Field propulsion
Field propulsion comprises proposed and researched concepts and production technologies of spacecraft propulsion 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, charged particle streams such as the solar wind, and interactions with planetary magnetospheres and ionospheric environments. In narrower usage, the term also covers efforts to engineer field–matter coupling using electromagnetic propulsion 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 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.
Definition
The term field propulsion refers to propellant-less 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. Examples include solar sails, magnetic sails, and electrodynamic tethers, which couple with external photon, plasma, or magnetic fields instead of expelling onboard propellant. Various types of field propulsion concepts include mechanisms where motion results from environmental coupling rather than from carrying and ejecting propellant. Field propulsion is not a single technology but a spectrum of approaches, ranging from 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 IKAROS mission, which demonstrated propulsion by harnessing radiation pressure from sunlight. 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. 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 and magnetohydrodynamics, 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. By interacting with such external reservoirs, a spacecraft can "push off" the surrounding medium, converting environmental energy or momentum into acceleration. 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.
Conservation of momentum is a fundamental requirement of propulsion systems because momentum is always conserved. This conservation law is implicit in the published work of Isaac Newton and Galileo Galilei, but arises on a fundamental level from the spatial translation symmetry of the laws of physics, as given by Noether's theorem. 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 solar winds. Open systems comply with the conservation of momentum by transferring it to or from the surrounding environment. Environment-coupled approaches such as sails, tethers, or plasma-wave coupling remain possible if the method of external coupling is strong enough. Reviews of field propulsion concepts emphasize that any open system must exchange momentum with an external medium such as photons, plasma, or magnetic fields, while closed-system "reactionless" claims conflict with this framework.
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 thermodynamic laws. In contrast, such reactionless drives are hypothetical closed systems that claim to produce thrust without exchanging momentum with an external entity, thereby violating the conservation of momentum, and are widely regarded as inconsistent with established scientific principles. Momentum conservation is the fundamental boundary on all propulsion concepts. Academic reviews echo this conclusion, stating that propulsion systems which generate thrust without reaction mass or interaction with external fields are regarded as inconsistent with the present framework of physics. Any propulsion system that purports to generate net thrust in a closed system without external interaction challenges this principle and is considered physically untenable under the Standard Model of physics, and would require physics beyond the Standard Model to be viable.
History of research and programs
Traditional rocketry has dominated aerospace propulsion in the 20th and early 21st centuries. Beginning in the 1960s as spaceflight programs expanded, contractor studies for the U.S. Air Force and NASA organized advanced and theorized advanced propulsion concepts under three main headings: Thermal, Field, and Photon, so that unconventional ideas for spaceflight could be compared within a common framework.NASA JPL Beam/Field Concepts 1983" /> Within this taxonomy, "field" referred broadly to approaches that might exchange momentum or energy with external reservoirs, such as plasmas, magnetic fields, or directed energy sources, and therefore contrasted with both conventional rockets and nuclear-thermal designs. These early surveys tended to treat such concepts as long-range prospects rather than near-term flight systems, but they kept the terminology of "field" propulsion alive in successive planning cycles.During the 1960s through the 1990s, electric and electromagnetic propulsion matured experimentally, with some systems flying in limited operational roles even as they continued to rely on propellant despite their strong field components. By contrast, the more speculative end of the spectrum such as concepts that couple to the environment without carrying reaction mass, remained in the research phase. A 1972 report from the Air Force Rocket Propulsion Laboratory, followed by Jet Propulsion Laboratory studies in 1975 and 1982, formalized this division by publishing roadmaps that again divided advanced concepts into Thermal, Field, and Photon classes. These reports emphasized "infinite specific impulse" systems that would obtain energy or working fluid from the ambient environment, and suggested that new advances in lasers and superconductors might breathe new life into earlier discarded concepts such as laser propulsion or ramjets.
By the late 1990s, NASA’s Breakthrough Propulsion Physics program framed research around the goals of propulsion with no propellant mass, maximum physically possible transit speeds, breakthrough energy sources, and emphasized empirical testability. It also raised the question of whether any propellantless effects could exist without violating conservation of momentum and energy. Later NASA Institute for Advanced Concepts studies continued in the same mold, examining whether Alfvén wave coupling or other plasma interactions might provide quasi-propellantless thrust. Across all of these efforts, surveys at the physics frontier acknowledged the conceptual appeal of field propulsion but also stressed the unresolved consistency issues that arise when no clear external momentum channel can be identified. By STS-75 in 1996 and LightSail 1 and LightSail 2 between 2015 and 2019, functional field propulsion systems were active in outer space.
Scope, terminology, and programmatic efforts
Published technical surveys and program documents use "field" or field-adjacent language in different ways. Contractor studies for NASA grouped "advanced" options under headings such as Thermal Propulsion, Field Propulsion, and Photon Propulsion, with "field" covering externally powered and field-interactive concepts beyond conventional rocketry. NASA's Breakthrough Propulsion Physics effort set research goals that explicitly included "propulsion that requires no propellant mass," maximum physically possible transit speeds, and breakthrough energy methods to power such devices, framing the field propulsion question in terms of fundamental physics limits and testable claims. Framed with an emphasis on empirical testability, the BPP stated three goals: propulsion that requires no propellant mass, transit at the maximum speeds physically possible, and breakthrough energy sources to power such devices. Separately, NIAC funded studies on using ambient plasmas and magnetic fields to generate thrust without expelling onboard propellant, including Alfvén-wave coupling concepts.In practice, the viability of any open field-coupled concept depends on coupling strength to the surrounding environment. For example, momentum exchange with the solar wind or a magnetosphere scales with local plasma density, magnetic-field magnitude, and wave/field interaction efficiency; in weak or highly variable environments, thrust and control authority are correspondingly limited. These constraints contrast with classical chemical and conventional electric rockets, whose performance is governed primarily by onboard propellant and its energy, reflecting fundamental engineering limits on achievable exhaust velocity and energy density. Electromagnetic propulsion reviews describe solid-propellant pulsed plasma, magnetoplasmadynamic systems, and pulsed inductive thrusters as electromagnetic spaceflight technologies. Physics-frontier program statements set three goals that included "propulsion that requires no propellant mass," maximum physically possible transit speeds, and breakthrough energy sources. Later NIAC work examined momentum exchange with ambient plasmas and magnetic fields as propellantless or quasi-propellantless mechanisms. Hypothetical field propulsion systems, in contrast, are framed in the literature as propellantless but encounter dependence on external media and unresolved consistency with conservation laws.