If you've forgotten all about Spirit and Opportunity, here's a reminder: they landed on Mars in 2003, and were designed to be operational for just 3 months. Spirit lasted 7 years (it died last fall) and Opportunity is still going strong 8 years later. Much of the credit for this unexpected longevity is due to the design team, but some of it is just plain luck: it turns out that the dust devils that frequently whisk across the Martian surface keep the explorers' solar panels relatively free of dust. Prior to the actual mission, engineers were expecting the red Martian dust to slowly accumulate on the solar panels. This would slowly reduce their power output until finally the explorers would run out of juice. The entire team was surprised – very pleasantly! – that the unpredictable dust devils have been coming along frequently enough to blow the panels clean well before the explorers ran out of juice...
