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.

So, past the bustling little world of Mars, where they lived dedicated to the idea that they would successfully terraform their world in no more than forty thousand years. They seem to regard this as fine. As long as it could be done, it should be done, and would be done; and so the work was good.

The crucial difference here, it seemed to us, between Mars and the terraforming project we had left back on Iris, was that Mars is very near Earth. Its human settlers were constantly going to Earth for what they called their sabbatical, and receiving shipments of Terran goods and materials. And these infusions of Earth meant that they were always escaping the zoo devolution problem. Iris did not have those infusions, and never would; and it was notable (though in fact we had forgotten to note it, in the press of events) that we had not heard anything from the Iris settlers for over twenty-two years. Possibly a very bad sign, although it would benefit from a discussion with Aram and Badim, and the rest of the humans sleeping on board, to interpret more fully what it might mean. But certainly there was one explanation for the silence that was simply very bad.

Then downward to the sun again, down down down, feeling the pull, speeding up, heating up. In for another nail-biting scorcher of a pass, although this time without the ball and chain of the magnetic drag hauling back on us as we went; but it lasted considerably longer, as we were now traveling only 4 percent as fast as we had been going on that first terrifying pass. This time it would take us five and a half days, but we stayed farther out, and only heated up to the same 1,100 degrees on the exterior, and held there; and when we came out of that pass, we were headed for Saturn. No more of mad roaring Jupiter, if we could avoid it, and this round we could. Each leg of the cat’s cradle we were making would be different now.

Round and round the system we flew, slower and slower. We had very little fuel left. We were a kind of complicated artificial comet. Our trajectory clarified before us as we went. We passed by many inhabited planetary and asteroidal bodies. For quite a few years, the people in the solar system did not seem to get used to us; we were still a marvel of the age, a sight to be seen, a great anomaly, a visitation, as if from another plane of reality. That was the Tau Ceti effect, the starship effect. We were not meant to come back.

Slower, slower, slower. Each pass a deceleration to be calculated, and the new speed employed in the calculation determining the next pass-by. Always our planned trajectory extended many passes out into the future, although there was a lacuna growing out there, a time when our fuel ran out, or say that it grew too low to be used, as we were saving some little last bit for some last purpose. Because there was a time coming when the arrangement of the planets in their orbits was going to present us with an insoluble problem. Cross that bridge when you come to it; yes, but what if there is no bridge? That was the ongoing question. But for now, as the passes kept passing, each easier to manage, each with a slightly larger target window, it continued to be a problem out there at the edge of the calculable, always beyond the ever-receding horizon of calculable passes. Some of them required more fuel than others, some none at all. The timing was all. As always.