Over the last ten years, there has been a consistent growth in the number of small satellites being launched into Earth’s orbit, with a significant increase from 39 launches in 2011 to 1,202 in 2020. This trend is expected to continue as satellite companies like OneWeb, SpaceX, and Amazon have ambitious plans to deploy tens of thousands of satellites in the future to improve internet access worldwide.
Small satellites are typically powered by electric propulsion systems that use expensive gas propellants such as xenon or krypton. However, the supply of these gases is limited. To address this issue, Trevor Lafleur, a plasma physicist and principal engineer at ThrustMe, a French space company, and his team are working on developing a cheaper and more sustainable propellant. This propellant will allow satellite constellations to maneuver through orbit safely and efficiently. Xenon, the heaviest nonreactive noble gas, is commonly used as a propellant because it has a low ionization threshold, which means it requires less energy to ionize. However, with a cost of $3,000 per kilogram, it is an expensive option. Moreover, it is a finite resource, accounting for less than one part per 10 million in Earth’s atmosphere. This is a concern, especially considering the gas’s widespread use in industries such as lighting and imaging.
SpaceX’s Starlink satellites rely on Krypton for propulsion, which is currently in high demand. Despite being less expensive than xenon, Krypton is not as effective. Additionally, a significant portion of the global supply of Krypton is utilized for thermal insulation in windows, a necessity that is predicted to increase as the planet warms and construction companies strive to minimize energy waste in larger structures. Due to these factors, Lafleur suggests that the small-satellite industry may need to seek alternative options.
Iodine has been introduced as a potential alternative to xenon. This mineral, naturally occurring in various sources including soil and seaweed, is readily available – its global production surpasses that of xenon by 500 times and it is also significantly cheaper, with a cost that is about 100 times lower. According to Lafleur, iodine can easily be utilized on multiple satellites while also meeting the demands of other industries. Despite having a similar mass as xenon, iodine has a storage density that is three times higher than xenon and nine times higher than krypton, both of which are stored as compressed gases in tanks. Iodine can be compactly stored in an unpressurized tank, making it easier to miniaturize. In tests, iodine has proved to be twice as efficient as xenon. Furthermore, it is safer to transport and can be delivered pre-fueled, eliminating the need for buyers to source propellants from local suppliers. As stated by Lafleur, it can simply be plugged in for use.
During the month of February in 2021, ThrustMe successfully completed the inaugural in-orbit trial of an iodine ion propulsion system. The NPT30-I2 thruster was activated 11 times while in space to assess its functionality and to adjust the position of the Spacety satellite. The findings of this experiment were published by ThrustMe in the journal Nature in November 2021.
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Ion thrusters utilize a method known as ionization, extracting electrons from an atom in order to produce thrust. Instead of ionizing valuable noble gases, ThrustMe’s propulsion system (refer to main image) transforms solid iodine into a plasma, which can subsequently be ionized and discharged as thrust.
The process involves heating solid iodine in a specialized tank until it changes into a gas through sublimation. This gas then enters a discharge chamber surrounded by a radio-frequency (RF) inductive antenna, which functions like an induction stovetop to heat the gas and convert it into a plasma. According to Lafleur, this results in a mixture of electrons and positive ions. The next step involves using a set of high-voltage grids to extract the positive ions from the mixture and propel them forward to generate thrust.
While iodine may seem like the perfect solution, it does have its drawbacks. Due to its highly corrosive nature, the team at ThrustMe had to protect the critical components of the thruster with specialized coatings that are compatible with iodine. These coatings are made from ceramic and polymer, as mentioned by a source from Popular Mechanics. Additionally, it takes approximately 10 minutes for the system to reach the necessary temperature to convert solid iodine into plasma. This delay can hinder the satellite’s ability to make swift orbit adjustments. However, Lafleur believes that this issue will be resolved as the system gains more power.
ThrustMe intends to expand its designs to cater to a diverse range of missions. According to Lafleur, the company is currently developing a bigger version of their existing propulsion system, along with a design that combines several thrusters. These advancements have the potential to propel a spacecraft to destinations such as the moon, Mars, or even further into the unknown.