"THAT SHOULD BE all for now, friend Caliban," said Prospero, standing in front of his office comm center, deep in the bowels of Valhalla. Caliban's image was on the screen, beamed from Depot via a shielded hardline link. "I believe we are now on a pace to effect a full evacuation of the citizenry here in Valhalla, should the need arise."

"I would frankly be astonished if it did not arise, friend Prospero," Caliban replied. He was in the New Law robots' offices in Depot, watching over operations there while Prospero was in Valhalla.

Prospero considered his friend carefully. There was little that could be judged from a robot's body language, but either Prospero was imagining things, or else Caliban was becoming increasingly nervous, increasingly on edge. Well, that was to be expected, given the situation. "I take it you believe that they will indeed divert the comet? Have you offered our protests, and our arguments against the project?"

"I have attempted to do so. I have even gathered petitions signed by humans opposed to the project, and done what I could to ally us with human groups against the comet diversion. But even those humans most violently against the comet impact will have nothing to do with me. It would seem they have concluded that association with the New Law robots would do them more political harm than good."

"That is not surprising, but it is certainly dispiriting," said Prospero. "Very well. If they will not listen to us, and if our voice is not an asset in the chorus of opposition, let others lead the fight against the comet. We will concentrate on preparing our citizens to escape. I have examined your proposed list of allocated evacuation destinations. " The human authorities had assigned various destinations to various groups of evacuees, hoping to maintain some sort of coherence and order to the massively complicated operation. Needless to say, the New Law Robots had not exactly drawn the most desirable assigned destinations. "You have rated Site 236 as having the highest safety margin."

"Yes. It is the most geologically stable of our assigned sites, and is likely to suffer the least infall of debris, and the least severe post-impact weather."

"Very well," said Prospero. "Prepare that site to receive sixty percent of our heavy equipment, and whatever proportion of our citizenry can be accommodated there as well, up to a maximum of sixty percent. We will disperse the remainder to other sites, to avoid our being wiped out altogether by some sort of unlucky accident at 236. While I agree the odds of heavy damage there are low, if chance puts a large comet fragment or a large piece of secondary debris in the path of 236, I would just as soon we were not all there. And you will arrange for ten percent of our equipment and population to be sent to Site 149."

"But 149 is the most exposed and dangerous of all our assigned sites! I had advised that we send no New Law robots there at all."

"I saw that recommendation," said Prospero. "I must say that it surprised me. There are times when you lack all vision. I would suggest that you look not only at the map of this world as it is now, but a map of the world as it will be. Prospero out." Prospero cut the connection and turned to the New Law robot on the other side of the room. "Well now, Lacon. Do you see now why I no longer wholly trust friend Caliban?" he asked.

"No, sir, I do not."

Prospero regarded his new prot��g�� with something close to disappointment. Lancon-03 was as tall and angular and alert-looking as any New Law robot, but even so there were times when she seemed completely incapable of advanced or subtle reasoning. If Caliban was proving to be a more and more unsuitable second in command, Prospero was beginning to wonder if Lancon-03 was going to be any better. "The map, Lacon, the map. If the comet fragments strike in the predicted locations, and the expected changes in local geography and sea level take place, Site 149 is going to be within a few kilometers of the new shoreline, right on top of what ought to be the best harborage for three thousand kilometers. It will be the largest port in this hemisphere-and the New Law robots will control it. We will be there. We will lay claim to it, not only as an assigned evacuation site, but because we are on it, in possession."

"But you put many New Law robots at risk by sending them to such a place," Lancon-03 objected.

"I expose a few to slight danger for the greater good of all. But I do more than that," he said.

Prospero turned toward the view window that took up most of one wall of his office. Prospero looked down on the interior of Valhalla, on the brightly-lit streets, on the graceful arcing ramps that led from one level to another, on the busy robots hurrying along with their belongings from one place to another, preparing to leave this graceful, tranquil city under its sky of stone. This city was all they had, the fruit of their own labor, the greatest achievement of the New Law robots. And the humans were preparing to smash it down into nothing, to wipe it out as if it had never existed, if doing so would be to their advantage. There was a lesson there for Prospero.

"I propose," he said, "to take as much advantage as possible of whatever opportunities are presented to me by this disaster."

IT WAS TIME.

After the endless hours of checks and counterchecks, after endless dress rehearsals, after wringing any number of bugs out of the system, all of it was done. And it was time.

Governor Alvar Kresh paced back and forth behind his console, and looked up again, for the thousandth, the ten thousandth time, at the two hemispheres on their pedestals, the two control center units, the two oracles who could predict, and even shape, the future-if one dared to let them.

Kresh felt as if he had spent his whole life in this room, and the rest of the universe was little more than a vague and distant dream. He smiled wearily. Unit Dee no doubt felt much the same way. To her, this whole world was a dream, though one of mathematical sharpness and clarity.

