Vectors

by Charles Sheffield

  By the time that Larry Marston came back with Jen Hasse and Alicia Kano, I doubt it I looked any more cheerful than Arnold Velasquez, down there at Tether Control. I sketched out the problem to the two newcomers; we had what looked like a hopeless situation on our hands.

  "We have seventy-one hours," I concluded. "The only question we need to answer is, what will we be doing at this end during that time? Tether Control can coordinate disaster planning for the position on Earth. Arnold has already ruled out the possibility of any actual help from Earth—there are no rockets there that could be ready in time."

  "What about the repair robots that you have on the cable?" asked Panosky, jumping into the conversation. "I thought they were all the way along its length."

  "They are," said Jen Hasse. "But they're special purpose, not general purpose. We couldn't use one to look for a radioactive signal on a bucket, if that's what you're thinking of. Even if they had the right sensors for it, we'd need a week to reprogram them for the job."

  "We don't have a week," said Alicia quietly. "We have seventy-one hours." She was small and dark-haired, and never raised her voice much above the minimum level needed to reach her audience—but I had grown to rely on her brains more than anything else on the station.

  "Seventy-one hours, if we act now," I said. "We've already agreed that we don't have time to sit here and wait for that bucket with the bomb to arrive—the terrorists must have planned it that way."

  "I know." Alicia did not raise her voice. "Sitting and waiting won't do it. But the total travel time of a carrier from the surface up to synchronous orbit, or back down again, is a little less than a hundred and twenty hours. That means that the bucket carrying the bomb will be at least half-way here in sixty hours. And a bucket that started down from here in the next few hours—"

  "—would have to pass the bucket with the bomb on the way up, before the deadline," broke in Hasse. He was already over at the Control Board, looking at the carrier schedule. He shook his head. "There's nothing scheduled for a passenger bucket, in the next twenty-four hours. It's all cargo going down."

  "We're not looking for luxury." I went across to look at the schedule. "There are a couple of ore buckets with heavy metals scheduled for the next three hours. They'll have plenty of space in the top of them, and they're just forty minutes apart from each other. We could squeeze somebody in one or both of them, provided they were properly suited up. It wouldn't be a picnic, sitting in suits for three days, but we could do it."

  "So how would we get at the bomb, even if we did that?" asked Larry. "It would be on the other side of the Beanstalk from us, passing at a relative velocity of six hundred kilometers an hour. We couldn't do more than wave to it as it went by, even if we knew just which bucket was carrying the bomb."

  "That's the tricky piece." I looked at Jen Hasse. "Do you have enough control over the mass driver system, to slow everything almost to a halt whenever an inbound and an outbound bucket pass each other?"

  He was looking doubtful, rubbing his nose thoughtfully. "Maybe. Trouble is, I'd have to do it nearly a hundred times, if you want to slow down for every pass. And it would take me twenty minutes to stop and start each one. I don't think we have that much time. What do you have in mind?"

  I went across to the model of the Beanstalk that we kept on the Control Room table. We often found that we could illustrate things with it in a minute that would have taken thousands of words to describe.

  "Suppose we were here, starting down in a bucket," I said. I put my hand on the model of the station, thirty-five thousand kilometers above the surface of the Earth in synchronous orbit. "And suppose that the bucket we want to get to, the one with the bomb, is here, on the way up. We put somebody in the inbound bucket, and it starts on down."

  I began to turn the drive train, so that the buckets began to move up and down along the length of the Beanstalk.

  "The people in the inbound bucket carry a radiation counter," I went on. "We'd have to put it on a long arm, so that it cleared all the other stuff on the Stalk, and reached around to get near the upbound buckets. We can do that, I'm sure—if we can't, we don't deserve to call ourselves engineers. We stop at each outbound carrier, and test for radioactivity. There should be enough of that from the fission trigger of the bomb, so that we'll easily pick up a count when we reach the right bucket. Then you, Jan, hold the drive train in the halt position. We leave the inbound bucket, swing around the Stalk, and get into the other carrier. Then we try and disarm the bomb. I've had some experience with that."

  "You mean we get out and actually climb around the Beanstalk?" asked Larry. He didn't sound pleased at the prospect.

  "Right. It shouldn't be too bad," I said. "We can anchor ourselves with lines to the ore bucket, so we can't fall."

  Even as I was speaking, I realized that it didn't sound too plausible. Climbing around the outside of the Beanstalk in a space-suit, twenty thousand kilometers or more up, dangling on a line connected to an ore bucket—and then trying to take apart a fusion bomb wearing gloves. No wonder Larry didn't like the sound of that assignment. I wasn't surprised when Arnold Velasquez chipped in over the circuit connecting us to Tether Control.

  "Sorry, Jack, but that won't work—even if you could do it. You didn't let me read the full message from the terrorists. One of their conditions is that we mustn't stop the bucket train on the Stalk in the next three days. I think they were afraid that we would reverse the direction of the buckets, and bring the bomb back down to Earth to disarm it. I guess they don't realize that the Stalk wasn't designed to run in reverse."

