Donald Moffitt - Genesis 02

impressing the girl. “If most of the mass of the system is in the big disks,” he said belligerently, “how come the sun hasn’t drifted toward one or another of them over a period of time—just as it would toward one side of a ‘Jao’s Bubble’? Excuse me— ‘Jao’s shell. ’ The same thing would apply—the attraction of the other two disks would decrease with distance, and it would keep getting worse!”
    He must have been a physics apprentice. He stole a glance at the girl and went on in a classroom voice. “In a synthetic system like this one, which is essentially three big masses mutually revolving at the points of an equilaterial triangle, a mass occupying the center can’t move above or below the equatorial plane because of the combined pull of the three major components.” He stared a challenge. “But it can and will move within the plane!”
    “I didn’t say most of the mass of the system was in the big disks,” Jao said kindly. “I said the big disks contain most of the planetary mass. Actually, the only mass about the same as a good-size gas giant—maybe a few tenths of one percent of the mass of the G-type star in the middle. So they’re in orbit around it in the normal way.”
    “But that would mean—”
    “Right. We know they’re very thin—maybe as little as fifty miles across the rim. But even so, with a diameter measured in orbital distances, that would give them a volume of maybe four thousand times the volume of your run-of-the-mill gas giant. So they’re lighter than they have any right to be.”
    The boy did some quick figuring in his head. “Four thou—but that would make them lighter than air!”
    “Correct. About three and a third times lighter. In fact, they have a density of only about twice that of helium, on average.”
    “But that’s impossible.”
    “I said on average.”
    Smeth bustled over. “One might posulate that they’re hollow, or honeycombed, or a gas enclosed by a membrane. Or made of a rigid, infinite-length polymer with properties we can’t imagine.”
    “What could be that light—and strong enough to maintain its shape over interplanetary distances?” the boy said.
    Jao stared out the window at the strange floating circles that had taken the place of most of the sky. His face was flushed with excitement.
    When he finally spoke, it was in Bram’s direction. “We’ll have to land on one of them to find out, won’t we, Captain?”
     
    Bram kissed Mim good-bye, feeling self-conscious in front of all the spectators. A crowd of about two thousand was jammed into the cavernous hangar, waiting to see the takeoff, and the rest of the population of the tree must have been watching on their holo sets. Bram could see the camera crew perched high on the spidery platform of an interbranch shuttle vehicle, where they had an overall view.
    “Be careful,” Mim said, pressing herself against the tough hide of his vacuum suit. “I wish you weren’t going this trip.”
    He embraced her one-armed, his bubble helmet tucked under the other arm. “The year-captain’s expected to lead the way,” he said. “That’s why they elect him. But don’t worry. Lydis is the best landing craft pilot we have—and it’s not going to be like landing, anyway. It’ll be more like docking with a nonrotating branch. She’s practiced it in the simulator a hundred times.”
    “But it’s spinning.”
    “So slowly at the rim that it makes practically no difference. You’re thinking in terms of a body like Yggdrasil, with a diameter only a few hundred miles across. In this case, the spin isn’t there to provide gravity. It cancels it. So when we match for it, we’ll touch down as lightly as a leaf.”
    “I’d still feel a whole lot better if I knew you were landing on the flat side.”
    A few feet away, under the skeletal arch of a landing leg, Jao left off nuzzling a clinging Ang looked across her golden head toward them.
    “That’d be a lot trickier, Mim, even though it looks simpler,” Jao boomed past Ang’s ear, making her wince. “Your normal instincts don’t apply on a body as bizarre as that. Neither do your first mathematical assumptions about up and down. Landing anywhere between the hub and the rim on a disk-shaped body would give Lydis some complicated gravitational gradients to cope with. The vertical component and the horizontal component don’t behave the same way in relation to the center of gravity. And then there’d be the added