Iodine as a propellant for electric propulsion systems: Current state-of-the-art and remaining challenges

High-performance electric propulsion systems have historically used xenon as a propellant because of its relatively high atomic mass and its chemical inertness. However, xenon is rare and global production is both limited and susceptible to strong market fluctuations: particularly in recent years due to several geopolitical events. Furthermore, given the rapid growth of the space industry, demand for xenon may anyway soon outpace production. An emerging alternative propellant is iodine, which despite having been proposed for electric propulsion as early as 1961, has only recently been demonstrated in space in 2020. Iodine is almost 100x cheaper than xenon and global production output is about 500x higher. In addition, iodine can be stored unpressurized as a solid, allowing a significant decrease in the mass and volume of storage tanks. But the strong electron affinity and unique properties and chemistry of iodine creates several challenges associated with material compatibility, propellant flow control, and plasma discharge physics. Here, we present an overview of iodine electric propulsion and discuss the current state-of-the-art, while also highlighting remaining technical barriers to widespread adoption and future research needs.