---
title: "Field propulsion"
chunk: 8/9
source: "https://en.wikipedia.org/wiki/Field_propulsion"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T03:55:18.670066+00:00"
instance: "kb-cron"
---

These systems generate thrust by exchanging momentum with external fields (magnetic, plasma, or photon), without expelling onboard reaction mass. Solar sails are a propellant-less propulsion method that produces thrust from solar photon pressure, rather than by expelling reaction mass. As with other environment coupled concepts, sail performance depends on local solar pressure: the interstellar probe concept uses a very close solar flyby to take advantage of "increased solar flux" and the resultant "increased solar photon pressure", and scaling to a 160,000 m2 sail would require advances in sail materials, deployment, and attitude control systems.
Sailcraft engineering couples ultra-light structures to stringent pointing and thermal constraints. Once deployed, thrust is almost normal to the sail, so small attitude changes steer the thrust vector. Performance evolves with materials science and control: lower areal density (mass per unit sail area) directly increases acceleration, and by tilting the sail the small continuous thrust can be steered for precise trajectory shaping. 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. Typical sail films have reflective front coats and high-emissivity back coats; wrinkling and billowing reduce efficiency. Forward (Journal of Spacecraft and Rockets, 1984) outlined a proposed method of how solar-system-based laser systems and a roughly 1,000 km light-focusing Fresnel lens system could propel thin-film sails to ~0.11% of the speed of light, enabling an unmanned flyby of Alpha Centauri in approximately 40 years. 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.
Analyses of magnetic sail concepts indicate thrust arises from deflecting the solar wind around a spacecraft-supported magnetic field, with performance set by the distance at which solar-wind pressure balances the sail's magnetic pressure; larger effective magnetic cross-sections increase momentum transfer but require large-radius, high-current superconducting coils. 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. Magsails have also been proposed for interstellar missions, where interaction with the interstellar medium provides propellantless terminal deceleration into a destination solar system. Key engineering challenges include the mass and size of the superconducting loop and the constraints imposed by achievable superconducting currents and magnetic fields. The design tradeoffs emphasize achieving a large effective magnetic cross-section for the superconducting loop while keeping its mass low. Magnetospheric plasma propulsion (M2P2) is a NIAC proposal by Robert Winglee, in which plasma injection inflates a magnetic bubble that couples with the solar wind. It is considered a variant of magnetic sails.
The most studied examples are electrodynamic tethers (EDT), which generate Lorentz-force-based drag or thrust by coupling a long current-carrying conductor to a planetary magnetic field, thereby exchanging momentum with a planetary magnetosphere or ionosphere to enable propellantless drag or thrust in suitable environments (e.g., low Earth orbit), and fall under broad definitions of field propulsion due to their use of external fields for momentum exchange. In operation, a conductive tether moving through a planetary magnetic field experiences a motional electromotive force, a voltage induced by its motion through the field; 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. As open systems, they conserve momentum by reaction with the ambient plasma and magnetic field. Electrodynamic tethers have been deployed in several space tether missions, including the TSS-1, TSS-1R, and Plasma Motor Generator (PMG) experiments. Electrodynamic tethers can also generate electrical power at the expense of orbital energy.
Related electrostatic sail concepts also entered in-space technology-demonstration phases in the 2020s. NASA's small-spacecraft propulsion survey described the electric sail and the closely related plasma brake as relatively immature environment-coupled propulsion technologies, and noted that AuroraSat-1, launched on May 5, 2022, served as a technology demonstration mission for a Plasma Brake module. In 2025, Aalto University in Finland reported the launch of Foresail-1p carrying a Plasma Brake experiment intended to enable the first-ever space measurements of Coulomb drag, in which a charged tether interacts with surrounding plasma to change a satellite's orbit.

=== Development and testing ===
These are concepts under active engineering development or testing that adapt field-based acceleration or coupling principles for new operational regimes. As in the historical survey literature discussed above, this section includes some systems that fall outside the narrower propellantless-only sense of field propulsion, especially externally powered concepts and terrestrial field-matter coupling applications.

==== Beamed-energy and externally powered thrust ====

Microwave electrothermal thrusters use microwave energy, potentially externally supplied, to heat a fluid propellant. When powered externally, it falls under beamed-energy propulsion with mass acceleration via directed fields. Laser ablation propulsion uses pulsed laser energy to ablate onboard material into a plasma jet; although it expels mass, the energy source is external, placing it within beamed-energy propulsion approaches. Photonic laser thrusters are a photon-pressure system that relies on externally beamed lasers instead of sunlight.
Leik Myrabo's beamed-energy Lightcraft program, spanning several decades, employed a projected-power, combined-cycle MHD system designed to reconfigure across multiple flight regimes. Czysz and Bruno also highlighted the concept's very low onboard propellant requirement, writing that it had "the least onboard propellants of any system". Myrabo's architecture was described as scalable by siting the projector on Earth, in orbit, or on the Moon, explicitly noting propulsion implications for geosynchronous orbit, the Moon, and nearby planetary/moon systems. Research has been limited to laboratory testing and subscale atmospheric Lightcraft demonstrations, with orbital proposals remaining unflown.

==== Field-interaction in atmosphere or dense media ====