 The rest would be reaction mass for the mass-driver engine, which could be arranged to be the spent rocket stages used to launch the payload.Assuming that 100% of the returned asteroid was useful, and that the asteroid miner itself couldn't be reused, that represents nearly a 95% reduction in launch costs.However, the true merits of such a method would depend on a thorough mineral survey of the candidate asteroids; thus far, we have only estimates of their composition.