It has gone into a "safe mode" with its solar panels facing back at the sun, giving controllers intermittent communication with the craft as it spins.

CNN: Planet-hunting Kepler spacecraft in trouble

In a statement, Nasa said the problem had been detected on Tuesday, when the telescope went into a pre-programmed "safe mode" which kicks in "if the observatory has trouble knowing where it should point", Mr Grunsfeld told AFP news agency.

BBC: Nasa's Kepler telescope hobbled by faulty wheel