Aurora - страница 73

On the way out to Uranus, we tried to come to grips by way of modeling with the fact that our pass-by of that lightly banded and ringed giant was going to be different, because it rotates transversely to the plane of the ecliptic; its axis of rotation is such that it rolls around the sun like a ball, a strange anomaly in the solar system, an anomaly the cause of which a cursory inspection of the literature suggested was still poorly understood. What it meant now for us was that if we did the usual aerobraking, which indeed we had to do, as it was necessary for our continuing effort at deceleration, we would be punching through several of the planet’s latitudinal bands, created by winds each rushing in the opposite direction to those above and below it, as on Jupiter; and so at each border between bands there would be a similar area of wind shear and atmospheric turbulence, well represented by the wild band borders of great Jupiter. Perhaps not a good idea!

We had a bit more time than before to model this problem, although we still looked quite fast to the people of the solar system, who were used to these crossings taking years. Although there was also a class of very fast ferries that could jaunt around the system, if they found they had a really pressing need for speed. Fuel and other costs made these quick trips very rare, and yet it did give the locals a basis for comparison, which is why we had been such a marvel at first, coming in faster than anything. Now we were normalized in terms of their idea of speed, fast but not extraordinarily so. It might also have been true that the novelty of our return was also wearing off, and we were becoming just another odd feature of life in the solar system. We hoped so.

Soon enough Uranus approached, its narrow faint ring making it clear that we were going to round it pole to pole, and though the ring was no problem to dodge, nor the little fragment moons, the models had confirmed that we needed to be very cautious with our aerobraking, staying as high in the Uranian atmosphere as we could while still coming out of the turn headed toward Neptune, after a sharp right curve.

So in we came, and Uranus grew in the now-familiar way, looking mauve and lavender and mother-of-pearl, and we hit the upper atmosphere and at first it was the same as always, a sharp deceleration, ramping up to 1 g, not at all bad, and then WHAM WHAM WHAM WHAM, it was as if we were running through doorways without opening the doors, terrific smacks that increased in shuddering with each impact. Things broke, animals and people died, probably of heart attacks, six people this time, Arn, Arip, Judy, Oola, Rose, and Tomas, and it really was becoming unclear whether we could sustain many more such concussive slaps, it was startling how much a wind shear wall could obstruct one, a little left-right punch followed instantly by a right-left punch, when happily we were out of the atmosphere again before anything more damaging occurred, and were again on course, and on our way out to Neptune.

Which meant we were coming to the crux. Push had come to shove. Again we would come in, dodge the negligible rings, make a dip into the upper atmosphere of this cool blue beauty, reminiscent in appearance to Tau Ceti’s Planet F, we found. But this time our turn had to be almost a U-turn (perhaps the source of the use of the letter U in the gravity assist equations?), not quite a true U-turn, but 151 degrees, quite a wraparound, a V-turn, not easy at all, and at 113 kilometers a second. That meant a deeper dive into the atmosphere, and more tidal forces, and more g-forces. Aerobraking would again shake us; it would feel like we were a rat in the jaws of a terrier, perhaps. But if we succeeded, then we would be headed back in, downsystem toward the sun again, quite considerably slowed down, and in a pattern that would seem to allow us to continue our decelerative cat’s cradle, pinballing around the solar system from gravity handle to gravity handle, at least for as long as our fuel held out to make course corrections. We were running low on fuel.

So: Neptune. Cool blue-green, lots of water ice and methane. Gossamer ring, barely visible. Not much sunlight out here. Well beyond the habitable zone of any life-form known. A slow place. Interesting to have given it a submarine name; it seemed somehow appropriate, in the usual mushy metaphorical way, in that impressionistic, vague, feeling kind of way.

