Thousands of satellites are launched every year and it is critically important we can move them once they reach space. UNSW Canberra researchers are developing new ways to do this.
We have become increasingly reliant on satellites for everyday tasks. Using GPS for mapping or tracking routes, telecommunications, weather forecasting and emergency warnings all rely on satellites.
Thousands of satellites are launched into space every year, but once they are in orbit the need to move and position them does not end. This is where onboard propulsion systems come in. However, which substance is best suited as a propellant for these propulsion systems remains an ongoing challenge for the space industry.
Dr Trevor Lafleur from UNSW Canberra Space and his colleagues have recently completed the most comprehensive review to date of iodine, and its emergence as an attractive alternative propellant for electric space propulsion systems.
Once in space, satellites usually require an onboard propulsion system to change orbits and perform manoeuvres. They are used for de-orbiting at the end of the mission, emergency collision avoidance to prevent collisions with space debris or other satellites, maintaining or changing altitude, compensating for aerodynamic drag, and a range of other functions. So, propulsion systems are a critical component of modern satellites that enable in-orbit mobility, while helping to ensure the long-term sustainability of space.
Almost all propulsion systems used today are effectively classified as rockets. This means that the system carries its own onboard propellant, and a thrust force is generated by ejecting this propellant away from the satellite at high velocity.
Space propulsion can typically be classified as either chemical propulsion, or electric propulsion. We are all familiar with footage of large rockets spewing out flames and smoke as they take off from Earth, perhaps carrying astronauts or equipment to space – this is chemical propulsion. These rockets heat propellants to very high temperatures (in some cases to more than 3000 degrees Celsius), the resulting propellant gas is ejected at supersonic speeds out of a nozzle and this, in turn, generates thrust.
Chemical propulsion systems can generate a very high thrust and are the only viable method known today for launching satellites into space.
However, once satellites have reached orbit we increasingly look to electric propulsion to move the satellites as needed. Electric propulsion systems use electrical power, typically generated from solar panels or stored in batteries, to accelerate propellant via electric or magnetic fields. However, to facilitate this, the propellant usually needs to first be ionized. That means we must strip electrons from the propellant atoms or molecules, so that we end up with a “soup” (known as plasma) of positively charged ions and negatively charged electrons. With these charged particles, we can then apply electric and magnetic fields to control them. One of the major benefits of electric propulsion is that by using electric fields for acceleration, we can greatly increase the speed at which the propellant is ejected. Thus, less propellant is needed to perform manoeuvres in space. But the disadvantage is that the thrust force is very weak. If you put a piece of paper on your hand, that is roughly the force level that many electric propulsion systems can provide. Thus, electric propulsion systems only work in space and cannot be used to launch satellites.
Because of its attractive properties, xenon has traditionally been the propellant of choice for many high-performance electric propulsion systems. Firstly, it is a noble gas that is non-toxic and is not chemically reactive with most materials. It is also easy to ionise xenon, and it exhibits very attractive storage properties. In fact, when sufficiently pressurised, it has a higher storage density than even liquid water. This means a smaller storage tank size is needed, which is again an advantage for spacecraft.
However, xenon presents several significant challenges. It is a relatively rare gas and production is through fractional distillation of air: a process that is time consuming and energy intensive. For every 1000 metric tons of oxygen produced, only about 1 kilogram (kg) of xenon is obtained. Consequently, xenon is relatively expensive, costing around $5000 per kg, and global production is limited to around 50–60 metric tons per year.
With as many as 2000 satellites forecast to be launched per year moving forward, and with the vast majority requiring onboard propulsion, estimates suggest that space industry demand for xenon alone may outpace global supply. This ignores the demand for xenon from other sectors such as the semiconductor, medical, and lighting industries. Additionally, Russia and Ukraine produce 25-30% of the world’s xenon and prices surged significantly after the war there broke out.
Iodine is an element many people would encounter on a regular basis, whether that is in topical antiseptics like Betadine, in iodised table salt or in pregnancy supplements; it is critical to human health. It also has many properties that make it attractive for use in space.
Iodine is a has a similar atomic mass to xenon and requires slightly less energy to ionise. As a result, it can achieve similar or even better performance than xenon. The cost of iodine is approximately 100 times lower than xenon, and global production is almost 500–600 times higher. With such a large global output, iodine has the potential to more than satisfy current and future space industry demand.
Iodine is typically a solid at ambient conditions. It therefore has a higher storage density than xenon and does not require pressurisation. This significantly reduces the size and mass of the storage tank and overall propulsion system. These cost reducing factors make iodine particularly attractive for small satellites and satellite constellations.
Being stored as a solid complicates how the propellant is delivered and the control of propellant flow to the thruster. Gaseous propellants can simply be directed to the thruster by opening a valve. However, iodine needs to be heated to just above 100 degrees Celsius so that it sublimates – that is, transitions from a solid to a gas without first becoming a liquid. Therefore, a system that can heat the iodine before ejecting it via the thruster is needed.
A second problem with iodine is that it is significantly more chemically reactive than xenon and can corrode many common materials such as iron or aluminium. This means system design and material selection is critical.
Iodine has a very complex plasma chemistry compared to xenon, meaning that when you attempt to manipulate it, many different types of reactions can occur. Fundamental understanding of iodine plasmas is still relatively immature compared with other substances like xenon and requires further research – something UNSW Canberra is contributing to.
Currently, there are more than 100 iodine electric propulsion systems in space. However, most of them only operate at low power, below 100 watts, whereas larger satellites require 1-10 kilowatts to generate greater thrust. Further work is needed to adapt iodine for use in more powerful thrusters.
Finally, many electric propulsion systems generate thrust by accelerating positive ions. But to prevent the spacecraft from becoming negatively charged, a separate device known as a neutraliser is needed. This neutraliser emits electrons to ensure charge balance. Stable use of high-performance neutraliser technologies with iodine has not yet been achieved due to its extreme chemical reactivity.
At UNSW Canberra Space, we are performing research focused not only on new propulsion technologies, but also alternative propellants, such as iodine. This all feeds into our broader research priority associated with space safety, security, and sustainability. We currently have several projects related to iodine and better understanding its fundamental principles.
We have collaborated with the University of Michigan to conduct a world-first audit of iodine propulsion systems from initial design to performance in space. This is an important step in providing confidence to the broader community regarding iodine propulsion technology.
With our partners in France at Ecole Polytechnique, CNRS, and Safran via their joint laboratory COMHET, we have developed one of the most advanced iodine plasma chemistry models that exists in the world today. We are using this work to help analyse laboratory measurements and in numerical simulations to aid the development of future propulsion systems.