temperatures a hundred degrees below freezing, when you’re sitting in a toasty cabin, but it can be dangerous to forget.
While I was doing this I looked out at the life dome rising in the distance. I could pick out people sledding down a hill and further away a crowd in a snowball fight. A scramble like that is more fun on Ganymede than on Earth; somebody a hundred yards away can pick you off with an accurate shot, because low gravity extends the range of your throwing arm. We don’t have anything really spectacular on Ganymede in the way of recreation— nothing like the caverns on Luna, where people can fly around in updrafts, using wings strapped to their backs—but what there is has a lot of zip. For a moment I wished I was out there, in that isolated Earthlike environment, tossing a snowball, instead of piloting a Walker up to the ice fields. I mulled over what Zak had said last night. Then I cut the thought short; it was too late to back out now.
I engaged the engine and the Cat lurched forward. The legs moved methodically, finding the level of the ground and adjusting to it; the gyros kept us upright and shock absorbers cushioned our cabin against the rocking and swaying.
I clicked on the Cat’s magnetic screen. The dome area has buried superconductors honeycombing the area, creating a magnetic web. As the Cat left the fringes of that field, we needed more protection from the steady rain of energetic protons. They sleet down on Ganymede from the Van Allen belts. A few hours without protection would fry us. Cat’s walls contain superconducting threads carrying high currents. They produce a strong magnetic field outside, which turns incoming charged particles and deflects them.
I took us away from the base at a steady thirty klicks an hour; it would be slower when we hit rough country. The morning sun came slanting in as we moved along the eastern rim of the valley; I switched on the polarizers in our windows to keep down the glare.
Maybe I should explain about morning on Ganymede. It’s a complicated business. The hardest thing to adjust to when you first land here is the simple fact that Ganymede is a moon, not a planet. It’s tied to Jupiter with invisible gravitational apron strings that keep it tide-locked, one face toward Jupiter, always. Meanwhile it revolves around Jove and in turn the Fat One orbits around the sun. The situation is pretty much the same as the Earth-Luna system: Luna shows the same face to Earth and revolves around it in about twenty-eight days, so the lunar “day”—one complete day-night cycle—is twenty-eight days long. Ganymede revolves around Jove in a fraction more than a week, so its “day” is seven Earth days long; the sun is in the sky three and a half days, every week.
This makes a pretty complicated week, believe me. The base has legislated that sunrise occurs at Saturday midnight; it’s arbitrary, but it makes for a symmetric week and symmetry is like catnip to scientists. We were starting out Sunday morning and the sun would be in the sky until Wednesday afternoon.
All this time Jove squats square in the middle of the sky, like a striped watermelon. At the moment the sun was streaming through the ports of the Cat and I had to polarize them to cut the glare. Yuri looked up from his map and said, “By the way, that little maneuver back there didn’t slip by me.”
“What?”
“Don’t play dumb. I heard you try to talk Vandez into putting you in charge. It’s a good thing he saw through you.”
“Well, I don’t know,” I said slowly. “It seemed to me as long as you didn’t know much about a Walker you shouldn’t be running one.”
“What is there to know? I picked up all I need in a few hours. Here, get out of the seat.”
I stopped the Cat and, Yuri slid into the driver’s chair. We had reached the end of the valley and were heading over a low rise. Here and there ammonia ice clung to the shadows.
Yuri started us forward, staying close to the usual path. The whole trick of guiding a Walker is to keep the legs from having to move very far up and down on each step. It’s easier for the machine to inch up a grade than to charge over it.
So the first thing Yuri did was march us directly up the hill. The legs started straining to keep our cabin level and a whining sound filled the air. The Cat teetered, lunged forward, stopped and died.
“Hey!” Yuri said.
“You shouldn’t be surprised,” I said. “She’s just doing what any self-respecting machine does when it’s asked to perform the impossible. She’s gone on strike. The automatic governor cut in.”
Yuri said something incoherent and got up. I took over again and backed us off slowly. Then I nudged the Cat around the base of the hill until I found the signs of a winding path previous Walkers had left. Within fifteen minutes we were in the next valley, its hills lit with the rosy glow of the sun filtering through a thin ammonia cloud overhead.
Chapter 7
We made good time Sunday. I did most of the driving. Yuri gradually picked up the knack of guiding the Cat—I suspected Captain Vandez had him practice only on flat, even ground around the base the day before. The Captain was a spaceman, after all, and groundhog work wasn’t his piece of pie.
We slept overnight in the shadow of a thin, tall peak widely used as a landmark; the map called
Before we turned in for the night I located the way station that was our destination, and camped us next to it. The station wasn’t much: just an automatic chemical separator and a set of sampling devices. The chemical plant is vital. It collects water frost that condenses on its outstretched plates—usually a pint a day. The water is automatically stored. Whenever a Walker comes by the driver hooks up his air and water tanks to the station’s water reserves and replenishes his supplies.
Where does the station get the air? That’s the trick: water is hydrogen and oxygen, so if you could break it up and throw away the hydrogen gas, you would have all the breathable oxygen you need. The Walker does just that— it passes an electric current through water to vaporize it, and then bleeds off the oxygen.
Air isn’t just oxygen; the first spacemen found it a lot safer to use a lot of inert nitrogen, too, to keep down the risk of fires. The station extracts nitrogen from the ammonia in the ice all around it, automatically, and has it ready to mix with the oxygen.
Before we bedded down I connected the air and water hoses from the Walker, clicked a toggle switch on the control board over to FILLING MODE, and forgot about it.
Monday morning the sun was a third of the way across the black sky and Jupiter’s banded crescent resembled a Cheshire cat’s grin. Yuri fixed a quick-heating breakfast and we set out.
Our route now ran due north. We wove through wrinkled valleys of tumbled stone and pink snowdrifts, keeping an eye open for anything unusual. We were heading for the edge of the “tropical” zone of Ganymede—the belt around the equator where the rock is exposed. Beyond that band lie the blue ice fields. The temperature drops steadily as you leave the equator. If the Walker would take us that far (and if we were crazy) we could have found mountain passes near the north pole nearly two hundred degrees centigrade below freezing.
Slowly, steadily, man is pushing the “tropical” zone toward the poles. Ganymede is warming up.
We avoided the areas near the fusion plants. The big reactors throw out heat and gas at an enormous rate. The ice around them melts and forms churning rivers. The warm water carries heat to neighboring areas and they start melting, too.
There’s a limit to the method, though. If you’re not careful, your fusion plants will melt their way into Ganymede and get drowned. Ganymede is a big snowball, not a solid world at all. It’s mostly water. There’s an ice crust about seventy klicks thick, with rock scattered through it like raisins in a pudding. Below that crust Ganymede is slush, a milkshake of water and ammonia and pebbles. There’s a solid core, far down inside, with enough uranium in it to keep the slush from freezing.
So the fusion plants don’t sit in one place. They’re big caterpillars, crawling endlessly outward from the equator. Their computer programs make them seek the surest footing over the outcroppings of rock—only they run on tracks, not feet. We saw one about midday, creeping over a ridgeline in the distance, making about a hundred meters in an hour, sucking in ice and spewing an ammonia-water creek out
