For space farming and for large space fleets, Space Colonies required materials harnessed from any source other than Earth. The Moon supplied materials at 3% of Earth costs, but dependence on one supplier left colonies vulnerable. By the late 23rd century, colonies had adapted intelligent robot vehicles to mine deep space. These robots traveled into the asteroid belt and flung asteroids back to the colonies at a steady rate, delivering raw materials at a cost much lower than if they were brought from the Earth surface. Using hollowed asteroids as a foundation, Cities created large orbital farms, capable of supporting thousands.