Donald Moffitt - Genesis 02

gravitating here even though Yggdrasil’s slow rotation periodically turned the scenery on its head. The holo still ran at the opposite end of the lounge, but even though it showed close-ups, there was no added detail to make it worth watching.
    “It works out to an ingeniously timed energy trap,” Jao was burbling to anyone who would listen. His burly form was at the center of a knot of people, the nearer ones in danger of getting clipped by his waving hands.
    “Listen to him,” Smeth grumbled to Bram. “You’d think he was taking credit for it himself. It’s nothing like the continuous bubble he theorized about.”
    “… though the timing’s decayed somewhat after seventy-four million years,” Jao went on. “Otherwise, we wouldn’t be seeing the disks by so much leaked light, and we never would have seen the star itself.”
    A pretty admirer who must have stretched Jao’s uxoriousness to the limit spoke up. “I know you explained it before, Jao, but it’s awfully confusing. It gives me a headache just to think about it.”
    “It’s beautiful, beautiful!” Jao boomed. “Look, there are four shells of disks—an outer and inner shell in equatorial orbit, and an outer and inner shell in polar orbit. The polar shells are the itsy ones on the inside, and their main job is simply to reflect all radiation into the equatorial plane.”
    “I understand that, but …”
    “Each shell consists of three disks whose diameters are equal to the radii of their distance from the sun. Actually, it’s their centers of gravity that’re in that orbit. But they don’t swivel. Each one of them has exactly enough spin to make its day equal to its year, so that the flat side always faces sunward.” He frowned. “Except that one of the inner ones once got a nudge from something that messed up its synchronization—probably a solar flare. That’s why the sun was able to pop out in the equatorial plane when we were in the cometary belt.”
    “But why don’t they all just crash into one another?” his admirer wailed.
    “Look—each set of three consists of disks whose centers of gravity are at the points of an equilateral triangle, thus occupying the same orbit in a state of equilibrium. It’s a very stable arrangement. And the fact—now, get this—the fact that the diameter of each disk is equal to the radius of its orbit means that the zone of interception of the inner set is equal to the zone of interception of the outer set—so that when you project that cone, it’s like having a solid fence of six disks, all tangent to each other.” He smiled benignly. “Except that you don’t have to worry about them crashing together. To say nothing of all the mass you save.”
    “By using littler disks?”
    He nodded. “Almost all the mass of the energy trap is in the three big ninety-million-mile disks. The next shell inward—the one that orbits at thirty-six million miles— contains only about a sixth as much mass. Call it four twenty-fifths. And the ratio holds as you keep diminishing—so that all of the inner disks put together add up to less than the mass of one more big disk. Original Man was very clever. He made his fence out of geometry instead of mass.”
    “It’s a fence with a lot of gaps, though, isn’t it?” said a smart aleck who looked as if he were the boyfriend of the girl or aspired to be.
    “Not as much as you think,” Jao said indulgently. “Let’s figure it out. Hey, Smeth, what’s the formula for a hy-pocycloid—never mind, I’ll graph it.”
    He grabbed for the touch pad he had dangling from a chain around his neck and poked at it with thick fingers. An electric-blue circle grew on its photoplastic surface, followed by a horizontal line that bisected it, then two curves with the same radius as the original circle that sprouted from the ends of the line and met at the top. Little boxes began to subdivide the resulting figures, getting smaller and smaller until the eye could no longer separate them. The negatively curved triangle in the center differentiated itself with a change of color. Jao’s fingers asked the touch pad a couple of questions, and he read off the answer.
    “Yah,” he said. “The equatorial fence intercepts about seventy percent of the solar energy that comes its way. So does the polar shell. Together they cover somewhat more than one-fifth of the surface of an imaginary sphere enclosing the sun at any radius. I guess that was good enough to do the