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He had picked the spot he was aiming for. There was a jagged hunk of rock sticking out that looked as though it would make a good handhold. Right nearby, there was a fairly smooth spot that would do to brake his “fall”. He struck it with his palm and took up the slight shock with his elbow while his other hand grasped the outcropping.

He had not pushed himself very hard. There is not much weathering on the surface of an asteroid. Micro-meteorites soften the contours of the rock a little over the millions of millennia, but not much, since the debris in the Belt all has roughly the same velocity. Collisions do occur, but they aren’t the violent smashes that make the brilliant meteor displays of Earth. (And there is still a standing argument among the men of the Belt as to whether that sort of action can be called “weathering”.) Most of the collisions tend to cause fracturing of the surface, which results in jagged edges. A man in a vacuum suit does not push himself against a surface like that with any great velocity.

* * * * *

St. Simon knew to a nicety that he could propel himself against a bed of nails and broken glass at just the right velocity to be able to stop himself without so much as scratching his glove. And he could see that there was no ragged stuff on the spot he had selected. The slanting rays of the sun would have made them stand out in relief.

Now he was clinging to the surface of the mountain of rock like a bug on the side of a cliff. On a nickel-iron asteroid, he could have walked around on the surface, using the magnetic soles of his vacuum suit. But silicate rock is notably lacking in response to that attractive force. No soul, maybe.

But directly and indirectly, that lack of response to magnetic forces was the reason for St. Simon’s crawling around on the surface of that asteroid. Directly, because there was no other way he could move about on a nonmetallic asteroid. Indirectly, because there was no way the big space tugs could get a grip on such an asteroid, either.

The nickel-iron brutes were a dead cinch to haul off to the smelters. All a space tug had to do was latch on to one of them with a magnetic grapple and start hauling. There was no such simple answer for the silicate rocks.

The nickel-iron asteroids were necessary. They supplied the building material and the major export of the Belt cities. They averaged around eighty to ninety per cent iron, anywhere from five to twenty per cent nickel, and perhaps half a per cent cobalt, with smatterings of phosphorous, sulfur, carbon, copper, and chromium. Necessary–but not sufficient.

The silicate rocks ran only about twenty-five per cent iron–in the form of nonmagnetic compounds. They averaged eighteen per cent silicon, fourteen per cent magnesium, between one and one point five per cent each of aluminum, nickel, and calcium, and good-sized dollops of sodium, chromium, phosphorous, manganese, cobalt, potassium, and titanium.

But more important than these, as far as the immediate needs of the Belt cities were concerned, was a big, whopping thirty-six per cent oxygen. In the Belt cities, they had soon learned that, physically speaking, the stuff of life was not bread. And no matter how carefully oxygen is conserved, no process is one hundred per cent efficient. There will be leakage into space, and that which is lost must be replaced.

There is plenty of oxygen locked up in those silicates; the problem is towing them to the processing plants where the stuff can be extracted.

Captain St. Simon’s job was simple. All he had to do was sink an anchor into the asteroid so that the space tugs could get a grip on it. Once he had done that, the rest of the job was up to the tug crew.