Just about every two years, the planet Mars makes its closest approach to Earth at a distance of around 36 million miles. Thats when we pack our robotic emissaries off to the Red Planet, timing their launches to spend the least effort to get there. These probes may pave the way for human explorers and, perhaps permanent settlers who will dig deeper still in search of answers to our most pressing question: Did Mars develop far enough, and stay that way long enough, for life to arise?