Anatomy of an ion engine

Source: NASA

Most conventional systems use high pressure or temperature to push a gas through a rocket nozzle. The action of the gas leaving the nozzle causes a reaction that pushes the craft in the opposite direction. This is what causes a balloon to fly around when the end is opened and the stretched rubber squeezes the air out. Ion propulsion works on the same principle, but the method of pushing the gas out is unique.

The inert gas xenon, which is similar to helium and neon but heavier, is used as propellant. The composition of xenon is simple: each atom consists of a tiny and dense nucleus surrounded by a cloud of electrons. The nucleus is 54 positively charged protons plus about 76 neutral neutrons. (Xenon gas is a mixture of 9 isotopes, meaning there are 9 different values for the number of neutrons. From a low of 70 to a high of 82, the number of neutrons makes only very modest differences in the behaviors of the atoms.) The 54 positive charges in the nucleus are precisely balanced by 54 negatively charged electrons, rendering the atom electrically neutral – until the ion propulsion system gets in the act.

Inside the ion thruster, an electron beam, somewhat like the beam that illuminates the screen in a television, bombards the xenon atoms. When this beam knocks an electron out of an atom, the result is an electrically unbalanced atom: 54 positive charges and 53 negative charges. Now with a net electrical charge of 1 unit, such an atom is known as an “ion.” Because it is electrically charged, the xenon ion can feel the effect of an electrical field, which is simply a voltage. So the thruster applies more than 1000 volts to accelerate the xenon ions, expelling them at speeds as high as 40 kilometers/second (144,000 kilometers/hour). Each ion, tiny though it is, pushes back on the thruster as it leaves, and this reaction force is what propels the spacecraft. The ions are shot from the thruster at roughly 10 times the speed of the propellants expelled by rockets on typical spacecraft, and this is the source of ion propulsion’s extraordinary efficacy.

All else being equal, for the same amount of propellant, a spacecraft equipped with ion propulsion can achieve 10 times the speed of a craft outfitted with normal propulsion, or a spacecraft with ion propulsion can carry far less propellant to accomplish the same job as a spacecraft using more standard propulsion. This translates into a capability for NASA to undertake extremely ambitious missions such as Dawn.

The rate at which xenon is flowed through the thruster is very low. At the highest throttle level, the system uses only about 3.25 milligrams/second, so 24 hours of continuous thrusting would expend only 10 ounces of xenon. Because the xenon is used so frugally, the corresponding thrust is very gentle.

The force of the ion thruster on the spacecraft is comparable to the weight of a single sheet of paper. So here is an ion propulsion experiment you may conduct safely at home: hold a piece of paper in your hand, and you will feel the same force that the ion thruster exerts. Because the fuel efficiency is so great, the thruster can provide its push not for a few minutes, like most engines, but rather for months or even years. In the weightless and frictionless conditions of spaceflight, the effect of this thrust can gradually build up to allow the spacecraft to achieve very very high speed. Ion propulsion delivers acceleration with patience.