NASA used a more complete overnight shutdown than normal to turn off a heater that runs even when it's not required because of a faulty switch. The energy drain has been limiting Opportunity to one or two hours of daily activity, but going to ``deep sleep'' mode ran the risk of ruining a cold-sensitive science instrument.

"Opportunity successfully executed our second deep sleep of the mission," Chris Lewicki said in the recorded flight director's daily update from NASA's Jet Propulsion Laboratory.

The "deep sleep" was tested once before being put into regular use Thursday. It could have wrecked an instrument called the miniature thermal emission spectrometer -- Mini-TES for short -- if temperatures had fallen low enough.

"We survived the night just fine with that and it saved the energy that we expected to," Lewicki said. "More importantly our Mini-TES instrument, which we put in danger by the deep sleep operation, did survive the night and we hope it will survive more of these. But we're getting dangerously close to the cold temperatures that it isn't able to survive through."

Opportunity and its twin, Spirit, recharge their batteries with sunlight, but that has been decreasing as winter deepens in Mars' southern hemisphere. Dust accumulation on the solar panels is also decreasing solar generation of electricity. Spirit, however, doesn't need ``deep sleep'' because it doesn't have a runaway heater.

Both rovers have completed their primary missions since landing in January but are continuing to explore and may last for several more months.