“I do.”

“Then here goes. Standard color codes.”

John’s personal suit switched to remote visual mode. The real external world remained visible as a faint superimposed star field. Fatalities early in the construction of Sky City had led to a decision that the immediate environment must never be completely excluded in favor of remote image data.

The simulation employed a standard vantage point for shield display, a position in space a million kilometers from Earth and fixed in the plane of the ecliptic. The planet sat at the middle of the field of view, a blue sphere almost twice the size of the full Moon as seen from Earth. The space shield formed a long, narrow cone, slightly flared at the end like a trumpet. Invisible to human eyes, it had been stylized in the simulation to show as a web of light blue against a black background. The cone was widest closest to Earth, with the central axis pointed directly away from the planet at about forty-five degrees to the ecliptic. In that direction lay Alpha Centauri, impossibly remote yet dictating every human priority.

The effectiveness of the shield at a given time depended on the balance of two factors: the shield’s stage of completion, and the energy and composition of incident particle flux. “Hot spots,” locations where the shield was inadequate to redirect the charged particles, were delineated in vivid hot pink. Trouble locations where the situation was improving sat as green islands within the hot spots. Places where things were getting worse showed as stigmata of flaring orange.

The default time rate for the simulation was forty-two thousand to one. At that setting the Moon provided the observer with a natural feel for the pulse of events. Once a minute the sunlit white marble made its four-hundred-thousand-kilometer sweep around Earth, oscillating four or five degrees above and below the plane of the ecliptic.

At that rate, the display of the period from the present to the time of maximum flux energy would take a hundred minutes; long before that John would be at Sky City. He called for a tripling of simulation display rate and settled back to watch. Already he was having premonitions. Will Davis would never suggest such a viewing unless the implications were severe.

In any case, John would have caught the problem even without prior warning. He remembered the results of the simulation from four months ago. Superimposed on the blue web you would always see a scattering of pink patches, within which the flecks of green and orange came and went apparently at random.

Today was different. Eight minutes into the display — less than two years ahead in real time — the pattern began to change. Pink patches spread. Within them he saw few flecks of green. The orange glow of worsening problems dominated.

Something had changed, rapidly and for the worse. It surely had to be something more than the number of uncharged particles, which John still judged acceptably low. He kept watching until the simulation reached the time of maximum flux. By then the space shield had become a sieve. In such a future, Earth would suffer far more than during the initial wave of twenty-seven years ago.

His imagination, heightened by the boost, flashed to scenes of disaster made vivid by early childhood. The first effects of the Alpha Centauri supernova had been calving of the Antarctic ice shelf, blizzards at the equator, tidal waves, and nested sets of tornadoes. When the gamma pulse hit, planes dropped from the sky, telephones and television and radio became useless, and mayhem, starvation, and disease took over.

That surprise would not happen again — the gamma pulse would not be repeated. But there was scope for new and worse disasters. His boosted consciousness threw images at him with awful clarity.

The upper atmosphere had shielded Earth from the direct effects of the gamma-ray pulse. It would not do the same for the particle storm now on its way. The charged nuclei, predicted to be mostly bare protons, would come like a hail of tiny bullets, smashing into the air at relativistic speed. Most would be absorbed on the way down, producing a cascade of forward-scattered mesons, electrons, and gamma rays. Some of those would in turn be absorbed and scattered, some would continue.

The atmosphere would turn into a cloud of charged nitrogen and oxygen ions, opaque at every wavelength from hard ultraviolet to short-wave infrared. In the darkness below, a deadly blend of radiation and particles would hit the surface. That final mixture would depend on the type and energy spectrum of the incident particles.

Marine organisms would be lucky, at least at first. Water was a fine absorber, and within a few feet of the surface it would quench the main radiation storm. Troubles would begin at the bottom of the food chain, when plankton and algae lacked solar radiation for photosynthesis. Starvation would work its way steadily up, to the krill, to the vertebrate fishes and crustaceans, finally to the seals and dolphins and whales.

Long before that, all land forms would be in trouble. Water was a good absorber for the torrent of high-energy radiation and particles, but unfortunately most plants and animals were at least seventy percent water. Microscopic explosions would happen in a billion cells at once, disrupting protein synthesis and demolishing the delicate nucleotide sequences in the germ plasm. Old organisms would sicken and die, embryos would form with mutation levels high enough to kill, and all under a smoking sky of reactive ions and toxic rains.

Things would improve — slowly. In perhaps a year, when the main storm was over, sunlight would bleed back in and the air would turn clear and clean. Before that, the species die-off might be worse than anything seen during the Permian or Cretaceous/Triassic mass extinctions. Humans ought to survive, but there could be another halving in population, matching that of twenty-seven years ago. Said that way, it didn’t sound too bad. Say it as two billion people dying in awful ways, and it became unthinkable.

John’s head and stomach were churning. The shield team’s job was to make sure that the worst you could imagine didn’t happen. But was the team up to the task, even without the new problems?

He turned off the visual feed from Cusp Station. Into the foreground sprang the sunlit upper face of the “space pill,” the thin disk of Sky City, a kilometer across and a hundred and ninety meters deep. He was closing on the nerve center for space operations, and he must prepare himself for arrival.

The trouble was, his brain refused to cooperate. The images of Earth’s once and future woes distracted him, boiling and bubbling through his mind. John glanced at the condition monitor and understood what was happening. These were Neirling boost side effects, annoying but not unusual. He squeezed his eyes shut and concentrated. After a few minutes the disturbing mind pictures began to fade.

A law of diminishing returns applied to the Neirling boosts. Boost once, and you gained a clean twelve hours of wakefulness and heightened attention. Boost again, before you crashed, and you had maybe eight more hours, with some loss of judgment at the end of it and a danger of making decisions you would later regret. If you tried to stay awake too long in second boost, your consciousness turned off without warning like a failed light bulb.

And third boosts?

They were done only in life-or-death circumstances. The user had at most five good hours, followed by an unavoidable twelve-hour crash. You woke feeling fine. However, observers insisted that the person who came out of a three-in-a-row Neirling boost was not the person who went in. Psychological tests were inconclusive.

John did not intend to add to the body of available evidence. He would go to Bruno Colombo’s office and listen to what the Sky City director had to say about the “new problem” — presumably the simulations. After that he would find a place where he could hide away.