When launching interplanetary probes from the surface of Earth, carrying all energy needed for the long-duration mission, payload quantities are necessarily extremely limited, due to the basis mass limitations described theoretically by the rocket equation. One alternative to transport more mass on interplanetary trajectories is to use up nearly all of the upper stage propellant on launch, and then refill propellants in Earth orbit before firing the rocket to escape velocity for a heliocentric trajectory. These propellants could be stored on orbit at a propellant depot, or carried to orbit in a propellant tanker to be directly transferred to the interplanetary spacecraft. For returning mass to Earth, a related option is to mine raw materials from a solar system celestial object, refine, process, and store the reaction products (propellant) on the Solar System body until such time as a vehicle needs to be loaded for launch.