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Field propulsion comprisesproposed and researched concepts andproduction technologies ofspacecraft propulsion in whichthrust is generated bycoupling a vehicle to externalfields or ambientmedia rather than by expelling onboardpropellant. In this broad sense, field propulsion schemes are thermodynamicallyopen systems that exchangemomentum orenergy with their surroundings; for example, a field propulsion system may couple itself tophoton streams,radiation,magnetized plasma, or planetarymagnetospheres. Familiar exemplars includesolar sails,electrodynamic tethers, andmagnetic sails. By contrast, hypotheticalreactionless drives areclosed systems that would claim to produce net thrust without any external interaction, widely regarded as violating thelaw of conservation of momentum and thestandard model ofphysics.
Withinaerospace engineering research, the label spans both established and proposed approaches that "push off" external reservoirs:photonic pressure fromsunlight (sails),charged particle streams such as thesolar wind (magsails and relatedmagnetic structures), and interactions with planetarymagnetospheres andionospheric environments (electrodynamic tethers). In narrower usage, the term also covers efforts to engineer field–matter coupling usingelectromagnetic propulsion (e.g.,electrohydrodynamics andmagnetohydrodynamics) as well as speculative mechanisms that draw ongeneral relativity,quantum field theory, orzero-point energy ideas to alter effectiveinertia or to couple directly to non-particulatefields of space.
Several elements of field-coupled propulsion have been successfully demonstrated in thelaboratory, field tests, and inlow Earth orbit—most notably, sails and tethers. No field propulsion method has yet been validated as a practical primary propulsion system forinterplanetary orinterstellar 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 carriedrocket propellant cost, mass, and weight) continues to motivate exploratory work. The topic remains active in targeted programs such asNASA’s formerBreakthrough Propulsion Physics Program as well as in studies by nationalspace agencies, academic research groups, andindustry organizations that investigatepropellantless or externally powered alternatives to conventionalrocket engines andelectric propulsion systems.
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The broad definition offield propulsion refers topropulsion systems in whichthrust arises from interactions with externalfields or ambientmedia, rather than from the sustained expulsion of onboardreaction mass or reliance on solidchemical fuels. In this framing, familiar exemplars includesolar sails,magnetic sails, andelectrodynamic tethers, which couple with externalphoton,plasma, ormagnetic fields instead of expellingonboard propellant.[1]: 1–2 Various types of field propulsion concepts include mechanisms where motion results from environmental coupling rather than from carrying and ejecting propellant.[2]: 215–216 Some field propulsionreviews note thatopen systems exchangemomentum or energy with external media and that proposals ofclosed-system 'reactionless drive' propulsion are viewed with skepticism because they conflict withconservation of momentum.[3]: 2 Field propulsion is not a single technology but a spectrum of approaches, ranging from mature concepts that have beentested in flight tohighly speculative theoretical constructs. Momentum conservation is the fundamental boundary on all propulsion concepts.[3]: 2 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.[2]: 215–216, 219 Environment-coupled approaches such as sails, tethers, or plasma-wave coupling remain possible if the method of external coupling is strong enough.[1]: 1–2, 11–12
Broad definitions often includesolar sail systems, such as theJapan Aerospace Exploration Agency's (JAXA)IKAROS mission, which demonstrated propulsion by harnessingradiation pressure from sunlight.[1] Examples include systems that attempt to draw on thephoton field of sunlight, thecharged particles of thesolar wind, or themagnetic fields of planetary environments.[1]: 1–2 In a similar spirit,magnetic sail concepts proposed by Dana Andrews andRobert Zubrin envision the use of large magnetic fields to couple with the solar wind and thereby transfer momentum to the spacecraft.[1]: 1–2 [4]: 197 Narrower definitions, however, focus on experimentalelectromagnetic propulsion mechanisms, includingelectrohydrodynamics (EHD) andmagnetohydrodynamics (MHD), as well as more speculative proposals that invokegeneral relativity,quantum field theory, orzero-point energy as possible pathways to modifyinertia or couple directly to thestructured quantum vacuum.[2]: 215–216, 219 By interacting with such external reservoirs, a spacecraft can "push off" the surrounding medium, converting environmental energy or momentum intoacceleration.[2]: 216–217 In contrast, conventionalrockets achieve motion by expelling mass.[5]: 5–6 Most commonly, this is thecombustion output fromchemical propellants to generate thrust viaNewton's third law, which is the familiarrocket launch withexplosive flame and smoke beneath it.[5]: 5–6
