NASA's Dawn spacecraft encountered a glitch while preparing for its scheduled observation of Ceres on April 29 from an "opposition" position, directly between the dwarf planet's mysterious Occator Crater and the sun.
The malfunction will not significantly impact Dawn's rest of the extended mission, NASA said in a statement on Wednesday.
One of Dawn's two remaining reaction wheels stopped functioning on April 23. By electrically changing the speed at which these gyroscope-like devices spin, Dawn controls its orientation in the zero-gravity, frictionless conditions of space.
The team discovered the situation during a scheduled communications session on April 24, diagnosed the problem, and returned the spacecraft to its standard flight configuration, still with hydrazine control, on April 25.
The failure occurred after Dawn completed its five-hour segment of ion thrusting on April 22 to adjust its orbit, but before the shorter manoeuvre scheduled for April 23-24. The orbit will still allow Dawn to perform its opposition measurements.
The spacecraft is outfitted with four reaction wheels. It experienced failures of one of the wheels in 2010, a year before it entered orbit around Vesta, and another in 2012, as it was completing its exploration of that fascinating world.
When a third reaction wheel stopped working this week, the spacecraft correctly responded by entering one of its safe modes and assigning control of its orientation to its hydrazine thrusters.
The reaction wheel's malfunctioning will not significantly impact the rest of the extended mission at Ceres, according to NASA.
Dawn, launched in 2007, completed its prime mission in June 2016, and is now in an extended mission. It has been studying Ceres for more than two years, and before that, the spacecraft orbited giant asteroid Vesta, sending back valuable data and images.