The cloud, in other words, became not just a physical cloud of flying objects in space but a computational cloud as well, a free-floating, self-regulating Internet. The function of Parambulator was to give its users an Olympian perspective on all that was happening in that network, and at some level all you really needed to know about it was that scary things were shown in red. Ivy looked at it now, more in curiosity than in alarm, since they had been practicing maneuvers for weeks and she thought she knew what to expect. Whenever Izzy fired her thrusters and changed her params, red propagated through the scatter plots like a drop of blood in a glass of water. All the free arklets, and all the ones connected to bolos or to heptads or triads, now needed to evaluate their params and see whether they were in danger of colliding with Izzy. Or—almost as bad—of drifting away so far that they could never get back to the swarm, a condition shown by a yellow dot in the display. It was a simple matter for any given arklet to plot a new course that would avoid both of those fates. Much more complicated was for three hundred arklets to do it at the same time without banging into each other. So a kind of negotiation had to take place, based not on awaiting commands from Izzy but on observing what “nearby” arklets were doing and coordinating the firing of thrusters with them to minimize the amount of red showing up on the plot.

It was necessary to place the word “nearby” in scare quotes because it had a different meaning in this swarm than it did to a bird in a flock. To a bird, nearby meant just that. To things maneuvering in the six-dimensional parameter space of orbital mechanics, “nearby” meant “any set of params that is potentially interesting to me in the next few minutes,” and it could apply to objects that were currently too far away to be noticed. Once that was accounted for, however, the arklets could do as birds did when flying in flocks. In the simulations that they had seen shortly after the concept had been proposed, it had looked astonishingly like the behavior of schooling fish. And the reality of it, which had only been implemented in the last few months of round-the-clock launches from Kourou, Baikonur, Canaveral, et al., answered well to those simulations. It just happened more slowly in real time.

It was happening now, in response to Izzy’s course change. The red only spread so far, then began to recede, first fraying around the edges, then dying off in patches. A few dots went yellow, then corrected themselves as they caught up. Ivy’s expectation, based on the last few months’ tests and exercises, was that the last few red dots would turn white very soon and cease to be a concern. But this didn’t happen. Some remained stubbornly red. Spinning the plot around, looking at it in various modes, she zeroed in on those dots and queried them. Almost all of them were cargo modules or passenger capsules that had been launched during the Splurge: the last-minute effort made by all the spacefaring nations of the world to launch every last rocket they had capable of reaching orbit.

Her phone buzzed. A message had come back from Cal; his boat must have resurfaced.

What’s that supposed to mean?

He had only just now seen her last text.

It means we are no longer engaged.

That seemed a little blunt, so she added, You need to find some nice mermaid.

After a minute he answered {crying} I was going to do the same. Your odds considerably better.

She answered Bullcrap, which was an old joke between them. When she had first met him at Annapolis, he had been such a straight arrow that he was unable to speak the word “bullshit.”

SAB = Straight Arrow Babe came back.

SAB is sad :( Why did you dive?

Big surface wave came through. Bad news for East Coast.

Who tells you? Do you have a chain? Meaning chain of command.

One rung left above me. Then, after a pause, POTUS has gone dark.

She typed in Thank God for that and hesitated before sending it. But the world was coming to an end; she didn’t have to worry about repercussions. She hit Send.

She’d never talked to Cal about what had happened on Day 700: the fuel-air devices, the nuclear warhead. But she was certain it had been his finger on the button.

May God have mercy on her soul, Cal answered, and she knew the subtext: and may He have mercy on mine.

This exchange of messages was interrupted by one from Markus: need u.

She pocketed the phone to free her hands for movement through Izzy, maneuvered through the maze of habitation modules to the Stack, and headed aft, bound for the Tank. The trip down the Stack took no time at all. A week ago she would have had to maneuver around people clumped in twos and threes for conversation. Since Markus had declared PSAPS, this had changed; one of his edicts had been that the Stack must be kept clear for rapid movement of essential personnel. Right now it was as empty as she’d ever seen it. Down in the Zvezda module she saw some comings and goings, and recognized, for a moment, the spiky profile of Moira’s hair. She would be busy making preparations to disperse the Human Genetic Archive to the cloud, a project that in and of itself was at least as complicated as anything happening with swarms and params. Essential personnel indeed.

Luisa popped into view down in H1 and propelled herself up the Stack like she meant business. After nearly colliding with one of Moira’s helpers, she let her momentum carry her up into Zarya, then stopped hard at the entrance to the tube that led to the Woo-Woo Pod. She looked into it for a few moments, evaluating, then made a decision and pulled herself into it.

Ivy passed by the same location a few moments later, slowed for a moment, and glanced down the length of the tube. It was possible to see straight down its length, across the spherical Pod, and through its windows to the Earth. Normally this meant the blue light of the oceans and the white light of clouds and ice caps. Sometimes, a lot of green when they were passing over well-watered parts of the world, or some yellow when over the Sahara.

Right now the light was orange because the Earth was on fire.

People were screaming down there in the Pod. Luisa must have been sent there to calm people down. Ivy was almost drawn in by a sort of magnetic power of fascination. Earth looked as if some god had attacked it with a welder’s torch, slashing away at it and leaving thin trails of incandescence. Some of these were red and steady: things burning on the ground. Others were blinding bluish-white and evanescent: trails drawn through the atmosphere by meteorites.

She fancied she could almost feel the warmth radiating from the planet.

Markus needed her. She couldn’t help the screaming people down in the Pod. She turned her head aft and pushed on.

Hovering in the entrance to the genetic storage modules, Moira was ticking off items on her tablet, listening, dead faced, to something on a large pair of headphones. She noticed Ivy. She peeled a headphone away from one ear and aimed it at her. Ivy recognized a cappella music, medieval polyphony. “King’s College is holding up rather well,” she said. “Do you know the piece?”

“I’m certain I’ve heard it before, but I can’t place it,” Ivy said.

“Allegri’s ‘Miserere mei, Deus,’” Moira said. Thanks to the Morg’s insistence that she take Latin, Ivy knew what it meant: Have mercy on me, O God.

“It’s beautiful.”

“They would sing it at Tenebrae, in the wee hours, as they extinguished the candles one by one.”

“Thank you, Moira.”

“Thank you, Ivy.”

A minute later she was in T3. As always, she stood flat-footed for a moment to get the feel of simulated gravity, then headed toward the Farm and the Tank. Passing through the utility section she considered getting herself a cup of coffee. Then she felt shock and shame over the fact that she was thinking about coffee while her planet was being set on fire.