Conservation of momentum is a fundamental requirement of propulsion systems because in experiments momentum is always conserved. This conservation law is implicit in the published work ofIsaac Newton andGalileo Galilei, but arises on a fundamental level from the spatialtranslation symmetry of the laws of physics, as given byNoether's theorem.[6] For instance, MHD drives accelerateconductive fluids usingelectromagnetic fields, resulting inthrust through theLorentz force, with momentum conserved via interaction with external media, such as theinterplanetary orinterstellar media, or thesolar wind.[7][1] Open systems comply with theconservation of momentum by transferring it to or from the surrounding environment.[2]: 216–217
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.[2][1][3] In contrast,reactionless drives are hypotheticalclosed 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 establishedscientific principles.[2]: 216–217 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.[1]: 7–10 These constraints contrast with classical chemical and conventionalelectric rockets, whose performance is governed primarily by onboard propellant and its energy, reflecting fundamentalengineering limits on achievableexhaust velocity andenergy density.[5]: 39–40 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.[3]: 2 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 theStandard Model of physics, and would likely requirephysics beyond the Standard Model to be viable.[6]
Traditional rocketry has dominatedaerospace propulsion in the 20th and early 21st centuries. Beginning in the 1960s as spaceflight programs expanded, contractor studies for theU.S. Air Force andNASA 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.[8]: 26 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.[8]: 26 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.[8]: 25
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.[7]: 1–2 [5]: 10–11, 623 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.[3]: 1–2 [2]: 215–216 A 1972 report from theAir Force Rocket Propulsion Laboratory, followed byJet Propulsion Laboratory studies in 1975 and 1982, formalized this division by publishing roadmaps that again divided advanced concepts into Thermal, Field, and Photon classes.[8]: 25–26 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.[8]: 25–26, 406
By the late 1990s, this style of classification had been taken up by NASA'sBreakthrough Propulsion Physics Program, which asked whether propellantless propulsion could be made consistent with conservation laws and whether momentum exchange with ambient plasmas or fields could substitute for mass ejection.[3][1] LaterNASA Institute for Advanced Concepts (NIAC) studies continued in the same mold, examining whetherAlfvén wave coupling or other plasma interactions might provide quasi-propellantless thrust.[1]: 1–2 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.[3]: 2 BySTS-75 in 1996 andLightSail 1 andLightSail 2 between 2015 and 2019, functional field propulsion systems were active inouter space.
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.[8] Electromagnetic propulsion surveys treat plasma/ion engines (including pulsed plasma, magnetoplasmadynamic, and pulsed-inductive thrusters) as electric propulsion, some of which employ strong electromagnetic fields but still expel reaction mass.[7] Physics-frontier program statements define an aspirational target class as "propulsion that requires no propellant mass," motivating studies of momentum exchange via ambient fields and other non-chemical pathways.[3]
NASA's Breakthrough Propulsion Physics (BPP) effort (established 1996) 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.[3] Separately, NIAC funded studies on using ambient plasmas and magnetic fields (e.g., solar wind, magnetospheres) to generate thrust without expelling onboard propellant, including Alfvén-wave coupling concepts.[1]: 1–2 Framed explicitly as a way to separate testable physics from non-viable claims, the BPP articulated three headline goals—propulsion with no propellant mass, maximum physically possible transit speeds, and breakthrough energy sources—and used those goals to scope candidate studies across sails, tethers, and more speculative field-interaction ideas.[3]
A NASA contractor survey ofbeam-powered propulsion in "Advanced Beamed-Energy and Field Propulsion Concepts" argued that emerging missions would require propulsion capabilities exceeding chemical rockets and examined systems where power is transmitted to the vehicle (laser/microwave/particle beams), decoupling onboard energy from thrust production.[8] The report organized prospects into thermal, field, and photon classes and identified enabling technologies (e.g., higher-current superconductors, potential room-temperature superconductors) as potential gates to field-interactive designs.[8] It also noted historic funding swings and prior AFRPL/JPL surveys, situating "field" concepts within long-running advanced propulsion roadmapping rather than near-term flight systems.[8] The study emphasized a return to the unrestricted creativity and "free-thinking" that characterized propulsion research in the late 1950s and early 1960s.[8]
