In a space first, scientists test ion thrusters powered by iodine

For a few years now, ion propulsion technology's sci-fi mechanics have raised the standard for flying spacecraft, replacing fiery rocket tails as the new in-thing. Ion propulsion can be about 10 times faster than normal fuel and can continuously run for prolonged periods of time, gaining a wicked amount of speed along the way. 

One drawback, however, is it's typically employed with xenon thrusters. JAXA's Hayabusa2 mission used the classic xenon as a fuel. Xenon, a heavy noble gas, is exceptionally rare on Earth, pricey and difficult to maintain. That's why French aerospace company ThrustMe is pursuing a plan for better ion propulsion tech. They suggest using iodine instead of xenon.

Iodine is incredibly prevalent, inexpensive and easy to store with minimal effort. Seaweed, printing ink, dairy and even table salt are rich in the non-reactive element. 

"Iodine is significantly more abundant and cheaper than xenon and has the added advantage that it can be stored unpressurized as a solid," Dmytro Rafalskyi, CTO and co-founder of ThrustMe, said in a statement. 

Along with his team, Rafalskyi developed a working propulsion system with an iodine ion thruster, called the NPT30-I2. It includes all of ion technology's needed subsystems and fits within a single package of roughly 10x10x10 centimeters (about 4x4x4 inches).

Using iodine as fuel for spacecraft has been toyed with in the past. But what sets ThrustMe apart is it actually sent a satellite into space with its device -- and the operation was a success. It published its results on Wednesday in the journal Nature.

"Although iodine is viewed as a game-changing propellant and has been investigated by companies, universities and space agencies around the world," the team writes in their paper, "no system has previously been tested in space."

ThrustMe integrated its system into the Beihangkoshi-1 research satellite, which is operated by the global space company Spacety. The craft was launched into orbit by a Long March 6 rocket on Nov. 6, 2020. Since then, the ThrustMe team has been scrutinizing every aspect of the propulsion system, and according to its observations, everything worked as expected.

"Having our results peer-reviewed and publically accessible provides the community with further confidence and helps to create a benchmark within the industry," Ane Aanesland, CEO and co-founder of ThrustMe, said in a statement.

The breakthrough of ion propulsion

For a spacecraft to move forward, it needs to propel something backward. This notion is the classic Newtonian sentiment "every action has an equal and opposite reaction." Normal combustion engines ignite chemicals in fuel, creating gas (and wonderful flames), that is pushed out the exhaust at the bottom of the spacecraft. This propels it upward. 

The issue with that technique of propulsion for spacecraft is it only works in short bursts -- the "action" of the engine needs to always be "on" for the spacecraft's opposite "reaction."

Ion thrusters are a bit different. Instead of jetting out an intense stream of fuel, they take advantage of a concept called ionization. The process works something like this:

Every atom of every element has some amount of electrons, particles with a negative charge, and protons, particles with a positive charge. When an atom is neutral, typically its stable state, its number of electrons equals its number of protons. That creates a net zero charge for the atom. 

When an atom is ionized, the electron amount changes -- what ion propulsion systems are after. 

An ion thruster works to take and remove electrons from a bunch of atoms, turning them into charged "ions." Once there's an even number of negative ions and positive ions, a neutral environment called a plasma is created. The key with plasma is it responds to electric or magnetic fields. 

Ion thrusters have magnetic grids inside to induce fields and ultimately expel the positive ions. Ions propelled at exceedingly fast speeds out of the back of the spacecraft push the spacecraft forward. A bonus point? The thrusters use significantly less "fuel" than chemical rockets.

Even though the push is super light at first -- one NASA scientist describes the initial propulsion to be as light as a gentle blow -- it picks up speed over time. Over days, months or years, the velocity compounds upon itself.

Xenon is normally used for the operation because as it's a noble gas, it's easier to pluck off or pop on electrons. It also already lives in a gaseous state, which helps move the plasma-induction process along. But, as per ThrustMe, iodine atoms are a fair competitor.

Is iodine better?

Iodine has a few of its own limitations. The main reason scientists haven't gone through with building iodine thrusters in the past is that the element is too corrosive in a solid state. But to keep iodine as a player in the thruster game, storing it as a solid is necessary because that enables the cheapest maintenance. 

The team writes that "iodine has a high electronegativity that can lead to corrosion with many common materials" and that "vibrations during launch and spacecraft motion once in orbit can cause solid iodine to break into pieces, which may damage the propulsion system or lead to poor thermal contact during heating."

But on the bright side, the crew got around that hurdle by using ceramic containers to carry its iodine atoms in the thrusters, which appeared to help. They also found the solid iodine could easily be heated to become a gas and begin the ion propulsion process.

It should also be noted that other companies are looking to novel compounds for ion thrusters as well, such as SpaceX. The researchers say SpaceX chose krypton as a propellant for Starlink satellites. "However," they write in the paper, "krypton has a higher ionization threshold and lower atomic mass than both xenon and iodine, and the required propulsion system power increases by more than 25% to achieve the same thrust level."

All in all, more research is likely needed to determine the viability of iodine-based thrusters, particularly because this is the first demonstration of the device, but ThrustMe's successful mini-satellite spaceflight could be a solid first step.