So Gaius would be launched on a geyser-powered steam catapult, into regions of space where Triton’s gravitational pull would slow the ship’s outward flight from 2.3 to 0.8 kilometers per second. Under other conditions, without aid of chemical or nuclear propellants, a mere blast of nitrogen steam was just not powerful enough, and Gaius would continue to decelerate, until at last its rate of climb leveled off at 0.0 kilometers per second and began clicking backward to —0.1, —0.2, —0.3….

But according to plan, Neptune would be moving into the refugees’ path, yanking them away from Triton. Slowly, the ship would begin to pick up speed, to fall up, tail first, toward a planet with seventeen times the Earth’s mass. That was when all those tanks of chemical propellant would be needed. And if the braking maneuver over the north pole did not get them killed, and if they managed to target the pole without falling down instead upon the hypersonic winds of the equator, then the plan Lenny had chosen would allow them to draw all the energy they needed by drifting up and down through the Neptunian thermocline—half blimp, half submarine—till the end of time.

The plan Lenny had chosen, and to which they had all agreed, would force them, until they invented procedures for extracting carbon dioxide and silicon atoms from the atmosphere and weaving them into new building materials, to share living quarters barely as large as a one-bedroom apartment. They would be allowed one shower every three days, in water recycled through urine stills. Their food and air would be provided by a four-meter-square box of wet microtubes filled with genetically redesigned algae. In essence, they would be living on artificially flavored slime.

The ship, designed for diving into high-pressure environments, could not even provide the luxury of a window. But this did not matter. Soon there would be very little to see. Neptune was so far removed from the Sun that once Gaius penetrated twenty meters below the cloud tops, all natural light would be cut off.

So, as their pads told them that the last Bardo cone was firmly in place and that the steam catapult was charged and ready, they just stood on the nitrogen flood plain, all four of them, looking around for a very long time. There were worms in the sky. On the horizon, directly in front of the rising hemisphere of Neptune, seven geyser-spawned plumes twisted more than ten kilometers into the heavens, ranging from dusty yellow to peach, depending on the slant of the Sun’s rays. Here was unearthly beauty, made more unearthly and more beautiful by the fact that they would never see the like of it again.

At Vinny’s feet, water and nitrogen—substances he once drank and breathed on Earth—had become the substance of Triton’s geology. These were the youngest rocks on the moon’s surface, and yet they were astonishingly old. When the cryovolcanoes had spewed the last of them, the remotest reptilian ancestors whose DNA flowed in his veins had not yet appeared on Earthly shores. Vinny lifted the last rock he would ever see and tried to feel the heft of time in the palm of his hand, to understand it in terms comprehensible in the flash of a human lifetime. He tried a thought experiment, knowing that if he started counting forward from the second he was born, he would be thirty-three years old by the time one billion seconds had passed. And as many as a billion years had passed since the nitrogen rock he held in his glove had solidified. In all that time, he told himself, Triton had never known the stirrings of life. The sea upon which he stood had never lived … until now. Vinny was still running his thought experiment when the horizon, already dark by Earthly eyes, grew even dimmer, as if the distant Sun were passing behind a veil of cloud. But there were no clouds in Triton’s sky, and the whole of Neptune had gone into shadow at the same moment. Vinny felt Sharon tugging at his elbow. She was pointing excitedly behind him.