Advanced propulsion systems may overcome the limitations of chemical rockets by accessing more energetic physical principles, particularly those involving directed-energy and field interactions.[8] Beamed-energy propulsion concepts eliminate the need for onboard energy sources by transmitting power from remote stations using lasers, microwaves, or particle beams to induce thrust remotely.[8] Such systems decouple the vehicle from traditional propellant constraints and enable the possibility of high specific impulse with relatively high thrust-to-weight ratios.[8] Theoretical development includes mechanisms for transferring momentum via electromagnetic coupling or by interacting with the physical structure of the vacuum, a line of reasoning that parallels field propulsion concepts.[8] Some approaches explore the utilization of atmospheric or ambient materials as virtual reaction mass or interaction medium, pushing beyond the limitations of mass ejection propulsion.[8] Proposals also include advanced electrostatic and MHD-based concepts that could leverage charged particle interactions with atmospheric fields to produce directed motion.[8]
The AFRPL report proposed that propulsion researchers should prioritize "infinite specific impulse" (Isp) systems that obtain both working fluid and energy from the ambient environment, offering exceptional performance potential.[8] For field propulsion, specific technological breakthroughs such as higher current density superconductors, metallic hydrogen, or room temperature superconductors were identified as potentially enabling innovations.[8] The study argued that improvements in technologies like high-power lasers or new energy transfer methods could revitalize previously discarded propulsion ideas, including laser propulsion and infinite-Isp ramjets.[8]
Forward (1984) extended beamed-sail studies to the interstellar scale, suggesting that phased solar-system lasers could impart sustained acceleration to ultralight sails across astronomical distances.[9] He calculated that such a system might accelerate a probe to ~0.11 c and reach Alpha Centauri in about four decades, bringing the timescale of an interstellar flyby to within a human lifetime.[9]
A NASA electromagnetic-propulsion review by Myers identifies three main types of electromagnetic propulsion systems: pulsed plasma thrusters (PPTs), magnetoplasmadynamic thrusters (MPD), and pulsed-inductive thrusters (PIT). PPTs had already flown for attitude/drag makeup; MPD devices had space heritage in experimental regimes; and PITs sought to reduce electrode-erosion limits by inductive coupling.[7] The review emphasized benefits (high specific impulse, precise impulse bits, robustness) alongside unresolved issues (efficiency, cathode wear, spacecraft power/integration), presenting these as electric-propulsion architectures that still accelerate propellant even when strong fields are central to their operation.[7]
Pulsed plasma thrusters (PPTs) represent the earliest class of electromagnetic propulsion systems to achieve operational deployment in space.[7] Developed in the 1960s, these thrusters generate thrust by ablating solid propellant with an arc discharge and accelerating the resulting plasma via Lorentz forces.[7] Unlike later concepts relying on inductive or steady-state operation, PPTs utilize compact, low-power, pulsed configurations suitable for satellite positioning and drag compensation.[7] The Soviet Zond-2 Mars mission in 1964 marked the first planetary use of electric propulsion, followed by successive U.S. deployments culminating in the Nova satellite series.[7] PPTs remain relevant for low-mass spacecraft requiring precise impulse bits, though scalability to higher power levels has presented technical challenges.[7]
Magnetoplasmadynamic thrusters (MPDTs) are another major class of electromagnetic propulsion systems investigated for both quasi-steady and steady-state spaceflight applications.[7] Operating through the Lorentz force generated by the interaction of discharge currents with self-induced or externally applied magnetic fields, MPD thrusters were among the first electric propulsion devices to fly in space.[7] Initial experimental campaigns explored both self-field and applied-field configurations, with development advancing through international programs aimed at supporting high-power planetary missions.[7] While technological challenges remain—including efficiency optimization and cathode erosion—MPD thrusters continue to serve as a key pathway toward scalable, high-thrust, long-duration propulsion architectures.[7]
Pulsed inductive thrusters (PITs) are a form of electromagnetic propulsion developed to overcome the erosion and lifetime limitations of electrode-based systems.[7] By inducing plasma currents through time-varying magnetic fields, PITs accelerate neutral propellants without requiring physical contact between conductors and plasma.[7] The concept originated in the late 1960s and evolved through successive experimental designs focused on performance scaling, circuit optimization, and propellant compatibility.[7] Although no PIT system has flown in space, the thruster class remains of interest due to its potential for high-efficiency, long-duration propulsion with minimal material degradation, particularly in missions requiring flexible propellant selection and reduced contamination risk.[7]
