Riding Light and Gravity: Propellantless Tech Aims for Deep-Space Travel

A comprehensive review surveys propellantless propulsion methods—gravity assists, solar sails, magnetic sails, and electric sails—that draw momentum from planets, sunlight, and the solar wind to move spacecraft without onboard propellant. Gravity assists are proven but require favorable planetary alignments, solar sails offer steady thrust with large, delicate membranes, magnetic and electric sails promise higher acceleration but depend on technologies not yet realized. No single solution fits all missions, but together these approaches could transform solar-system and interstellar exploration, reducing or eliminating the need to carry propellant.