  "Damnation. What else do they have in that message?" I asked. "What can they do if we decide to stop the bucket drive anyway? How can they even tell that we're doing it?"

  "We have to assume that they have a plant in here at Tether Control," replied Velasquez. "After all, they managed to get a bomb onto the Stalk in spite of all our security. They say they'll explode the bomb if we make any attempt to slow or stop the bucket train, and we simply can't afford to take the risk of doing that. We have to assume they can monitor what's going on with the Stalk drive train."

  There was a long, dismal silence, which Alicia finally broke.

  "So that seems to leave us with only one alternative," she said thoughtfully. Then she grimaced and pouted her mouth. "It's a two-bucket operation, and I don't even like to think about it—even though I had a grandmother who was a circus trapeze artiste."

  She was leading in to something, and it wasn't like her to make a big build-up.

  "That bad, eh?" I said.

  "That bad, if we're lucky," she said. "If we're unlucky, I guess we'd all be dead in a month or two anyway, as the recycling runs down. For this to work, we need a good way of dissipating a lot of kinetic energy—something like a damped mechanical spring would do it. And we need a good way of sticking to the side of the Beanstalk. Then, we use two ore buckets—forty minutes apart would be all right —like this . . ."

  She went over to the model of the Beanstalk. We watched her with mounting uneasiness as she outlined her idea. It sounded crazy. The only trouble was, it was that or nothing. Making choices in those circumstances is not difficult.

  * * *

  One good thing about space maintenance work—you develop versatility. If you can't wait to locate something down on Earth, then waste another week or so to have it shipped up to you, you get into the habit of making it for yourself. In an hour or so, we had a sensitive detector ready, welded on to a long extensible arm on the side of a bucket. When it was deployed, it would reach clear around the Beanstalk, missing all the drive train and repair station fittings, and hang in close to the outbound buckets. Jen had fitted it with a gadget that moved the detector rapidly upwards at the moment of closest approach of an upbound carrier, to increase the length of time available for getting a measurement of radioactivity. He swore that it would work on the fly, and have a better than ninety-nine percent chance of telling us which outbound bucket containe
d the bomb—even with a relative fly-by speed of six hundred kilometers an hour.

  I didn't have time to argue the point, and in any case Jen was the expert. I also couldn't dispute his claim that he was easily the best qualified person to operate the gadget. He and Larry Marston, both fully suited-up, climbed into the ore bucket. We had to leave the ore in there, because the mass balance between in-going and outbound buckets was closely calculated to give good stability to the Beanstalk. It made for a lumpy seat, but no one complained. Alicia and I watched as the bucket was moved into the feeder system, accelerated up to the correct speed, attached to the drive train, and dropped rapidly out of sight down the side of the Beanstalk.

  "That's the easy part," she said. "They drop with the bucket, checking the upbound ones as they come by for radioactivity, and that's all they have to do."

  "Unless they can't detect any signal," I said. "Then the bomb goes off, and they have the world's biggest roller-coaster ride. Twenty thousand kilometers of it, with the big thrill at the end."

  "They'd never reach the surface," replied Alicia absent-mindedly. "They'll frizzle up in the atmosphere long before they get there. Or maybe they won't. I wonder what the terminal velocity would be if you hung onto the Stalk cable?"

  As she spoke, she was calmly examining an odd device that had been produced with impossible haste in the machine shop on the station's outer rim. It looked like an old-fashioned parachute harness, but instead of the main chute the lines led to a wheel about a meter across. From the opposite edge of the wheel, a doped silicon rope led to a hefty magnetic grapnel. Another similar arrangement was by her side.

  "Here," she said to me. "Get yours on over your suit, and let's make sure we both know how to handle them. If you miss with the grapple, it'll be messy."

  I looked at my watch. "We don't have time for any dry run. In the next fifteen minutes we have to get our suits on, over to the ore buckets, and into these harnesses. Anyway, I don't think rehearsals here inside the station mean too much when we get to the real thing."

  We looked at each other for a moment, then began to suit up. It's not easy to estimate odds for something that has never been done before, but I didn't give us more than one chance in a hundred of coming out of it safely. Suits and harnesses on, we went and sat without speaking in the ore bucket.

  I saw that we were sitting on a high-value shipment—silver and platinum, from one of the Belt mining operations. It wasn't comfortable, but we were certainly traveling in expensive company. Was it King Midas who complained that a golden throne is not right for restful sitting?

  No matter what the final outcome, we were in for an unpleasant trip. Our suits had barely enough capacity for a six-day journey. They had no recycling capacity, and if we had to go all the way to the halfway point we would be descending for almost sixty hours. We had used up three hours to the deadline, getting ready to go, so that would leave us only nine hours to do something about the bomb when we reached it. I suppose that it was just as bad or worse for Hasse and Marston. After they'd done their bit with the detector, there wasn't a thing they could do except sit in their bucket and wait, either for a message from us or an explosion far above them.

  "Everything all right down there, Larry?" I asked, testing the radio link with them for the umpteenth time.

  "Can't tell." He sounded strained. "We've passed three buckets so far, outbound ones, and we've had no signal from the detector. I guess that's as planned, but it would be nice to know it's working all right."

