The High Frontier Principle (Dr. Gerard K. O'Neill):
Launching heavy payloads from Earth's deep gravitational well demands massive rocket fuel ratios and spews thousands of tons of greenhouse gases and alumina directly into the fragile upper atmosphere. By using lunar mass-drivers (electric catapults) and redirecting small carbonaceous/metallic asteroids into high lunar orbit, humanity can assemble space habitats and solar power stations with zero Earth emissions and over 5x payload energy efficiency.