You'd have no reason at all to want to visit Mercury. It's a bleak, dry, blowtorched world, the roasted hazel nut of the solar system. It's a place that is simultaneously hot enough to melt lead and cold enough to liquefy helium. It has no air, no water and the only weather it experiences are ferocious magnetic tornadoes. But Mercury is also one of the niftiest, toughest and—oddly—loveliest planets we've ever known.