Oct. 11, 1997 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 be well beyond 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. 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, blue haze visible from the back of the spacecraft as it catapults through space.
DS1's xenon ion engine, which fires electrically charged atoms from its thrusters, is just 29.9 centimeters (11.8 inches) in diameter. It is powered by more than 2,000 watts from large solar arrays provided by the Ballistic Missile Defense Organization.
The actual thrust comes from accelerating and expelling positively charged atoms, called ions, starting with only about 22.7 milligrams (20-thousandths of a pound) of thrust. While the charged atoms are fired in great numbers out the thruster at more than 112,654 kilometers (70,000 miles) per hour, their cumulative mass is so low that the spacecraft moves only millimeters per second in its early stages of flight. However, it can eventually build up to 112,976 kilometers (70,200 miles) per hour, compared to just 16,737 kilometers (10,400 miles) per hour for the fastest chemical propulsion engines.
After DS1 is launched by an expendable rocket with sufficient power to escape Earth's gravity, it will orbit the Sun at the same speed as Earth. With the ion engine's power, the spacecraft's velocity will increase over time to more than 35,405 kilometers (22,000 miles) per hour, fast enough to rendezvous with a comet or asteroid.
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.
For further details about the DS1 mission, visit http://nmp.jpl.nasa.gov/ds1/.
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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