Asteroids as Fuel
Asteroids are not only a source of metals and minerals, there are also asteroids with large amounts of water. From water we can make rocket fuel. That means instead of bringing fuel to the asteroids, we can manufacture it there. To bring 1 kg of space craft to an asteroid only takes 4.3 kg of fuel. If fuel is manufactured there, it could bring back any amount of material back to earth orbit at the same cost - 4.3 kg of fuel per kilogram of material. Sending 1 kg of space craft requires 4.3 kg of fuel, it then generates 430 kg of fuel for every 100 kg of payload to return to Earth orbit. Part of this payload could be water making it possible to generate fuel from Earth orbit without sending it up from Earth at a cost of 27 kg of fuel for every 1 kg of fuel in orbit. Bottom line : By establishing fuel generators on asteroids, we have nearly unlimited resources available from the asteroid belt.