Scientists at the NASA Jet Propulsion Laboratory (JPL), in Pasadena, California, are currently investigating an error that occurred aboard the Dawn spacecraft. The orbiter is currently flying around Vesta, the largest asteroid in the solar system.
On August 8, computers aboard the satellite turned off one of Dawn's reaction wheels, an instrument essential for establishing the vehicle's position and orientation in space. Mission controllers are now investigating the source of the error and the status of the device itself.
This particular reaction wheel is not the only pointing device on Dawn, but rather a part of a larger system. However, its contribution to orienting the satellite correctly is significant. At this point, the spacecraft is using its thrusters to maintain a steady orientation towards Earth.
This position facilitates communications between Dawn and the NASA Deep Space Network, and makes it easier for JPL engineers to conduct diagnostics and figure out what went wrong. According to early results, the rest of the satellite appears to be in good working order.
The spacecraft arrived in orbit around the massive asteroid on July 15, 2011, and has been studying its geology and geochemistry ever since. Its primary science mission concluded on July 25, 2012. Over the course of its studies, Dawn changed four separate orbits around the space rock.
Currently, it is slowly spiraling away from Vesta, preparing for departure. “The Vesta mission has been spectacularly successful, and we are looking forward to the exciting Ceres mission ahead of us,” JPL Dawn project manager, Robert Mase, explains.
Ceres is a protoplanet located in the Inner Asteroid Belt (IAB), like Vesta. It represents Dawn's second target, and the final destination of the mission. The spacecraft is scheduled to enter orbit around the small world in early 2015, after navigating through a thick maze of asteroids.
At this time, the ion propulsion system powering Dawn is shut down, so that engineers can address the reaction wheel problem. As soon as that is taken care of, the system will be reengaged, and the spacecraft will resume its slow escape from Vesta's gravitational pull.
Unlike conventional thrusters, the ion engines fire continuously, producing only limited amounts of thrust. It takes roughly 4 days to accelerate the spacecraft from 0 to 60 miles (97 kilometers) per hour, but the performances are extremely reliable.