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

It has the little engine that could, and the pint- sized power plant on board NASA's Deep Space 1 probe has been doing it longer and more efficiently than anything ever launched. The spacecraft, designed to test new technologies, has run its unique propulsion system for more than 200 days (4800 hours).

"The ion propulsion engine on Deep Space 1 has now accumulated more operating time in space than any other propulsion system in the history of the space program," said John Brophy, manager of the NASA Solar Electric Propulsion Technology Applications Readiness project, at the agency's Jet Propulsion Laboratory (JPL) in Pasadena, CA.

Unlike the fireworks of most chemical rockets using solid or liquid fuels, the ion drive emits only an eerie blue glow as ionized (electrically charged) atoms of xenon are pushed out of the engine. Xenon is the same gas found in photo flash tubes and many lighthouse bulbs.

The almost imperceptible thrust from the system is equivalent to the pressure exerted by a sheet of paper held in the palm of your hand. The ion engine is very slow to pick up speed, but over the long haul it can deliver 10 times as much thrust per pound of fuel as more traditional rockets.

Previous ion propulsion systems, like those found on some communications satellites, were not used as the main engines, but only to keep the satellites on track. Deep Space 1 is the first spacecraft to use this important technology as its primary means of propulsion. The NASA Space Electric Rocket Test 2, launched into Earth orbit in 1970, had the previous record for ion propulsion, thrusting for about 161 days.

"The importance of ion propulsion is its great efficiency," says Dr. Marc Rayman, project manager for Deep Space 1. "It uses very little propellant, and that means it weighs less so it can use a less expensive launch vehicle and ultimately go much faster than other spacecraft."

The ion particles travel out at about 68,000 miles per hour. However, Deep Space 1 doesn't move that fast in the other direction, because it's much heavier than the ion particles. By the end of the mission, the ion engine will have changed the spacecraft's speed by about 6,800 mph (over 11,000 kph).

"This opens the solar system to many future exciting missions which otherwise would have been unaffordable or even impossible," added Dr. Rayman.

xenon ion engine, photographed through a port of the vacuum chamber where it was being tested at NASA's Jet Propulsion Laboratory, shows the faint blue glow of charged atoms being emitted from the engine. Photo: NASA/JPL

The technology is so efficient that it only consumes about 3.5 ounces (100 g) of xenon per day, taking about four days to expend just one pound (0.4 kg).

The only other system that has operated longer is a ground- based replica of the spacecraft's engine. The ongoing extended-life test, being done in a vacuum test chamber at JPL, has run its ion propulsion system for almost 500 days (12,000 hours) and is scheduled to complete nearly 625 days (15,000 hours) by the end of the year.

The Deep Space 1 ion engine could have a total operating time of more than 583 days (14,000 hours) by the end of its mission in the fall of 2001.

With its primary mission to serve as a technology demonstrator - testing ion propulsion and 11 other advanced technologies - successfully completed in September 1999, Deep Space 1 is now headed for a rendezvous with Comet Borrelly. NASA extended the mission, taking advantage of the ion propulsion and other systems to target a risky, but exciting, encounter with the comet in September 2001.

But early in this bonus mission Deep Space 1 suffered a serious setback with the loss of its star tracker navigation system. Rather than abandon the project, NASA engineers managed a deep-space rescue. They sent new software, on-the-fly, turning an onboard camera into a navigation instrument - all while Deep Space 1 was 200 million miles from Earth.

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, DC. The California Institute of Technology in Pasadena manages JPL for NASA.