Getting around in space is expensive in terms of actual money and resources.
First, you have to get your payload into orbit atop a rocket. Then — whether or not you’re working with a satellite or a spacecraft destined for another world — you have to make course corrections and possible accelerations, all using the limited amount of propellant you brought with you.
Remember Newton’s third law of motion? If you want to move forward in space, you have to throw something backward.
Exactly what that something is and how you throw it has a lot to do with your bottom line: Chemical rocket engines generate powerful thrust for short periods of time but require a lot of fuel and an oxidizer that you have to launch into space. And this can get pricey.
Meanwhile, electric propulsion systems, like ion thrusters, use electric fields to expel ionized gas to generate a lower thrust — but very efficiently. But they require far, far less propellant than chemical rockets. This is why many satellites now use ion thrusters, as do some deep space missions, such as the Japanese Space Agency’s asteroid sample return missions, Hayabusa 1 and 2.
Still, price comes up again. Most ion thrusters use xenon as a propellant, and xenon is an expensive gas with many competing terrestrial uses. As the number of satellites and operators grows, the cost of xenon could make ion thrusters unsustainable. Plus, xenon ion thrusters are too large to be used by many smaller satellites, such as CubeSats.
Instead of xenon, the engine uses iodine, which is more efficient than xenon, cheaper, and takes up far less space. It’s an engine type that could sustainably power a lot more spacecraft more efficiently, and spacecraft of all sizes to boot.
“The system is significantly cheaper to produce compared to xenon-based,” ThrustMe Chief Technology Officer Dmytro Rafalskyi tells Inverse. “It's also way cheaper to use — it doesn't require any filling procedure before the launch, and also doesn't need special certification because of the tank pressure.”
What’s new — You may have heard of iodine before: it’s a mineral found in some foods and in nature, and is essential for the production of thyroid hormones. It’s found in trace amounts in water, soil, and the air.
In this case, we’re dealing with iodine-based plasma. It’s created “by electron impact ionization using a radio-frequency (RF) inductive antenna, and positive plasma ions are extracted and accelerated by the grids to high speeds to produce thrust,” the study team writes.
Iodine ion thrusters are not totally new. They date back almost 50 years — as an idea. But ThrustMe’s innovations make the engines a reality. Rafalskyi and colleagues not only describe the iodine thruster engine ThrustMe designed — they built it and flew it successfully in space in November 2020.
Launched atop a Long March 6 rocket in November 2020, the Beihangkongshi-1 satellite (operated by the small satellite company Spacety) was able to use ThrustMe’s iodine thruster to alter its attitude by 200 to 400 meters during each of 11 test firings.
The flight test helped solve one of the two major challenges facing ThrustMe in developing an iodine propellant ion thruster, Rafalskyi says: a lack of data on how iodine would function in practice.
“The problem of lacking the data on iodine is really important, as many specific physical and chemical properties are unknown,” he says. “We had to do a lot of research for studying that — we have created a database of material interactions and corrosion under iodine.”
Corrosion is a serious issue in engines that operate for thousands of hours expelling a hot soup of ionized gas. One reason for the widespread use of xenon in ion thrusters — aside from the fact that its relatively heavy atomic mass, which increases thrust efficiency — is that it’s a noble gas, and does not react with engine components.
But while xenon must be stored under pressure as a gas, iodine can be stored as an unpressurized solid, enabling miniaturization impossible with pressured gas propellants.
This does, however, present another challenge: figuring how to utilize a corrosive and solid fuel that must be turned into a gas. A solid piece of iodine could crack or break up, ruining fuel flow.
ThrustMe’s design solves this by storing the iodine in a porous ceramic block, using heaters to sublimate it into gas, and a radio frequency antenna to ionize the gas before accelerating it out the back of the thruster using an electrically charged grid.
It’s complicated, but still more efficient and compact than using xenon propellant. With iodine’s greater atomic mass, it’s a more efficient thruster overall.
That could open electric spacecraft propulsion up to operators that don’t have access today, Rafalskyi says.
The big picture— On one hand, iodine thrusters could enable new constellations of efficiently maneuvered small satellites, and put the rapidly growing space industry on more sustainable footing. Almost 1,300 satellites were launched in 2020 alone.
On the other hand, Rafalskyi says, iodine thrusters can also power larger spacecraft with greater efficiency, including deep space planetary science missions.
“We have delivered 10 plus systems to the clients worldwide this year.”
“The size of the satellite is not limited at all,” he says, as the “larger the satellite is, as more electrical power is available and therefore larger and more efficient electric propulsion system can be used onboard.”
That could include crewed missions— at least in principle. Even with iodine as a propellant, most thrusters take too long to get a spacecraft up to the speed that would reduce a crew's exposure to harmful radiation, Rafalskyi says.
But once future spacecraft with much larger power sources are available, then ion thrusters could become the human deep spaceflight engine of choice.
What’s next?— The next steps for ThrustMe will be to launch additional test engines to research greater performance from their iodine ion thrusters.
But in the meantime, they’re busy making their first-generation engine available to those who want it.
“We have delivered 10 plus systems to the clients worldwide this year, using our pilot production line,” Rafalskyi says. “Now we are working to finalize the production line and reach the capacity of a few hundred units per year.”
Abstract: Propulsion is a critical subsystem of many spacecraft. For efficient propellant usage, electric propulsion systems based on the electrostatic acceleration of ions formed during electron impact ionization of a gas are particularly attractive. At present, xenon is used almost exclusively as an ionizable propellant for space propulsion. However, xenon is rare, it must be stored under high pressure and commercial production is expensive. Here we demonstrate a propulsion system that uses iodine propellant and we present in-orbit results of this new technology. Diatomic iodine is stored as a solid and sublimated at low temperatures. A plasma is then produced with a radio-frequency inductive antenna, and we show that the ionization efficiency is enhanced compared with xenon. Both atomic and molecular iodine ions are accelerated by high-voltage grids to generate thrust, and a highly collimated beam can be produced with substantial iodine dissociation. The propulsion system has been successfully operated in space onboard a small satellite with manoeuvres confirmed using satellite tracking data. 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. For example, iodine enables substantial system miniaturization and simplification, which provides small satellites and satellite constellations with new capabilities for deployment, collision avoidance, end-of-life disposal and space exploration.