Copyright © 1983, by David Brin. All rights reserved. No duplication or resale without permission.

     Imagine six very long parallel wires, hanging in space, always aimed toward the surface of the Earth 500 kilometers below.
     At both ends the wires are anchored to flat rows of giant cylinders -- forty in the upper layer, A Deck; and sixteen in the lower, B Deck. An elevator, consisting of two welded tanks, moves between the two ends, carrying people and supplies both ways.
     I've lost count of the number of times I've explained the curious structure to visitors. I've compared it to a double-ended child's swing, or a bolo turning exactly once always high. It's been called a skyhook, and even a bean-stalk, though the idea's nowhere near as ambitious as the ground-to-geosynchronous space-elevators of science fiction fame.
     The main purpose of the design is simply to keep the tanks from falling. The two massive ends of the Farm act like a dipole in the gradient of the Earth's gravitational field, so each deck winds up orbiting edge-forward, like a flat plate skimming. This reduces the drag caused by the upper fringes of the atmosphere, extending our orbital lifetime.
     The scheme is simple, neat, and it works. Of course the arrangement doesn't prevent all orbital decay. It takes a little thrust from our aluminum engines, from time to time, to make up the difference.
     Since our center of mass is traveling in a circular orbit, the lower deck has to move much slower than it "should" to remain at its height. The tethers keep it suspended, as it were.
     The upper deck, in turn, is dragged along faster than it would normally go, at its height. It would fly away into a high ellipse if the cables ever let go.
     That's why we feel a small artificial gravity at each end, directed away from the center of mass. It creates the ponds in my garden, and helps prevent the body decay of pure weightlessness.

     We had our meeting in the lounge. Susan Sorbanes, our business manager, took her place to my left, Don Ishido to my right. There were no chairs, but we stood at rest in the feeble gravity, a table made of spun aluminum fibers between us and the federal officials. Our backs were to the giant quartz window.
     Across the table, Colonel Robert Bahnz, the new DOD representative, floated impassively. He had said hardly a word, apparently content to leave the talking to Henry Woke, the NASA official who had come up in Pacifica with him. Bahnz stood at a slight angle, which had to take a certain amount of work. Was it his way of showing his contempt for the Tank Farm's famous gravity, so unlike the free-fall conditions in the government's shiny little Space Stations?
     "So you people have decided to hit us on two fronts at once, Dr. Woke?" Susan spoke softly, but her voice had a cutting edge. "You're going to attack the Farm's man-rating, and you're cutting back on our share of the residual propellants and water."
      Woke was a middle-aged bureaucrat who must have convinced himself long ago that space visits were a route to advancement in NASA. I could tell by his faint green pallor that he was doped up against space sickness.
     "Now, Dr. Sorbanes," he said. "Safety's been an issue ever since a crewman fell from B Deck two years ago. As a quasi-federal institution, Colombo Station must adhere to man-rating policy. That's all we are interested in."
     "We've had a good safety record for ten years, except for that one incident," Susan replied. "And Congress gave us exemptions back in '89, you'll remember."
     "Yes, but those exemptions expire this year. And I think you'll find this Congress less willing to take chances with the safety of its citizens in orbit."
     "I don't see why we have to go the gold-plated route NASA and DOD used in the Space Stations," Susan said acidly. "All that approach accomplished was to slow you down by a decade, and almost turn the country off on space for good!"
     Woke shook his head. "Perhaps, Dr. Sorbanes. Indeed, it's because NASA has seen the value of the Tank Farm approach that we had last year's unfortunate misunderstanding regarding tank deliveries. Since Stations Two and Three began operating their own propellant recovery units and aluminum smelters, we've found that we need the leftover tanks as much as you do. We're all going to have to share. That's what it comes down to."
     Don Ishido shook his head. "That's a load of bull! Our contract only guarantees us a third of the tanks launched, in return for which we use the slingshot effect to boost government and commercial cargoes into higher orbits, and provide shuttles like Pacifica with temporary angular momentum loans. That leaves you with two thirds of the tanks to do with as you wish!
     "Let's face it. It's not the tanks that are causing the problem. It's you stealing our water!"
     I cleared my throat. It was time to step in, before this broke down completely.
     "I think what Mr. Ishido means, Dr. Woke, is that Colombo Station depends on delivery of at least fifty tons of residual propellants a year, for life support, chemistry, and especially to provide oxydizer for our aluminum engines. Without those engines, our orbit will decay, and we'll be forced to use the extremely inefficient method of flinging away tanks to maintain altitude. The Farm will cease accumulating mass, and our value to our investors will disappear ... this just as we were about to show a real profit for the first time."
     Woke shrugged. "Of course we have no intention of cutting off the water and oxygen you need to maintain life support. No one even considered such a thing."
     Damn right, I thought. Nothing would alienate the public like that. But trimming our ration, forcing us to spend tanks as fast as we get them -- they could pull that off without trouble.
     Yeah. We had almost closed a deal with some big Earthside chemical houses to produce large amounts of low-g biochemicals on B Deck, when NASA Station Two undercut us by $2 million. But the killer had really been the rumors over our water situation. The investors had shied away from the uncertainty.
     It hurt like hell. We were just short of making it. We had gobs of solar power, but the Earthsiders couldn't agree on how to receive it. With water and our giant tanks we could run a tremendous chemical plant, but timid companies stopped just short of buying in. We'd planned to set up a space hotel and sell vacations for scores of tourists at a time, but we were stymied by this "man-rating" straw man.
     Our ecological recycling system had us ninety-five percent independent of Earth resupply. Our smelter was operational and waiting for customers: We had developed the aluminum engine.
     But all anyone wanted to buy was the slingshot effect. We were a glorified switching yard in orbit. And the new government clearly wanted us to stay just that.
     Woke kept up his soothing apologia. I had heard it all before. I wasn't the one to fight him, anyway. That was up to our lawyers back in Washington. My job was to come up with miracles. And right now they appeared to be in short supply.

