She hooked a safety line around a pipe that went from the carousel hub to the airlock, and faced one of the last panels. It was
silver, a meter square, made of two sheets of thin foil sandwiched together. She touched the screwdriver to one corner and the device clucked as it found the slot. The counterweight rotated. It gulped the loose screw before it could drift away.
Three more times and the panel floated away from the layer of anti-meteorite foam beneath. Cirocco held it and turned to face the sun, conducting her own informal puncture survey. Three tiny, bright lights marked where the sheet had been hit by grains of meteoritic dust.
The panel was held rigid by wires along the edges. She bent two of these in the middle. After the fifth fold it was small enough to fit in the thigh pocket of her suit. She fastened the flap, then moved to the next panel.
Time was at a premium. Whenever possible they combined two chores, so the end of the ship's day found Cirocco reclining on her bunk while Calvin gave her a weekly physical and Gaby showed her the latest picture of Themis. The room was crowded.
"It's not a photo.," Gaby was saying. "It's, a computer- enhanced theoretical image. And it's in infra-red, which seems to be the best spectrum."
Cirocco raised herself on one elbow, careful not to dislodge any of Calvin's electrodes. She chewed on the end of the thermometer until he frowned at her.
The print showed a fat wagon wheel surrounded by broad- based, bright red triangular areas. There were six red areas on the inside of the wheel, but they were smaller, and square.
"The big triangles on the outside are the hottest parts," Gaby said. "I figure they're part of the temperature control system. They soak up heat from the sun or bleed off the excess."
"Houston already decided that," Cirocco pointed out. She glanced at the television camera near the ceiling. Ground control was monitoring them. If they thought of something Cirocco would hear of it in a few hours, asleep or not.
The wheel analogy was almost literally true, except for the heating or cooling fins Gaby had indicated. There was a hub in the center, and it had a hole which could have taken an axle if
Themis had actually been a wagon wheel. Radiating from the hub were six thick spokes which flared gradually just before joining the outer portion of the wheel. Between each pair of spokes was one of the bright, square areas.
"This is what's new," Gaby said. "Those squares are angled. They're what 1 originally saw; the six points of light. They're flat, or they'd scatter a lot more light. As it is they only reflect light to Earth if they're at just the right angle, and that's rare."
'What kind of angle?" Cirocco - lisped. Calvin took the thermometer out of her mouth.
"Okay. Light comes in parallel to the axis, from this angle." She moved an extended finger toward the print. "The mirrors are set to deflect the light ninety degrees, into the wheel roof." She touched the paper with her finger, turned the finger, and indicated an area between two spokes.
"This part of the wheel is hotter than the rest, but not so hot that it could be soaking up all the heat it gets. It's not reflecting it or absorbing it, so it's transmitting it. It's transparent or trans- lucent. it lets most of the light go through to whatever's underneath. Does that suggest anything to you?"
Cirocco looked up from her careful examination. "What do you mean?"
"Okay. We know the wheel is hollow. Maybe the spokes are, too. Anyway, picture the wheel. It's like a car tire, big and fat and flat on the bottom to give more living space. Centrifugal force pushes you away from the hub."
"I've got all that," Cirocco said, slightly amused. Gaby could he so intense when explaining something.
"Right. So when you're standing on the inside of the wheel, you're either under a spoke, or under a reflector, right?"
"Yeah? Oh, yeah. So-" "So it's always either daytime or nighttime at any particular spot. The spokes are rigidly attached, the reflectors don't move, and neither can the skylights. So it has to be that way. Permanent day or permanent night. Why do you think they'd build it that way?"
"To answer that, we'd need to meet them. Their needs must he different from ours." She looked back at the picture. She had to keep reminding herself of the size of the thing. Thirteen hundred meters in diameter, 4000 around the outer rim. The prospect of meeting the beings who built such a thing was worrying her more each day.
"All right. 1 can wait." Gaby was not that interested in Them- is as a spacecraft. To her it was a fascinating problem in observation.
Cirocco again looked at the picture.
"The hub," she began, then bit her lip. That camera was still running, and she didn't want to say anything too hastily.
"What about it?"
"Well, it's the only place you could dock with the thing. The only part that's motionless."
"Not the way it is now. That hole in the middle is pretty big. The first time you reach anything solid, it's moving at a pretty good clip. 1 can calculate-"
"Never mind. It's not important right now. The point is, only at the very dead center of rotation could you dock with Themis without a great deal of trouble. 1 sure wouldn't want to try it."
"So? "
"So there must be a compelling reason why there's no docking facilities visible there. Something important enough to sacrifice that location, some reason for leaving a big hole in the center."
"Engine," Calvin said. Cirocco glanced at him, got a glimpse of his brown eyes before he turned back to his work.
"That was my thought. A real big fusion ramscoop. The machinery is in the hub, electromagnetic field generators to funnel the interstellar hydrogen into the center, where it gets burned."
Gaby shrugged. "Makes sense. But what about docking?" "Well, leaving the thing would be easy enough. just drop out a hole in the bottom and get escape velocity for free, plus some to fool around with. But there ought to he some sort of dingus that would telescope out to the center of rotation when the engine isn't running, to pick up scout ships. The main engine has to he there. The only other way would be to space engines around the rim. I'd want three, at least. More would be better."
She turned to face the camera. "Send me what you can about hydrogen ramscoop engines," she said. "See if you can give me some idea of what to look for if Themis has one".
"You'll have to take your shirt off," Calvin said.
Cirocco reached up and switched off the camera, leaving the sound on. Calvin thumped her back and listened to the results while Cirocco and Gaby continued to study the picture of Themis. They came up with no new insights until Gaby brought UP the matter of the cables.
"As far as 1 can tell, they form a circle about midway between the hub and the rim. They support the top edges of the reflecting panels, sort of like the rigging on a sailing ship."
"What about these?" Cirocco asked, indicating the area between two of the spokes. "Any idea what they're for?"
"Nope. There's six of them, and they run midway between the spokes from the hub to the rim, radially. They pass through the reflects panels, if that tells you anything."
"Not exactly. But if there's any more of these things, maybe smaller ones, we should look for them. These cables are about- what did you say? Three kilometers around?"
"More like five."
"Okay. So one that's just a tiny thing-say about as big around as ringmaster-might be invisible to us for a long time, especially if it's as black as the rest of Themis. Gene's going to be nosing around there in the SEM. I'd hate for him to hit one."
"I'll get the computer on it," Gaby said. Calvin began packing his equipment.
"As disgustingly healthy as usual," he said. "You people never give me a break. If 1 don't try out that five-million-dollar hospital how am 1 going to make them believe they got their money's worth?"
"You want me to break somebody's arm?" Cirocco suggested.
"Nah. I already did that, back in medical school."
"Broke one, or fixed it?"
Calvin laughed. "Appendix. Now there's something I'd like to try. You don't hardly get busted appendixes anymore."