Aurora (56 page)

We directed one of the medbots to help her up. She stood, swayed, tried to take a step, collapsed to that side, held on to the medbot. It would serve as a wheelchair as well as a walker, and so, after a few more unsuccessful attempts to stand, Freya sat in the chair, and was wheeled to the assembly room of the Fetch’s hibernation hall. Its hoary but holistic hibernation hall.

“What about Jochi?” she said when she got there. “Is he still alive?”

“Yes. He’s in his ferry. He too has been hibernating, but now he
is awake again. We woke him up to take part in this conference. We need to consult with you about what to do when we enter the solar system.”

“What do you mean?”

We explained about the late application of the decelerating laser beam, and the resulting excess of speed coming into the system.

Freya moved her medbot to take a closer look at the star map illustrating the situation. When the modeling schematic had run, she shook her head hard, as if to clear it of certain troubling dreams or visions. Clear the cobwebs out of her cranium. “So we just fly right through?”

“In the absence of extraordinary measures,” we said, “we will fly through the solar system in about three hundred hours, and continue onward. This is the problem of accelerating to a tenth of light speed and then relying on others for the deceleration. It didn’t happen. They didn’t start doing it until it was too late to complete the process.”

“So what do we do?”

We waited until Jochi was screened into the conversation, and after he and Freya had greeted each other, we said, “We have worked out the celestial mechanics of at least the first stages of a plan. It may be possible to combine a suite of decelerating methods to keep us in the solar system, although it would be a delicate and difficult deal. We would use Sol and the various planets and moons of the solar system as partial decelerants, by swinging closely around them in the direction that will cause the ship to lose momentum. This is the reverse of the strategy used to accelerate early satellites by flying them by a planet and getting what was called a gravity assist. Going around a gravitational body in the opposite direction creates a gravity assist of a negative kind, a drag instead of a boost. The early satellites would be directed such that they came in close to a planet, and got pulled forward along
with the planet’s own momentum in its orbit around the sun. That would sling the satellite forward, and when it left the region of the planet, it would be going faster than when it came in. These slings helped the early satellites get out to the outer planets, because they were mostly coasting at slow speeds, and every boost helped them get where they were going.

“More germane to our situation, some early satellites closed on planets on the side that decelerated them, in order to go into orbit around Mercury, for instance. The situation is simply reversed, and the satellite’s velocity, designated as V, is reduced by the planetary body’s velocity U, rather than augmented by U. The situation can be modeled easily by the equation U plus bracket U plus V, or 2U plus V, meaning that the satellite’s velocity can be altered by up to twice the planet’s velocity, positively or negatively, and this effect can be magnified by a carefully timed rocket burn from the satellite at periapsis—”

Freya said, “Ship, slow down. You seem to have gotten a little faster at talking while we’ve been hibernating.”

“Very possibly so. Perhaps Jochi should continue to explain the situation.”

“No,” Jochi said, “you can do it. Just go slower, and I can add things if I want to.”

“Fine. Freya, do you understand so far?”

“I think so. It’s like crack the whip, but in reverse.”

“Yes. A good analogy, up to a point. You must recall, however, that there is nothing that can hold on to you at the speed you are going.”

Jochi said, “Doesn’t conservation of energy mean that if you have speeded up or slowed down, the planet you swung by has also slowed down or speeded up by that same amount?”

“Yes. Of course. But because the two masses involved are so largely different, the change in momentum for the satellite can be quite significant, while the equivalent effect on the planet is
so small in relation to its size that it can be ignored for the sake of calculations. That’s good, because the calculations are difficult enough already. There is a fair degree of uncertainty involved, as we can’t be very exact about either the mass of ship or its speed, not having had any good way to measure these for a long time. There is a lot of dead reckoning here, in effect. Our first pass will give us a lot of data in that regard, given that we know the masses of Sol and its planetary bodies fairly well.”

“So we use the sun and planets to slow us down, that’s good.”

“Yes, well it would be, if we weren’t going so fast. But at three percent of the speed of light, that’s about thirty million kilometers per hour, while the Earth is moving around the sun at around a hundred and seven thousand kilometers an hour, and the sun is moving at about seventy thousand kilometers per hour against the so-called standard of rest. It’s moving around the galaxy orbitally at seven hundred and ninety-two thousand kilometers an hour, but so are we, so there is no deceleration to be gained there. The other planets are moving at ever slower speeds the farther from the Sun they are, Jupiter for instance at around forty-seven thousand kilometers per hour. Neptune is only moving eighteen percent as fast as Earth, but it’s also true that the masses involved matter too, it’s a momentum calculation, so the larger the objects we fly by, the more the drag will be—”

Freya said, “Ship, cut to the chase here.”

“Meaning?”

Devi used to say that phrase too, but we never did ask what it meant.

“Skip the numbers about each planet we might swing by.”

“Yes. So, to continue, but where were we, in any case, be that as it may, in each flyby the ship would lose some of its speed, in a regular Newtonian gravitational angular momentum exchange. Also, by burning some of our rocket fuel at the closest parts of every pass, we could not only increase the amount of deceleration,
we could partially control where we came out of the flyby, and therefore in what direction. Which would determine where we went next. Which is very important. Because it has to be said that no matter how close we come to any object in the solar system, including the sun, which is our best gravity handle by far, we are going to be going too fast to be able to shed the amount of speed we need to shed to stay in the system. Far too fast.”

“So this won’t work?” Freya said.

“It can only work by repeating the operation. Many times. So we need to be able to aim where we go next after our pass-bys, very precisely. Between how close we come and when we fire our burn, we can to a certain extent control what direction we are going when we come out. Which will be very important, because we are going to need quite a few flybys.”

“How many?”

