The reaction wheel difficulty surfaced during a scheduled Aug. 9 communications check of the spacecraft by JPL controllers. The spacecraft’s ion propulsion system was turned off in response and is scheduled to be reactivated on Aug. 17.

The “hybrid control” system software was developed by JPL and Orbital Sciences Corp., which developed Dawn, and installed on the spacecraft in April 2011 in case it was needed at Vesta, Rayman says.

Even before the latest difficulty, the probe’s reaction wheels were to be deactivated in September for the 30-month journey to Ceres.

“We had intended to spend the interplanetary cruise to Ceres developing the detailed plans for operations there,” Rayman says. “We will include hybrid control in the mix. The work we have done so far shows that hybrid control is indeed effective. So if we have to use it, I expect we will still be able to accomplish an exciting and rewarding exploration.”