Something inside him had changed. Or was it merely that something had been burned out, destroyed, when he watched Kaelor destroy himself! Surely the last of the old Davlo had died with Kaelor? Had anything, anyone, taken the old Davlo’s place, or was he just an empty shell of a man, going through the motions?

No. Never mind. Think about other things. Think about the plan to move the comet.

Davlo’s initial plan had been to use a fairly standard high-yield nuclear bomb, but the Settler-designed detonation thrusters were a vast improvement on that idea. In essence, a d-thruster was a nuclear bomb set off inside a powerful force field formed in the shape of a huge rocket nozzle. The force field directed the force of the explosion into the proper direction, in effect producing a shaped charge that was far more efficient and far more controllable.

Other explosive charges were being rigged as well, of course. Once the comet had been redirected into its intercept course with Inferno, it would still be quite some distance away from the planet. It would take it just over thirty-two days to move from the point in space where the initial course change was made to its intercept with Inferno.

Just before arrival at the planet, the comet would be broken up into smaller pieces by explosive cutting charges, each piece to be directed toward a different point on the surface. Each fragment would have its own smaller, non-nuclear propulsion system and attitude control system.

And that was the part that worried Davlo. That was the greatest danger in the plan. In theory, at least, it might be possible for human operators and standard computer systems to manage the complexities of the operation. But the current plan called for Grieg to be broken up into twelve fragments, and it was far from certain that all the cutting charges would shear the massive body into pieces of precisely the intended size. Besides which, there were bound to be thousands of smaller fragments produced by the blasts of the cutting charges. Most would be too small to do any damage.

But all it would take was a fragment smashing into a thruster at the wrong moment, or for a fragment to end up being larger or smaller than expected, and then the whole careful sequence of events could go out of control. There were enough spare thrusters to serve as backups, so that if some of the thrusters on a given fragment were destroyed, the rest would be able to do the job. Indeed, there were no ifs in the question. Some part of the established plan was going to go wrong-it was just that no one could be sure which part. It would require immediate, real-time management of the operation to deal with the inevitable problems.

Managing the terminal phase of the operation would mean dealing with thousands of operations simultaneously. It would require juggling the twelve fragments at once, keeping them out of each other’s way while guiding them down to their intended impact sites, while dealing with the cloud of debris produced by the cutting charges.

No matter what theory said, in practice, the job was beyond humans, beyond any combination of human and computers. The only entity able to deal with it all would have to have the decision-making ability of a human combined with the computational speed and accuracy of a computer-in short, a robot.

Nor would just any robot do. The task was too complex for any standard robot to contend with. Even just handling the hundreds of sensory input channels would overwhelm a normal positronic brain.

The one, the only, possible way to control the terminal phase was to hand the job over to Units Dee and Dum.

And that, of course, meant putting a Three-Law robot, and her computerized counterpart, in charge.

And if Kaelor had killed himself rather than cooperate with the comet intercept, how the devil was Dee actually going to run the operation without losing her mind-or point-blank refusing to do the job?

THE SAME SORT of question was very much on Alvar Kresh’s mind as he and Fredda settled into their aircar for the brief flight from the Winter Residence to the Terraforming Center. Their days had settled into a routine with startling speed. Get up, go to the center, spend the day sorting out the details of the planet’s fate, then go home to the Residence for dinner and a good night’s sleep, or at least an attempt at sleep, before getting up to do it all again the next day.

Somehow, he hadn’t expected there to be so many decisions for him to make, so much hands-on work for him to do. For all the power and capacity and sophistication of the Terraforming Center and the twin Control Units, there were some decisions that no robot or other human could make, disputes that only the governor had the authority to settle. And besides, there were a lot of humans out there who were not going to take orders, however sensible, from a robot. And there were things that Kresh knew that Dee and Dum did not-how best to handle this local leader, which prices for emergency supplies he could expect to bargain down and which he could not, where he could ask a favor, where he could call one in, how far people could be pushed if need be, and when to give up.

But everything was routed through the Terraforming Center. It had soon become clear to Kresh that he would have had to relocate his command operations at the Center if it hadn’t started there to begin with.

Fredda followed him into the aircar and sat down in the seat next to him. Donald took his place at the controls, did a safety check, lifted off, and headed for the center.

So far, the preparations for the comet diversion were going quite well. But he could not stop worrying. It was never far from his mind that Dee believed all Inferno to be a simulation. Whether or that was likely to be help or hindrance he still could not decide. “So what do you think?” he asked his wife.

Fredda looked at him with an amused smile. “About what? It’s a little hard to offer my opinion unless I get a few more clues than that.”

“Sorry. I’m a little preoccupied. Do you think Dee and Dum are going to be able to control this operation?”

“I don’t know,” said Fredda. “I spend every day monitoring Dee, watching her behavior, trying to understand her. But there’s a very basic barrier I can’t get around. She doesn’t think any of this is real. I can understand the logic behind telling her the world is imaginary, but I must admit I question the wisdom of the decision. So much depends on her getting things exactly right-and yet, to her, it is all a game. She’s so casual about it all, as if the whole situation had been set up solely for her amusement.”

“From her point of view, it was all set up for her amusement,” said Kresh. “As far as she is concerned, the world of Inferno is just a puzzle for her to solve-or declare insoluble.” He was silent for a moment before he spoke again. “I’d have to agree with you about her attitude,” he said, “but at the same time, I’d have to say the quality of her work has been impeccable. She may not take it seriously, but she does it seriously. Maybe that’s all that counts.”

“I hope so,” said Fredda, “because I don’t know what the devil we do if we decide we don’t trust her. In theory, we could pull the plug and let Unit Dum take up the slack. But I don’t think that’s really possible anymore. The two of them are too interlinked, too interconnected. They rely on each other too much for us to pull one of them abruptly off-line.”

“And Dee is in charge,” Kresh suggested. “It seems to me she just uses Dum as a sort of auxiliary calculating device.”

“No,” said Fredda, quite sharply. “That assumption is an easy trap to fall into. She does run the show when it comes to human interaction-that much is obvious. But that is the smallest fraction of their work. In everything else, they are coequal. There are some areas where Dum very definitely takes the lead-such as computational speed. Yes, he’s just a dumb machine, a mindless computer system with a crude personality simulator to serve as an interface. But he’s carrying a lot of the load. We not only need both of them-we can’t have one without the other.”