NASA's Breakthrough Propulsion Physics (BPP) memo framed research questions at the limits of physics—no-propellant propulsion, ultimate transit speeds, and breakthrough energy production—explicitly to sort physically testable ideas from non-viable claims.[3]: 1 Minami and Musha survey a class of proposals that posit a "substantial physical structure" of spacetime/vacuum (macroscopic in GR, microscopic in QFT) and investigate whether asymmetries in massless fields or vacuum-medium interactions could yield thrust without mass expulsion; they report that such systems remained theoretical and controversial, with ongoing but inconclusive experiments and consistency concerns with momentum conservation.[2]
This dual framework situates field propulsion concepts at the intersection of general relativity—which treats spacetime as a dynamic geometry—and quantum field theory, where the vacuum hosts fluctuating fields and latent energy.[2] Several mechanisms have been theorized to achieve such coupling, including vacuum polarization, spacetime curvature manipulation, and asymmetrical force distributions generated by interactions with massless fields.[2] Though none have been experimentally validated, laboratory-scale studies have sought force generation analogs such as zero-point energy fluctuations and electrogravitic interactions, with results remaining inconclusive and difficult to replicate.[2] A central theoretical challenge is the reconciliation of these concepts with conservation of momentum, since without a clearly identified external momentum exchange medium such mechanisms would violate fundamental physical laws.[2]
A NIAC Phase I study evaluated "ambient plasma wave propulsion," focusing on momentum exchange with existing space environments (solar wind, magnetospheres) via waves such as Alfvén modes, alongside sails and electrodynamic tethers, as candidates for propellantless or quasi-propellantless thrust.[1]: 1–2 The authors highlight both the appeal (no onboard reaction mass) and the limits (technical immaturity, power/sensitivity shortfalls) of such concepts for significant maneuvering.[1]: 11–12
One group of field propulsion concepts comprises environment-coupled systems that utilize their surroundings to produce thrust, includingsolar sails,magnetic sails, and, with certain restrictions,electrodynamic tethers, which use the solar wind or ambient magnetic fields to generate thrust; in an example design, a magnetic sail uses a loop of superconducting cable to create a magnetic field that deflects solar wind plasma and imparts momentum to the attached spacecraft.[1]: 1–2 [4]: 197 A second category covers electrically driven devices in which strong electromagnetic fields dominate the acceleration physics, but in which momentum closure still proceeds through exhaust of a carried propellant; representative examples include pulsed plasma thrusters (PPTs), magnetoplasmadynamic thrusters (MPD), and pulsed-inductive thrusters (PIT).[7] A further and more speculative class invokes direct interactions with a structured vacuum or with spacetime geometry, proposing thrust without any expulsion of mass, an idea surveyed in the general relativity and quantum field theory literature but not empirically validated.[2] Finally, some historical NASA surveys also groupedbeamed-energy propulsion into "field" or field-adjacent categories, because the energy supply is external to the vehicle and power is transmitted by laser, microwave, or particle beams rather than carried onboard.[8]
This layered taxonomy reflects the way that contractor reports and program reviews organized the subject during the late twentieth century. Air Force Rocket Propulsion Laboratory and Jet Propulsion Laboratory roadmaps in the 1970s and 1980s placed advanced systems into three headings—Thermal, Field, and Photon—explicitly separating concepts that drew energy or momentum from their environments from those that remained tethered to onboard propellant stores.[8]
In contemporary technical reviews it is common to reserve the term "field propulsion" for schemes that exchange momentum with external reservoirs, since those remain consistent with conservation of momentum when an identifiable medium supplies the counterforce. By contrast, devices such as PPT, MPD, and PIT thrusters—although dominated by internal electromagnetic fields—are placed within the broader family of electric propulsion because their exhaust streams provide the reaction mass that enforces momentum balance.[7][1] Academic surveys distinguish environment-coupled concepts from electric-propulsion devices that expel carried propellant, separating speculative field-coupled ideas from near-term electric technologies.[1]: 1–2 [7]: 1–2
Historical roadmaps also considered beamed-energy propulsion "field-adjacent," since power is supplied from outside the vehicle (laser, microwave, or particle beams) and converted to thrust onboard, decoupling energy supply from carried mass.[8] Such schemes are not propellantless if they ablate or heat onboard reaction mass, but the externalization of power places them near "field" concepts in advanced-propulsion surveys.[8][citation needed]
