NASA engineers are working to identify a problem with the asteroid-bound Dawn spacecraft after part of its attitude control system shut down.
The malfunction occurred June 17 and knocked out one of Dawn's four attitude control reaction wheels used to orient the spacecraft as it flies toward the two biggest asteroids in the solar system.
Dawn's mission is aimed at exploring the asteroid Vesta in 2011 and 2012 and dwarf planet Ceres which is also the solar system's largest asteroid in 2015.
Currently, the problem is not expected to throw Dawn off its asteroid rendezvous schedule, NASA mission managers said.
"We're looking at the data carefully in order to understand what the long-term prospects are for this reaction wheel," said Marc Rayman, Dawn's chief engineer at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "In the meantime, we're continuing our planned activities with the spacecraft."
Typically three of the four wheels spin at any given time. Different spinning speeds allow the spacecraft to hold steady or rotate as needed.
The problem arose when "all members of the team but the insomniacs and the spacecraft itself were slumbering," Rayman wrote in a Sunday mission blog entry. Automatic sensors detected an unusual excess of friction building up on reaction wheel No. 4, and shut it down accordingly.
Engineers have mostly ruled out the friction buildup being the result of a new software update, reactivation of the spacecraft or a recent velocity change.
Such a problem would normally have prompted Dawn to activate its unused reaction wheel as a replacement. But all four reaction wheels had already been operating because the spacecraft had entered safe mode.
NASA engineers had also periodically given each reaction wheel a rest during part of Dawn's mission, but will now simply keep wheels 1, 2 and 3 going, mission managers said.
For now, Dawn continues toward its target asteroid at more than 9,800 mph (15,850 kph).
The Dawn mission launched in September 2007 and has already traveled 213 million miles (342 million km) from Earth as of June 27, and has 30 million miles (48 million km) more to go before reaching the asteroid Vesta in August 2011.
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