French aerospace company ThrustMe has successfully tested an iodine-powered spacecraft for the first time, showing that this fuel is a viable alternative to the more expensive and difficult-to-store xenon.
The satellite industry is booming: over the next decade, experts predict that up to 24,000 satellites will be launched into orbit, with most of them requiring a propulsion system to manoeuvre. Electric propulsion systems are a good choice because of their high fuel efficiency. These generate thrust by using electrical energy to accelerate the ions of a propellant gas.
Currently, most systems are powered by the noble gas xenon. Bur xenon is rare, expensive (at approximately $3,000 per kg), and must be packed into high-pressure tanks to fit on a satellite. It’s also in demand for other industries such as medicine, lighting and semiconductors.
Ground-based tests have shown that iodine could be a good alternative. Now, as reported in a paper in Nature, researchers have made a successful test of iodine in orbit for the first time.
“Iodine is significantly more abundant and cheaper than xenon, and has the added advantage that it can be stored unpressurised as a solid,” says Dmytro Rafalskyi, lead author of the paper and co-founder of ThrustMe, the company that developed the system.
Iodine also transforms directly from a solid into a gas when heated (ie, it sublimates), so iodine crystals can be placed straight into the thruster without the need for bulky high-pressure tanks to store them. Solar energy can then provide the one watt of power needed to heat the crystals to become gas.
Plus, the propulsion system is tiny, fitting within a package of roughly 10 cubic centimetres.
ThrustMe’s system was integrated into a 20kg CubeSat satellite, Beihangkongshi-1, operated by China’s Spacety. It was launched by a Long March 6 rocket on the 6 November 2020, and the results show that the system performed successfully.
“We anticipate that these results will accelerate the adoption of alternative propellants within the space industry and demonstrate the potential of iodine for a wide range of space missions,” Rafalskyi and colleagues write in their paper.
In an accompanying News & Views article, independent aerospace engineering experts Igor Levchenko and Kateryna Bazaka call the system “not only remarkably simple, light and inexpensive, but also efficient”.
They say it could be a “gamechanger” for small satellites, including those that need propulsion systems to form flexible networks, known as constellations.
“For large satellite constellations, such as the 42,000-satellite Starlink system planned by aerospace-manufacturer SpaceX in Hawthorne, California, changing the propellant from xenon or krypton to iodine would lead to multi-million-dollar savings,” they write.
“Further savings could come from simplifying the propellant’s storage and supply technology, which would also save money by decreasing the mass of the thruster.”
Levchenko and Bazaka also note that cheap iodine-powered propulsion systems could also be used to reduce the cost of in-orbit manufacturing.
“For example, the research company Varda Space Industries in Torrance, California, is building the world’s first commercial zero-gravity industrial park in space. The facility will manufacture products that are difficult to build on the surface of Earth owing to the effects of gravity, such as 3D-printed arteries and hearts, and certain pharmacological drugs.”
But there are still a few hurdles to overcome before iodine-based propulsion systems become the norm, including the fact that iodine is highly corrosive, so the metal and electronics components of a satellite must be protected.
Solid iodine also requires around 10 minutes to be heated to a high enough temperature to sublimate into a gas, which means a satellite might be less responsive in orbit.
But nevertheless, Levchenko and Bazaka say that this “is an impressive contribution to the rapidly changing landscape of electric propulsion technologies”.