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Blog-a-Day #6 -- Ion My Way!

We talked about light sails last time. To be honest, light sails will be very effective for space probes or other unscrewed spacecraft. In my opinion, less so for moving people about the solar system. The further you get from the sun, the less sunlight will hit your sail thanks to the Inverse Square Law, which is almost as pesky as the Rocket Equation. Also, I’ve never been able to figure out how you slow down and stop. It’s not like there’s another sun out there pushing back on your sail.

What other alternatives are there that work no matter how far away you are and which direction you need to go? And how does the picture of the young girl who is about to send those wispy seeds flying relate to spaceflight? One that comes to mind, is an old idea that has actually flown in space on many satellites and a couple of deep space probes. It’s called an ion drive. I first heard about ion drives when I read 2001 A Space Odyssey by Arthur C. Clarke.

The spaceship, Discovery 1, carries a crew of two who are awake and several others who are in hibernation, oh and the AI named HAL. It still takes several months to get to Jupiter, but given that Clarke was an astrophysicist—and the inventor of the concept of geostationary communications satellites—we can assume the physics and math of the ion drive are solid.

What is an ion drive? An ion drive is an electrically-powered thruster which takes a gas, preferably one with a lot of electrons, and strips some of those electrons from the gas atoms creating positively-charged ions. Does anybody remember old tube televisions? You know, those big heavy boxes with low-resolution pictures? They were built around a cathode ray tube which used strong electric fields to accelerate negatively-charged electrons at a screen covered with phosphors. Well, you can think of an ion engine as an anode ray tube. The principle is the same, but the polarity of the charges and fields are reversed.

Unlike a rocket, which is propelled by expanding superheated gas ejected through a nozzle, ion drives are cold, easily controlled, and surprisingly efficient. Xenon gas that has had electrons removed by bombarding it with other electrons, then passes through two grids, one positively charged and the other negatively charged. The charge gradient between the grids kicks the atoms up to very high speeds. Thanks to Sir Isaac again, when something rushes out of the engine, its momentum is conserved by pushing the engine in the opposite direction. That’s the whole action/reaction thing.

The Deep Space 1 probe travelled to the outer solar system, flying by an asteroid and a comet. Deep Space 1 and its successor Dawn craft proved that, although the thrust generated is very small, very high speeds can be achieved because the engine works continuously for weeks or even months, consuming only about two hundred pounds of propellant gas. Ion engines are scalable by sticking them together like a honeycomb. So, with sufficient propellant and electrical power, a crewed spacecraft to Mars or beyond would certainly be feasible.

A crew of hearty explorer/colonists might be willing to spend decades getting to a nearby star, propelled by ion engines. I can think of many story possibilities using this scenario. In fact, I’ve started sketching one out about a pair of very special twins.

Rob Johnson

May 21, 2020

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