Soggdon was there with him, and Fredda, and Donald, and all the others, the roomful of experts and technicians and specialists and advisors who had seemed to appear out of nowhere, unbidden, drawn in by nothing more or less than the crisis itself. But, in the final analysis, there was no point in any of them being there. He had heard what they all had to say, and considered their opinions, weighed all the pros and cons again and again. There was nothing more any of them could tell him that he did not know already. Not even anything more than Dum and Dee might say.

In the midst of all of them, he was alone. The one person who, by rights, should have been there, was not. But Davlo Lentrall was still with the comet diversion fleet. The first and most important phase of the fleet's work was now done. Now they had only to monitor the comet, track it, watch the telemetry.

Assuming they had to do even that. If he, Alvar Kresh, planetary governor of Inferno, decided to say no, to turn his back and walk away, Comet Grieg would go sailing off into the darkness, not to be seen for another two centuries. There would not be much point in watching its telemetry in such a case.

Nor was there much point in considering the possibility of such a case ever coming to pass. Alvar Kresh knew what he had to do. There was very little point in pretending otherwise. How could he possibly walk away from it all now, after so much had happened? How could he say no, and spend the rest of his life watching the planet slowly decay, spend the rest of his life asking himself what ij; telling himself if only?

He had to go forward. He had no real choice.

And that was the part that terrified him.

Forcing himself to be calm, he picked up the headset and put it on. "Unit Dee, Unit Dum," he said. "This is the governor."

"Yesss, Governnorrr," the unison voice replied. It startled Kresh to hear the two speaking together once again. It had been quite some time since they had done that. Was it because Dee recognized the gravity of the event? Some sort of effort at ceremony? Or was it for some other reason, or at random, for no reason at all, or because Dee was continuing to brood and wonder, and becoming less and less stable as she did so?

"I have reached my decision," he said. But he did not speak the words yet. Could he trust Dee with the job? Perhaps he should take the control of the maneuver away from Dee and Dum, tell the comet diversion task force to perform the bum manually.

But no. Better to let Dee have the practice, make sure all of her control connections to the comet's attitude control and thruster systems were working. They would have to use her either to control terminal descent or Last Ditch. Better to let her have a test drive, as it were. There was a nice long window for the comet diversion bum. By adjusting the thrust of the initial bum and the attitude of the comet, they could perform the bum any time in the next twelve hours. If some connection failed, if the bum was inaccurate, they would have time to fix it, or decide to abort and perform an emergency lateral bum to throw the comet well clear of the planet. Not so in the rapid-fire sequence of the terminal-phase breakup of the comet. Best to test as much of the system as possible now. This was the easy part. The hard stuff would come later.

And if he didn't trust Dee, he shouldn't let the comet diversion happen at all.

"I hereby order you to perform the planned diversion maneuver on Comet Grieg," he said, and the room was deathly quiet.

"Verrry wellll, Governorrr," said the unison voice. "Weee shalll commmmence the fffinall countdown in fourteen minutes, thirteen seconds. Tttthe burnnn willll commence one hourrr later."

"Thank you, Dee. Thank you, Dum," said Kresh. He took the headset off and sat down heavily in his console chair.

"By all the forgotten gods," he said. "What have I done?"

FREDDA AND ALVAR went outside, and Alvar, at least, was very much surprised to discover that it was full night. How long had it been since he had last left the control room? Twelve hours? A day and a half? Three days? He felt sure that if he concentrated hard enough, he could work it out, remember the last time he had come out, the last time he had gone in.

But there was not much point to the exercise. It was over, and he was out, and that was all that really mattered.

Fredda took him by the hand and led him away from the cold sterility of the Terraforming Center, out away from the lifeless stresscrete of the parking lots, and out onto the cool green lawns that surrounded the Center. "Look," she said, pointing up into the western sky. "There it is."

Kresh looked up in surprise. "I'll be damned," he said. "So it is." He had never seen Comet Grieg before. There it was, a fat, featureless golden dot hanging in the darkness. It had no tail, showed no features, but there it was. It seemed incredible that something that obvious should have been so hard to find. But he knew he was seeing the highly reflective sunshade parasol, and he knew the comet was moving fast, straight toward them. Considered as a question of logic, it made perfect sense that it would get closer, and appear larger and brighter as it did so. But still, somehow, it was a shock to see it up there.

He had seen its image endless times in the pictures beamed back from the diversion task force. He had seen it modeled, dissected, shown in false color detail, symbolized by a formless dot in an orbit simulation-but he had never seen the thing itself. There was something jarring, startling, in seeing the comet firsthand, in receiving direct, sensory, personal proof that it was real, that it was no simulation, no abstraction, but a flying mountain of ice and stone that he had ordered dropped onto this world.

Fredda led him out onto the cool, soft, grass and sat down. He sat down beside her and leaned back on his arms, and felt the dampness of the grass on his pants and arms. He could smell the clean, cold, earthy tang of the dirt, and a gentle breeze tickled the nape of his neck. "Let's watch it from here," said Alvar.