  "You shouldn't expect anything for at least thirty-six hours," said Alicia.

  "I know that. But it's impossible for us not to look at the detector whenever we pass an outbound bucket. Logically, we should be sleeping now and saving our attention for the most likely time of encounter—but neither one of us seems able to do it."

  "Don't assume that the terrorists are all that logical, either," I said. "Remember, we are the ones who decided that they must have started the bomb on its way only a few hours ago. It's possible they put it into a bucket three or four days ago, and made up the deadline for some other reason. We think we can disarm that bomb, but they may not agree—and they may be right. All we may manage to do is advance the time of the explosion when we try and open up the casing."

  As I spoke, I felt our bucket begin to accelerate. We were heading along the feeder and approaching the bucket drive train. After a few seconds, we were outside the station, dropping down the Beanstalk after Jen and Larry.

  We sat there in silence for a while. I'd been up and down the Stalk many times, and so had Alicia, but always in passenger modules. The psychologists had decided that people rode those a lot better when they were windowless. The cargo bucket had no windows either, but we had left the hatch open, to simplify communications with the other bucket and to enable us to climb out if and when the time came. We would have to close it when we were outside, or the aerodynamic pressures would spoil bucket stability when it finally entered the atmosphere—three hundred kilometers an hour isn't that fast, but it's a respectable speed for travel at full atmospheric pressure.

  Our bucket was about four meters wide and three deep. It carried a load of seven hundred tons, so our extra mass was negligible. I stood at its edge and looked up, then down. The psychologists were quite right. Windows were a bad idea.

  Above us, the Beanstalk rose up and up, occulting the backdrop of stars. It went past the synchronous station, which was still clearly visible as a blob on the stalk, then went on further up, invisible, to the solar power satellite and the great ballast weight, a hundred and five thousand kilometers above the surface of the Earth. On the Stalk itself, I could see the shielded superconductors that ran its full length, from the power satellite down to Tether Control in Quito. We were falling steadily, our rate precisely controlled by the linear synchronous motors that set the accelerations through pulsed magnetic fields. The power for that was drawn from the same superconducting cables. In the event of an electrical power failure, the buckets were designed to 'freeze' to the side of the Stalk with mechanical coupling. We had to build the system that way, because about once a year we had some kind of power interruption—usually from small meteorites, not big enough to trigger the main detector system, but large enough to penetrate the shields and mess up the power transmission.

  It was looking down, though, that produced the real effect. I felt my heart begin to pump harder, and I was gripping at the side of the bucket with my space-suit gloves. When you are in a rocket-propelled ship, you don't get any real feeling of height. Earth is another part of the Universe, something independent of you. But from our position, moving along the side of the Beanstalk, I had quite a different feeling. We were connected to the planet. I could see the Stalk, dwindling smaller and smaller down to the Earth below. I had a very clear feeling that I could fall all the way down it, down to the big blue-white globe at its foot. Although I had lived up at the station quite happily for over five years, I suddenly began to worry about the strength of the main cable. It was a ridiculous concern. There was a safety factor of ten built into its design, far more than a rational engineer would use for anything. It was more likely that the bottom would fall out of our ore bucket, than that the support cable for the Beanstalk would break. I was kicking myself for my illogical fears, until I noticed Alicia also peering out at the Beanstalk, as though trying to see past the clutter of equipment there to the cable itself. I wasn't the only one thinking wild thoughts.

  "You certainly get a different look at things from here," I said, trying to change the mood. "Did you ever see anything like that before?"

  She shook her head ponderously—the suits weren't made for agility of movement.

  "Not up here, I haven't," she replied. "But I once went up to the top of the towers of the Golden Gate Bridge in San Francisco, and looked at the support cables for that. It was the same sort of feeling. I began to wonder if they could take the strain. That was just for a bridge, not even a big one. What will happen if w
e don't make it, and they blow up the Beanstalk?"

  I shrugged, inside my suit, then realized that she couldn't see the movement. "This is the only bridge to space that we've got. We'll be out of the bridge business, and back in the ferry-boat business. They'll have to start sending stuff up by rockets again. Shipments won't be a thousandth of what they are now, until another Stalk can be built. That will take thirty years, starting without this one to help us—even if the Colonies survive all right, and work on nothing else. We don't have to worry about that, though. We won't be there to hassle with it."

  She nodded. "We were in such a hurry to get away it never occurred to me that we'd be sitting here for a couple of days with nothing to do but worry. Any ideas?"

  "Yes. While you were making the reel and grapnel, I thought about that. The only thing that's worth our attention right now is a better understanding of the geometry of the Stalk. We need to know exactly where to position ourselves, where we'll set the grapnels, and what our dynamics will be as we move. I've asked Ricardo to send us schematics and lay-outs over the suit videos. He's picking out ones that show the drive train, the placing of the superconductors, and the unmanned repair stations. I've also asked him to deactivate all the repair robots. It's better for us to risk a failure on the maintenance side than have one of the monitoring robots wandering along the Stalk and mixing in with what we're trying to do."