HOUSTON — The Dawn mission team at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., have slowed the probe’s scheduled Aug. 25 gravitational escape from Vesta until early September while controllers checked out high friction readings from a second spacecraft reaction wheel.

The condition is not expected to affect the second leg of its eight-year main belt asteroid mission to Ceres, or data collection at the dwarf planet.

The difficulty, which triggered a software shutdown of the spacecraft pointing device on Aug. 8, should not interfere with Dawn’s scheduled arrival at Ceres in February 2015, according to Marc Rayman, NASA’s Dawn chief engineer and mission director.

Dawn is equipped with four reaction wheels, though orbital data-gathering operations normally require three. The first Dawn reaction wheel loss, in June 2010, prompted the development of a “hybrid control” strategy that relies on two reaction wheels and hydrazine thrusters on the probe for data-gathering by the spacecraft’s collection of four U.S., German and Italian cameras and spectrometers.

The spacecraft will slip from Dawn’s gravitational clutch on Sept. 5, 11 days later than originally scheduled, Rayman told Aviation Week Aug. 15 via e-mail.

The ambitious $466 million, eight-year Dawn mission was launched on Sept. 27, 2007, marking the first attempt to send one spacecraft to multiple planetary system bodies for orbital studies.

The ion-propelled spacecraft reached Vesta on July 16, 2011, to compile topographical as well as elemental and mineral composition maps, chart the magnetic field and search for moons.

Dawn’s Vesta mission phase drew to a close on July 25, when the xenon-fueled ion propulsion was activated to gradually raise the spacecraft out of the asteroid’s gravity well.

“By that time, we had exceeded all of our requirements and achieved a significant amount of bonus science, including mapping Vesta at five times better resolution than planned,” Rayman says.

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.”