NASA has recovered the Kepler Space Telescope from 'emergency mode'

>Normally Kepler (in K2 mode) uses the pressure from sunlight combined with the two remaining reaction wheels to maintain orientation. It still does need periodic thruster usage but the heavy lifting is done by solar pressure and reaction wheels which makes the propellent usage very efficient. However it is a complicated and precarious balancing act. It needs full instrumentation, computer operation, and periodic updates from Earth to work.

>When it goes into emergency mode it falls back on 100% station keeping thrusters because that is simpler although far more expensive in terms of fuel. They don't know exactly why it went into emergency mode but for whatever reason Kepler believed it could not maintain orientation without it.

>In emergency mode it has to expand propellent because without some station keeping it would begin to tumble uncontrollably. If you have a spacecraft millions of kms away from Earth, tumbling out of control with its communication array no longer pointed at Earth you will probably never regain control. So it is a last ditch effort to maintain proper orientation on the hope that command & control update can fix the problem. It begins "looking" for an command & control signal from Earth (using propellent to orient the spacecraft). If/when it finds it, it then tries to keep that orientation using 100% station keeping thrusters regardless of fuel consumption. It will continue to do so until standard operation is restored or it runs out of fuel.

http://www.nasa.gov/feature/mission-manager-update-kepler-re...