In 1978 Donald Kessler and Burton Cour-Palais published a paper arguing that satellites would eventually collide often enough to form a belt of debris, and that the debris would go on to hit more satellites. A colleague, John Gabbard, attached Kessler’s name to the idea. The first accidental collision between two intact spacecraft came on 10 February 2009, about 790 km over Siberia: a working Iridium and a dead Cosmos, meeting at more than ten kilometres a second. What I keep turning over is the speed of the rest of it. Films show the cascade as one terrible afternoon, but the people who study it describe something slow, governed by how crowded each altitude is. A chain reaction that slow has no day on which it starts. It would look like a slightly worse year, and then another, and the argument about whether it had begun could last as long as the reaction.