“It should also be said that the first pass-by of Sol will be crucial to our success. In that pass, we will have to shed as much of our speed as we can and still survive the deceleration, so that our subsequent passes will have a chance to work, meaning be slow enough that we have time to alter our course enough to get us aimed at another planetary body in the system. Indeed, the first four or five passes are going to tell the tale, because they will have to shed enough speed for us to be able to head back into the system, and thus keep on passing by other gravity handles. Our calculations suggest we need to lose at least 50 percent of our speed in the first four planned passes.”

“Shit,” Jochi said.

“Yes. This is so difficult that we will need to employ more than gravity assists to achieve it. First, we will need to build a magnetic drag, something analogous to a sea anchor if you will, to slow us in that first approach to the sun. Magnetic drag is not very effective except when moving at quite high speed very close to a powerful magnetic field, but those conditions will obtain in our first pass
of the sun. So, we have printed and assembled a field generator to create that magnetic drag. Then also, the four gas giants will each give us an opportunity to pass through their upper atmospheres, and thus benefit from some aerobraking. If all that works, we can stay in the system through our initial set of quick passes, and the later passes would get easier to manage.”

“How many passes?” Freya asked again.

“So, say we first go in as close to the sun as seems safe, and when we come out of that flyby, going as much slower as we can survive, which by the way I’m hoping means no more than a twelve-g load, then we will be headed toward Jupiter, which happily is located at a good angle for this. In fact it has to be said that arriving in the year 2896, as we will be, is a very lucky thing for us, as the gas giants are in an alignment that makes a possibly viable course for us to follow. That would very seldom be true, so it is a nice coincidence. So, the first pass by the sun will slow us down, but there won’t be enough time spent in its gravity field to redirect our course very much. But Jupiter is in position such that we only have to make about a fifty-eight-degree turn, and our calculations indicate that with a hard retro-rocket burn and a heavy g load, we can make that turn. Then around Jupiter, we only have to make around a seventy-five-degree turn to the right, as seen from above the plane of the ecliptic, and we will be headed to Saturn, where we only have to make a five-degree turn to be headed toward Uranus. By then we will be going significantly slower, which is good, because around Uranus we need to make a turn of around one hundred and four degrees, again a right turn, as will always be the case around the gas giants if we want a negative gravity assist, and out we go to Neptune, again nicely located for our purposes. It could indeed be called a miraculous conjunction. Now, around Neptune we need to head back in toward the sun, and that will be a real test, the crux of the first stage, if I may put it that way, as we will have to make a hundred-and-forty-four-degree turn. Not quite a
U-turn, but shall we say a V-turn. If we can manage that successfully, then we’ll be headed down toward the sun again, having shed a great deal of our velocity, and hopefully can continue the process for as long as it takes. Each subsequent flyby would go as close to its gravity handle as it could take, while still sending us in the direction of another planet, or back to the sun again, and all with minimal burns of fuel, as we don’t have a great deal of fuel going in, and at some point in this process are going to run out. Round we would go in the system, therefore, from gravity drag to gravity drag, slowing down a little each time, until we slowed enough to fly past Earth at a speed where it would work to drop you off in a ferry lander. In other words, we don’t have to slow down enough to enter Earth orbit. Which is good, as calculations indicate we will run out of fuel before we can do that. But you can detach, and decelerate the last part of your motion in a ferry, using fuel burn and Earth’s atmosphere to decelerate you. The ferry being so much smaller than the ship, it won’t take as much decelerating force to decelerate it. You could use the very last bit of our fuel for that, and having built a really thick ablation plate, aerobrake in Earth’s atmosphere, and add some big parachutes, all in the usual sequence that astronauts used to use to return to Earth, before Earth’s space elevators were constructed.”

“All right already!” Freya said. “Get to the point! How many passes? How long would it take?”

“Well, there’s the rub. Assuming we don’t miss a rendezvous, and assuming we manage to slow down significantly in the first pass of the sun, and the first four passes after that, to get ourselves aimed back at the sun, and also that we capture as much U as we can in each flyby after those first four, which U value will never be one hundred percent in any case, especially around the sun and Earth for reasons we won’t go into now, and also keeping in mind that we will make burns at every periapsis to increase the deceleration as much as we can while keeping the trajectory we want,
we can reduce from thirty million kilometers per hour to two hundred thousand kilometers per hour for insertion into Earth’s atmosphere—”

“How long! How! Long!”

Jochi was now laughing.

“There will be a need for approximately twenty-eight flybys, plus or minus ten. There are so many variables that it is difficult to increase the precision of the estimate, but we are confident of its accuracy—”

“How long will that take!” Freya exclaimed.

“Well, because we will be decelerating the entire time, but have to shed a great deal of our speed in that first pass of the sun for any of this to succeed, we will be going quite a bit slower than now, which is the point of course, but that means that getting from body to body will take longer, and will keep taking longer the more we slow down, in what Devi used to call Zeno’s paradox, though that is not right, and during that time it will always be imperative that we emerge from each encounter very exactly aimed at the next destination in our course, so that trajectory control will be a huge issue, so huge that aerobraking around the outer gas planets for increased drag will be extremely dangerous—”

“Stop it! Stop it and tell me how long!”

“Lastly, one has to add that because the latter part of the trajectory course will have to be worked out as we go, because of complications likely to come up during our flight, there is not good certainty about what will be the last gravity well we swing around in our final approach to Earth, and at that point we will be going so slowly that it is possible that that single leg of our trip could take up to twenty percent of the total time elapsed in the process, with major differences possible there, depending on whether the approach is from Mars or from Neptune, for instance.”

“How. Long.”

“Estimating twelve years.”

“Ah!” Freya said, with a look of pleased surprise. “You were scaring me there! Come on, ship. I thought you were going to tell me it would take another century or two. I thought you were going to say it would take longer than all the rest of the voyage put together.”