Beyond simple ablation or thermal boost,[8] roadmaps examined hybrid schemes in which a vehicle carries a small amount of working fluid that is heated or photodesorbed by a remote beam,[8] allowing thrust throttling while still decoupling most of the energy supply from onboard mass.[8] Such studies typically trade transmitter complexity and pointing requirements against vehicle mass and thermal limits;[8] mirrors, rectennas, or absorber cavities must survive high power densities while maintaining optical figure or RF efficiency across long ranges.[8] Mission analyses in this class often treat the beaming infrastructure as reusable capital equipment, amortized across multiple flights,[8] whereas the vehicles are lightweight, short-lived stages optimized for acceleration corridors.[8]
Several devices central to electromagnetic propulsion rely on strong fields yet remain conventional in the momentum sense because they accelerate carried propellant.[5]: 647–649 Representative families include pulsed plasma thrusters (PPTs), magnetoplasmadynamic thrusters (MPD), and pulsed-inductive thrusters (PIT), each with distinct trade-offs in lifetime, efficiency, and power scaling; PPTs have flown for attitude and drag makeup, MPD has flight heritage in experimental regimes, and PIT remains ground-tested.[7]: 1–2
Within the electric-propulsion family, these devices illustrate how strong fields can dominate the internal acceleration physics while momentum closure still proceeds through exhaust.[7]: 5–8 In programmatic roadmaps, these technologies frequently serve as baselines for comparison with environment-coupled concepts, anchoring expectations for power-to-thrust ratios, lifetime, and system mass at mission-relevant scales.[5]: 648–649 [7]: 5–8
Although not presently in wide use for space, there exist proven terrestrial examples of field propulsion in which electromagnetic fields act upon a conducting medium such as seawater or plasma for propulsion, known collectively as magnetohydrodynamics (MHD). MHD is similar in operation to electric motors, however rather than using moving parts or metal conductors, fluid or plasma conductors are employed. The EMS-1 and more recently theYamato 1[10] are examples of such electromagnetic Field propulsion systems, first described in 1994.[11]
Electrohydrodynamics (EHD) is another method where electrically charged fluids are accelerated for propulsion and flow control; laboratory and flight demonstrations includedevices driven by corona discharge.[12][13]: 532–535
NIAC studies have examined using radio-frequency waves (e.g., Alfvén waves) launched from a vehicle to couple with ambient plasma and magnetic fields, transferring momentum to the environment and producing propellantless thrust. The 2011 Phase I assessment found the approach technically immature but potentially enabling if sensitivity and power challenges can be overcome.[1]
Electrodynamic tethers exchange momentum with a planetary magnetosphere/ionosphere via Lorentz forces on a long current-carrying conductor, enabling drag or thrust without propellant in suitable environments (e.g., low Earth orbit).[1] As open systems, they conserve momentum by reaction with the ambient plasma and magnetic field.[1] In operation, a conductive tether moving through a planetary magnetic field experiences a motional electromotive force; closing the circuit through the ambient ionosphere allows current to flow, and the resulting Lorentz force can provide either drag (for deorbit) or, with external power injection, thrust along specific orbital geometries.[1] Beyond orbit changes, tethers can also be used for in-situ power generation at the expense of orbital energy, though practical systems must address current collection (e.g., plasma contactors), arcing, attitude control, and vulnerability to micrometeoroids or space debris.[1]
Magnetic sails generate thrust by coupling a spacecraft-supported magnetic field to the solar wind, transferring momentum from the ambient charged particle flow to the vehicle.[4]: 197 Analyses of magnetic sail concepts indicate thrust arises from deflecting the solar wind around a spacecraft-supported magnetic field, with performance set by the stand-off distance at which solar-wind dynamic pressure balances the sail's magnetic pressure; larger effective magnetic cross-sections increase momentum transfer but require large-radius, high-current superconducting coils.[4]: 197–199 Magnetic sails include essentially propellantless acceleration and deceleration by interaction with the solar wind and interstellar medium, since the sail exchanges momentum with the surrounding plasma instead of expelling onboard propellant.[4]: 197–203 Key engineering challenges include the mass and size of the superconducting loop and the constraints imposed by achievable superconducting currents and magnetic fields.[4]: 197–199 Mission studies of magnetic sails show that they can perform heliocentric transfers between circular orbits by using the solar wind for outbound acceleration and inbound braking.[4]: 197–199 Magsails have also been proposed for interstellar missions, where interaction with the interstellar medium provides propellantless terminal deceleration into a destination solar system.[4]: 201–203 The design tradeoffs emphasize achieving a large effective magnetic cross-section for the superconducting loop while keeping its mass low.[4]: 199
