almost shadowless illumination over the whole interior. But beyond that, over the rim of the most distant terrace, were darkness and mystery.
In his mind’s eye, Calvert knew perfectly well what lay there. First there was the flat circular plain over a kilometer across. Trisecting it into three equal parts, and looking much like broad railroad tracks, were three wide ladders, their rungs recessed into the surface so that they would provide no obstruction to anything sliding over them. Since the arrangement was completely symmetrical, there was no reason to choose one ladder rather than another, that nearest to air lock Alpha had been selected purely as a matter of convenience.
Though the rungs of the ladders were uncomfortably far apart, that presented no problem. Even at the rim of the hub, half a kilometer from the axis, gravity was still barely one-thirtieth of Earth’s. Although they were carrying almost a hundred kilos of equipment and life-support gear, they would be able to move easily hand over hand.
Commander Norton and the back-up team accompanied them along the guide ropes that had been stretched from air lock Alpha to the rim of the crater. Then, beyond the range of the floodlights, the darkness of Rama lay before them. All that could be seen in the dancing beams of the helmet lights was the first few hundred meters of the ladder, dwindling away across a flat and otherwise featureless plain.
And now, Mercer told himself, I have to make my first decision. Am I going up that ladder, or down it?
The question was not a trivial one. They were still essentially in zero gravity, and the brain could select any reference system it pleased. By a simple effort of will, Mercer could convince himself that he was looking out across a horizontal plain or up the face of a vertical wall or over the edge of a sheer cliff. Not a few astronauts had experienced grave psychological problems by choosing the wrong co-ordinates when they started on a complicated job.
Mercer was determined to go head-first, for any other mode of locomotion would be awkward. Moreover, that way he could more easily see what was in front of him. For the first few hundred meters, therefore, he would imagine he was climbing upward; only when the increasing pull of gravity made it impossible to maintain the illusion would he switch his mental directions one hundred and eighty degrees.
He grasped the first rung and gently propelled himself along the ladder. Movement was as easy as swimming along the sea bed—easier, in fact, for there was no backward drag of water. It was so easy that there was a temptation to go too fast, but Mercer was much too experienced to hurry in a situation as novel as this.
In his earphones he could hear the regular breathing of his two companions. He needed no other proof that they were in good shape, and wasted no time in conversation. Though he was tempted to look back, he decided not to risk it until they had reached the platform at the end of the ladder.
The rungs were spaced a uniform half-meter apart, and for the first portion of the climb Mercer missed the alternate ones. But he counted them carefully, and at around two hundred noticed the first distinct sensations of weight. The spin of Rama was starting to make itself felt.
At rung four hundred, he estimated that his apparent weight was about five kilos, or about eleven pounds. This was no problem, but it was getting hard to pretend that he was climbing when he was being firmly dragged upward.
The five hundredth rung seemed a good place to pause. He could feel the muscles in his arms responding to the unaccustomed exercise, even though Rama was now doing all the work and he had merely to guide himself.
“Everything OK, Skipper,” he reported. “We’re just passing the halfway mark. Joe, Will, any problems?”
“I’m fine. What are you stopping for?” Calvert answered.
“Same here,” added Myron. “But watch out for the Coriolis force. It’s starting to build up.”
So Mercer had already noticed. When he let go of the rungs, he had a distinct tendency to drift off to the right. He knew perfectly well that this was merely the effect of Rama’s spin, but it seemed as if some mysterious force was gently pushing him away from the ladder.
Perhaps it was time to start going feet-first, now that “down” was beginning to have a physical meaning. He would run the risk of