Iodine Thruster Demonstration Opens Way For More Powerful, Maneuverable Spacecraft

An iodine electric propulsion system firing in a vacuum chamber as seen looking downstream of a flight model. Systems like this had been demonstrated on Earth, but only now shown to work in space. Image credit: ThrustMe

The successful demonstration of an iodine electric thruster in space suggests the world is close to cheaper, more powerful ways to move satellites in orbit and spacecraft exploring other worlds. With low Earth orbit having become a much more dangerous place after last week’s blowing up of the Cosmos 1408 satellite, improved evasion capacity will be much appreciated.

In space, Newtonian action and reaction aren’t obscured by friction or air resistance, so to move a spacecraft you just need to fire a propellant in the opposite direction you want to go. Electric propulsion thrusters are more efficient than those powered by chemical reactions and are in increasingly widespread use. However, they’ve been hampered by less than ideal propellants.

Iodine has been proposed for 50 years as a more suitable propellant than the xenon and krypton currently in use, but some obstacles have stood in the way. French firm ThrustMe’s successful first demonstration of an iodine thruster in space has been reported in Nature.

The experiment took place a year ago when a 20 kilogram (44 pound) CubeSat satellite boosted its altitude by 200-400 meters (656-1,312 feet) on each of 11 firings.

Thrusters like this are a long way from providing the swift bursts of power that would allow a craft to take off from a planet or large moon. Once in space, however, they are much more efficient than rockets for making adjustments and have also been used for slow build-ups of speed such as on asteroid exploration missions.

ThrustMe's flight model of the NPT30-I2 iodine electric propulsion system firing in a vacuum chamber as seen from the side. Image credit: ThrustMe

Propellant isn’t the same as fuel – in theory, any ionized material could be used but most substances present problems of one form or another. Xenon is currently the dominant propellant, in part because of its low ionization energy. However, it’s expensive on Earth, almost impossible to obtain elsewhere in the Solar System, and needs to be stored in high-pressure tanks, whose weight needs to be launched. Krypton, whatever Superman may tell you, is cheaper, but has some similar problems.

Iodine has an even lower ionization energy than xenon, but can be stored as a solid and turned to gas with just a little heating. Plenty of people have noticed iodine’s potential and studied its practicality on Earth. Unfortunately, iodine is very reactive, posing a threat to metal parts in a thruster and, when stored as a solid, tends to break up in ways that could interfere with its flow.

ThrustMe had previously demonstrated the practicality on Earth of storing iodine in an aluminum oxide ceramic block prior to heating. The Nature paper confirms the idea works in space, where it matters, as well. Once heated, the iodine was ionized using a radio-frequency inductive antenna, accelerating a chain reaction and producing ions that were then accelerated using an 800-1,300 Volt electric field. The resulting propellant has a better power to weight value than xenon.

An early evening under a dark sky confirms how much more common satellites are becoming. In 2011, only 39 new satellites were launched, an editorial accompanying the paper notes, in 2020 that number was 1,202. To avoid bumping into each other those satellites need the capacity to control their orbits cheaply, something ThrustMe’s iodine engine may provide.

ThrustMe acknowledges xenon still offers faster start times, an advantage in emergencies.

While cheap and efficient, electric thrusters are still much less powerful than traditional rockets, presenting an obstacle to crewed missions, which require heavy vehicles and can’t afford to wait. Nevertheless, there are still hopes this will be the technology that takes humans to Mars.

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