Many years ago, this magazine was owned by the Walt Disney Corporation, and I would sometimes get one of the company’s earworm songs caught in my head: “It’s a Small World,” the musical accompaniment to the ride of the same name at Disney World in Florida. That song has popped up in my brain again recently, but in a very different and more majestic context. We are entering a new stage in the exploration of the solar system, one that inverts the theme of much that came before. Big is out and