The most closely watched, and likely the slowest, road trip in history hit its halfway mark Tuesday as NASA’s Spirit rover continued its drive to a nearby Martian crater. Spirit added another 100 feet in its travels Tuesday; it has about 400 feet to go before it reaches the “Bonneville” crater. Scientists hope to drive the rover below the crater’s rim to examine deeper layers of rock. In recent days, the rover has investigated a 3-inch-deep trench dug with its own wheels and taken readings of its surroundings within Mars’ Connecticut-sized Gusev Crater region.