Threshold: Chapter Sixteen

    "So how much power are we talking about here?" Jackie leaned forward, studying the modifications which reflected the wide-flung rings that had been visible behind Odin when the huge E.U. vessel was approaching. "Accelerating a ten thousand ton vessel at a hundredth of a g is a million newtons. That’s not so much. But I know that building the driver for the mass-beam was a major effort and it uses a lot more than that."

    Horst Eberhart smiled. "You know the NERVA drive well, and some others, but I’m guessing not so much the mass-beam?"

    She glanced at him sharply, but then saw that the smile was a purely friendly one, the smile of one engineer to another saying: I’ve got some cool stuff to show you! It was not at all sarcastic or derisive.

    And that’s what he’s here for, she reminded herself. Whatever cloak-and-dagger might be going on, here at least they could be straightforward. And that’s a really, really nice smile. "No, I didn’t study anything on mass-beams except the basic concept: throw stuff from way over here to hit something there, to push it on its way."

    "Yes, that’s the basic idea. But as you say, the devil is in the details. That million newtons is what has to actually be delivered to the ship."

    "Inefficiency in the driver makes that larger in actual cost?"

    He shook his head.

    Jackie frowned. "No, I’m missing a fundamental issue, and I know I’m going to kick myself when you tell me."

    "Probably. I did." He activated a little animation showing a simplified mass-beam driver and a cartoon ship. The driver threw little balls at the ship and bounced them off a plate on the ship’s base. "See, I throw the mass at the ship, it bounces off, ship moves forward. Keeps moving forward. Then I have to throw next mass, but ship is moving faster now. The best momentum transfer is when the speed of the mass is about twice the speed of the ship, so to make the best transfer throw my mass must be a little faster this time—"

    Jackie smacked herself on the forehead. "Of course! When you start the ship going, you only have to accelerate the particles to twenty centimeters per second, but by the time you hit thirty kilometers a second you need to be throwing them at sixty kilometers a second. Basically since you’re using them as the ship’s fuel, you have to do what the ship would have done if it was carrying them along—spend the energy to first accelerate the fuel to your speed, then give it that extra oomph for the hundredth-G. So that means that you’re using..." She did some quick calculations. "Wow! I get over a hundred gigawatts constant?"

    "You are good," Horst said, his tone very respectful. "You picked up on the whole thing much faster than I did. I can’t tell you the exact numbers for our assembly, because that’s restricted information. Which is silly, in my opinion, since if we push Odin to the limit you will know the numbers anyway."

    "So you could actually get a little more acceleration speed out of it to begin with by cranking up the wattage, though it’d be less efficient."

    "Not much point, though. Constant acceleration builds up quickly enough. Good for launching things and getting them out of the way, I suppose."

    "How exactly do you stop, though? It’s like a solar sail that way."

    He shook his head. "Not quite, Ms. Secord."

    "Please, call me Jackie."

    He flashed a white-toothed grin. "And please call me Horst. The idea is to build many collectors and accelerators and put them around the solar system, eventually. But if you know where you are going, you can send some slow stuff ahead of you. Catch as much of the regular beam as you can to store for fuel, use that, then—"

    Jackie laughed. "Oh, that’s clever! You can either bounce the particles off, or catch them as fuel. So after you empty your tanks on the first decel, you start slowing down using the slower cloud of stuff sent before you left. That refills your tanks, so you can do another stop burn, after already doing one and having the slow cloud slow you up more. Hmmm... and the slow cloud can be more concentrated—massive—because you have the energy to send the stuff faster. So you send more stuff slower. There’d be a lot of fancy tradeoffs, but I can see that would work nicely. How do you keep the beam focused, though? At a few hundred million miles I’d think it would be, well, a lot of kilometers across."

    "I thought that too. But look, here is an image of one of the fuel particles."

    The image that popped up was surprisingly recognizable. "Faerie Dust? You’re sending tons of Faerie Dust?"

    "Faerie…? Oh, yes, Mr. Baker’s nickname for it. Not nearly so complex. It has to be simple. Tons of fancily-designed material would be expensive, and you need many, many tons. There is a special plant dedicated to the whole operation. Very simple overall design, with just enough capability to home on target signal."

    "Signal? Oh, I see. You use something like a laser pointed towards the driver, and the particles use that as a homing beacon; they can use sunlight and the solar wind to steer themselves towards you. Very nice. And they’ll really concentrate down this far?" She indicated the large ring designs, which extended to a maximum diameter of about four times Nobel’s habitat ring, or somewhat over a kilometer across.

    "Yes. Not a problem. The swarm of particles tries to stay concentrated. Uses very little power and only needs small corrections every few hours."

    Jackie studied some of the parameters. "It’s still going to be expensive. I find it hard to believe so much has happened that makes all this possible."

    "A few years makes a big difference. You left Mars two years ago for Ceres."

    "God. It really has been that long." She shook her head. "If they’re making superconductor cable that large, it’ll be a great investment. Right now I guess your accelerator must spend a lot of time idle, so if other people were paying you for it that would defray costs."

    He shrugged, something very noticeable with his wide shoulders. He seemed far more athletic than the average engineer. That piqued Jackie’s interest, which had already been aroused. She was quite athletic herself and always had been.

    "The money side of it doesn’t interest me," he said, "but I would guess so. Do you want to go over installation details?"

    "No, not right now, Horst. I know more than enough now to be able to inform Dr. Glendale and let him know that I think it’d be a great idea. Until I find out whether we’re going for it, details would just be more of a tease than anything else. Besides, we’ve been at this for five hours."

    He blinked. "Really that long?"

    "Check for yourself." An impulse came to her, which she began to stifle from automatic reflex. Then...

    Well, why not?

    "We could quit for the day. Nobel just finished downloading our copies of the new films you brought with you. Would you like to watch one with me?"

    She bestowed her own white-toothed grin on him. Which, she knew, was quite a gleaming affair, especially against her complexion. Her mother had been Indian in her ancestry—American Indian, mostly Choctaw. Judging from the evidence at least some of that ancestry had been of African origin, too, although that showed in her dark skin, not her features.

    Horst Eberhart’s eyes widened, and for a minute she thought he’d refuse. But then she realized his hesitation was simply the natural shyness of someone who, like herself, was normally cautious in personal relations. She thought of it as the Cursed Wallflower Syndrome.

    "Well… yes," he said. "I would enjoy that very much."

    She shut down the engineering station and floated up, grabbing his arm. "Then let’s be off." She felt a delicious pang of self-conscious worry and anticipation she hadn’t felt in years.