It was a routine command. The return trip along the extended spine of the space shield and the transfer trajectory to Sky City would require none of John’s attention. His personal suit informed him that a one-gee boost with midpoint turnover would carry him the hundred-thousand-plus kilometers from Cusp Station, at the outermost limit of the. shield, to his usual docking port on Sky City in one hundred and sixteen minutes.

One hundred and sixteen minutes, in extended boost mode and with little to do but indulge in worried speculation. Couldn’t Bruno Colombo have offered a word or two to indicate the nature of the difficulty? Something like, they had a different problem and not just another problem? Or had there been another death, another disappearance, another frightful episode in a lengthening sequence?

Probably not. But the murders were getting to him, as they were to everyone living on Sky City.

The suit was accelerating him smoothly along the axis of the space shield. The shield was eighty-five percent complete, but for that you had to take the word of the computers. There was nothing to be seen.

John looked out to the side anyway, at right angles to his direction of motion. Of course, he saw nothing. The three-billion-ton mass of the shield was there, but it spread out to form a narrow cone almost a hundred thousand kilometers long and eighteen thousand kilometers across at the base. The matter in the shield, about a gram of it per square meter, formed a delicate spiderweb of superconducting fibers, load filaments, node sensors, thrustors, and computing tubules. It could stand considerable tension forces, but an ounce of compression anywhere would make it buckle. The structure made small but constant adjustments in relative geometry and overall position to prevent that. The computational problem was continuous and complex. The shield had to maintain a fixed shape and position in the presence of time-varying forces: Earth attraction, lunar, solar, and planetary perturbations; solar wind and radiation pressure. And, of course, it must deal with the variable but steadily increasing sleet of charged particles that provided the shield’s whole reason for existing.

John looked down past his feet. For reasons of personal comfort, suits usually accelerated you headfirst. Until turnover time that meant Cusp Station lay directly below him, at the apex of the cone, while Sky City was above his head.

Sky City, which for the past seven months had been prowled by an insane -

No. Not that. Those thoughts were the effects of the second Neirling boost on his nervous system. He had to get his attention fixed on something tangible.

Cusp Station was still visible below, a bright cluster of green and orange lights. No word from Will and the others. Presumably they were still chewing on the technical problems.

But Cusp Station was not what he wanted to see. He added an apodizing disk to the suit’s optical sensor, removing the unwanted glare of the station, and scanned the area close to it. After a few seconds he located his target and locked on.

And there it was. An undistinguished second-magnitude star, one of hundreds of that brightness in the sky. Yet a star like no other.

For all of recorded history Alpha Centauri had been the third brightest star in the heavens: Rigel Kentaurus, a splendid visual binary with a third and faint companion, Proxima Centauri.

Thirty-one years ago, the Alpha Centauri system had exploded as a mighty supernova. But because Alpha Centauri was about four and a third light-years from the Sun, no one on Earth knew what had happened until twenty-seven years ago, when early in 2026 a torrent of light reached the solar system. For several months Alpha Centauri blazed down on Earth like a second Sun.

The cascade of light had created freak weather and global devastation. The flux of high-energy gamma rays, a few weeks later, knocked out all microcircuits and everything that depended on them. Civilization shuddered, gasped, staggered — and thought it had survived.

Until it realized that the Alpha Centauri supernova, unpredicted by all scientists and proclaimed impossible by most, had not finished with the Earth. The star had waned, until the faint remnant provided a poor object for visual observation. But most of the energy of a supernova is not given off as light or gamma rays. It resides in the sleet of electrically charged subatomic particles thrown out during the stellar explosion. That particle storm, fast-moving as it is, travels much more slowly than light. In the case of Alpha Centauri, the densest cloud was calculated to move at about nine percent of light speed. Travel time from Alpha Centauri: about forty-eight years. Arrival date for the storm center: forty-four years after the first blaze. Say, in 2070.

Earth would need protection. But there were further complications, introduced by the particle velocity distribution. To find when a particle storm has its biggest effect, you must look not at simple particle density but at the product of particle flux density and particle kinetic energy. The most energetic and destructive particles are the ones that travel fastest. Hence the worst time would be earlier than 2070. It was predicted as 2061.

No. The worst time is here and now, with murder most foul on Sky City . . .

The suit’s acceleration ended abruptly. The skyscape swirled dizzily, bringing John shuddering back to reality.

Turnover time. Halfway to Sky City. Still an hour to go. An awful situation. The Neirling boost was designed for when you were desperately busy with more work than you could handle. Employ it when you had nothing to do, you were like a motor running against zero load. Your brain spun faster and faster until it burned out its bearings. And in this case John knew exactly where his brain would run until it blew: on the subject that he was trying to keep out of his mind, the awful deaths.

He had to find something to do.

He transmitted to Cusp Station. “Will? It’s been an hour and I’ve not heard a word from you. What’s going on?”

It was a few seconds before he heard a reply. The delay wasn’t travel time; that was negligible over such a short distance. He must be interrupting something.

“We’ve been solving problems, what do you think?” Will Davis’s voice held a touch of surprise and perhaps reproach. “Not like some people. We’re done with what we were working on when you left, it’s on the way to your office for you to approve. Lauren’s down and out; she crashed a few minutes ago. What’s wrong, John? Feeling lonely?”

“Not lonely. Insane would be better. I had to give myself that second boost, otherwise I’d be sleeping when I get to Headquarters. But now I’m going crazy out here with only the stars for company. I need something to do. Link me in for your visuals, will you?”

“I could, but you won’t get much from seeing me and the others just sitting here. I can give you something a lot better to chew on. It’s what we’re stewing on ourselves, when we’d all be better off sleeping. Yesterday the deep space network received a new Sniffer profile, and nobody likes the looks of it. The flux profile for uncharged particles from Alpha Centauri has changed. Increased.”

“Bad news.” John didn’t need to say more. The electromagnetic field created by the finished shield would change the trajectory of charged particles. They would be diverted, passing around Earth and leaving it untouched. But neutral, uncharged particles were not affected by the field. They would get through and smash into the shield. Too much of that, and the fragile barrier would be destroyed. Then everything would get through, charged and uncharged, to hit an unprotected Earth.

“Also, we just received a new simulation,” Will went on. “It came in from the analysis team back on Earth after you left. We assume that they incorporated the new Sniffer data, though it’s hard to see how they could have done it so fast. Either way, they’ve worked out what will happen from now to the time of maximum particle flux if we don’t make changes to the construction schedule. Remember the protocol for changing display time rates?”