DOOB COULDN’T GUESS HOW MANY TIMES DURING HIS LIFE HE HAD noted a cottony tuft of cloud in a blue sky, then looked up hours later to discover that it had developed into a bank of clouds that covered the sun and told of a change in the weather. Such phenomena happened too slowly for the mind to discern them as happening at all. During the last hours of A+1.335, something like that occurred in the cloud of lunar debris that had been hanging in the sky for the last seven hundred days. Later they would watch the movies of it in time-lapse, compressing a day’s changes into a minute of video, and it would look like an explosion. Or an epidemic of explosions. If you watched the video carefully enough, frame by frame, you could see it progress from one part of the cloud to the next as the Eight Ball shot through. Like a particle lancing through a cloud chamber, it was invisible save for the trail of consequences it left in its wake. A few months earlier it might have passed through without touching anything, but today the density of rocks in the cloud was such that it could not avoid smashing into some of them on its way through. Doob, making a crude statistical calculation, put the likely number of collisions at ten, plus or minus five. Not a large number in a cloud that now contained millions of rocks, but enough to push the system, trembling on the precipice of an exponential explosion, over the edge. Around its unseen track the White Sky took form and fury. The cloud bloomed and evoluted like cream in coffee, spreading and paling, though from place to place one could see fresh bursts as rocks hurled out in earlier collisions found distant targets and touched off smaller chain reactions of their own. In places it took on a cellular structure as curved detonation fronts spread, contacted others, and merged into lacy foams of white arcs. It had an austere, monochromatic beauty about it. There was no fire and no light other than what cold sunlight the rocks bounced back to the eye. Later, when they began to enter the atmosphere, there would be fire and plenty of it. But for now the world was ending in a fractal blooming of dust and gravel, an apocalypse in a gravel quarry.