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Ion engine propulsion, a futuristic form of spacecraft propulsion referred to in science fiction novels and films for decades, is one step closer to becoming a reality. On September 25, JPL completed an 8,000-hour endurance test of a prototype xenon ion engine, providing a green light for the engine's first- ever application to a deep space mission next summer.

Ion propulsion, also known as solar electric propulsion, is set to be used on Deep Space 1 (DS1), the first launch of the New Millennium program, a series of missions designed to test new technologies so that they can be confidently used on science missions of the 21st century. DS1, which will fly by Mars, an asteroid and a comet while validating a dozen technologies, is scheduled to launch on July 1.

"This marks an exciting step in deep space exploration," explains Jack Stocky, manager of the NASA Solar Electric Propulsion Technology Application Readiness (NSTAR) program, which is developing ion propulsion for use on a variety of missions. "After years of speculation about the potential of this form of propulsion, we are finally nearing the day when we can validate solar electric propulsion as the propulsion system of choice for tomorrow's most distant missions."

The most extensively instrumented endurance test of an ion engine ever performed, the test, which began on June 17, 1996, verified the engine's life expectancy, which has proven to exceed the needs of the DS1 mission, while demonstrating performance levels that exceeded all expectations. Conducted in the space- like environment of JPL's vacuum chamber, the test was designed to run full power for several days, then shut off and restarted, a stressing process repeated until 8,000 hours of operation were accumulated.

Ion propulsion provides only the tiniest amount of thrust, roughly equivalent to the pressure of a single sheet of paper held in the palm of the hand - approximately 10,000 times smaller than the thrust of the main engines on typical planetary spacecraft. Its magic lies in its staying power, for this low thrust slowly changes the craft's velocity from low to high speed, making it ideal for long missions. Compared to traditional chemical propellants, solar electric propulsion provides tremendous savings for future deep space and Earth-orbiting missions with great velocity-change (delta v) requirements.

Xenon, a heavy, inert gas used as fuel for the DS1 experiment, is converted into an eerie, faint, blue haze visible from the back of the spacecraft as it catapults through space.

DS1's ion engine, which fires electrically charged xenon atoms from its thrusters, is just 29.9 centimeters (11.8 inches) in diameter. It is powered by another of the mission's dozen technologies: large solar arrays generating more than 2,000 watts, provided by the Ballistic Missile Defense Organization.

The actual thrust comes from electrically accelerating and expelling the positively charged atoms, called ions. While the ions are fired in great numbers out the thruster at more than 100,000 kilometers (68,000 miles) per hour, their mass is so low that the engine produces an exquisitely gentle thrust of only 90 millinewtons (20-thousandsths of a pound). Thus, it requires patience to accumulate the great speeds which ion propulsion is capable of achieving.

The total propellant used may be about 10 times less than would be required for a conventional chemical propulsion system, roughly equivalent to having one's car get 300 miles per gallon. This efficiency in turn allows NASA to explore the solar system with much smaller spaceraft and less expensive rockets. On DS1, the xenon propellant will increase the spacecraft speed by about 3.6 kilometers (2.2 miles) per second, fast enough to reach a variety of fascinating destinations in the solar system.

A few weeks after DS1 is launched by an expendable rocket with sufficient power to escape Earth's gravity, the ion propulsion system will be turned on to begin slowly but surely increasing the spacecraft's speed.

In addition to the engine itself, being assembled by the Hughes Electron Dynamics Division, Torrance, CA, NSTAR is also delivering a power processing unit, digital control interface unit, propellant storage and control system, and a diagnostics system. The latter will aid in the understanding of how this novel system performs in space.

For further details about the DS1 mission, visit

Development of the xenon ion engine is supported by NASA's Offices of Space Science and Aeronautics, Washington, D.C. NASA's Jet Propulsion Laboratory is a division of the California Institute of Technology, Pasadena, CA.

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