Rob hesitated, but the urge to explain was too strong.

“Just as you did. You have a rotating cable out in a free orbit — thousands of kilometers of it.” He leaned forward, at the same time as Regulo moved his chair farther away from the desk.

“Now suppose you want to move a space pod from the Belt to the Moon,” Rob went on. “You make it rendezvous with the center of the cable, where the powersat sits. The center of mass of the cable would be moving in a free-fall orbit, travelling about the sane speed as the pod, so you use hardly any reaction mass to make the rendezvous. You don’t need much acceleration from the pod’s drives, either, just a fraction of a gee will be enough. Once you have the pod at the middle of the cable, you let it move out along the drive train. As the pod moves from the center it feels a centripetal acceleration. You need to use the drive train on the cable to restrain it. When it reaches the end of the cable, you release it to move in free fall. You’ve given it a big velocity boost. But the trouble from the point of view of a human on the pod is the acceleration. Out at the end of the cable, it’s huge. I looked at a couple of examples. A cable four thousand kilometers long, with end velocity of twenty-four kilometers a second, would give thirty gees at each end. That’s what killed the Goblins.”

“They were unlucky.” Regulo had moved his chair farther and farther from the desk, until it was almost back to the wall. “If you like, you could even say that it was Caliban’s fault. He received no inputs on space operations for passenger transfer, and intelligence can’t replace experience. He put the space pod to a cable rendezvous with a cargo Slingshot — one with high accelerations, never intended for people.”

“Do you have Slingshots for passengers?” Rob moved forward right up to the desk.

“We built the first two, just a month ago. I could find out which cable your Goblins used easily enough, by checking the angular momentum of all of them. Each time we use a Slingshot we naturally increase or decrease its angular momentum.” Regulo stood up, his back to the wall. “We lose angular momentum when we throw a cargo in toward the Sun, and pick it up when we catch something thrown in from Mars or the Belt. Provided we move the same mass of materials in and out, the whole system balances — just like the beanstalk back on Earth. I would have given you details of the Slingshot as soon as we had Lutetia under control. You’ve got the idea, but you’ll be surprised when you see how much we can cut off transit times.

“Well, enough of that.” Regulo’s voice changed in timbre, becoming gruffer and more intense. “The Slingshot was used in a way I never expected. It killed two of the `Goblins,’ if you’re correct. But what about the rest of it? Joseph was secretly performing some kind of social experiment here, that’s what you’re telling us. If he had a self-sustaining colony they would have been through many generations in thirty years. It makes me wonder what type of social structure they could have evolved. Did Joseph tell you what he was trying to achieve in his colony, before Caliban got him?”

“Not a thing.” Rob stood up. “Morel didn’t intend to tell me anything. He was supremely logical, and logical people don’t bother to explain things to a dead man. I had one other factor to consider while I was locked in that lab. Morel wasn’t an anthropologist. He didn’t have the slightest interest in social structures. He never told me what he was doing. But you see, Regulo, I know it anyway.”

“Aye.” Regulo’s voice was as calm as ever. “I was afraid of that. The second that you came in here, I thought that the game might be over.”

He waved a thin hand at the control panel. “While you were talking, I gave the signal for the maintenance crews to make emergency departure from Atlantis. They’re clear now, wondering what the devil is going on. See the ship?”

On the display, a large freighter hovered beside Atlantis. Near it, filling the screen, the swollen balloon of Lutetia hung, its surface white-hot and smoking with escaping volatiles.

“I have to ask you one more thing,” Regulo went on, “though I think I know the answer. I suppose that it would be a waste of time to offer you part of Regulo Enterprises?”

Rob shook his head. The movement sent a flare of pain down his left arm.

“I thought not.” Regulo’s hands were behind him against the wall. A panel slid open to reveal a dimly lit corridor. “We respect money, you and I, but it has never been the main drive for either of us.” He sighed. “It’s a pity. We could have done great things as a team.”

“I know. Great things.” Rob’s voice was scarcely loud enough to hear. “To work with you, Regulo, for that I’d have given everything I own. But this is different. There are some rules that I can’t break.” He cleared his throat and spoke more loudly. “It’s over.”

“Not everything.” Regulo stepped back through the opening. Rob and Corrie did not move. “When you came in, I suspected that Atlantis was finished one way or another. So while were were talking I set the controls for collision with Lutetia. We have a few more minutes before impact.” He pointed again to the screen, to Lutetia’s swelling bulk. “After that, it’s no more Atlantis. No more Morel, no Goblins, no Caliban, no Sycorax. No evidence to support anything that you said. Follow me, both of you, or there will be no Rob Merlin and no Cornelia.”

The panel began to close.

“I’ll hold the ship for you.” There was a plea in Regulo’s bright eyes. “Hurry. I have to destroy Atlantis, but I can’t stand the thought of losing either of you.”

While the wall panel was still closing, Corrie ran rapidly around the desk and began to examine the settings on the controls. Rob dragged himself wearily across to join her.

“What’s the maximum drive setting for Atlantis?” He could feel pulses of pain running up his arm and through his whole body.

“About a thirtieth of a gee.” Without waiting to consult Rob, Corrie was throwing in new settings. “That’s not the point. The outer surface will fail at much less than that. I don’t think we dare try for more than a hundredth of a gee.”

“What happens if the outer membrane bursts?”

“The aquasphere would flood the drives. We’ll burn up in Lutetia.”

Rob moved to the display console and switched in a camera to show the exterior of Atlantis.

“Don’t use that drive unit, Corrie. It’s the best one for the direction of thrust that we need, but we’d fry Regulo. He’ll be coming out of that shaft. Take the next two drives and balance their thrusts. It will be close enough to tangential, we won’t lose more than a few percent effectiveness.”

He leaned across the desk, wincing as his left hand touched it. “Give us a fiftieth of a gee.”

“That’s too high. We’re only rated for half that.”

“Do it — and pray that Regulo over-engineered his products.”

There was a small but perceptible jolt as the two drives cut in. The image of Lutetia did not move on the screen.

“It’s not working, Rob.”

“Give it time. Accelerations take a while before you see the effects.” He was watching a second display, but it blurred as he stared at it. His eyes were refusing to focus. “Good thing we didn’t use that first drive, Corrie. Here comes Regulo, out of the shaft.”

A small, white-suited figure emerged from the exit tunnel closest to the waiting ship.

“He’ll go across to the ship, Rob.”

“Let him. We can’t stop him.”

“What happens if we can’t save Atlantis?”

Rob shrugged. “Tough on us, good for Regulo. He was right, without the Goblins or Caliban there will be no evidence. Even if we escaped, he still has all the money and influence. No one would ever believe me.”

Strain gauge readings from the skin of the aquasphere were well past the safety limits. Under the steady acceleration, a billion tons of water wanted to stay behind.