Winter is bearing down on the southern hemisphere of Mars, where the days are growing shorter and colder, and the dimming Sun seems to be running out of energy. That means it’s time for the two robotic rovers there, Spirit and Opportunity, to be going into a kind of hibernation. Not that any season anywhere on Mars is ever balmy. By the start of winter in the middle of next month, temperatures are forecast to fall as low as minus 150 degrees Fahrenheit and never rise above zero, a range only a few degrees lower than current autumnal levels.