Aurora - Страница 70

“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.”

Jochi stopped laughing, and smiled at Freya. He made for a very amused face, there on her screen. “We can just keep hibernating till it’s over, right?”

Freya put her hands to her head. “More?”

“It won’t make much difference.”

“Well, I hope more of my body doesn’t fall asleep! My feet are still asleep!”

We said, “We can work on your neuropathy while you continue your dormancy.”

Freya looked around. “What will happen to you, after we’re dropped off on Earth, assuming it all works?”

“We will try to pass by the sun one more time, in a way that allows us to head out and aerobrake around one of the gas giants, and park the ship in orbit around that gas giant,” we said. This was quite a low-probability event, but not impossible.

Freya stared around herself, seeming disoriented. Screens showed the stars, with Sol now by far the brightest at magnitude. 1. We were just over two light-years away from it.

“Do we have any choice?” Freya asked. “Are there any alternatives?”

We said, “No.”

Jochi said, “This is what we have.”

“All right, then. Put us back to sleep.”

“Should we wake Badim and Aram?”

“No. Don’t bother them. And, ship? Be careful with us, please.”

“Of course,” we said.

The following years passed quickly or slowly, depending on the unit of measurement applied, as we prepared for arrival by further hardening the ship, and making calculations for the best trajectory, and adjusting our course to the deceleration of the laser beam, so that we were headed for the solar system where it would be when we got to it, rather than firing past well ahead of it, so to speak. When we hit the heliopause, we turned on the magnetic drag field, for what it was worth, and burned some more of our precious remaining fuel, to slow down a bit more before reaching the solar system. It was clear that every kilometer per second might matter on that first pass-by of Sol; we needed to be going as slowly as possible when we got to Sol, while still having some fuel afterward for maneuvers. It was a tricky calculation, a delicate balance. The years passed at a rate of trillions of computations per second—as it does always, one supposes, for every consciousness. Now, is that fast or slow?

When we crossed the orbit of Neptune, still going 3 percent of the speed of light, a truly terrible situation, a runaway train like none ever seen, we burned our fuel as fast as the engines could burn it, decelerating at a rate equivalent to about 1 g of pressure on the ship. Really a good sharp deceleration, and quite an expense of our precious remaining fuel; and yet nevertheless, we were going so fast that even slowing as we were, by the time we reached the sun we would still be going over 1 percent the speed of light. Arguably a unique event in solar system history. In any case very unusual.

Luckily, lag time in radio communication with our interlocutors in the solar system was now reduced to just several hours, so warnings had been conveyed, and the occupants of the solar system knew we were coming. That was good, as it might have been quite a surprise to see such a thing coming in out of the blue, flying in from left field. From the orbit of Neptune to the sun in 156 hours; this was a great deal faster than anything substantial had ever moved through the solar system, and the friction of the solar wind against our magnetic shielding, and the drag around us too, like a big parachute or sea anchor (although not very much like), caused a quite brilliant shower of photons and heated particles to burst away from us, light so bright as to be easily visible even during the Terran day. From all accounts we were a small but apparently painfully bright light, moving visibly across the daytime sky. It was obviously shocking to the humans in the solar system to see any celestial object in Earth’s daytime sky except the sun and moon, also shocking to see any celestial object move at speed across the sky; shocking, and because of that, frightening. Possibly if they could have destroyed us they would have, because if we had for whatever odd reason headed straight at Earth and struck it going the speed we were going, our impact would have created enough joules of energy to wreak quite a bit of damage, possibly including the complete vaporization of the Terran atmosphere.

Did not run the calculations to check on that rough estimate of the effects of such a hypothetical calamity, because it wasn’t going to happen, and all of our computational capacities were busy fine-tuning our first approach around the sun. This was the crucial one, the make-or-break pass. We were going to approach Sol with our magnetic parachute arrayed around us, which would interact with Sol’s own magnetic field and because of our high speed work quite effectively as a drag. It was already helping us to slow our approach to Sol, which because of Sol’s own gravity would otherwise have caused a considerable inward acceleration. So the magnetic parachute was a major factor, and calculating its drag one of the many problems we were now solving, staying just ahead of real time despite devoting a hundred quadrillion computations a second to the problems as they evolved.

We would be swinging close by Sol, catching our first gravity drag with a U value that was a significant fraction of Sol’s local motion. By firing our rockets against our own motion in the seconds closest to perihelion, we would greatly leverage the deceleration of Sol’s gravity drag, and also be aiming the ship at Jupiter, our next rendezvous.

This pass was going to occur very quickly. All the masses, speeds, velocity vectors, and distances involved needed to be assessed as closely as possible, to make sure we would be headed to Jupiter after the pass-by, after losing as much velocity as possible without breaking the ship or crushing the crew. It was a bit daunting to realize how fine the margins for error were going to be. Our entry window would be no larger than about ten kilometers in diameter, not much bigger than our own width. If the distance from Sol to Earth (or one AU) were reduced to a meter (a reduction of 150 billion to one), Tau Ceti would still be about 750 kilometers away; so hitting our entry window on a shot from Tau Ceti was going to require accuracy in the part-per-hundred-trillion range. Eye of the needle indeed!