We were still going very fast, but it was a long way to go, so we had 459 days to set things up. The diameter of our approach window was smaller than ever, given the need for such a sharp turn; vanishingly small; really hitting the mark precisely on the nose of our capture plate would be best, so we were setting the trajectory window at a hundred meters in diameter, which after all the distance crossed was rather extraordinary to consider: but even so, a hundred meters was a bit too big a window; really a single meter, a geometrical point, would be best.

In we came. Hit the mark. Started the pass, knuckles white.

Aerobraking comparatively smooth, compared to the hammering of Uranus. A rapid vibration, an occluding of vision in the upper clouds, a few minutes of blind trembling, of intense anxiety, nail-biting suspense; and out again, after another 1-g press, which this time was more than ever a matter of tidal forces, as we swung around so far. V-turn!

And out of the pass, headed toward the sun. Downsystem. Looped in. Caught. Back.

If each of our five passes was called a one-in-a-million throw, which was a very conservative estimate of the odds, then all five together made it a one-in-an-octillion chance. Amazing—literally—in that we had indeed been threading a maze. Little joke there.

And so down to the sun again, going slower than ever, although still 106 kilometers per second. But the next pass of the sun would put a good heavy brake on that, and on we would go, slower each time, working through a version of Zeno’s paradox that fortunately was not truly a perpetual halving, but would come to an endpoint, thus our own happy ending to a very severe halting problem.

On the way back in, we passed near Mars, which was interesting. There were so many stations there that it was no longer a scientific facility only, but something more like Luna, or the Saturnian system, or the Europa-Ganymede-Callisto complex: a kind of nascent confederation of city-states, buried in cliffsides and under domed craters, but each outpost quite various in design and purpose, and altogether more than just an outpost of Earth, though it was still that too. Early dreams of terraforming Mars quickly, and thus having a second Earth to walk around on, had come to grief, mainly because of four physical factors overlooked in the first flush of optimistic plans: the surface of Mars was almost entirely covered by perchlorate salts, a form of chlorine salts that had given Devi fits as well, as only a few parts per billion gave humans terrible thyroid problems, and could not be endured. So that was bad. Of course it was true that many microorganisms could easily handle the perchlorates, and in their eating and excreting consume and alter them to safer substances; but until that happened, the surface was toxic to humans. Worse yet, there turned out to be only a few parts per million of nitrates in the Martian soil and regolith, an odd feature of its original low elemental endowment of nitrogen, the reason for which was still a source of debate, but meanwhile, no nitrates, and thus no nitrogen available for the creation of an atmosphere. And so the terraforming plans were faced with a radical insufficiency. Then third, it was becoming clear that the fines on the Martian surface, having been milled by billions of years of drifting in the winds, were so much finer than dust particles on Earth that it was extremely difficult to keep them out of stations, machines, and human lungs; and they wreaked damage on all three. Again, once microorganisms had carpeted the surface, and fixed the fines by bonding them into layers of desert pavement, and also as the surface got hydrated and the fines became bogged in muds and clays, that problem too would be solved. And last, the lack of a strong magnetic field meant that a thick atmosphere was really needed to intercept radiation from space, before the surface would be very safe for humans to be on.

So, none of these problems were pure stoppers, but they were big slower-downers. Concerning the nitrogen lack, the Martians were negotiating with the Saturnians to import nitrogen from Titan’s atmosphere, as it had become clear that Titan, for its own terraforming plans, had too much nitrogen. Transfer of that much nitrogen would be a Titanic chore, ha-ha, but again, not impossible.

The upshot of all this was that the terraforming of Mars was still on the table and a subject of huge enthusiasm for many humans, particularly the Martians, although really, in strict numerical terms, even more of these enthusiasts lived on Earth, which seemed in fact to be home to enthusiasts of all kinds, for any project imaginable, judging by the roar of radio voices coming from it, almost like an articulated version of Jupiter’s mighty radioactive yawp. Oh yes, Earth was still the center of all enthusiasm, all madness; the settlements scattered elsewhere in the solar system were outliers. They were expressions of Terrans’ will, and vision, and desire.