In-orbit demonstration of an iodine electric propulsion system

The study addresses the need for alternative propellants in electric propulsion due to xenon’s scarcity, high cost, and requirement for high-pressure storage. Electric propulsion offers high exhaust velocities and efficient propellant usage for spacecraft manoeuvres (orbit transfer, station-keeping, collision avoidance, and deorbit). Xenon has been the predominant propellant, but increasing demand from satellite megaconstellations and other industries risks supply constraints. Iodine is proposed as a more abundant, lower-cost, solid-storable alternative with lower ionization thresholds and higher molecular mass (I2) than xenon, potentially improving ionization efficiency and thrust-to-power. The research goal is to develop, qualify, and demonstrate in orbit an iodine-fueled gridded ion thruster (NPT30-12) and compare its performance against xenon, validating its viability and benefits for small satellites and constellations.

The paper situates the work within the context of established electric propulsion (ion and Hall thrusters) used on numerous missions (e.g., SMART-1, Dawn, Hayabusa/Hayabusa2). It reviews constraints tied to xenon: rarity in the atmosphere, production cost and limitations, and competing applications (lighting/imaging, anesthesia, semiconductor etching). With the growth of satellite megaconstellations, xenon demand may exceed supply. Prior research on iodine as an alternative propellant has been undertaken by companies, universities, and agencies, including iodine Hall and RF ion thrusters. Modeling and experiments have suggested iodine’s lower ionization thresholds and favorable collisional processes could yield higher ionization efficiency than xenon. However, before this work, no iodine propulsion system had been demonstrated in space. This study provides the first in-orbit demonstration, bridging prior ground-based studies and flight operation.

System design and architecture: The NPT30-12 is an iodine-fueled gridded ion propulsion system integrating propellant storage/delivery, an inductively coupled RF plasma source, high-voltage multi-aperture accelerator grids, an electron-emitting neutralizer, power processing, and passive thermal management. Solid diatomic iodine is stored unpressurized in a tank directly upstream of the plasma source tube. Heaters sublimate iodine to achieve a controlled saturation pressure of ~2–6 kPa, with tank temperature maintained at 80–100 °C to avoid local melting. A small orifice between tank and source tube passively stops propellant flow when off as iodine re-deposits.

Materials and contamination control: Due to iodine’s high electronegativity and corrosivity, the source tube and interfaces use technical ceramics (aluminum oxide, zirconium oxide), and vulnerable metal surfaces are polymer-coated. To mitigate mechanical fragmentation and ensure thermal contact, iodine is embedded within a 95%-porosity alumina ceramic block (tank-to-propellant mass fraction 54%). Iodine is melted and infused into the porous block during assembly, then solidified.

Thermal management: Waste heat from plasma and power electronics is conducted to the storage tank and frame, minimizing dedicated heater power (<1 W additional during steady-state). Remaining heat is radiated or conducted to the spacecraft.

Plasma generation and acceleration: An RF inductive antenna generates plasma by electron impact ionization. Both I+ and I2+ (and possibly I3+) form via direct dissociative ionization and two-step pathways. Positive ions are extracted and accelerated through grids at 800–1,300 V, reaching velocities ~40 km/s. A downstream cathode filament thermionically emits electrons for beam neutralization.

Ground testing and diagnostics: Prior to flight, ground testing characterized plasma and plume. Time-of-flight (TOF) electrostatic diagnostics measured mass-to-charge spectra to determine beam composition (I+, I2+, I3+) vs RF power. Ion-beam current vs RF power was measured for iodine and, via temporary modification, xenon, for comparison. Propellant mass flow rate was inferred from pre/post-operation system mass measurements. Beam divergence half-angle was measured using an automated array of electrostatic probes; grid design was optimized for focusing and low divergence. Thrust was measured directly on a thrust balance and indirectly estimated from beam current, grid voltage, plume divergence, and species composition using F = αγ I_beam sqrt(2 Mi V / qi), where γ = cos θdiv and α accounts for species mass-to-charge contributions.

Flight demonstration: A flight model was integrated on the 12U CubeSat Beihangkongshi-1 (Spacety; ~20 kg), launched 6 Nov 2020 to ~480 km Sun-synchronous orbit (Long March 6). In-orbit firings (11 events over the reported period) tested ignition, repeatability, and attitude-controlled thrust vectoring. Each firing lasted ~80–90 min (including 10–20 min warm-up/ignition). Orbit changes were analyzed via mean semi-major axis using onboard GPS (≈20 m accuracy), independent SSN tracking (satellite catalog 46838), numerical simulations with GMAT, and theoretical models. Telemetry recorded thrust and total power during firings, along with ion beam current, neutralizer electron current, and acceleration grid current to verify neutralization and system health.

