Unlike conventional chemical rockets that burn at a fast rate over a short period of time to generate huge thrust, ion drives run at a steady rate over a long period to generate a high velocity.
The ions travel at 31.5 kilometres per second (19.6 miles per second) and produce 0.5 Newtons (0.1 pounds) of thrust. This force is equivalent to the weight of a few coins dropped into your hand, whereas chemical rockets can produce hundreds of thousands of Newtons of thrust. Despite the low amount of thrust, ions can accelerate a spacecraft to 90,000 metres per second (over 320,000 kilometres per hour or 200,000 miles per hour).
Currently, NASA favours xenon as a propellant gas for ion propulsion systems. One type of xenon-fuelled ion engine works by heating a hollow discharge cathode. This pushes a stream of electrons out of the cathode and towards the discharge chamber walls.
The xenon propulsion gas is magnetized and forced into the chamber, and the flow of electrons that are stripped off the xenon atoms creates highly excited positive ions. An electric charge is then passed through a metal grid at the back of the chamber, which makes the positive xenon ions rush through the grid to accelerate the spacecraft.
The main drawback with the type of gridded electrostatic ion thruster described above is that the grid will eventually be degraded and destroyed. This isn’t a major problem as NASA has successfully run its NASA Evolutionary Xenon Thruster (NEXT) for over 43,000 hours to simulate five years of continual operation. This is four times more than the time needed to accelerate a spacecraft to the asteroid belt and beyond, and shows that it could power spacecraft to explore comets, asteroids, the outer planets and their moons.
Besides the NEXT engine, NASA’s Glenn Research Center, Cleveland, Ohio, has also produced the High Power Electric Propulsion (HiPEP) ion engine. This uses magnetic fields and microwaves to heat the atoms in the propellant, to create a plasma. The ions are then taken from the plasma to power the ‘craft. Another revolutionary engine is the NASA-457M Hall thruster engine, which is ten times more powerful than any other ion thruster ever built.
Since ion drives reduce the need to carry large fuel loads into space and are durable, they are ideal thruster systems for keeping large geostationary communication satellites in position as well as for sending a new generation of unmanned spacecraft to the Solar System’s outer planets.