The Millenium Falcon dives into a swooping corkscrew, its ion engines flaring, as a star destroyer, with its own gigantic blue ion engine, is in hot pursuit, supported by a half dozen tie fighters, themselves accelerated by the power of ions.

When I think "ion engines", this is where my mind goes – straight to science fiction – space ships engaging their ion engines, lifting off from a terrestrial landing platform, shooting into the sky and straight out into space.

Well, I've got good new and bad news.

The bad news is that, in reality, ion engines are not quite as robust as science fiction would have us believe.

The good news is that I'm able to say "in reality" at all – because ion engines are real, baby. Right now ion engines are revolutionizing travel within our solar system, but one day an ion engine may take at least some small part of humanity out to the stars.

Today we're going to deal with how ion engines work. Next time we'll talk about current and future applications of ion engine technology.

How Does An Ion Engine Work

Broad strokes, an ion engine uses electricity - either from solar power or from an onboard nuclear generator - to ionize individual atoms of propellant, usually xenon, and shoot those ionized atoms out the back of the engine to achieve thrust.

There's a lot to unpack here, so let's get into the weeds a little.

What are ions?

Ions are atoms that have more or less electrons than they have protons.

Wait, what is an atom again?

Lets harken back to those heady days in high school chemistry class. I'll give you the high school basics. Atoms are the physical building blocks of the universe. This photo contains some simplified renditions of atoms. Here are the (very) basic rules you need to know to understand ion engines:

Each atom has a nucleus, which consists of protons and, usually, neutrons.
The nucleus of an atom is, usually, encircled by at least 1 electron.
Each of the above particles, electrons, protons and neutrons, has a charge associated with it:
- neutrons = neutral charge
- protons = positive charge
- electrons = negative charge
The number of protons in an atom cannot change, otherwise you have a different kind of atom entirely. (E.g. nuclear fusion).
However, the number of electrons circling an atom can change, and often does, at least temporarily.

So what are Ions again?

When an atom has an equal number of electrons as it has protons, the atom is said to have a neutral charge. This is because protons have a positive charge, electrons have a negative charge, and when there are the same number of both, the two cancel each other's charges out.

An Ion is an atom that has either more electrons than protons or fewer electrons than protons.

Take a look at the photo to the right. It provides an example using a hydrogen atom, which is the simplest demonstration because hydrogen has only a single proton in its nucleus. Look at the photo:

The hydrogen atom in the center of the photo has a single electron circling its single proton. One negative charge + one positive charge equals a neutral charge. This is a neutral hydrogen atom and not an ion.
The hydrogen atom on the left has no electron. One positive charge equals a positively charged hydrogen atom, which is an ion. A positively charged ion is referred to as a cation
The hydrogen atom on the right has two electrons around its single proton. One positive charge + two negative charges = a negatively charged hydrogen atom, which is also an ion. Negatively charged ions are referred to as "anions."

This process of adding or removing ions is called "Ionization"

OK, I'll bite - what the hell is ionization?

I'm so glad you asked!

Ionization is the process by which electrons are added or removed from an atom. We, meaning human beings, use ionization as a tool all the time.

For example, take the common flourescent light bulb.

If you're reading this in an office, you may be under flourescent light right now. That light is made possible by ionization.

Flourescent bulbs are amazing chemical creations that I want to do a whole detailed post on in the future. But for today, we'll try to keep it concise:

The light you see coming out of a flourescent bulb is made possible by passing electricity (electrons) through cathodes. Those cathodes heat up, which releases free electrons into argon gas in the form of heat. The neutral argon takes on these free electrons and becoming negatively charged. These argon anions heat up into a glowing plasma, which in turn causes an ionizing chain reaction with mercury in the bulb. This chain reaction of ionization creates UV, which interacts with a phosphor coating, ionizing the phosphor and emitting, finally, visible light. Fluorescent light is the result of the ionization of three different materials in an ongoing chain reaction.

Cool... so, how does that move a spaceship?

An ion engine takes a noble gas, currently xenon, and ionizes it using a cathode, in much the same way as happens in a fluorescent bulb. Then, using either electrostatic or electromagnetic force, the xenon ions are sped up and and shot out the back of the ship as a the blue plasma you see to the right.

These xenon ions shooting out the back of the engine exerts a forward pressure on the ship itself - sort of like a rocket, but with a hell of lot less thrust.

A chemical rocket will expend a large amount of energy in a very short amount of time. This is why rockets are very good for, say, getting a spaceship off of the earth and into space.

Rockets use up their fuel super fast, super energetically and create a gigantic amount of thrust, very quickly. However rocket engines are not actually very efficient. (For a more detailed breakdown of space propulsion efficiencies google "specific impulse") Rockets require a lot of propellant relative to the overall amount of propulsion they provide.

Ion engines, on the other hand, are currently the most efficient propellant-based space engines that exist. However, ion engines also have a much lower thrust than chemical rockets. In fact, the meager thrust of an ion engine could never get a spaceship off the planet earth.

But, once a ship is in space, where there is no atmosphere or gravity to fight against, an ion engine really shines thanks to the conservation of momentum. Without the friction of an atmosphere or the pull of gravity to impede it, an Ion engine can eventually accelerate to great speeds while using far less propellant then a conventional rocket. It just takes a long time.

In the case of the Dawn spacecraft, which propelled itself exclusively on ion engines, the ship eventually reached 60km/s, but took many months to get that fast.

But we'll save the Dawn mission, and future applications of ion engines, for the next post. For now just remember:

TL/DR:

Ions are negatively or positively charged atoms with more or less electrons than protons.
Atoms become ions through the process of ionization.
Ion engines use ionization to ionize xenon gas and then shoot those ions out the back of the ship.
This is a very efficient process, but the thrust is very small.
Therefore, although ion engines can achieve great speeds, they do so, very slowly, very consistently, over a very long time, especially compared to conventional chemical rocket engines.

This is, as always, an over simplification of how ion engines work. I've left out a lot of details - especially the specifics of the different methods ion engines use to accelerate the xenon ions once they are created. I encourage you to look into that in more detail if you want, but for my purposes it's only important you understand the broad strokes.

Next time: The Dawn mission and The Future Of Ion Engines.

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Photo Sources
[1]By NASA Great Images in NASA Description Public domain, via Wikimedia Commons
[2][6]By Jet Propulsion Laboratory Public domain, via Wikimedia Commons
[3]By BruceBlaus (Own work) CC BY 3.0, via Wikimedia Commons]
[4]By Jkwchui Own work CC BY-SA 3.0, via Wikimedia Commons
[5]Christian Taube CC BY-SA 2.0, via Wikimedia Commons
[7]Public Domain, US Military Photo
[8]By National Aeronautics and Space Administration Public domain, via Wikimedia Commons

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So it worked the first time. So what? A single yunk means nothing. An experiment of yunkung with only one yunk? Please - it's ridiculous. No, more tests must be done - more yunk must be uncovered...

Piqued #10: Ion Engines 101: The Basics