Aurora

by Kim Stanley Robinson

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 ca
n 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 t
o 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!

  And it was going to be a hot and heavy pass. The heat was the lesser problem, as we would be near the sun for such a short time. During that time, however, the combination of the deceleration and the tidal forces exerted while swinging fifty-eight degrees around the sun would combine to a brief force of about 10 g’s. After study of the problem, we had first tried to construct the trajectory with the idea of holding to a maximum of 5 g’s, but in fact getting headed toward Jupiter, given our incoming trajectory, required risking a higher g-force. We were happy that we had spent the last century reconfiguring the ship to a much more robust arrangement, structurally very sound, in theory; but there was little we could do for our people, who were going to have to experience what was going to be a potentially rather traumatic, indeed possibly fatal, squishing. Cosmonauts and test pilots had briefly endured gravitational forces of up to 45 g’s, but these were specialists bracing themselves for the experience, while the hibernauts were going to be taken unawares. Hopefully they would not all be squished like bugs. We did not like to be subjecting them to such an event, but judged it was either that or a subsequent death by starvation, and what we had seen of their approach to starvation indicated that would not be a good way to die. As it was, our attempt to stay in the system represented at least a possibility of survival.

  Unfortunately, our first approach to the sun had to be adjusted by a pass-by of Earth first, this not to slow us down at all, but merely to help our angling toward Sol. Luck of the draw, really: alignment of the planets in this CE year 2896, year 351 ship time, was actually one of the few alignments that allowed for even a theoretical chance of this maneuver succeeding. So, first up, a close pass of Earth at 30 million kilometers per hour. It seemed likely people there were going to be alarmed.