The six-wheeled rover inches over the gravel of a wind-swept hillside, carefully avoiding boulders and precipices in a slow-motion hunt for a rock that struck the fancy of distant planetary scientists. But there’s a problem. One of the steel-cleated wheels is dragging like a broken shopping cart, and the engineers in Pasadena can’t figure out what’s wrong. They beam commands to budge the faraway robot back and forth, left and right. No luck.