Pasadena, Calif. ? One of the six wheels on the Mars rover Spirit has stopped working and the solar-powered robot must propel itself up a slope to catch enough sunshine to keep operating, NASA said Friday.

The right front wheel previously had an episode of balkiness but last week the motor that turns the wheel stopped working, the space agency’s Jet Propulsion Laboratory in Pasadena said in a statement.

Spirit is trying to reach a position where it can get as much sunlight as possible during winter. But while the point of minimum sunshine is more than 100 days away, there already is only enough to power about one hour of driving on flat ground per day, JPL said.

The rover was 390 feet from a spot on the north-facing side of a feature called McCool Hill on Friday, where it could spend the southern-hemisphere winter with its solar panels angled toward the sun, JPL said.