Pamelia P. Caswell
NASA Lewis Research Center
ION PROPULSION SYSTEM ON DEEP SPACE 1 RUNNING SMOOTHLY
The ion propulsion system developed by NASA Lewis Research Center engineers for NASA's Deep Space 1 spacecraft has been thrusting continuously for over 300 hours since it came to life on November 24, 1998. This marks the longest, uninterrupted engine thrusting time of a deep space propulsion system.
"The system has far exceeded the mission goal for continuous thrusting. We are very pleased with this successful demonstration of ion propulsion technology for planetary missions," says Vince Rawlin, Deep Space 1's test coordinator at NASA Lewis.
The Deep Space 1 mission team first powered up the engine on November 10, but the system shut itself off after running for 4-1/2 minutes. NASA Lewis engineers believe that the engine shut itself off because of metallic grit or other contamination between the two grids at the rear of the engine.
"This kind of shut-off is a design feature of the propulsion system to protect it from serious damage," said Michael Patterson, thruster design engineer in the On-board Propulsion Branch at NASA Lewis. "There is a fix for getting rid of debris, but the mission team wanted more data before implementing it."
On November 24, when the mission team sent commands to the engine to turn itself on to collect more diagnostic data, NASA Lewis engineers were not surprised when the engine started without incident. It is likely that changes in grid temperatures as the spacecraft conducted other technology validation tasks loosened the debris, and powering-up the thruster may have vaporized the remains.
For the past week and a half, the propulsion system and its solar power supply have been put through a series of checkout tests to verify operation at various throttle levels. Now that the propulsion system has passed its checkout tests, the technology demonstration part of the mission is nearly complete, and Deep Space 1's science mission, an encounter with asteroid 1992KD next July, has begun.
Deep Space 1 is the first NASA spacecraft to use an ion engine as its primary propulsion source. Its use culminates forty years of ion propulsion technology development at NASA Lewis.
"Over those years, we've helped in the development of the smaller ion engines as well as other electric propulsion engines, such as arcjet thrusters, which are being used on commercial communications satellites for stationkeeping," said James Sovey, Lewis project manager for DS1's ion propulsion system. NASA's emphasis on shorter, less costly projects made mission planners look more seriously at this higher risk technology.
The fuel used in Deep Space 1's ion engine is xenon, a colorless, odorless, and tasteless gas more than 4-1/2 times heavier than air. When the ion engine is running, electrons are emitted from a cathode into a chamber ringed with magnets, much like the cathode in a TV picture tube or computer monitor. The electrons strike atoms of xenon, knocking away one of the electrons orbiting the atom's nucleus. This leaves each atom one electron short, giving it a net positive charge and making it into an ion.
At the rear of the chamber is a pair of metal grids that are charged with 1,280 volts of electric potential. The force of this electric field exerts a strong "electrostatic" pull on the xenon ions - much like the way that bits of lint are pulled to a pocket comb that has been given a static electricity charge by rubbing it on wool. The electrostatic force in the ion engine's chamber causes the xenon ions to shoot past at a speed of more than 60,000 miles per hour (100,000 kilometers per hour), continuing right on out the back of the engine and into space. A second electron-emitting cathode, downstream of the grids, neutralizes the positive charge of the ion beam to keep the spacecraft neutral with respect to its environment.
At full throttle, the ion engine could consume about 2,300 watts of electrical power and put out 1/50th of a pound (90 millinewtons) of thrust. This is comparable to the force exerted by a single sheet of paper resting on the palm of a hand.
In 1992, NASA Lewis and NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA, began developing flight hardware for Deep Space 1. NASA Lewis developed the prototype ion engine and the power-processing unit that converts energy captured by solar arrays into power for the engine. An 8,000 hour endurance test conducted inside a JPL vacuum chamber verified that the engine and its systems would meet the requirements of the Deep Space 1 mission.
Additional information about the Deep Space 1 mission and Lewis can be found at:
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