An artist's concept of the Stardust spacecraft during interstellar dust collection. Photo: NASA

A thruster problem has been encountered by the Stardust comet sample return mission as the craft speeds along a perfect course after getting a boost from Earth's gravity last month.

Ground engineers are looking at an issue with Stardust's attitude control thrusters. Telemetry data received at Earth ground stations indicate that software aboard the craft are commanding unnecessary thruster jet firings.

The probe's orientation, or attitude, is tightly controlled to within four degrees on all three axes to allow communications with Earth. When the attitude drifts near the edge of this communications zone, on-board computers issue orders to fire maneuvering jets to re-start the motion in the opposite direction. However, systems are now instructing the thrusters to fire once again, apparently failing to understand that the spacecraft is now moving in the appropriate direction.

This is not an immediate problem, but in the long run, the issue could cause propellant shortages. That is why engineers are conducting increased analysis on returned data to attempt to weed out one of these multiple firings. As of Friday, engineers were still evaluating the telemetry.

Meanwhile, an engine firing to tweak Stardust's course after the successful Earth gravity-assist in January was recently cancelled after controllers calculated that the maneuver was not needed. The trajectory correction firing was originally conceived to fine-tune any small variations from the planned flight route that could be incurred by the highly dynamic Earth fly-by.

An artist's concept of Stardust during's flyby of Earth. Photo: NASA/JPL

The probe also switched communications duties from the low-gain antenna to the more powerful medium-gain antenna, officially marking the end of the Earth fly-by phase.

Also, the Cometary and Interstellar Dust Analyzer (CIDA) is operating once again after being shut down in October to keep Stardust inside thermal constraints through the Earth fly-by phase of the mission. The CIDA is standing by for interplanetary dust particles to impact the collection device. Once a grain is captured, the instrument studies the particle and transmits the results back to Earth.

Stardust recently celebrated its second year in space after launch atop a Delta 2 rocket from Cape Canaveral on February 7, 1999. The probe is due for a close encounter with comet Wild 2 in 2004, where it will collect cometary dust particles for return to Earth.