
Minerals and volatiles will be mined from an asteroid or spent comet then taken back to Earth or used in space for space-construction materials. Materials that could be mined or extracted include iron, nickel, titanium for construction, water and oxygen to sustain the lives of prospector-astronauts on site, as well as hydrogen and oxygen for use as rocket fuel. In space exploration, using resources gathered whilst on a journey is referred to as in-situ resource utilization.

Novel "Edison's Conquest of Mars"