Alpha-Decay Alpha-Particle Propulsion for Deep-Space Missions: A First-Order Feasibility Study

Abstract

Alpha decay ejects MeV-scale alpha particles at ∼10^7 m/s, implying an extremely high effective exhaust velocity and specific impulse. This paper studies whether that exhaust can be used as a practical thrust source for deep-space missions without external electrical power by directly allowing decay products to leave a spacecraft in a preferred direction. The key obstacle is not exhaust velocity but momentum directionality and energy deposition: alpha emission is nearly isotropic and alpha particles have very short ranges in solids, so most decay energy becomes heat unless the radioactive material is structured as ultra-thin films. We develop a thin-film “decay sail” model, implement a reproducible Monte Carlo simulation (no external dependencies) to estimate an effective momentum coupling coefficient η(d/R), and perform isotope scaling studies for Po-210, Th-228, Pu-238, and Am-241. For a representative geometry d/R≈0.5 and an areal mass of 10 mg/cm², our model predicts an initial thrust density of ∼3.2×10^(-4) N/m² for Po-210 (tens of nN/cm²), consistent with published MCNP-based estimates for similar configurations. The same geometry implies kilowatt-to-tens-of-kilowatt per m² heat fluxes for short-half-life isotopes and mA/m²-scale charging currents unless charge neutralization is provided. We conclude that alpha-decay propulsion is generally thrust-poor but power-rich, best suited to niche missions that can tolerate extremely low thrust, very large emitter area, and significant thermal/charging engineering.