“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.”

“No. Twelve years, we reckon, plus or minus eight years.”