NASA's Kepler spacecraft hobbled by faulty wheel

NASA's prolific planet-hunting Kepler spacecraft that has already discovered some of the most intriguing exoplanets known has most likely lost the capacity to carry out its mission, the space agency said.
Kepler relies on an array of flywheels, or reaction-wheel assemblies, to stabilise the pointing of its telescope toward a field of stars in the Milky Way.
The spacecraft needs three of its four reaction wheels in working order to carry out its exoplanetary mission, and the spacecraft had already lost one wheel in July 2012.
Now, a second wheel appears to have failed, and unless it can be revived the spacecraft's search for extra-solar worlds may be over, 'Scientific American' reported.

"Basically we need three wheels in service to give us the pointing precision we need to find planets," Kepler principal investigator Bill Borucki of the NASA Ames Research Center, said.
"Without three wheels it's unclear whether we could do anything of that order," Borucki said.

"At our semi-weekly contact on Tuesday, May 14, 2013, we found the Kepler spacecraft once again in safe mode. As was the case earlier this month, this was a Thruster-Controlled Safe Mode. The root cause is not yet known, however the proximate cause appears to be an attitude error," NASA said in a statement.
"We attempted to return to reaction wheel control as the spacecraft rotated into communication. But reaction wheel 4 remained at full torque while the spin rate dropped to zero.

"This is a clear indication that there has been internal failure within the reaction wheel, likely a structural failure of the wheel bearing. The spacecraft was then transitioned back to Thruster-Controlled Safe Mode," said NASA.
The spacecraft is stable and safe, if still burning fuel. In its current mode, our fuel will last for several months. Point Rest State would extend that period to years, NASA said.
We will take the next several days and weeks to assess our options and develop new command products. These options are likely to include steps to attempt to recover wheel functionality and to investigate the utility of a hybrid mode, using both wheels and thrusters, it said.

With the failure of a second reaction wheel, it's unlikely that the spacecraft will be able to return to the high pointing accuracy that enables its high-precision photometry. However, no decision has been made to end data collection.
The space agency said Kepler had successfully completed its primary three-and-a-half year mission and entered an extended mission phase in November 2012.
Kepler, a USD 600 million mission, was launched in 2009 on a search for other planets. So far, it has found 2,700 candidates, including a handful that may be habitable worlds.