New iodine-based plasma thruster tested in orbit

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Image of a jet of glowing material emerging from a metal box.
Enlarge / A demo model of the brand new thruster in operation.

ThrustMe

Most individuals are most likely accustomed to iodine via its position as a disinfectant. But if you happen to stayed awake via highschool chemistry, then you’ll have seen an indication the place powdered iodine was heated. Because its melting and boiling factors are very shut collectively at atmospheric pressures, iodine will readily kind a purple gasoline when heated. At decrease pressures, it will go instantly from strong to gasoline, a course of referred to as sublimation.

That, because it seems, might make it the proper gas for a type of extremely environment friendly spacecraft propulsion {hardware} referred to as ion thrusters. While it has been thought of a promising candidate for some time, a industrial firm referred to as ThrustMe is now reporting that it has demonstrated an iodine-powered ion thruster in house for the primary time.

Ion energy

Rockets depend on chemical reactions to expel a big mass of fabric as rapidly as doable, permitting them to generate sufficient thrust to carry one thing into house. But that is not the best method to generate thrust—we find yourself buying and selling effectivity in order to get the speedy expulsion wanted to beat gravity. Once in house, that want for velocity goes away; we are able to use extra environment friendly technique of expelling materials, since a slower price of acceleration is appropriate for shifting issues between completely different orbits.

The present effectivity champion is the ion thruster, which has now been used on a number of spacecraft. It works by utilizing electrical energy (usually generated by photo voltaic panels) to strip an electron off a impartial atom, creating an ion. An electrified grid then makes use of electromagnetic interactions to expel these from the spacecraft at excessive velocity, creating thrust. The ions find yourself being expelled at speeds that may be an order of magnitude greater than a chemical propellant can produce.

Only a comparatively small quantity of fabric will be accelerated directly, so this will’t generate something near the quantity of thrust produced in a brief time frame by a chemical rocket. But it makes use of far much less materials to provide the identical quantity of thrust and, given sufficient time, can simply produce an equal acceleration. Put in another way, if you happen to will be affected person about your acceleration, an ion engine can do the equal quantity in a kind that makes use of much less mass and fewer house. And these are two essential issues in spacecraft.

Critical to creating this work on a spacecraft’s vitality finances is a fabric that may be ionized with out requiring a lot vitality. Right now, the fabric of alternative is xenon, a gasoline that is simple to ionize and resides a number of rows down the periodic desk, that means that every of its ions is comparatively heavy. But xenon has its downsides. It’s comparatively uncommon (it is just one half per 10 million in our environment) and have to be saved in high-pressure containers, which cancel out a few of the weight financial savings.

Enter iodine

Iodine looks like an excellent different. It’s proper subsequent to xenon on the periodic desk and usually exists as a molecule composed of two iodine atoms, so it has the potential to provide extra thrust per merchandise expelled. It’s even simpler to ionize than xenon, taking 10 p.c much less vitality to lose an electron. And, in contrast to xenon, it fortunately exists as a strong underneath related situations, making storage far less complicated. Just a little bit of heating will convert it to the gasoline wanted for the ion engine to work.

The massive draw back is that it is corrosive, which pressured ThrustMe to make use of ceramics for a lot of the materials that it will come into contact with.

The thruster design included a gas reservoir full of strong iodine that might be heated with resistance heaters powered by photo voltaic panels. The iodine itself was inside a porous aluminum oxide materials that saved it from fragmenting from the vibrations it skilled throughout launch (the aluminum oxide is 95 p.c open house, so it did not subtract a lot gas storage). The tank is related to an ionization chamber through a small tube; when the system was cooled after use, sufficient iodine would solidify in this tube to seal off the gas from the surface world.

Once in the ionization chamber, the iodine gasoline is bombarded with electrons, which can knock different electrons off, making a plasma. The close by electrical grid then accelerated the constructive ions out of this plasma, creating thrust. Electrons have been extracted from the plasma and injected into the ion beam to maintain all the pieces electrically impartial.

Heat extractors have been connected to the electronics and the partitions of the iodine tube, with the warmth recirculated into the iodine gas whereas the thruster was firing. This saved the facility necessities for vaporizing the iodine right down to a single Watt as soon as the thruster reached regular state.

The complete setup was extremely compact, taking over about the identical quantity of house as a ten centimeter-per-side dice, and weighed just one.2 kilograms. And, by some measures, it outperformed a xenon-based thruster by 50 p.c.

Space-based demo

Working {hardware} was flown on a 12-unit cubesat weighing about 20 kilograms referred to as Beihangkongshi-1. And, over the past two years or so, the thruster has been used a number of instances to deal with shifting the satellite tv for pc to keep away from potential collisions. Satellite monitoring and on-board thruster monitoring present that the iodine-based thruster labored simply in addition to it had throughout testing on Earth.

It’s vital to repeat that the quantity of precise thrust is tiny—about 0.8 milliNewtons whereas in operation. But the thruster might simply preserve that for nicely over an hour, offering sufficient thrust to maneuver it into an orbit that was just a few hundred meters greater. So, whereas it might by no means put something into orbit, ThrustMe’s {hardware} can undoubtedly transfer issues round in orbit fairly nicely.

The massive limitation once more is the velocity. It solely strikes slowly, and it takes about 10 minutes to warmth the iodine sufficient for the thruster to begin working. If an emergency maneuver was wanted, this would not reduce it. But, assuming no one’s blowing up a satellite in your neighborhood, a lot of the dangers for satellites will be recognized nicely in advance.

Nature, 2021. DOI: 10.1038/s41586-021-04015-y  (About DOIs).

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