They had known, from very early in the planning of the Cloud Ark, that the Hard Rain would heat up the air— all of the air, all over the world. When air got hotter, it expanded. The atmosphere had only one direction in which it could expand: out into space. So, whatever drag Izzy felt from the traces of air at its accustomed altitude of some four hundred kilometers was bound to get worse as the atmosphere reached upward. How hot the air would get, how much it would expand, and how heavy the drag would get were questions of colossal import that, however, simply could not be answered until the Hard Rain actually started. As Doob always put it, the experiment of blowing up the moon had never been attempted before. The most they could do was wait and perform observations. Which was exactly what they had been doing ever since the Hard Rain had begun. But Markus had been distracted for most of that time, and was only now absorbing the latest results.