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Solar sails produce continuous, low-thrust acceleration by exchanging momentum with incident photons from the Sun; because the counter-momentum is supplied by the external photon field, they fall under broad definitions of field-coupled propulsion rather than reaction-mass ejection.[1] As with other environment-coupled concepts, performance depends on the available flux—in this case solar irradiance and sail reflectivity—and falls with distance from the Sun, motivating large, lightweight structures and precise attitude control schemes.[1]
Sailcraft engineering couples ultra-light structures to stringent pointing and thermal constraints.[14]: 2990, 2995 [9]: 188 Square and heliogyro designs use thin film sails on deployable booms; reliable deployment of large, low-mass structures and thin films is a key challenge.[14]: 2991, 3004–3005 Typical sail films have reflective front coats and high-emissivity back coats; wrinkling and billowing reduce efficiency.[14]: 2993–2995 Once deployed, thrust is almost normal to the sail, so small attitude changes steer the thrust vector.[14]: 2990–2991
Performance evolves withmaterials science and control: lower areal density directly increases acceleration,[9]: 188 and bycanting the sail the small continuous thrust can be steered for precisetrajectory shaping.[14]: 2990 Forward (Journal of Spacecraft and Rockets, 1984) outlined a proposed method of how solar-system-based laser systems and a ~1,000 km diameterFresnel zone "para-lens" could propel thin-film sails to ~0.11 c, enabling an unmanned flyby ofAlpha Centauri in approximately 40 years.[9]: 187, 193 In Forward's proposal, a two-stage sail system in which a massive ring sail reflects laser light back onto a detached payload sail, enabling the unmanned spacecraft to rendezvous and brake within the Alpha Centauri system.[9]: 193–194
Minami and Musha frame field propulsion at the physics frontier as interaction with a "substantial physical structure" of space, drawing ongeneral relativity at macroscopic scales andquantum field theory at microscopic scales.[2]: 215–216 They conclude that future engineering technologies for space travel will most likely require some form of field propulsion toexcite properties oflocalized regions in space.[2]: 220 Surveyed mechanisms include vacuum polarization, engineered spacetime curvature, and zero-point-field interactions; none have been experimentally validated, and all face unresolved consistency issues with momentum conservation.[3]: 2 Minami and Musha distinguish between two field propulsion concepts: one framed in terms of general relativity and one in terms of quantum field theory.[2]: 215–220
In the general relativistic field propulsion system,space-time is considered to be an elastic field similar to rubber, which means space itself can be treated as an infinite elastic body.[15]: 20–21 In Minami and Musha's framing, propulsive force arises from interaction with a physical structure of space instead of from expelling reaction mass.[2]: 216–217 According toquantum field theory andquantum electrodynamics, thequantum vacuum is modeled as a nonradiating electromagnetic background, existing in azero-point state, the minimum energy allowed by the theory.[15]: 24–25 Using this on adielectric material could, via the resultingLorentz force on bound charges, affect the inertia of the mass and that way create an acceleration of the material without creating stress or strain inside the material.[2]
A wide range of space propulsion methods have been proposed or demonstrated that fit within broad definitions of field propulsion. These systems do not rely on conventional chemical rockets, but instead seek to generate thrust by interacting with external media or applying directed energy fields. The following list highlights representative concepts.
Electromagnetic systems use electric and magnetic fields to accelerate plasma or ions.
These systems accelerate onboard or environmental particles using electromagnetic, electrostatic, or directed energy fields. Some may still require onboard mass or atmospheric medium.
These systems generate thrust by exchanging momentum with external fields (magnetic, plasma, or photon), without expelling onboard reaction mass.
This article incorporatespublic domain material from websites or documents of theUnited States government.
{{cite journal}}: CS1 maint: article number as page number (link)The cathode-less electron cyclotron resonance ion engines, μ10, propelled the Hayabusa asteroid explorer, launched in May 2003, which is focused on demonstrating the technology needed for a sample return from an asteroid.
An ion thruster consists of an ion source and a neutralizer both of which utilize microwave discharge with electron cyclotron resonance at a frequency of 4.25 GHz.
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