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, they 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 and 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 factor of centrifugal force tending to make us slide outward, though we don’t think it’d exceed the diagonal gravitational vector tending to pin us down. To say nothing of all sorts of unpredictable edge effects to get past before we could cross to those interesting structures on the rim. No, Mim, this is the simplest way. We’ve got it all worked out.”
Bram had felt Mim stiffen at Jao’s mention of “sliding outward” and “edge effects.” He turned it into a joke. “What Jao’s really worried about is having to hike across a ninety-million-mile plain to get to where we’re going.”
She smiled gamely. “I guess I don’t understand physics.”
“That’s all right, Mim, I don’t understand Bach,” Jao said.
“Don’t worry, we’ll be very careful,” Bram told her, “and we’ll be locked in to Jun Davd and his computer the whole time.
The third member of the exploration team, a dour geologist, named Enry, pushed his way through the well-wishers and said apologetically to Bram, “Lydis says she’s about ready. Says it’s time to get these people out of here and climb aboard.”
Enry stood there, stolidly waiting. He was a blocky, square-jawed man who long ago had been a touch associate with a geology touch group on the Father World. Though the Father World no longer existed, Enry had never given up his speciality; he pored over the old Nar records in the library and published a monograph every quarter century or so. He was the nearest thing to an expert the tree possessed, and he handled himself well physically in the null-gravity sports at the trunk’s center. Bram had thought of him immediately when choosing the exploration team.
“All right,” Bram said. He gave Mim a final peck that turned into something more as their lips touched again, then went with Enry to pry Jao loose from Ang.
A warning blast came from a two-tone bass whistle. Exasperated monitors wearing headbadges rushed back and forth, trying to shoo lingering spectators out of the drop area.
“Behind the ropes, behind the ropes! Everybody behind the ropes! Other side of the air curtain track!” The crowd moved as sluggishly as sap. “Keep it moving, keep it moving, unless you want to breathe vacuum!”
Bram got Enry and Jao started up the landing leg ladder with their gear and was preparing to climb it himself when he became aware of a disturbance at the fringe of the retreating crowd. A small, agile figure was darting past the monitors, getting chased by them, and darting back into the forbidden area. The interloper evaded a pursuer and made a beeline for the base of the ladder.
Bram saw corn-yellow hair flying and green eyes on either side of an upturned nose and recognized his great-great-great-granddaughter. “Ame!” he exclaimed. “What are you doing here?”
“I’m going with you,” she announced. “Here, take this.”
She unslung a lumpy shoulder bag and thrust it at him. The clinking sound of some kind of equipment came from within.
“You can’t,” he said. “We don’t know what we may run into. Anyway, it’s only a scouting trip. You’ll have plenty of opportunities to go along on the later landings, like everybody else.”
“That’s the point, Bram-tsu. You ought to have a palentogist along on your first survey, and I’m the only authentic specimen you’ve got.” She grinned engagingly at him. “Besides, I’ve turned myself into a pretty fair geologist, so I’ll carry my weight.”