But now the dust has become so thick that the Mars Exploration Rover team is reporting that this could mean the end for the Martian Methuselahs. The nearly opaque dust haze has dropped solar energy levels to record lows. Opportunity has it worse than Spirit 99% of the direct sunlight to Opportunity's solar panels is blocked. The rovers always require a little power, either from the solar panels or from the rovers' rechargable batteries, for internal heaters. Without the heaters, some parts would get too cold and would permanently break.
Right now the plan is to hunker down, save energy wherever possible, and try to ride out the storm. The engineers are being cautious with Spirit as well as Opportunity because dust storms tend to migrate and can become global. As of today Spirit and Opportunity have lived 1,170 and 1,150 Martian days beyond their planned 90-day lifetimes.
We'll keep you posted on how things are going.