It was with a heavy heart that NASA announced this week that the Mars rover named Opportunity was dead at 15. The size of a small go-kart, Opportunity touched down on the red soil of Mars in 2004, expected to serve NASA for a mere three months. Instead, the plucky robot geologist continued on its expedition mission across the surface of Mars for 15 years, best known for collecting hard evidence that Mars once had liquid surface water. Opportunity was exploring, apropos enough, Perseverance Valley when a dust storm blocked the sun, choking off the rover’s solar power. That was it. The longest-lived, widest-ranging Mars explorer was gone. “It has given us a larger world,” said John Callas of the Jet Propulsion Laboratory. “Mars is now part of our neighborhood.”