Ion propulsion—the key to deep space exploration

Ion propulsion—the key to deep space exploration
The comforting blue glow of an ion drive. Credit: NASA

When we think of space travel, we tend to picture a massive rocket blasting off from Earth, with huge blast streams of fire and smoke coming out the bottom, as the enormous machine struggles to escape Earth's gravity. Rockets are our only option for escaping Earth's gravity well—for now. But once a spacecraft has broken its gravitational bond with Earth, we have other options for powering them. Ion propulsion, long dreamed of in science fiction, is now used to send probes and spacecraft on long journeys through space.

NASA first began researching ion propulsion in the 1950's. In 1998, ion propulsion was successfully used as the main propulsion system on a spacecraft, powering the Deep Space 1 (DS1) on its mission to the asteroid 9969 Braille and Comet Borrelly. DS1 was designed not only to visit an asteroid and a comet, but to test twelve advanced, high-risk technologies, chief among them the ion propulsion system itself.

Ion propulsion systems generate a tiny amount of thrust. Hold nine quarters in your hand, feel Earth's gravity pull on them, and you have an idea how little thrust they generate. They can't be used for launching spacecraft from bodies with strong gravity. Their strength lies in continuing to generate thrust over time. This means that they can achieve very high top speeds. Ion thrusters can propel spacecraft to speeds over 320,000 kp/h (200,000 mph), but they must be in operation for a long time to achieve that speed.

An ion is an atom or a molecule that has either lost or gained an electron, and therefore has an electrical charge. So ionization is the process of giving a charge to an atom or a molecule, by adding or removing electrons. Once charged, an ion will want to move in relation to a magnetic field. That's at the heart of ion drives. But certain atoms are better suited for this. NASA's ion drives typically use xenon, an inert gas, because there's no risk of explosion.

Ion propulsion—the key to deep space exploration
Detail of an ion drive. Image: NASA Glenn Research Center. Credit: Vectorization by Chabacano

In an ion drive, the xenon isn't a fuel. It isn't combusted, and it has no inherent properties that make it useful as a fuel. The energy source for an ion drive has to come from somewhere else. This source can be electricity from solar cells, or electricity generated from decay heat from a nuclear material.

Ions are created by bombarding the xenon gas with high energy electrons. Once charged, these ions are drawn through a pair of electrostatic grids—called lenses—by their charges, and are expelled out of the chamber, producing thrust. This discharge is called the , and it is again injected with electrons, to neutralize its charge.

Unlike a traditional chemical rocket, where its thrust is limited by how much fuel it can carry and burn, the thrust generated by an ion drive is only limited by the strength of its electrical source. The amount of propellant a craft can carry, in this case xenon, is a secondary concern. NASA's Dawn spacecraft used only 10 ounces of xenon propellant—that's less than a soda can—for 27 hours of operation.

In theory, there is no limit to the strength of the electrical source powering the drive, and work is being done to develop even more powerful ion thrusters than we currently have. In 2012, NASA's Evolutionary Xenon Thruster (NEXT) operated at 7000w for over 43,000 hours, in comparison to the ion drive on DS1 that used only 2100w. NEXT, and designs that will surpass it in the future, will allow to go on extended missions to multiple asteroids, comets, the outer planets, and their moons.

Missions using ion propulsion include NASA's Dawn mission, the Japanese Hayabusa mission to asteroid 25143 Itokawa, and the upcoming ESA missions Bepicolombo, which will head to Mercury in 2017, and LISA Pathfinder, which will study low frequency gravitational waves.

With the constant improvement in systems, this list will only grow.

Ion propulsion—the key to deep space exploration
NASA Evolutionary Xenon Thruster. Credit: NASA

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Nov 04, 2015
NASA's ion drives typically use xenon, an inert gas, because there's no risk of explosion
But that's where the thrust is. I mean, you're looking for lots of thrust with low propellant consumption. The next logical step is to create some kind of hybrid ion thruster that vents controlled explosions for constant acceleration over long durations. If 1G of constant acceleration is achieved, then a Mars journey, including deceleration and orbital injection, should not take longer than a week.

Nov 04, 2015
Perhaps they could hoover up the Xenon once it has left the engine. That way they could go forever.

Nov 04, 2015
"Perhaps they could hoover up the Xenon..."

In a hard vacuum ? Sorry, not enough coffee...

Nov 05, 2015
"Perhaps they could hoover up the Xenon once it has left the engine."

That would be similar to putting a sail on your boat and blowing into it expecting the boat to move.

Nov 05, 2015
The next logical step is to create some kind of hybrid ion thruster that vents controlled explosions for constant acceleration over long durations.

You really don't want explosions in this type of drive (the negative grids are rather fine and already a point of some wear). Anyhow, the speed you get the fuel to using ion drives is a LOT higher than you get from an explosion exhaust. So any explosion contribution would be negligible at a significant wear-and-tear addition.

JRi
Nov 07, 2015
"Perhaps they could hoover up the Xenon once it has left the engine."

That would be similar to putting a sail on your boat and blowing into it expecting the boat to move.


This has been proved in Mythbusters to work, check https://www.youtu...TzMQWjo.

Not so with the attempt to collect your Xenon back in space.

Nov 07, 2015
This has been proved in Mythbusters to work, check https://www.youtu...MTzMQWjo


Newton says No! Mythbusters say Yes, Yes, Yes. Now it's time to prove Einstein wrong guys.

Nov 08, 2015
You really don't want explosions in this type of drive (the negative grids are rather fine and already a point of some wear).
I certainly wasn't suggesting using existing ion propulsion systems. I would use a redesigned anode system to deliver positive ions into an ion chamber, and subject those ions to a reactant. It's a whole new ball game.

Nov 12, 2015
Further on that subject, by way of example, the simplest way to increase thrust is to inject electrons directly into the ion chamber. Most of the electrons will complete the ion's electron shells, increasing their overall volume substantially, thereby providing increased thrust.

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