The limitations of finite fuel and the rigors of space travel demand a propulsion system that maximizes the thrust-to-power ratio. For electrically powered propulsion systems, xenon’s low ionization threshold and its high atomic mass have made it the industry’s preferred choice. But its rarity, its expense, and competing Earth-based applications have prompted researchers to look for more sustainable alternatives.

One such promising candidate is iodine, which has an atomic mass similar to xenon and a lower ionization threshold. Researchers have begun to investigate iodine’s potential. Most recently, Dmytro Rafalskyi from the French aerospace company ThrustMe and his colleagues have showcased the new propellant’s feasibility in space.

Unlike xenon, which is often stored as a supercritical fluid at pressures of 10–20 MPa, iodine can be kept as an unpressurized solid. Freed from the need to pressurize the fuel, the researchers designed a propulsion system that consists of an iodine storage tank piped directly to an inductively coupled plasma source. Nearby heaters sublimate the iodine, and a RF antenna ionizes the iodine gas to form a plasma. In that state, a set of charged grids then extracts and accelerates the iodine ions to produce thrust.

The propulsion system was tested with a 20 kg, toaster-oven-sized satellite, which was launched into orbit in November 2020. Over 11 firing tests, the researchers observed an average thrust of 0.8 mN produced from 55 W of power. The average thrust of each firing altered the satellite’s altitude by 200–400 m. The figure below shows the variation in the satellite’s orbit during the course of the thrust tests (black arrows).

The researchers calculated that their iodine propulsion system has a fuel efficiency about 50% better than a similar xenon-based one. Another advantage of iodine is its small footprint. The prototype iodine system weighs a little more than 1 kg and fits into a cube with a volume of about 1000 cm³. A xenon-based propulsion system, on the other hand, is about double in size and weighs 1.5 times as much. (D. Rafalskyi et al., Nature 599, 411, 2021.)