NASA asteroid mission has guidance problem

A NASA spacecraft on a mission to study the giant asteroid Vesta and dwarf planet Ceres has powered down part of a guidance system, the space agency reported. A reaction wheel, part of a system that helps the spacecraft point precisely, was shut down automatically by the spacecraft with data beamed back to Earth suggesting the wheel had developed excessive friction, a release from NASA Jet Propulsion Laboratory in Pasadena, Calif., reported Monday. Another Dawn reaction wheel had been shut down in similar circumstances in June 2010, the release said. During its flight to Vesta, the JPL team demonstrated they could, if necessary, complete the second part of the mission, the cruise to Ceres, without the use of reaction wheels. Vesta, one of the largest asteroids in the solar system, and Ceres, the only dwarf planet in the inner solar system, are both located in the so-called asteroid belt, roughly between the orbits of the planets Mars and Jupiter. Dawn has finished it science mission at Vesta, which it has been orbiting since July 15, 2011, and is slowly moving away for the journey to Ceres, using its thrusters to both move and to orient itself toward Earth for communications, JPL said. Aside from the reaction wheel issue, the rest of the spacecraft is otherwise healthy, the Dawn team said. "The Vesta mission has been spectacularly successful, and we are looking forward to the exciting Ceres mission ahead of us," Robert Mase, Dawn project manager, said.