Field propulsion refers to spacecraft propulsion proposed and researched concepts and production technologies in which thrust is generated by coupling a vehicle to external fields or ambient media rather than by expelling onboard propellant. In this broad sense, field propulsion schemes are thermodynamically open systems that exchange momentum or energy with their surroundings; for example, a field propulsion system may couple itself to photon streams, radiation, magnetized plasma, or planetary magnetospheres. Familiar exemplars include solar sails, electrodynamic tethers, and magnetic sails. By contrast, hypothetical reactionless drives are closed systems that would claim to produce net thrust without any external interaction, widely regarded as violating the law of conservation of momentum and the standard model of physics.

Within aerospace engineering research, the label spans both established and proposed approaches that "push off" external reservoirs: photonic pressure from sunlight (sails), charged particle streams such as the solar wind (magsails and related magnetic structures), and interactions with planetary magnetospheres and ionospheric environments (electrodynamic tethers). In narrower usage, the term also covers efforts to engineer field–matter coupling using electromagnetic propulsion (e.g., electrohydrodynamics and magnetohydrodynamics) as well as speculative mechanisms that draw on general relativity, quantum field theory, or zero-point energy ideas to alter effective inertia or to couple directly to non-particulate fields of space.

Several elements of field-coupled propulsion have been successfully demonstrated in the laboratory, field tests, and in low Earth orbit—most notably, sails and tethers. No field propulsion method has yet been validated as a practical primary propulsion system for interplanetary or interstellar missions, and are currently known to be limited to orbital operations. Even so, the prospect of exchanging momentum with external energy or matter reservoirs (and thereby reducing carried rocket propellant cost, mass, and weight) continues to motivate exploratory work. The topic remains active in targeted programs such as NASA’s former Breakthrough Propulsion Physics Program as well as in studies by national space agencies, academic research groups, and industry organizations that investigate propellantless or externally powered alternatives to conventional rocket engines and electric propulsion systems.

The broad definition of field propulsion refers to propulsion systems in which thrust arises from interactions with external fields or ambient media, rather than from the sustained expulsion of onboard reaction mass or reliance on solid chemical fuels. In this framing, familiar exemplars include solar sails, magnetic sails, and electrodynamic tethers, which couple with external photon, plasma, or magnetic fields instead of expelling onboard propellant. Some field propulsion reviews note that open systems exchange momentum or energy with external media and that proposals of closed-system 'reactionless drive' propulsion are viewed with skepticism because they conflict with conservation of momentum. In contrast, conventional rockets achieve motion by expelling mass. Most commonly, this is the combustion output from chemical propellants to generate thrust via Newton's third law, which is the familiar rocket launch with explosive flame and smoke beneath it. That method has dominated aerospace propulsion since the advent of gunpowder in ancient warfare during the 11th century, initial rocketry concepts in the 14th century, and the internal combustion engine for aviation in the 20th century.

Field propulsion is not a single technology but a spectrum of approaches, ranging from relatively mature concepts that have been tested in flight to highly speculative theoretical constructs. Broad definitions often include solar sail systems, such as the Japan Aerospace Exploration Agency’s (JAXA) IKAROS mission, which demonstrated propulsion by harnessing radiation pressure from sunlight. In a similar spirit, magnetic sail concepts proposed by Dana Andrews and Robert Zubrin envision the use of large magnetic fields to couple with the solar wind and thereby transfer momentum to the spacecraft. Narrower definitions, however, focus on experimental electromagnetic propulsion mechanisms—including electrohydrodynamics (EHD) and magnetohydrodynamics (MHD)—as well as more speculative proposals that invoke general relativity, quantum field theory, or zero-point energy as possible pathways to modify inertia or couple directly to the structured quantum vacuum.

The category of various types of field propulsion concepts investigate mechanisms where motion results from environmental coupling rather than from carrying and ejecting propellant. Examples include systems that attempt to draw on the photon field of sunlight, the charged particles of the solar wind, or the magnetic fields of planetary environments. By interacting with such external reservoirs, a spacecraft could in principle "push off" the surrounding medium, converting environmental energy or momentum into acceleration. Meanwhile, hypothetical reactionless drives and related unproven constructs occupy a set of more controversial spaces: they are framed as closed systems that would claim to generate thrust without any identifiable external interaction. Because they lack a counter-momentum reservoir (a reaction mass, or something to "push off" from), they are widely regarded in the scientific literature as violating myriad physical laws of science.

Although various terrestrial and laboratory-scale systems have provided partial demonstrations, such as solar sail experiments, no field propulsion method had been validated as a reliable, primary propulsion system for practical spaceflight as of 2011. Nonetheless, the topic continues to attract research attention, both because of the theoretical challenges it raises and because of the potential benefits if even one class of these systems could be made viable. Exploratory programs have included the former Breakthrough Propulsion Physics Program at NASA, as well as studies conducted under the auspices of other national space agencies, academic laboratories, space-related organizations, and private industry. Concurrently, options related to non-rocket spacelaunch are also under research.

Open systems comply with the conservation of momentum by transferring it to or from the surrounding environment. For instance, MHD drives accelerate conductive fluids using electromagnetic fields, resulting in thrust through the Lorentz force, with momentum conserved via interaction with external media, such as the interplanetary or interstellar media, or the solar wind.