An ion engine is an engine proposed for the propulsion of spacecraft.
How Does an Ion Engine Work?
Ion engines, also known as ion thrusters, are radically different to the chemical rockets that have traditionally been used to provide thrust for space vehicles. While the latter provide large amounts of thrust for a short period (by the burning of bulky chemical propellants), ion engines use various fuels to provide small amounts of thrust over a much longer period. This means that although ion engines accelerate slowly, a long enough period of acceleration allows them to reach great speeds. In addition, ion engines are very fuel efficient: the key issue for them is not fuel supply, but having enough electricity to burn fuel for long periods. For spacecraft travelling within the inner solar system, this electricity could be provided by solar panels; craft travelling outside this zone would require another source of electricity.
There are two main types: electrostatic ion engines (also known as gridded ion thrusters) and electromagnetic ion engines. They differ in the method used to accelerate the ions to provide thrust. The former type uses the Coulomb force; the latter uses the Lorentz force.
Applications of Ion Engines
Due to their small amount of initial thrust and long acceleration period, ion thrusters are not effective in propelling craft to leave a planet's surface. They are, however, much more effective than conventional propulsion methods for long journeys: it is estimated that whereas a mission to Mars would take approximately two-hundred and fifty days (one way) with conventional propulsion, an ion engine could effect the journey in as little as forty days. This is because of the constant acceleration possible with an ion engine: though it would start much more slowly, a ship it powered would eventually reach much greater speeds. In addition, a spaceship powered by an ion engine would be lighter, as it would not require heavy fuel tanks.
Present State of Ion Engine Development
Presently, ion engines are mostly theoretical, although some have been used to provide maneuvering thrusters for orbital satellites. It is expected, however, that once mankind starts to colonise the solar system it is the ion engine that will be the main method of propulsion for the spacecraft of mankind. NASA and a number of other organisations are researching this method of propulsion, however there is still some way to go before we see a craft like the TIE Fighter - or Twin Ion Engine Fighter - of Star Wars fame!
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