Despite my asking the HiRISE team to find and kill an apparent background process called HiOPACITY (heh, geek humor), the dust storms continue to give the rovers a tought time. Spirit experienced a record high tau of 4.738 on Sol 1265 with a corresponding solar array energy of 261 watt-hours. The most recent tau for Spirit on Sol 1269 was 4.322 with a solar array energy of 274 watt-hours.
Spirit still remains power positive, but things are tougher for Opportunity on the other side of the planet. Because we have been saving energy on Opportunity by not making science observations, the rover isn't generating waste heat. The waste heat normally goes into heating up the electronics box to keep the electronics warm. No activities means no waste heat means the temperature is dropping in the electronics box, about 1 degree C per sol. At a certain point, the rover will automatically turn on emergency heaters for the electronics, but they suck up a lot of power - power that Opportunity doesn't have right now. So we're now trying to strike a balance by having the rover stay on longer each sol to expend more heat, raising the electronics temperatures and avoiding the survival heater turn on. But the price is that the rover will now start draining power and risks tripping 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 communications. If there is not sufficient power, 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. At this point the ground team will be unable to help and Opportunity would be completely on her own. When skies clear Opportunity would wake up and service her normal communication passes (if within about 5 weeks) or listen each day to see if we are there trying to talk to her (if beyond about 5 weeks).
Oooooh, hang on little rover!!!!!!