Ion Drives

Previously, I talked about ions. Remember, they are atoms that have either more or less electrons than protons. For ion drives, positive ions, also called cations, are used to propel the craft ahead.

The ions are created in a chamber where the electrons are stripped from an atom or molecule (many times xenon or ammonia, depending on the thruster type), then are accelerated via an electrostatic or an electromagnetic field.

Ion drives have a very high specific impulse, which means they are very efficient engines. Specific impulse is the ratio between the spacecraft's thrust (the force at which the spacecraft is being pushed along) and the weight of the fuel used to create that thrust).

However, the thrust from an ion engine is small since ions themselves are rather tiny. Ion engines are not great from getting place to place quickly in the solar system. Typically, ion drives will be used for space stations for maneuvering or for deep space probes. Though the thrust is tiny, over time, that thrust can add up to a large velocity for the spacecraft.

Deep Space 1

Via NASA

Schematic of Deep Space 1

via NASA

Ions

An ion is a particle that is similar to an atom but has more or less electrons than protons in the nucleus. In most cases, ions are present in compounds that have a metal and a non-metal (like NaCl, common table salt), but can also be present in hot gases or plasmas, like the Sun.


If an ion has more electrons than protons, i.e. it is negatively charged, we call that an anion. In the above example (NaCl), the chlorine ion has one more electron than proton (18 electrons vs. 17 protons) and therefore, has a charge of -1 e (-1.602e-19 coulombs). If the ion has less electrons than protons, it is a cation. The sodium ion in NaCl has one less electron than proton, so has a charge of +1 e.


Why are ions are important? They are found in the spectra of stars, in nebulae, and as stated above many compounds that we use everyday. In the Sun or any star, the gas is so heated, that it can strip electrons from the atoms, ionizing the gas, converting it into a plasma.


There are some elements that do not ionize easily. We call these elements inert and they are found on the far right of a periodic table. These are the noble gases: Helium, Neon, Argon, Krypton, Xenon, and Radon. This does not mean that they cannot be ionized. Under normal temperatures and pressures, the outer electrons are not readily removed from their orbits. However, under extreme temperatures and pressures, electrons can be stripped from the outer shells. In fact, the alpha particle is the bare nucleus of a helium atom with a charge of +2 e. We find helium ions in the core of the Sun, as it is the final product of the proton-proton chain. Electrons do not easily combine with the bare nucleus to form a stable helium atom.


Next time, we will learn about ion engines and how they may be a future propulsion system for solar system travel.