Yesterday evening the Mars Exploration Rover mission posted an update that states "increasing concern" for the continued health of Opportunity. Although the skies are not quite as dark as they have been, dust is now beginning to settle on the solar panels, so "the rover is producing barely as much energy as it is using in a very-low-power regimen it has been following since July 18." Its low-power regimen has allowed it to remain power-positive until now, but because the rover has been inactive, its core temperature has been dropping by one degree Celsius per day. Now the temperature has dropped to -37°C, within 2° of triggering the power-on of survival heaters. Those heaters, if turned on, will quickly deplete Opportunity's remaining power. The only thing that the mission can do is to command more activities, a plan that will make Opportunity "power-negative," but which will warm up the rover enough to avoid the power-on of those heaters.
According to project manager John Callas, "This means there is a real risk that Opportunity will trip a low-power fault sometime during this plan. When a low-power fault is tripped, the rover's systems take the batteries off-line, putting the rover to sleep and then checking each sol to see if there is sufficient available energy to wake up and perform daily fault communications. If there is not sufficient energy, Opportunity will stay asleep. Depending on the weather conditions, Opportunity could stay asleep for days, weeks or even months, all the while trying to charge her batteries with whatever available sunlight there might be.