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

After a potentially mission-ending failure last fall, NASA's Deep Space 1 has been revived.

The mission to test a dozen futuristic technologies, including an ion propulsion engine, was completed last fall. But now the space agency has brought the craft back to life, sending the intrepid space probe toward a close encounter with Comet Borrelly in September 2001.

"Following a rescue mission of cosmic proportions, Deep Space 1 is in powered flight again," said Marc Rayman, the DS1 chief engineer. "Its advanced ion propulsion system is gently but persistently pushing the craft along as DS1 orbits the Sun."

DS1 nearly faded away into space history last November when a problem was found in the craft's star-tracker. The device determines the orientation of the spacecraft so its attitude control system can help control it.

After the problem was found, controllers at NASA's Jet Propulsion Laboratory in Pasadena, California, put DS1 into a safe mode to allow more time to examine the problem. Shortly thereafter, the star-tracker completely shut-down, leaving the spacecraft unable to control itself.

Since the failure occurred two months after the craft's primary mission, the testing of technologies through flying by asteroids and comets, many managers felt that the probe should be retired and completely shut down. However, engineers devised a way to resume operations by using the probe's camera instead of its star-tracker.

Engineers and scientists raced against the clock to try to get the craft back up and running by some time in July so it could begin maneuvers to fly by a comet next September.

The new system allows the craft's most sensitive camera to take pictures of stars to aid controllers on the ground and the probe's computers to regain control of its attitude control and propulsion systems. It works by using the camera to image a reference star.

The goal is to fire thrusters to try and keep the star at the intended place in the view of the camera.

After months of effort, DS1's ion propulsion and attitude control systems came to life on June 21. Data received later that day indicated all was going as planned. On June 23, the craft made a large maneuver to demonstrate the ability to orient itself toward deep-space, gather data, then turn toward Earth to transmit the new data back.

The spacecraft then turned back toward deep space to begin a regular week of thrusting toward the comet fly-by.

Deep Space 1 is currently 2.1 times as far from Earth as the Sun is and more than 820 times as far as the moon. At this distance of over 195 million miles, radio signals take more than 35 minutes to make the round trip.