An artist's concept of DS1's ion propulsion engine firing in space. Photo: NASA/KSC

The innovative engine now propelling NASA's Deep Space 1 spacecraft toward its ambitious September encounter with Comet Borrelly just won't give up, having now run for more than 10,000 hours -- 50 times beyond its originally required lifetime.

A working replica of the Deep Space 1 ion engine has logged in even more hours at NASA's Jet Propulsion Laboratory, Pasadena, Calif., where the mission is managed.

The spacecraft's engine was only required to complete 200 hours of operation in flight to prove itself a success. On March 21, it passed the 10,000-hour mark. It's expected to pass 14,000 hours by the end of its extended mission to Comet Borrelly.

The ion engine works by first removing an electron from the gas xenon, then using a pair of electrically charged grids to shoot the ionized gas out at more than 35,000 meters per second (78,000 miles per hour). The engine is one of a dozen important new technologies that the successful Deep Space 1 mission officially finished testing in 1999. Now that Deep Space 1 has been approved for a risky extended mission to Comet Borrelly, the long-lived ion engine will take the spacecraft near the comet. Similar ion engines may be used on future space missions, particularly missions to comets and asteroids where the ion engine's high fuel economy is important for precise navigation to the small bodies.

"The ground-based xenon ion engine has run for about 15,500 hours of testing time since the test began in early October 1998," said Dr. John Anderson of JPL, the ion engine test lead engineer. "That's more than 150 percent of the time it was designed to last."

"The results from Deep Space 1 and testing on the ground show that ion engines can be terrifically effective," said JPL's Dr. Marc Rayman, the project manager of Deep Space 1. "Now I'm looking forward to future spacecraft that use ion engines surpassing Deep Space 1's record as they undertake still more exciting missions."

Engineers partly attribute the secrets to the ion engine's long life to a slight increase in the flow of xenon through the engine early in the testing phase. "This reduced the amount of wear on the engine, and yet didn't significantly affect the engine's efficiency," said Dr. John Brophy, manger of NASA's Solar Electric Propulsion Technology Applications Readiness project.

Anderson began testing the ground-based ion engine when it was shipped to JPL from Hughes, which is now part of Boeing, in 1998. "We'd like to test it until the end of its life. Then we'll see how to make these engines last even longer," he said. He had also tested an earlier version of the ion engine, beginning in 1996.

The ion engine is tested for about 75 percent of the time over the two and a half years of the test, Anderson said, with other time spent on running diagnostic tests, and defrosting the xenon propellant that had become frozen in the vacuum system. At first, the engine was run at just more than half of its capacity, about 1.5 kilowatts, and then upped to full capacity, 2.3 kilowatts. The next phase of the test will be to run the engine at its lowest thrust level to demonstrate the engine's ability to run at low power near the end of its life, Anderson said.

Deep Space 1 has operated its ion engine between 520 watts and 1.9 kilowatts, in part depending upon the spacecraft's distance from the Sun during its flight in space. Deep Space 1's ion engine now also helps the spacecraft maintain its orientation relative to the stars, so it remains on for 99 percent of the time.

Deep Space 1 was launched in October 1998 as part of NASA's New Millennium Program, which is managed by JPL for NASA's Office of Space Science, Washington, D.C. The California Institute of Technology in Pasadena manages JPL for NASA.