If just getting to Mars is difficult, and most would agree it is, try finding a good parking spot once you arrive. More than 100 NASA engineers and scientists from around the country spent three years searching before settling on landing sites for Martian rovers Spirit and Opportunity. Some sites were too rocky; some too dusty. Others were too steep, too windy or too cold. Engineers considered several sites too risky for landing, with lots of “bad” rocks that could damage the rovers on impact.