The microwave-oven-sized Sojourner Rover rolls across Mars shortly after arriving at the planet on July 4, 1997. It was the first successful Mars landing in more than two decades, and it operated for almost three months. It measured the composition of the rocks and soil and found that liquid water had one flowed across the surface. A stationary lander, which snapped this image, measured Martian weather. [NASA/JPL]