Click here to see a time lapse video (4.4 MB Quicktime movie) of the DS1 Ion Thruster Compatibility Test taped on February 15, 1998. This testing was carried out on the DS1 spacecraft in the Solar Thermal Vacuum Chamber at JPL. Increasing the engine throttle level without increasing the available power caused the engine recycling in the video. The engine provides about 10 times the specific impulse (ratio of thrust to propellant used) of chemical propulsion. DS1 is the first spacecraft to use ion propulsion as the primary propulsion system. It is one of the 12 advanced technologies that was validated by DS1 during flight.
Rocket engines work by pushing propellant away from the spacecraft. The action of the propellant leaving the engine causes a reaction that pushes the spacecraft in the opposite direction. This is what causes a balloon to rush around as the air is allowed to escape; the air pushes on the balloon as it leaves.
An ion engine uses this same principle, but the great innovation is in how efficiently this happens. The gas xenon (which is like helium or neon, but heavier) flows into the ion engine, where it is given an electrical charge. Charged atoms are called ions. As soon as the xenon atoms become xenon ions, they can be pushed around by an electrical voltage. A pair of grids in the ion engine, electrified to almost 1300 volts, accelerates the ions to very high speed and shoots them out of the engine. As the ions race away from the engine, they push back on the spacecraft, propelling it in the opposite direction. [The electricity for this remarkable system can be provided by solar arrays, as on Deep Space 1 and Dawn, or by a nuclear power system, as on Project Prometheus. The principle of operation of the ion engine is the same.] The xenon ions travel at about 35 kilometers/second (77,000 miles/hour). This is about 10 times faster than the exhaust from conventional rocket engines, so the xenon gives about 10 times as much of a push to the spacecraft as chemical propellants do. That means that it takes only one tenth as much propellant for an ion engine to work as it does for a chemical propulsion system. To accomplish some of the more ambitious and exciting missions in the solar system, we simply cannot build and launch spacecraft large enough to carry the chemical propellants that would be needed for the mission. Ion propulsion is one of the ways to get around this problem.
Now the ion engines use only a very small amount of xenon at a time. That means that the thrust is very very low. If you rest a piece of paper on your hand, the paper pushes on your hand about as hard as the ion engine pushes on the spacecraft! It may take 4 days or more just to use up 1 kilogram (about 2 pounds) of xenon. Unlike chemical engines, which can be operated for minutes, or in extreme cases, for an hour or so, ion engines can be operated for years. The effect of the gentle thrust slowly builds up, eventually attaining speeds far beyond the reach of conventional propellants.
Deep Space 1, using less than 74 kg (163 pounds) of xenon, accelerated by about 4.3 kilometers/second (9600 miles/hour). This is greater than any spacecraft has ever been able to change its speed. (DS1 could have achieved still higher velocity, but mission controllers had objectives other than just going faster and faster, so they did not operate it to attain the maximum speed possible.) It thrusted for 678 days, far far longer than any propulsion system had ever been operated. [Dawn will surpass both of these records, and later missions using ion propulsion will do even more.]