They had gotten here by means of two large burns. The first, and smaller, had placed them into an ellipse that had shot beyond the orbit of the former moon. After several days of weightless coasting away from the Earth, they had succumbed to the force of gravity, looped lazily around, and begun to fall back again toward the burning planet. This had been timed in such a way that, about a day later, they would be overtaken by Ymir, coming in on a roughly parallel track. But Ymir was traveling much faster—coming in hot, as Sean had told them—basically because it had been falling in toward the Earth from an extremely high starting place, gathering speed relentlessly for weeks. Left unmolested, Ymir would come screaming in toward Earth with a relative velocity of some twelve thousand meters per second, make a hairpin turn just a few kilometers shy of a catastrophic encounter with the glowing atmosphere, then go hurtling back outward again, not to return for a couple of months. Eventually her orbit would decay to the point where she would get dragged down by the atmosphere and destroyed.