Ion Propulsion

Ion propulsion is a method of propulsion that uses electrical rather than chemical forces to generate thrust for a spacecraft. Although less powerful than chemical engines, ion propulsion engines are more efficient and can be used continuously for long periods, making them ideal for deep space missions. The concept of ion propulsion has existed for many years, but only recently have ion engine-driven spacecraft been flown.

Ion propulsion works by taking advantage of the very strong repulsive force between two objects with the same electric charge. A cathode emits a stream of electrons that collides with neutral atoms of xenon, a gaseous element, in a chamber. The collisions strip the xenon atoms of one or more electrons, converting these atoms into positively charged ions. The xenon ions drift toward a pair of grids, one positively charged and one negatively charged, in back of the chamber. Once the ions are between the grids, the repulsive force from the positively charged grid accelerates them out the back of the chamber at speeds of up to 30 kilometers (18.6 miles) per second. Once the xenon ions are free of the engine, another cathode fires electrons at them to neutralize them and prevent them from being attracted back to the engine. A variation of this design referred to as the "Hall effect thruster," uses a combination of electric and magnetic fields to accelerate ions.

A key advantage of ion propulsion is efficiency. The exhaust from an ion engine travels up to 10 times faster than does the exhaust from a chemical engine, generating far more thrust per pound of propellant. However, the thrust from an ion engine is very weak and cannot support the weight of the engine, let alone the rest of the spacecraft. This makes ion propulsion unsuitable for lifting spacecraft off the surface of Earth. In space, however, ion engines can run continuously for weeks, compared to minutes for chemical engines. These engines can build up significant thrust over time.

The American rocket pioneer Robert H. Goddard first proposed ion propulsion in 1906. Research started in earnest in the 1950s, and the first suborbital flight tests of ion engines took place in 1964. Although American interest in ion propulsion waned in the late 1960s, the Soviet Union continued to work in this area, flying Hall effect thrusters on a number of spacecraft in Earth orbit. These thrusters allowed the spacecraft to modify their orbits with less propellant than is the case with chemical engines. In the 1990s the American satellite manufacturer Hughes began to include ion thrusters on communications satellites, allowing them to stay in the proper orbit.

The most important test of ion propulsion in space has been the National Aeronautics and Space Administration's (NASA) Deep Space One (DS1) spacecraft. DS1 was launched in October 1998 to test a number of advanced technologies, including ion propulsion. A month after launch, and after some initial problems had been overcome, DS1 fired up its ion engine. Working for months at a time, the engine propelled DS1 past the asteroid Braille in July 1999 and the comet Borrelly in September 2001. The engine operated for over 15,000 hours, well over a year, during the mission.

see also Accessing Space (volume 1); Mars Missions (volume 4); Power, Methods of Generating (volume 4); Rocket Engines (volume 1).

Jeff Foust

Internet Resources

"Frequently Asked Questions about Ion Propulsion." NASA Jet Propulsion Laboratory. <http://www.nmp.jpl.nasa.gov/ds1/tech/ionpropfaq.html>.

"Ion Propulsion: Over 50 Years in the Making." NASA Marshall Space Flight Center. <http://www.spacescience.com/newhome/headlines/prop06apr99_2.htm>.

"How Does Solar Electric Propulsion (Ion Propulsion) Work?" Northwestern University. <http://www.qrg.ils.nwu.edu/projects/vss/docs/Propulsion/zoom-solar-ion.html>.