Spacecraft propulsion is crucial for maneuvers such as orbit transfers, collision avoidance, and end-of-life disposal. Electric propulsion systems offer significantly higher exhaust speeds than chemical propulsion, resulting in more efficient propellant use. Gridded ion and Hall thrusters are prominent examples, utilizing electron impact ionization of a gas to create a plasma and accelerate ions. While xenon is the current propellant of choice due to its low ionization threshold and high atomic mass, its rarity, high storage pressure requirements, and high cost pose limitations, especially with the rise of satellite mega-constellations increasing demand. This paper addresses these limitations by exploring iodine as a potential alternative propellant. Iodine is more abundant and cheaper than xenon, can be stored as a solid at ambient pressure, and has a lower ionization threshold, making it an attractive substitute. Despite prior research, no iodine-based electric propulsion system had been space-tested prior to this study. The paper describes the development and in-orbit testing of such a system, assessing its performance and feasibility for various space applications.

The paper cites numerous studies highlighting the limitations of xenon as a propellant for electric propulsion systems. Research on alternative propellants, including iodine, has been conducted by companies, universities, and space agencies. However, these prior studies had not progressed to in-orbit demonstrations. The paper reviews existing literature on gridded ion thrusters and the advantages of using iodine, such as its lower ionization threshold and higher relative mass compared to xenon. It emphasizes the need for a more sustainable and cost-effective solution to propel the growing number of satellites, particularly within mega-constellations.

The research involved the development and testing of the NPT30-12 iodine electric propulsion system (55 W nominal power, 0.8 mN thrust). The system uses solid diatomic iodine stored in a tank connected to an inductively coupled plasma source tube. Heaters sublimate the iodine, which then flows into the source tube. A radio-frequency (RF) inductive antenna creates an iodine plasma. Positive ions are extracted and accelerated by high-voltage grids, reaching speeds around 40 km/s. A cathode neutralizes the ion beam. Technical ceramics (aluminum oxide and zirconium oxide) were selected to mitigate iodine's corrosive properties; vulnerable metal surfaces were coated with polymer film. Iodine sublimation rate was controlled by regulating the tank temperature (80-100°C). To prevent damage from vibrations, the iodine was embedded in a porous aluminum oxide ceramic block. Waste heat was directed towards the storage tank for efficient thermal management. Ground testing employed time-of-flight spectrometry to characterize beam composition. Numerical plasma discharge modeling was used for comparison. In-orbit testing was performed on the Beihangkongshi-1 satellite, using GPS data, numerical simulations (GMAT), and independent tracking data (SSN) for maneuver confirmation. Thrust measurements were taken using both direct (thrust balance) and indirect (ion beam current, voltage, and divergence data) methods. Beam divergence was measured using an automated array of electrostatic probes. The system underwent extensive qualification testing to meet in-space and launch vehicle requirements.

Ground testing revealed that iodine's ionization efficiency is higher than xenon's, due to lower ionization thresholds and different collisional processes. A propellant mass utilization efficiency of approximately 60% was achieved for iodine (compared to 40% for xenon). The ion flux distribution measurements confirmed the presence of high-energy ions. Direct and indirect thrust measurements were consistent. The ion beam had low divergence (10-15°). The maximum thrust and specific impulse were approximately 1.3 mN and 2,500 s, respectively. In-orbit testing showed clear correlation between propulsion system firings and changes in the satellite's orbit, as observed through GPS, GMAT simulations, and SSN tracking data. Eleven firings resulted in a cumulative altitude change exceeding 3 km. Telemetry data from both ground and in-flight tests were comparable, confirming effective ion beam neutralization and consistent performance.

The results demonstrate iodine's viability as a replacement for xenon in electric propulsion systems, offering enhanced performance and significant advantages. The higher ionization efficiency of iodine reduces the energy required for plasma generation and leads to improved propellant utilization. The low beam divergence indicates effective ion focusing, enhancing thrust efficiency. The successful in-orbit maneuvers confirm the system's operational capability in the space environment. The miniaturization achieved with iodine simplifies integration into smaller satellites, opening new opportunities for various space applications.

This paper successfully demonstrates the in-orbit operation of an iodine-based electric propulsion system, highlighting its superior performance and potential for wide-scale adoption. Iodine offers a sustainable, cost-effective alternative to xenon, enabling miniaturization, simplification, and enhanced capabilities for diverse space missions. Future research could focus on further optimizing system efficiency, exploring alternative iodine storage methods, and expanding the application of iodine propulsion to larger spacecraft and more complex missions.

While the study demonstrates the feasibility and performance of the iodine propulsion system, further research is needed to evaluate its long-term reliability and performance in different orbital environments. The relatively small scale of the in-orbit test limits the generalizability of the findings to larger systems. The potential impact of iodine corrosion on long-duration missions also requires further investigation and mitigation strategies.