Fredda leaned over and kissed him on the cheek. "Good idea," she said, laughing gently. "Glad you thought of it."

"So maybe once in a while we think alike, you and I," said Alvar. "But right now I'm tired of thinking. Tired of deciding. At least that's all over-for the moment, anyway."

"For the moment," Fredda agreed. "Rest now. Rest now, away from all of them, and in a while we'll be able to see the comet shine."

"Yes," said Kresh, yawning mightily. He could feel all the tension, all the worry, draining out of him. It was done, for good or ill. "Rest. Rest awhile. And then I want to see them light the fuse under that comet."

But when, at last, the fat star in the sky blazed into glory, Governor Alvar Kresh was sound asleep, snoring gently.

DAVLO LENTRALL SHOVED his way a little closer to the porthole, and promptly got shoved back out of the way. At last he gave it up. Too many people were trying too hard to get near the too-small piece of glass.

In the old days, he would have expected them to defer to him. He would have reminded them that none of them would be here, that none of this would be happening, if not for him. Who but he had a better right to be near the glass? But now it simply amazed him that he had ever been capable of thinking that way. What right had he to anything?

Besides, they all had a right to be at the window. All of them, Spacers and Settlers, technicians and engineers and spaceworkers, specialists of every possible sort, had all worked impossible shifts, taken on impossible tasks on unmeetable schedules-and succeeded.

Davlo gave up and made his way toward the equally crowded cargo bay. They had set up huge repeater screens there. And there was at least some hope he would be able to see better from there.

The hope was realized the moment he set foot in the cargo bay. The main viewer screen had a view of Comet Grieg. There it was, huge and misshapen, a gleaming ball of rock and ice, hanging in the velvet darkness, sheathed in the shining gold of the parasol that was now draped over its pockmarked surface.

Once he would have felt nothing but sheer, vain pride in what he had caused to happen. But now, to look on that enormous object, and know that he had changed its fate, that his actions and those of others had converted a vagrant thought in the back of his mind into the huge and stunning piece of reality floating in the darkness, simply terrified him. What hubris. How could humans imagine themselves to have the competency, the wisdom, the right to attempt anything so grandiose?

He glanced at the countdown clock, and saw that they were getting close. Only a few seconds left.

Could they truly do this thing? Could they, would they, truly bring this flying mountain down onto their world? It seemed impossible. It seemed madness, suicidal.

A wave of panic swept over Davlo, hemmed in by the swarm of bodies. Someone in the front of the crowd began to chant out the countdown. "Twenty. Nineteen. Eighteen. "Another voice joined in, and another, and another, until the entire room full of people who had made this thing happen were shouting out the numbers in unison. "Seventeen! Sixteen! Fifteen!" the voices cried out, calling a bit louder with every number.

All except Davlo. Suddenly he was gripped by fear, by shame, by guilt. It could not work. It couldn't possibly. They were going to destroy the world. He had to stop them, stop this. It was a mistake, a horrible, terrifying mistake that could never be put right. Dropping a comet on a living world? No. No! He could not let them. He plunged forward, into the crowd, and tried to get to the front, call out a protest, a warning, but the crowd was too tight, the shouting voices too loud. He could not move forward, and he could not hear himself. "Ten!" they shouted. "Nine! Eight!"

But it could not be. It must not be. The dangers, the risks, were too great. The image of Kaelor, Kaelor at his death, Kaelor dying to prevent this thing flashed through his mind. "No!" he shouted out. "No! Stop!"

"Seven! Six! Five! Four!"

"Please! Stop!" he shouted out, though no one could hear. "It's me, Lentrall! It's a mistake! Stop!"

"THREE!"

"TWO!"

Davlo Lentrall sagged backward, let his body go limp. The bloody-minded arrogance of it all. How could he have dreamed that he, and he alone, could see the way? Now he had killed them all.

"ONE!"

"ZERO!" they all shouted.

"Zero," Lentrall whispered to himself in horror.

The screen flared into spectacular brightness, the light of a new sun blooming into life at the base of Comet Grieg. A tail, a jet of power and light and energy, lanced out from the detonation thruster, the powerful and intricate and clever device that allowed the comet to be aimed straight for its target, let it move toward the doom of the planet with far greater precision and efficiency. A tail of blazing, glowing plasma stabbed out into the darkness, and a shudder of power and motion rippled along the gleaming surface of the sunshade parasol. Rips and tears appeared in it as huge fragments of stone and ice tore free of the comet and smashed holes in the reflective plastic sheeting.

And the comet began to move, slid into its new heading, shifted into its new orbit and its new destiny. Down toward Inferno. No, thought Lentrall. No. He would have to stop it. He had to get down there first. He would have to get back to Inferno, and prevent the disaster he had set in motion. Somehow.

The burn ended, the jet of light died, the room erupted in cheers and shouts and applause, but Davlo Lentrall did not notice. He looked toward the screen, and saw nothing there but a monstrous weapon he had aimed at his own world.

What have I done? he asked himself.

What have I done?