Rob stared at the image in front of him, his imagination hyperactive. Regulo wouldn’t have gone to these lengths unless he had something very real in mind. It was just a question of sorting through all the possibilities and choosing the one with the commercial slant.
“What’s the composition of this rock?” he said suddenly.
“Metals, mostly — several different ones.”
Regulo waited expectantly. After a minute or two more, Rob nodded.
“I see it,” he said. “It all seems feasible, but I’d want to explore the details.”
“Well, man.” Regulo was suddenly impatient. “Come on, tell me how you think it ought to work.”
“All right.” Rob stood up and went closer to the screen. He pointed at the drives in the rock. “Let’s start with these. You set them to provide equal and opposite thrusts, one on each side of the asteroid. You fire them tangential to the surface, and you use their torque to set the rock spinning fast about an axis. The faster, the better, provided that the tungsten sheath around the whole thing can take the strain.”
“No problem at all with a small rock like this. We might have more to worry about when we get to something the size of Lutetia.”
“Let’s finish this one first.” Rob pointed again at the image. “I’ll assume you have the powersat in position by the housing there. You picked that placing so the powersat sits on the axis of rotation of the rock. It would be a messy calculation, but the principles are easy. Now you begin to feed power in to the rock, through a grid over its surface. A lot of power. For something much bigger than this, I don’t think a powersat will do it. You’ll need a fusion plant or a power kernel, otherwise the job will take forever.”
He squinted again at the configuration on the screen. “Are you sure that the rotation will be all right? I’d expect a stability problem. It will be difficult to keep a smooth rotation about a single axis as the shape changes. I assume you looked into that and have the answers?”
Regulo nodded. “I cut my teeth on that sort of problem, calculating the change in mass and moments of inertia as the volatiles boil out of an asteroid during solar swing-by. We’ll have small adjustments to make as we go, but I have those worked out. Keep going.”
“Alternating currents,” Rob said. “Big ones, through the middle of the asteroid. When you apply those from the power source, you’ll get eddy current heating inside the rock from hysteresis effects. If you put enough power into it, you’ll melt the whole thing. You’ll produce a spinning ball of molten metals and rock. Spinning fast. I assume you’ve looked at the shapes and structures for a stable rotation? You’ll want a Maclaurin ellipsoid, with an axis of symmetry, rather than a Jacobi ellipsoid with three unequal axes.”
“You will indeed.” Regulo’s face was intent, his eyes fixed unwinkingly on Rob. “I’ve looked at the stability of the rotating mass. It will be all right. What next?”
“The rotation produces an acceleration gradient inside the rotating ball. The heaviest metals will migrate to the outside, the lightest ones will be forced to lie inside and closest to the axis of spin.” Rob was visualizing the ball, shaping it before him with his hands. “It’s like a big centrifuge, separating out the layers of melted materials. All you need now is the final stage: the Spider. It sits out on the axis of rotation, at the opposite end from the main power source. But it has that long, specialized proboscis, so it can reach any point inside the asteroid. You insert it to the depth that you want, and draw off that layer of rock or metal. Then you extrude it directly through the Spider — I already made the modifications you asked for, to permit high-temp extrusion.”
“You did.” Regulo’s eyes were gleaming. “And we can do away with all that mess that we had to use for the beanstalk. Chernick and the Coal Moles was a neat idea, but it was still a patched-up solution. With direct extrusion we’ll see a terrific improvement in what we can do. Give me access to Lutetia and I’ll spin you a cable from here to Alpha Centauri, with any material in the asteroid. No more grubbing about for different metals. They’ll come pre-sorted by density.”
He grinned at Rob’s expression. “All right, maybe not Alpha Centauri. We could certainly spin a web right through the Solar System, if we can think of a good use for one.”
“I like that. A beanstalk, all the way from Mercury to Pluto.” Rob was silent for a moment, chewing at his lower lip. “Won’t work, though,” he said at last. “You could never get it stable.”
“True enough.” Regulo leaned over the desk and cut back to a full display of the asteroid. “I’m just indulging in a little random speculation. That’s how everything starts, though I must admit I don’t see any way of making that one work — yet. There are a couple of other things that you didn’t mention about this system. How would you stop it from slowing down and stopping the rotation? You’ll have frictional losses, effects of the solar magnetic field, all that sort of thing working against you.”
“After the drives are switched off? I’d expect those to be small effects, but anyway it should be easy enough to compensate for them. It won’t be a perfectly homogeneous figure of revolution, even when it’s melted. Stick a pulsed magnetic field on it, about the rotation axis. You won’t need much torque to keep the spin rate constant.”
Regulo grunted his approval. “Where were you twenty years ago, when we were designing the Icarus solar scoop? I could have used your head on that. Most people don’t seem to be able to think straight even when they have all the facts.”
“Twenty years ago? I’d just lost my first milk tooth.”
“Aye. God knows it, I’m getting old.” Regulo rubbed at his lined forehead with a thin, veined hand. “Twenty years ago, to me it’s like yesterday. One more thing for you to think about, then we’ll pack this in and do some work on the beanstalk. From what you’ve seen of this so far, do you see any problems when we go to a really big one? Say, when we spin up Lutetia?”
Rob shrugged. “Well, there’s one obvious problem. You can’t possibly extend the proboscis far enough to penetrate through to the center of something that big. So you’ll have to mine the heavy materials on the outside first, even if that’s not the way you’d prefer to do it. I can see cases where you might want to get at the lighter metals and the volatiles first.”
“I’ve worried about that one, too. At the moment I’m playing with the idea of zone melting, but I’m not completely happy with it.” Regulo watched and waited in silence, while Rob mulled over that problem.
“I see what you mean,” Rob said at last. “You’re assuming that the materials are scattered fairly uniformly through the whole body of the asteroid. That looks like a big assumption to me — unless you’ve checked it some other way?”
Regulo shook his head. “The theory of formation suggests that most of the volatiles will be on the outside. I would melt just the first couple of kilometers in from the surface, and mine there first. I think the Spider could tap that deeply without much trouble.”
“And leave the middle solid until you want to melt further?” Rob looked thoughtful. “I don’t have your experience on differential melting. The Spider can do it all right, that’s not the issue. But I’m still not comfortable with the idea. Let me think about this for a few days and see if I come up with anything better. It’s not efficient to switch the power on and off, and I would expect that zone melting will give you problems with rotational stability.”