Mars Exploration Rovers: Spirit and Opportunity (NASA, 2004)

The twin Mars Exploration Rovers, Spirit and Opportunity, rewrote our understanding of the Red Planet by proving it once had liquid water. Opportunity’s 14-year, 45-km traverse remains the longest surface mission in planetary exploration history. The rovers discovered hematite 'blueberries,' sulfate minerals, and layered rock formations—all definitive evidence of past water activity. This data transformed the search for life from speculation into a scientific priority. Spirit and Opportunity’s findings directly influenced the landing site for the Perseverance rover, making them the most consequential robotic missions since Viking.