Neat Tech

Scientist are looking at replacing Xenon propellant with Iodine for ion drive.

The advantage to Iodine is that it is a solid, and hence is far denser when stored, and does not required a pressure vessel to store.

Unlike the noble gas Xenon though, Iodine, a halogen, is rather corrosive, and so requires some attention to materials.

Basically, the Iodine is boiled off (at 184.3°C) and then ionized and accelerated to very high velocity to provide a highly efficient low thrust source:

Most people are probably familiar with iodine through its role as a disinfectant. But if you stayed awake through high school chemistry, then you may have seen a demonstration where powdered iodine was heated. Because its melting and boiling points are very close together at atmospheric pressures, iodine will readily form a purple gas when heated. At lower pressures, it'll go directly from solid to gas, a process called sublimation.

That, as it turns out, could make it the perfect fuel for a form of highly efficient spacecraft propulsion hardware called ion thrusters. While it has been considered a promising candidate for a while, a commercial company called ThrustMe is now reporting that it has demonstrated an iodine-powered ion thruster in space for the first time.

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The current efficiency champion is the ion thruster, which has now been used on a number of spacecraft. It works by using electricity (typically generated by solar panels) to strip an electron off a neutral atom, creating an ion. An electrified grid then uses electromagnetic interactions to expel these from the spacecraft at high speed, creating thrust. The ions end up being expelled at speeds that can be an order of magnitude higher than a chemical propellant can produce.

Only a relatively small amount of material can be accelerated at once, so this can't generate anything close to the amount of thrust produced in a short period of time by a chemical rocket. But it uses far less material to produce the same amount of thrust and, given enough time, can easily produce an equivalent acceleration. Put differently, if you can be patient about your acceleration, an ion engine can do the equivalent amount in a form that uses less mass and less space. And those are two very important considerations in spacecraft.

Critical to making this work on a spacecraft's energy budget is a material that can be ionized without requiring much energy. Right now, the material of choice is xenon, a gas that's easy to ionize and resides several rows down the periodic table, meaning that each of its ions is relatively heavy. But xenon has its downsides. It's relatively rare (it's only one part per 10 million in our atmosphere) and must be stored in high-pressure containers, which cancel out some of the weight savings.

Iodine seems like an ideal alternative. It's right next to xenon on the periodic table and normally exists as a molecule composed of two iodine atoms, so it has the potential to produce more thrust per item expelled. It's even easier to ionize than xenon, taking 10 percent less energy to lose an electron. And, unlike xenon, it happily exists as a solid under relevant conditions, making storage far simpler. Just a bit of heating will convert it to the gas needed for the ion engine to work.

The big downside is that it's corrosive, which forced ThrustMe to use ceramics for most of the material that it would come into contact with.

By way of comparison, an ion drive can provide a little under a millinewton of thrust with an ISP (fuel efficiency) of about somewhere between 1,500 and 10,000 seconds.

By comparison, the Space Shuttle Main Engine (SSME) produces about 2.3 meganetwons of thrust and an ISP of 363 seconds.

It's a lot less thrust, but it can provide thrust for weeks and months, and in the vacuum of space over long distances, say much beyond the moon, it will significantly shorten travel time.