• Ionization efficiency and beam current: For the same mass flow and RF power, iodine produced nearly 50% higher ion-beam current than xenon, consistent with iodine’s lower ionization thresholds (≈10.5 eV for I and 9.3 eV for I2+) versus xenon (≈12.1 eV for Xe+), reducing electron temperature and wall losses.
• Propellant mass utilization: Maximum propellant mass utilization efficiency ηm ≈ 60% for iodine (vs ≈40% for xenon under comparable conditions).
• Beam composition and dissociation: Dominant ions are I2+ and I+, with increasing RF power yielding higher iodine dissociation and a larger I+ fraction.
• Acceleration and energy: Ions accelerated at 800–1,300 V exhibited average energies near the net accelerating voltage; characteristic exhaust velocities are ~40 km/s.
• Beam collimation: Optimized grid design achieved low divergence half-angles of 10–15°. Iodine exhibited slightly lower divergence than xenon, attributed to higher ionization efficiency and fewer neutral collisions in the plume.
• Thrust and specific impulse: Within total power <65 W, measured thrust up to ~1.3 mN and specific impulse up to ~2,500 s. A performance map was established for different iodine mass flow rates.
• Total impulse capability: At maximum specific impulse, the system can deliver ~5,500 Ns total impulse (≈1,500 h burn time).
• In-orbit manoeuvres: Eleven firings (each ~80–90 min; 10–20 min warm-up) produced altitude increases of ~200–400 m per firing at ~0.8 mN thrust and ~55 W. Cumulative altitude increase exceeded 3 km during the reported period. Semi-major axis changes correlated with firing times and agreed among GPS, SSN tracking, GMAT simulations, and theoretical predictions.
• Neutralization and health: Telemetry confirmed sufficient beam neutralization in flight (electron emission current exceeded ion current) and consistency with ground test conditions.

The results demonstrate that iodine is a viable and advantageous alternative propellant to xenon for gridded ion propulsion. Higher ionization efficiency with iodine translates to increased beam current at a given RF power and mass flow, improved propellant utilization, and slightly reduced plume divergence. The flight demonstration validated end-to-end system performance in orbit, with maneuver magnitudes consistent with models and ground-based performance characterizations. Operationally, storing iodine as an unpressurized solid simplifies tankage, reduces handling complexity, and enables propulsion system miniaturization suitable for small satellites. The ability to conduct orbit maintenance to counter drag, perform collision avoidance maneuvers, and enable end-of-life disposal was shown on a 12U CubeSat platform. For larger satellites and constellations, transitioning to iodine could alleviate xenon supply pressures and simplify integration by removing high-pressure systems. Collectively, these findings address the core research goal: proving in space that iodine-propelled ion thrusters can match or exceed xenon-based performance while offering logistical and architectural advantages.

This work reports the development, qualification, and first in-orbit demonstration of a compact iodine-fueled gridded ion propulsion system (NPT30-12). Ground testing showed improved ionization efficiency, strong beam collimation, and competitive thrust and specific impulse at low power. In space, repeated firings produced orbit changes consistent with predictions, confirming operational viability and maneuvering capability on a small satellite. Iodine’s abundance, low storage complexity, and performance benefits suggest it can replace xenon across a broad range of missions, from smallsat station-keeping and collision avoidance to large-satellite applications and exploration.

Future research directions include: scaling to higher power and larger thrust classes; long-duration life testing and erosion/corrosion assessments with iodine across materials and components; optimization of grid geometry and RF coupling for different mass flow regimes; advanced propellant management and thermal architectures to further reduce heater power; comprehensive in-orbit validation across diverse spacecraft platforms and mission profiles; and exploration of alternative electronegative propellants and multi-propellant flexibility.

• The in-orbit results are from a single 12U CubeSat platform with limited total firing time over the reported period; long-term lifetime and degradation (e.g., grid erosion, material compatibility under extended iodine exposure) are not fully characterized here.
• Propellant mass flow was inferred via pre/post-operation mass measurements, introducing uncertainty relative to direct flow metrology.
• GPS-based orbit determination has finite accuracy (~20 m), and comparisons rely on modeling (GMAT) and SSN tracking; while consistent, these add model/data fusion uncertainties.
• Performance comparisons with xenon were conducted under modified ground-test conditions and may not capture all operational differences in flight.
• Iodine’s corrosivity necessitates specific materials and coatings; broader environmental impacts (e.g., contamination risk to neighboring spacecraft surfaces) are not explored in detail.