Rover engineers are growing increasingly concerned about the temperature of vital electronics on NASA's Mars Exploration Rover Opportunity while the rover stays nearly inactive due to a series of dust storms that has lasted for more than a month.

Dust in the atmosphere and dust settling onto Opportunity's solar panels challenges the ability of the solar panels to convert sunlight into enough electricity to supply the rover's needs. The most recent communication from Opportunity, received Monday, July 30, indicates that sunlight over the rover's Meridiani Planum location remains only slightly less obscured than during the dustiest days Opportunity survived in mid-July. With dust now accumulating on the solar panels, 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.

Keeping Opportunity's activity to a minimum has reduced the amount of energy going into the rover's electronics core, reducing the amount of heat that comes from the electronics components themselves during normal operation.

"The overnight low temperature of Opportunity's electronics module has been dropping since we implemented the very-low-power operation, even though the outside environment is actually warmer during this dust storm," said John Callas, rover project manager at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

That temperature has dropped to minus 35 degrees Fahrenheit (minus 37 Celsius), within about 3 Fahrenheit degrees (about 2 Celsius degrees) of triggering survival heaters to turn on. Those heaters could push the rover's total use of electricity higher than what the solar panels produce, soon depleting the batteries.

"This is energy Opportunity does not have to spare," he said.

To forestall the survival heaters from turning on, the rover team has altered Opportunity's daily regimen to keep the electronics active for a longer period each day. This, too, could put the rover through some negative-net-energy days if the sky does not begin to clear.

Callas said, "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."

Spirit, meanwhile, is also accumulating some dust on solar panels under a sky at Gusev Crater that remains nearly as dusty as the worst Spirit has recorded.

"We will continue to watch the situation on Mars and do all we can to assist our rovers in this ongoing battle against the environmental elements on the Red Planet," Callas said.