On its 221st birthday, the nation that revolutionized life on Earth with the automobile will put a motor vehicle on Mars. At 1:07 p.m. EDT on Friday, Independence Day, America’s Mars Pathfinder spacecraft will slam into the Red Planet, then bounce and roll to a halt in a cocoon of airbags. Then, over the weekend, a rover the size of a microwave oven will roll out onto the surface of Mars.