Electric Propulsion (EP) is a propulsion system designed specifically for space that uses electrical energy to accelerate propellant through various electrical or magnetic physical processes – essentially accelerating a gas that has been ionised in some way electrostatically. The advantage of such a system over conventional propulsion is that the propellant can escape at up to 20 times the speed of conventional propulsion, making EP much more efficient in terms of the mass of propellant required – reducing the mass of propellant that the device has to carry, ultimately leading to significant cost savings.
The propellant may vary for different propulsion systems using different solutions: sometimes even conventional propellants may be used, but typically noble gases such as xenon or argon are used, especially the former, which is used by most of these propulsion systems. Another disadvantage of xenon is that it has to be stored at high pressure. EPs are mainly used in satellites, which is one reason why these noble gases are rare and why the industrial production of such propellants is very expensive and not sustainable in the long term.
This is where iodine comes in, which is used in the EP developed by the French company ThrustMe and researchers at Nanyang Technological University in Singapore. The engine was tested in space last November on a 20 kg CubeSat satellite, and a paper reporting the results has just been published in Nature, reports Interesting Engineering. Compared to Xenon, iodine has several advantages: it is available in larger quantities, it is cheaper to produce and it is easier to store because in its elementary state iodine is composed of diatomic I2 molecules, a brownish-black solid that would be stored in the container. However, iodine sublimes easily – leaving the liquid state out, it turns into a gas at lower temperatures.
The news also reports that iodine is also more ionisation efficient than xenon – so it requires less energy to ionise, which means less propellant and less electrical energy is needed to power it. So the satellite can also be smaller, simpler – ultimately cheaper – and the reduction in mass can save money on launching these devices into space. In addition, smaller satellites make it easier to avoid collisions and reduce the mass of space debris.
The IE does mention that the method is not perfect: the iodine needs to be heated to some extent before use, so the engine will have warm-up and cool-down periods during which it cannot be used. And if EP were to be installed in larger devices, it would need quite a lot of energy.