Propellantless spacecraft propulsion
Propellantless spacecraft propulsion is any spacecraft propulsion system that accelerates the craft by interacting with the environment rather than expelling on-board propellant, which is the more traditional approach used by chemical and electric propulsion systems. Various physical phenomena can be exploited to achieve propulsion without propellant, most notably gravity, radiation pressure, solar wind, planetary magnetic fields or aerodynamic drag in the upper atmosphere.
Several propellantless technologies have been demonstrated in flight, while others are at varying stages of development. Aerobraking was first demonstrated around Earth by Hiten (1991), and has since been used by several interplanetary spacecraft. Solar sailing, while not widely used, has been demonstrated in flight by several spacecraft, including interplanetary ones. Space tethers of various types have also been flown on several occasions as far back as Gemini 11 (1966).
Field propulsion is a related term used by some authors to describe proposed propellantless propulsion systems, mostly in the domain of advanced propulsion concepts that would require significant theoretical or technological innovation.
Unlike reactionless propulsion, propellantless propulsion systems need not contradict the law of conservation of momentum since momentum can be exchanged with the environment.