     The crewcut DOD man, Bahnz, was staring at something over my shoulder. I shifted a little to look.
     Out on A Deck they were readying a Defense cargo for launch. They had peeled away the blue shrouding and set the cylinders near the edge of the deck. At the right moment the package would slip off into the starry field below us, falling away from Earth in a steep ellipse. At apogee a motor would cut in, carrying the spysat the rest of the way to geosynchronous orbit.
     Bahnz had a gleam in his eyes as he observed the preparations.
     You want my Farm, don't you? I thought. You peepers fought us in the beginning, but now you see we're the one thing keeping us ahead of the other nations in space. Now you want my Tank Farm for your own.
     Two years ago, they had tried to get us to store "strategic assets" in the A Deck tanks. I threatened to resign, and the Foundation found the guts to refuse. That's when the troubles had started.
     Bahnz noticed my look, and smiled a knowing smile.
     He thinks he holds all the aces, I thought. And he might be right.
     There were some old SF stories I read when I was a kid, about space colonies rebelling against Earth bureaucracies. I had a brief fantasy of leading my crew in a "tea party," and kicking these two jerks off our sovereign territory.
     Bahnz saw the peaceful smile on my face, and must have wondered what caused it.
     Of course the rebellion idea was absurd. It wasn't what any of us wanted, and it wasn't practical. We might be ninety-five percent free of Earth logistical support, but that last few percent would be with us for a hundred years. Anyway, without either water or new tanks every year, Mother Earth's atmosphere would quickly pull us down.
     While Don and Susan kept our side of the charade, I looked out the window, thinking.
     Next year would be solar maximum, when the coronal ion wind would come sleeting in from the active sun. The upper atmosphere would heat up and bloat outward, like a high tide dragging at our knees. At solar max we could lose twenty kilometers of altitude in a single year. Maybe much more.
     Our investors would be caving in within eighteen months. Even the Italians would soon be begging the U.S. administration to make a deal.
     For an instant I saw the Earth not as a broad vague mass overhead, but as a spinning globe of rock, rushing air, and water, of molten core and invisible fields, reaching out to grapple with the tides that filled space. It was eerie. I could almost feel the Tank Farm, like a double-ended kite, coursing through those invisible fields, its tethers cutting the lines of force -- like the slowly turning bushings of a dynamo.
     That was what young Emily Testa had compared it to. A dynamo. We could draw power from our motion if we ever had to -- buying electricity and paying for it in orbital momentum. It was a solution in search of a problem, for we already had all the power we needed.
     The image wouldn't leave my mind, though. I could almost see the double-ended kite, right there in front of me ... a dynamo. We didn't need a dynamo. What we needed was the opposite. What we needed was ...
     "I think we should recess," I said suddenly, interrupting Dr. Woke in the middle of a sentence. It didn't matter. My job wasn't diplomacy. It was miracle-working.
     "Susan, would you show our guests to some rooms? We'll all meet again over supper in my cabin, if that's okay with you gentlemen?"
     Woke nodded resignedly. I think he had hoped to go back down right away in Pacifica. Colonel Bahnz smiled. "Dr. Rutter, will you be serving Slingshot with dinner?"
     "It's traditional," I replied, anxious to get rid of the man.
     "Good. It's one of the reasons I came up today." Bahnz's grin seemed friendly enough, but there was an undertone to his voice that I understood only too well.
     I waited until they had left, then turned to Ishido. "Don, go fetch Emily Testa and meet me in the power room in five minutes."
     "Sure, chief. But what ...?"
     "There's something I want to try. Now shake a leg!"
     I kicked off down the hallway, looking for a computer terminal. I don't think I touched the floor twice in fifty yards.