So Kepler's reaction wheels are essential for pointing the spacecraft accurately and steadily.

CNN: Kepler or not, we'll find life in space

Controllers have been forced to use thrusters in addition to the last remaining reaction wheels in order to maintain a stable attitude.

BBC: NEWS | Science/Nature | Asteroid probe 'did touch down'

But last July one of the spacecraft's four reaction wheels broke down, leaving scientists aware that a further failure was likely and would prevent the telescope operating as it should.

BBC: Nasa's Kepler telescope hobbled by faulty wheel

The probe was built with four reaction wheels and needs three of them -- one for each axis -- to aim its telescope precisely at a distant star, deputy project manager Charlie Sobeck said.

CNN: Planet-hunting Kepler spacecraft in trouble