She was in an unusually cheerful and fickle mood. That was all the response that Rob could get. She replied to all his questions with cryptic, evasive answers, while the cruiser bore them steadily closer to Atlantis.
Rob remained peering into the scope, seeking more details of the mystery asteroid ahead. Following Regulo’s work, it had become a sphere of water rather less than two kilometers across. It was surrounded by a restraining membrane of tough flexible plastic, a trapping surface for solar heat. The aquasphere was pierced by twenty metal-lined shafts that served as structural braces and also provided access from the exterior of the asteroid to the central metal sphere where living quarters and laboratories were located. Other entry to the two-hundred-meter central biosphere came from the ports that connected the living quarters to the aquasphere. As they drew closer, Rob could see the silver gleam of heavy drive equipment positioned near the outer edge of each entry shaft. The whole ponderous assembly was rotating slowly about its center of mass. Small attitude jets positioned at a number of points on the surface showed how the rotation rate was controlled.
“I thought you were just joking about getting away from a nova,” said Rob. “Now, I’m not so sure. There are drives all over that thing and they look like big ones. Do you know what sort of acceleration he can get on it?”
Corrie was busy at the communicator, tuning in for their final arrival. “Not much at all,” she said. “There’s plenty of power, but the limiting factor is the strength of the support shafts and the surface membrane around the aquasphere. They take the main stresses when Atlantis is accelerated. The interior is nearly all liquid water, even allowing for the support shafts and interior structures. You need monster drives for any acceleration worth speaking of, because Atlantis masses about four billion tons. That takes some shifting. Regulo usually doesn’t try for more than a hundredth of a gee. He gets around, but it takes a while.”
They were creeping closer to one of the entry shafts, their angular rate matching to that of the asteroid. Close up, the surface had a dull, smooth finish, making Atlantis visible only as a black mass occulting the bright star field behind it.
“No wonder I couldn’t pick it up on the screen,” said Rob. “The surface is sitting there in full sunlight but there’s no radiation back-scatter at all. At least, there’s not enough to see.”
“There should be hardly anything at visible wavelengths.” Corrie was sitting next to him as they awaited final docking. “Morel designed it that way. The aquasphere has been made into a self-sustaining community of plants and animals. It uses all the light that it can get for photosynthesis. That’s why Regulo and Morel covered it with variable albedo materials. Nothing is reflected as visible light, and all the heat goes out through the side facing away from the Sun.”
“Sounds like a violation of the Second Law of Thermodynamics to me.” Rob was impatiently peering out of the side port, waiting for a glimpse of the interior. “You’re telling me I won’t be able to see anything at all from out here, then?”
“That’s right. Wait until we get inside, then you’ll see plenty. You can even take a swim through the interior if you want to.” She grinned at some secret joke. “I somehow doubt that you will. I certainly never have. I should have warned you of one other thing: be prepared for a fishy dinner. Regulo imports food when he feels like it, but he makes the point to new arrivals that he has a completely closed ecology operating in Atlantis. The human living quarters in the center are part of the overall balance, with reprocessed wastes going back into the aquasphere as nutrients. Of course, you lose a little mass when you move around the System, but Regulo replaces that occasionally from other asteroids.”
“Does Atlantis have any internal power sources? Big ones, I mean, to provide power and light.”
“There are a couple of fusion plants, and Regulo talks of adding a power kernel. Why?”
“I was thinking of your statement that Regulo hates the Sun. With this set-up, he’s independent of it. He could provide the light for photosynthesis in the aquasphere from his own power sources, and if he did that he could go as far away from the center of the System as he chose — out beyond the Halo, if he feels like it.”
“He’s talked of it; but he likes to know what research is going on, in the Belt and back on Earth. If it weren’t for that, I think he might take Atlantis a long way out. Maybe leave the System completely.” There was a slight bump, felt through the floor of the ship. “Feel that? We’re docked. We can go inside now. Regulo doesn’t believe in elaborate entry procedures. Anybody that he doesn’t want in Atlantis would never get this far. His computers will have checked the signature of this ship against the System ship listings when we were still a hundred thousand kilometers out.”
She stood up and led the way out of the main cabin and through the lock. The rotation rate of Atlantis was low, barely enough to give a feeling of weight. The ship had docked at the exterior surface of the asteroid, on the “equator” farthest from the axis of rotation of the sphere. A flexible umbilical led to the entry shaft. It had been attached automatically as the ship docked. As they passed into the main shaft, baffles sealed it behind them. Within thirty seconds the atmosphere in the interior was up to half a standard atmosphere, oxygen-rich and matched to that of the ship they had just left. Rob followed Corrie as she pulled herself easily along the broad, dark tunnel that led to the central metal sphere. About halfway along they halted at a second lock and removed their suits. Once they were ready to go on, Corrie led Rob to the side of the tube.
“I think I can show you something to match your Coal Moles,” she said. “You know, Morel and Regulo built a complete water-world here, and this is one of the viewing ports. You’ll find the same sort of thing all over the inner sphere. Take a look through there.”
She pointed to a transparent panel about two meters across set into the side of the lock chamber. Rob went to it and looked out. It took a few seconds to become accustomed to the scale and distance of what he was seeing. Then he grunted with surprise and leaned closer to the panel.
The water that filled the interior of Atlantis was very clear. He could see for at least a hundred meters into a green, shady interior, filled with huge and abundant plant growth. It clustered around a complex supporting grid in the form of a symmetrical series of spherical frames, like concentric shells. Between the spheres of vegetation, far away into the dim light, moving shapes were faintly visible. In rainbow colors, they turned, darted, or cruised lazily among the curtains of floating plant life. At the limit of vision, Rob fancied that he could see the phantom outline of something much bigger, a dark irregular shape outlined against the lighter green-blue background. As he watched, it drifted farther off and merged into the fronded luxuriant weeds.
He turned back to Corrie. “That looks like a fresh-water ecology out there, but I could swear that I’m seeing forms that only live in salt water back on Earth. Is it fresh, salt, or what?”
“It’s all fresh water. There was no easy way to find a mass of salt where and when they wanted it. They discovered salt deposits later on some of the asteroids, but by that time they were committed to most of the biological forms.” Corrie again began to lead the way toward the central structure. “You’re quite right about the mixture of life-forms. That has been one of Morel’s interests. Over the past twenty-five years he has been developing marine animals that can stand the transition from salt water to fresh. You’ll see how successful he has been when you have an opportunity to examine the aquasphere. It wasn’t an easy problem. Morel had to do a good deal of genetic engineering before he was satisfied with most of them.”