The Mars Pathfinder rover uses its alpha proton X-ray spectrometer to analyze the Yogi Rock on the surface of Mars
July 4, 1997 — There are good machines and bad machines — and then there are flat-out lovable machines. That's what the Mars Pathfinder rover was. The plan was to build a little wheeled robot about the size of a microwave oven, swaddle it in inflatable air bags, bounce it down on the surface of Mars, then deflate the bags and let it toddle away to explore. It was a punch line made real, and on the July 4, 1997, it happened. The rover not only worked as advertised; it opened the way for its bigger brothers Spirit and Opportunity, launched in 2003 and still chugging along on Mars long after Pathfinder winked out. Unlike lunar footprints, which last forever on the airless moon, the rovers' tire tracks are constantly blown away — one more reason to be glad they continue to make them.