was a real world, if a small one, with a geography complicated enough to have kept a generation of mapmakers busy. Its surface was so pocked with craterlets and covered in crags arid gullies and cracks that it was hard to study anyone feature on the surface before it got lost among all the others.
Comet Grieg was one of a special class of so-called “dark” comets. Inferno’s star system had plenty of normal comets, of the classic “dirty snowball” type composed primarily of water ice and other volatiles. But for reasons that still were not entirely understood, star systems with poorly developed planetary systems also seemed to produce a large number of dark comets—and Inferno shared its star with only two planets barely large enough to qualify as gas giants, a wizened little asteroid belt, and the usual sorts of deep space debris-comets, asteroids, planetesimals, and so on.
Called “dark” because they produced relatively small tails, and were composed of darker material, dark comets were closer to being asteroids encased in ice than anything else. Grieg had a particularly large proportion of stony material, but it contained plenty of water ice and other volatiles. A hazy nimbus of gas and dust and ice shards floated about the behemoth, bits of debris from the size of molecules up to the size of small aircars that had either been knocked loose by the natural heating and outgassing as the comet neared the sun, or else thrown clear by human interference.
A searchlight from a closer-in ship stabbed through the cloud of debris and struck the surface of Comet Grieg, flooding one small area on the surface with a light so bright, so clear, it did not seem to belong on such a darkened surface. A smooth and perfect cylindrical shape stuck up out of the comet’s surface. Davlo recognized it. It was one of the dozens of thrusters planted on the comet’s surface. He had helped calculate their placement, and played at least a small part in working out the firing sequence that had been used to eliminate the comet’s spin. It had been in a wobbling two-axis tumble when the task force had arrived. Now the spin had been restored and refined, and the comet’s nose was pointed straight at the sun.
But the sun would have no further chance to melt this comet. Davlo looked from Comet Grieg to the sunshade, a huge and insubstantial parasol floating in space a kilometer or so sunward of the comet, forming a permanent solar eclipse as seen from the surface of Grieg.
Left to its own devices, Grieg would have melted and boiled and sublimed away a substantial amount of material by now, forming a coma that would, in turn, have been blown back by the solar wind into a modest tail. But the sunshade stopped all that, and kept the comet in the deep freeze.
The parasol was itself being blown back by the solar wind, slowly drifting in toward the comet. In about another day or so it would come into contact with the comet, moving far too slowly for it to be called a crash. The parasol would drape itself around the comet like a small handkerchief dropped onto a large egg. It would tear in places, and the work crews would cut deliberate holes in it where it served their purposes, but that would be of no consequence. The parasol would reflect sunlight just as handily, losing only a few percentage points of its effectiveness.
Davlo Lentrall could not help but wonder what Kaelor would have thought of all this. He would have had some sardonic comment to make, no doubt, some dour turn of phrase that would capture the weaknesses in the plan in fewer words than anyone else. Or, Davlo wondered, was he making Kaelor too human? Kaelor had died in a futile attempt to prevent the comet capture. It stretched credulity to the breaking point to imagine he could be witness to the event, first hand, without the Three Laws taking hold of him, forcing him to desperate action. Davlo Lentrall was finding it more and more easy to understand desperation, and how it might drive someone to do something dangerous.
But one did not have to think on the grand scale to see this was no place for robots. Davlo looked out the port again, and spotted two tiny, space-suited figures moving some huge and unidentifiable piece of machinery about on the surface of the comet. A misplaced step, a crack in a faceplate, a shove to the machine that was a trifle too hard, and one or both of them would be dead. It was impossible to imagine any modem robot allowing humans to do anything so risky.
Davlo glanced at the wall chronometer, and realized that his break was nearly over. More out of duty than desire, he began to eat, the motion mechanical, the taste of the food unnoticed. Back to work. He would have help with the final check-calculations for the placement of the main detonation thrusters. It should have been humbling, galling even, for Dr. Davlo Lentrall, the man who had seen the potential of Comet Grieg, the man who had dreamed the dream and planned the plan, to be assigned a position as minor as assistant calculation engineer. Glory and accolades should have been his.
But, somehow, he no longer saw it that way. Others here, mostly the Settlers, were far more skilled at handling the detailed mathematics of moving a small world through space. He saw his position as a penance, and a fitting one. How brilliant and noble could his vision have been if his closest associate was willing to die in order to stop it? Davlo found himself embarrassed and ashamed whenever someone recognized him and congratulated him on his grand plan. Most of the crew had learned to avoid the subject, and, indeed, had learned to avoid Davlo.
But he had been sent here to do work, and he had agreed to do it. So he accepted the tasks he was given, and did them as best he could. Besides, work got his mind off things. He could worry about solving the equation, determining the proper thrust and orientation. Off-shift was the worst, nights spent staring into the darkness, thinking of all the ways things could go wrong. No, he wanted no congratulations.
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
