Sparkling with pure colors were the eight much longer arcs between them. Seen from a distance, each arc had a predominant color. Encoded in those colors was the history of their building, and in turn that of the human races during the last thousand years—the Fifth Millennium, the Millennium of the Ring. Prior to that—during the first four thousand years of the Hard Rain—space had been so dirty that the human races had been obliged to hunker down in the shelter of massive nickel-iron bodies such as Cleft, whose orbits were, of course, similar to that of the moon whose core had once comprised them—nine times farther away from Earth than the habitat ring was now. As Dubois Harris had foreseen, the orbit of the former moon had been a fine place—the only place, really—to restart a civilization, as long as hellfire was raining down on Earth. But to the extent that the human race, as a whole, was capable of having a plan, it was to return to Earth eventually. The Hard Rain diminished, gradually at first, and then, during the Fourth Millennium, more steeply as fleets of robots, issuing from their nickel-iron fastnesses like bats from caverns, began to sweep the skies clean, policing the rubble cloud, herding specks and pebbles together, and spiraling them down into disciplined orbits at geosynchronous altitude. Most of the work was accomplished using the pressure of sunlight, a weak form of propulsion that took hundreds of years to have its effect.