In writing Astronomical Returns (and teaching myself about space in general), the challenge I always come up against is learning and discussing the perfectly intermediate level of technical knowledge. As a lifelong space fan, I know way more than the average person, but with just my meager finance degree, I know nothing compared to an actual engineer or scientist. Fortunately for me, in the field of rocket fuel my problems were solved by a single book - Ignition: An Informal History of Liquid Rocket Propellants. I consider it the most informative book I've ever read, and I highly recommend it to you all!

Originally published in the 1970s, they just released a new reprint! It used to be quite rare, I had to dig deep in the UT Austin library to find it

When people think of rocket fuel, the image that comes to mind is usually a big booster firing massive chemical engines on the launchpad. But while conventional rockets are both powerful and necessary to escape Earth's gravity, they have their limitations. The fuel they carry is heavy and quickly exhausted, so once in orbit most spacecraft simply coast onward with minimal further course corrections; there's no way to produce constant acceleration/deceleration, the way we drive our cars on the highway. For that, an entirely new form of propulsion has been developed in the past few decades: ion drives! These futuristic engines provide a minuscule yet super-efficient thrust by using electrical currents to accelerate ions out the back end of the spacecraft, and though it sounds like science fiction, ion engines have been used since the late 1990s!

Looks like something straight out of your favorite science fiction movie!

Here's my best attempt at a 30-second ion engine crash course: rather than combusting fuel to produce exhaust and propel a spacecraft forward, an ion engine bombards a chamber full of gas with electrons. The electron stream knocks other electrons off the neutrally charged gas, producing a flow of positive ions that can be accelerated outward with an electrode, as well as more electrons that can continue the reaction. But while Ignition taught me everything I know about conventional rocket fuel and the pros and cons of various choices like kerosene, liquid hydrogen, and hypergolic fuels (learn more here), I realized I had no idea why we use the propellants we do for ion engines! Most ion drives use xenon - why is that the case, and have we considered anything else?

Looks complicated, but it's actually easy to picture with a simple animation - click this video!

In theory, we could use any gas that can be ionized, but noble gases like xenon are particularly appealing because they don't react with anything else (if you recall your AP Chemistry, it's because their outermost electron orbital is full!). But xenon isn't the only noble gas; if you check out the periodic table below, you'll see we've got a whole host of other options to pick from. What else makes xenon ideal?

Back when I was in high school chemistry, oganesson (Og) hadn't even been named yet!

Remember that our goal is to ionize the propellant, which means we need to knock an electron off each atom. As you go down the periodic table, the elements have more and more electrons, so they get farther and farther away from the nucleus and require less energy to displace, making heavier noble gases more suitable for ion drives. Additionally, heavier ions carry more momentum, producing more thrust for the spacecraft. But then why stop at xenon; why not use radon (Rn) or oganesson (Og) in our spacecraft? Unfortunately radon is radioactive, even the most stable isotope only has a half life of a couple of days. And oganesson? Well, that's just a downright wacky element, completely manmade, ridiculously unstable, and entirely impractical. So xenon it is!

Simplified electron diagrams of the first 5 noble gases. It takes about twice as much energy to ionize helium versus xenon (source)