propellantless

Propellantless propulsion refers to methods of generating thrust without expelling reaction mass. It relies on interactions with external fields, particles, or radiation to impart momentum to a spacecraft, in contrast to conventional rocket propulsion that carries and expels propellant.

Common approaches include photon-based propulsion and beamed-energy concepts. Photon propulsion uses momentum transfer from light to produce thrust, most notably in solar sails that capture momentum from sunlight. Beamed-energy propulsion envisions an external energy source directing momentum to a sail or absorber on the spacecraft. The most mature demonstrations of solar sails have included JAXA’s IKAROS and The Planetary Society’s LightSail missions, which have shown that continuous thrust is possible, though the resulting forces are very small and demand large, lightweight structures and extended flight times.

Other propellantless ideas rely on interactions with ambient plasma or magnetic fields. Electric sails (E-sails) use long charged tethers to deflect solar wind protons, generating thrust without onboard propellant. Magnetic sails (magsails) aim to use magnetic fields to accomplish a similar momentum transfer with the solar wind. Electrodynamic tethers can exchange momentum and electrical power with a planet’s magnetosphere and ionosphere, offering thrust or power without propellant. These concepts have undergone space-based testing, but practical, long-duration propulsion remains under investigation.

Status and outlook: Propellantless propulsion is not yet widely deployed. Solar sails and related concepts are the most developed, but face significant engineering and environmental challenges. In many cases, thrust levels are small and mission design hinges on favorable space conditions and massive, lightweight structures.