Montage of our solar system

A new NASA spacecraft engine that begins flight at less than a snail's pace but builds up enough speed to catch a comet will soon be used to push exploring spacecraft to the far reaches of the solar system.

A prototype of a xenon ion engine, which fires electrically charged atoms from its thruster, began a nearly year-long endurance test April 30 at NASA's Jet Propulsion Laboratory, Pasadena, CA.

Once validated by the test, a similar engine will power the first New Millennium mission, called Deep Space-1, to an asteroid and a comet in 1998. The comet will be West-Kohoutek-Ikemura and the asteroid will be McAuliffe, named after the school teacher Christa McAuliffe who died in the Challenger accident.

"NASA has been experimenting with ion drive engines for 30 years," said Jack Stocky, manager of the ion propulsion system project at JPL. "However, this test will be the most extensively instrumented endurance test of an ion engine ever performed."

In space, the 30-centimeter-diameter (11.8-inch) engine will use the heavy but inert xenon gas as fuel and be 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 the positively charged atoms, called ions. The thrusting action is similar to that of chemical propellant engines which expel burning gases, except that such engines can produce up to millions of pounds of thrust. The engines in rockets that lift the Space Shuttle, for instance, combine metal-warping heat with an earthshaking roar and quickly lift the shuttle to more than 7.6 kilometers per second (17,000 miles per hour).

An ion engine, however, starts with only about 20- thousandths of a pound of thrust. There's no roar, just an eerie blue glow. While the atoms, charged by an electric arc which removes one of the 54 electrons around its nucleus, are fired in great numbers out the thruster at more than 31 kilometers per second (70,000 miles an hour), their accumulative mass is so low, the spacecraft moves only millimeters per second in its early stages of flight.

Still, ion propulsion is more propellant efficient than chemical propulsion because it expels molecules from the engine at a much higher speed, Stocky said. A chemical propulsion engine has an exhaust velocity of 4.6 kilometers per second (10,400 miles per hour), while ion propulsion exhaust is 31.5 kilometers per second (70,200 miles per hour).

Built at NASA's Lewis Research Center, Cleveland, OH, the engine will be tested for 8,000 hours (330 days) in the space- like environment of JPL's vacuum chamber. "Ion engines have such low thrust they cannot operate in an atmosphere and have to be tested in a vacuum," said Dr. John Brophy, user validation assessment manager for the project. "JPL has the technical expertise and the cost-effective facility for the test." The test is designed to run full power for two days and then shut off for one hour and restart. This stressing process will be repeated until 8,000 hours of operation have been accumulated.

After Deep Space-1 is launched by an expendable rocket with sufficient power to escape Earth's gravity, it will be in orbit around the Sun moving at the same speed the Earth moves in its orbit. That means that relative to Earth, the spacecraft will not be moving at all. But, slowly, the low-thrust ion engine will increase and the spacecraft's velocity over time to greet its celestial target at more than 22,000 miles per hour, fast enough to rendezvous with a comet or asteroid. The prototype ion engine carries 80 kilograms (176 pounds) of xenon in a tank, which in flight would last from one to two years, depending on its destination and the amount of total thrusting required, Brophy said. Deep Space-1 will consume only 45 kilograms (99 pounds) of xenon during its mission.

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