NASA's Ceres-orbiting Dawn spacecraft has encountered a technical glitch while preparing to observe a mysterious crater on the dwarf planet.

However, the malfunctioning will not significantly impact the rest of the extended mission at Ceres, NASA said. The spacecraft was positioning itself directly between the dwarf planet's mysterious Occator Crater and the Sun, when one of its two remaining reaction wheels stopped functioning on April 23.

The observations from this position may yield new insights about the bright material in the centre of the crater. 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 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 reaction wheel's malfunctioning will not significantly impact the rest of the extended mission at Ceres, NASA said.

Dawn 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. Dawn was launched in the year 2007.

The Dawn operations team has been well prepared to deal with the loss of the reaction wheel, NASA said.
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.