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The ion propulsion system on NASA's Deep Space 1 spacecraft came to life Tuesday, November 24, and has continued running smoothly since.

The engine started up at 2:53 p.m. Pacific Standard Time in response to commands sent to the spacecraft. After running overnight in low-thrust mode, engineers commanded the engine to switch to higher-thrust modes today. The mission team plans to leave the engine running over the four-day Thanksgiving weekend.

The team originally powered up the engine November 10, but the system shut itself off after running for 4-1/2 minutes. When controllers sent commands to the engine to turn itself on Tuesday, they planned to collect more data on the status of the system but believed it was unlikely the engine would keep running.

"We are very pleased that the engine started and continued to thrust," said Dr. Marc Rayman, Deep Space 1's chief mission engineer and deputy mission manager at NASA's Jet Propulsion Laboratory, Pasadena, CA. "In fact, it has been running very smoothly over the first day of its operation."

Engineers believe that the engine probably shut itself off when it was started two weeks ago because of metallic grit or other contamination between the two high-voltage grids at the rear of the advanced engine. It is likely that changes in temperature as the spacecraft conducted other technology validation activities affected the flakes, and powering-up the thruster may have vaporized the remains.

"It's common for new ion engines on the ground or on Earth- orbiting spacecraft to shut themselves off a few times when they are first exercised," said Rayman. "We would not be surprised if the engine shut itself off again over the first few days or weeks that it runs.

"Deep Space 1's charter is to test new, advanced technologies," Rayman added. "If everything worked perfectly on the first try, it would be an indication we had not been sufficiently aggressive in selecting the technologies. Diagnosing the behavior we have seen is a very valuable part of Deep Space 1's objective of enabling future space science missions."

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 hollow bar called a cathode into a chamber ringed by magnets, much like the cathode in a TV picture tube or computer monitor. The electrons strike atoms of xenon, knocking away one of the 54 electrons orbiting each atom's nucleus. This leaves each atom one electron short, giving it a net positive charge -- making the atom what is known as an ion.

At the rear of the chamber is a pair of metal grids which are charged positive and negative, respectively, with up to 1,280 volts of electric potential. The force of this electric charge 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 on a dry day. The electrostatic force in the ion engine's chamber, however, is much more powerful, causing the xenon ions to shoot past at a speed of more than 100,000 kilometers per hour (60,000 miles per hour), continuing right on out the back of the engine and into space.

At full throttle, the ion engine would consume about 2,500 watts of electrical power and put out 90 millinewtons (1/50th of a pound) of thrust. This is comparable to the force exerted by a single sheet of paper resting on the palm of a hand.

When the engine was started Tuesday, it ran overnight thrusting at a power level of 500 watts. This morning engineers commanded it to thrust at a level of 885 watts, then at 1,300 watts. Engineers may decide to have the engine thrust at a lower level while it runs over the next few days.

The ion propulsion system flight hardware was built for Deep Space 1 by Hughes Electron Dynamics Division, Torrance, CA; Spectrum Astro Inc., Gilbert, AZ; Moog Inc., East Aurora, NY; and Physical Science Inc., Andover, MA. Development of the ion propulsion system was supported by NASA's Office of Space Science and Office of Aeronautics and Space Transportation Technology, Washington, DC. JPL is managed for NASA by the California Institute of Technology.

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