NASA's Deep Space 1 probe will receive its final radio command from Earth today, ending a three-year mission that tested new technologies and completed an extraordinary fly-by of comet Borrelly in September.

An artist's concept of DS1. Photo: NASA/JPL

The final transmission will be sent from Earth at around 3 p.m. EST, commanding the spacecraft to turn off its ion propulsion system, which has accumulated over 670 days of operating time. The engine has used around 90 percent of its xenon fuel supplies.

After Earth-bound controllers officially end the mission of Deep Space 1, the probe will continue to operate on its own in orbit around the Sun. The spacecraft's transmitter will also be left on, leaving open the option for officials in the future to contact the craft.

"American taxpayers can truly be proud of Deep Space 1," said Dr. Colleen Hartman, Director of NASA's Solar System Exploration Division, Washington, D.C. "It was originally designed to be an 11-month mission, but things were going so well that we kept it going for a few more years to continue testing its remarkable ion engine and, as a bonus, to get close-up images of a comet. By the time we turn its engines off tomorrow, Deep Space 1 will have earned an honored place in space exploration history."

After its launch on October 24, 1998, Deep Space 1 tested twelve new technologies that could be used on future spacecraft. Included in these twelve tests was the ion engine, a much more efficient alternative to traditional chemical propulsion systems. Possible uses for this newly proven technology include Mars sample return missions.

Another tested device on Deep Space 1 was an autonomous navigation system that utilized images of stars and other celestial objects to plot its course around the solar system. That took controllers on Earth out of the loop in the craft's navigation, marking another step in testing artificial intelligence aboard space probes. A similar system will be used on NASA's Deep Impact mission that will probe secrets of the nucleus of comet Tempel 1.

Other technologies flown on Deep Space 1 were a beacon monitor to summarize health status for ground controllers, lighter, cheaper, and more efficient solar arrays, new telecommunications systems, miniaturized electronics and spacecraft components, a system that can detect and fix spacecraft problems on its own, and a more efficient and smaller ion and electron spectrometer. "I'm not sad it's ending, I'm happy it accomplished so much," said Dr. Marc Rayman, Deep Space 1's project manager at the Jet Propulsion Laboratory. "I think it inspired many people who saw the mission as NASA and JPL at our best -- bold, exciting, resourceful and productive."

After nine months of putting the twelve technologies through their paces, Deep Space 1's primary mission was completed. In the remaining two months in the allotted eleven months for the primary mission, the probe flew just 26 kilometers above the surface of the asteroid Braille on June 28, 1999. In the high speed fly-by, the craft snapped pictures and measured the basic properties of the space rock.

In November 1999, the spacecraft's star tracker failed, leaving the probe with no way to accurately measure its orientation in space. Engineers were then forced to develop plans to continue the mission without a star tracker. That feat was accomplished in early 2000, making the probe available for an extended mission to comet Borrelly.

On September 22, 2001, after many hours of ion engine operating time, Deep Space 1 flew to within 1,349 miles from nucleus of comet Borrelly, gathering the best close-up images and data from a comet to date. Scientists believe that data yielded from the fly-by could revolutionize the study of comets.

In this highest resolution view of the icy, rocky nucleus of comet Borrelly, (about 45 meters or 150 feet per pixel) a variety of terrains and surface textures, mountains and fault structures, and darkened material are visible over the nucleus's surface. This was the final image of the nucleus of comet Borrelly, taken just 160 seconds before Deep Space 1's closest approach to it. Photo: NASA/JPL

"Deep Space 1 is a true success story," said Dr. Charles Elachi, the director the Jet Propulsion Laboratory in Pasadena, California. "We are proud that future generations of spacecraft will benefit from its accomplishments."

Deep Space 1 was launched on October 24, 1998, aboard a Delta 2 rocket from Cape Canaveral, Florida. The spacecraft weighed around 1,069 pounds at launch, and is now estimated to have a mass of approximately 900 pounds, subtracting the propellant that has been used over the past three years. The entire mission cost of Deep Space 1 comes to a total of $149.7 million.