Like a couple of camera-packing tourists, two robotic rovers are schlepping enough photographic gear to Mars to keep the folks back home in pictures for years. NASA hopes the first of its twin rovers, Spirit, will begin taking pictures within hours of its landing Saturday. Its sibling, Opportunity, should do the same after its arrival at the Red Planet three weeks later. During each of the 90-day missions, the unmanned shutterbugs could snap tens of thousands of photographs. The images, from the microscopic to the panoramic, should reveal the planet with unprecedented clarity.