Ion Propulsion: the Future of Space Travel
Throughout history, humans have had a continuous fascination with stars and the universe. From the Greek myth of Icarus to the modern sagas of Star Wars and Interstellar, almost every single civilization is ripe with tales of space adventures. As of the moment, one invention, first conceived in 1906 and built at NASA in 1959, could realize this collective dream: ion propulsion might be the technology that will bring us out of the solar system and to the stars.
What is Ion Propulsion?
Ion propulsion is a form of electric propulsion used for spacecrafts. The key difference between ion propulsion and chemical propulsion (think fuel burning rockets) is that ion thrusters eject ions instead of combustion gases to create thrust. An ion is an atom or molecule that is electrically charged as a result of gaining (anion) or losing (cation) electrons. Most ion thrusters ionize propellant by electron bombardment, where a high-energy electron hits a neutral propellant atom, releasing electrons from the propellant atom and thus creating a positively charged ion. A gas is hence produced that consists of equal numbers of positive and negative ions, which results in the gas having no overall charge. This is called a plasma: a substance that has some of the properties of a gas, but is affected by electric and magnetic fields, for instance lightning.
The most common propellant used is the noble gas xenon. It is easily ionized and has a high atomic mass, thus generating a high level of thrust when ions are accelerated. It also is inert and has a high storage density, making it suited for storing on spacecraft.
Ion thrusters are classified as either electrostatic or electromagnetic. The main difference is the method used to accelerate ions: electrostatic ion thrusters use Coulomb force (electrostatic force) and accelerate ions in the direction of the electric field, whereas electromagnetic ion thrusters use Lorentz force (electromagnetic force) to move ions. Most ion engines are electrostatic.
How does an Electrostatic Ion Thruster Work?
The primary parts of an ion propulsion system are the ion thruster, power processing unit (PPU), propellant management system (PMS) and digital control and interface unit (DCIU). The PPU converts electrical power — usually obtained through solar cells or a nuclear battery — into voltages needed for the cathodes to operate and currents needed to produce the ion beam. The PMS reduces the xenon pressure from the higher storage pressures in the tank to a level that can be used by the ion thruster components. Last but not least, the DCIU controls and monitors system performance and performs communications functions.
Operation is fairly straightforward. Neutral propellant (xenon) is injected into the discharge chamber. An electron gun, which is composed of cathode ray tubes, fires electrons at the xenon atoms, creating a plasma of positively and negatively charged ions. High-strength magnets prevent electrons from freely reaching the walls; this lengthens the time that electrons stay in the discharge chamber and thus increases the probability of an ionizing event.
The positively charged ions migrate towards the back of the discharge chamber, where high voltage grids grab the ions and accelerate them to speeds up to approximately 145, 000 km/h. The positively charged ions are expelled from the thruster as an ion beam, which produces thrust. The neutralizer — another hollow cathode — discharges an equal amount of electrons to make the total charge of the exhaust beam neutral.
Ion Propulsion in Use
Ion propulsion was used on the spacecraft Deep Space 1, which was the first spacecraft to flyby an asteroid and a comet. Ion thrusters are currently being used to keep over 100 geosynchronous Earth orbit communication satellites at their desired location. Three are also enabling the Dawn spacecraft to orbit the protoplanets Ceres and Vesta, between Mars and Jupiter.
The Future of Ion Propulsion
NASA is currently working on the NASA Evolutionary Xenon Thruster (NEXT). NEXT, a high-power ion propulsion system designed to reduce mission cost and trip time, operates at three times the power level of the ion thrusters of the Dawn spacecraft. It has operated continuously for 51, 000 hours (almost 6 years), which demonstrates the reliability of ion thrusters and proves that they are one of the best ways to power long-term deep-space missions. NASA is also currently developing the Annular Engine, which is potentially more powerful than NEXT and could achieve high thrust levels, allowing ion thrusters to be used in new ways and environments.
The ion propulsion system’s efficient use of fuel and electrical power enable spacecrafts to travel faster, cheaper, and farther than any other current propulsion technology. These thrusters have high specific impulses — ratio of thrust to the rate of propellant consumption — meaning that they require significantly less propellant for a given mission compared to chemical propulsion.They have demonstrated fuel efficiencies of up to 90%, compared to 35% for chemically-propelled rockets. Spacecrafts powered by these engines could reach a maximum speed of approximately 90, 000 meters per second (324,000km/h); in comparison, Space Shuttles have a top speed of about 8, 000 meters per second (30,000km/h).
However, there are disadvantages to ion thrusters. The main one is that although they are able to achieve high speeds, they have very low thrust (acceleration). Current ion thrusters can provide only 0.5 newtons of thrust, which is equivalent to the force you would feel by holding 10 quarters in your hand. In comparison, the thrust of a single Boeing 747–400 (the most common passenger plane model in service) engine is 242 kilonewtons. The small amounts of thrust generated by ion thrusters mean that a spacecraft powered by ion propulsion cannot lift off from Earth, as the engines do not generate enough force to overcome drag and gravity. In fact, any kind of air friction and gravitational force can slow or stop an ion propelled ship, and ion thrusters only work in space where there is no air and being far from planetary bodies limit the influence of gravity. (If you’re the captain of an ion propelled spaceship, plot yourself a route that passes by exactly zero planets and black holes.) While current ion engine technology is inadequate for interstellar travel, its fuel efficiency and reliability are promising. NASA’s continuing focus and research on this technology could produce spectacular and unforeseen developments.
