A team of researchers may have found the elusive component to making electric propulsion more mainstream in the near future. Interestingly, the component isn't some rare earth mineral but a common food supplement, iodine. A small satellite was put in orbit last year, using iodine as a propellant, according to this paper published in Nature today.
Traditionally, chemicals have been used as propellants for space vehicles. According to the European Space Agency, electric propulsion, where electrical power is used to accelerate the propellant is a much more efficient way of propulsion since it uses a lesser mass of propellant and can eject it up to twenty times faster. This can acutely reduce the mass of propellant spacecraft carry for orbit raising and other such maneuvers, further reducing the cost of launch price of these vehicles.
This method of propulsion is largely used for satellites used for applications such as Earth observation, navigation, and telecommunications. However, the current propellant of choice is xenon, an inert gas found in trace amounts on the planet. At times, krypton has also been used.
Michael Tate, Co-Founder & COO of Infinite Composites, a company engaged in making pressure vessels for holding propellants said, "Krypton and Xenon have very high density. Their pressure ranges typically from 3000 psi (200 bar) to 5000 psi (350 bar) which is ideal for electric propulsion systems because it’s all about “throwing” as much mass behind you as possible."
However, apart from being rare, the commercial production of the propellant is an expensive affair and unsustainable in the long term. A collaborative team of researchers comprising of French space firm, ThrustMe, and the technical experts from Nanyang Technological University in Singapore used iodine instead.
According to ThrustMe's website, iodine can be stored as solid fuel inside the spacecraft, unlike xenon that needs to be pressurized. When a moderate temperature is applied, the halogen element sublimates — gets converted into its gaseous form, skipping the liquid phase altogether. This makes it ideal for applications, where pressurized gases are needed.
On November 6, 2020, ThrustMe used iodine as a propellant on a 44 pound (20 kg) CubeSat and performed maneuvers to place it in its orbit. The Nature paper not only confirms the use of iodine as a propellant but also claims that it has a higher ionization efficiency.
"Ionization efficiency refers to how efficiently power can be used to ionize propellants into the plasma that will ultimately be ejected from the spacecraft," said Dr. Natalya Bailey, co-founder of Accion Systems that provides electric propulsion systems for satellites using proprietary ion electrospray thrusters.
István Lörincz, President & Cofounder at Morpheus Space that provides artificial intelligence-driven satellite propulsion systems adds, "The higher this efficiency is, the less propellant and electrical power is needed to operate the propulsion systems, which leads to smaller tanks, solar panels, and batteries, which leads to cheaper satellites and cheaper rocket launches."
This is also the claim the authors make in the paper that using iodine as a propellant can make satellite systems smaller, simpler, easier to deploy as well as dispose-off after their lifetime. Iodine is far cheaper to produce and more abundant than xenon on Earth. However, there are some downsides too.
Like Dr. Bailey points out, "Propellants like iodine must be heated prior to use resulting in a warm-up and cool-down periods. At a larger level, electric propulsion systems are power-intensive, prohibiting satellites from thrusting and using their primary payloads at the same time. This results in considerable downtime where satellites cannot do their intended mission."