low energy threshold for a small-scale harvesting operation, and air friction is far more forgiving at 200 km than at 100 km. Note that S. T. Demetriades pioneered in space propulsion, from the atomic oxygen ramjet (he proved it not feasible in the 1950s) to nuclear, ion, and plasma thrusters. He received the 2010 AIAA Award for Plasmadynamics and Lasers. Propellant depots Boeing has suggested a non-extractive propellant depot, or "space gas station," which accumulates material launched from the planet at low cost, allowing future lunar missions without the need for large launch vehicles like the Saturn V. MIT has recently proposed a similar plan which would store emergency fuel reserves left over from lunar missions. See also List of plasma (physics) articles Atmosphere-breathing electric propulsion References Spaceflight concepts Spacecraft propulsion Space access