The Fountains of Paradise - Arthur C. Clarke Page 0,20

set, but are fixed motionless in the sky. We, and our parents, and their parents have long taken for granted the synchronous satellites and space stations, which move above the equator at the same speed as the turning earth, and so hang forever above the same spot.

“The question Artsutanov asked himself had the childlike brilliance of true genius. A merely clever man could never have thought of it—or would have dismissed it instantly as absurd.

“If the laws of celestial mechanics make it possible for an object to stay fixed in the sky, might it not be possible to lower a cable down to the surface, and so to establish an elevator system linking earth to space?

“There was nothing wrong with the theory, but the practical problems were enormous. Calculations showed that no existing materials would be strong enough. The finest steel would snap under its own weight long before it could span the thirty-six thousand kilometers between earth and synchronous orbit.

“However, even the best steels were nowhere near the theoretical limits of strength. On a microscopic scale, materials had been created in the laboratory with far greater breaking strength. If they could be mass-produced, Artsutanov’s dream could become reality—and the economics of space transportation would be utterly transformed.

“Before the end of the twentieth century, super-strength materials—hyperfilaments—had begun to emerge from the laboratory. But they were extremely expensive, costing many times their weight in gold. Millions of tons would be needed to build a system that could carry all earth’s outbound traffic. So the dream remained a dream—until a few months ago.

“Now the deep-space factories can manufacture virtually unlimited quantities of hyperfilament. At last we can build the Space Elevator—or the Orbital Tower, as I prefer to call it. In a sense it is a tower, rising clear through the atmosphere, and far, far beyond….”

Morgan faded out, like a ghost that had been suddenly exorcised. He was replaced by a football-sized Earth, slowly revolving. Moving an arm’s breadth above it, and keeping always poised above the same spot on the equator, a flashing star marked the location of a synchronous satellite.

From the star, two thin lines of light started to extend—one directly down toward the earth, the other in exactly the opposite direction, out into space.

“When you build a bridge,” continued Morgan’s disembodied voice, “you start from the two ends and meet in the middle. With the Orbital Tower, it would be the exact opposite. You have to build upward and downward simultaneously from the synchronous satellite, according to a careful program. The trick is to keep the structure’s center of gravity always balanced at the stationary point. If you don’t, it will move into the wrong orbit, and start drifting slowly around the earth.”

The descending line of light reached the equator; at the same moment, the outward extension also ceased.

“The total height must be at least forty thousand kilometers, and the lowest hundred, going down through the atmosphere, may be the most critical part, because there the Tower may be subject to hurricanes. It won’t be stable until it’s securely anchored to the ground.

“And then, for the first time in history, we will have a stairway to heaven—a bridge to the stars. A simple elevator system, driven by cheap electricity, will replace the noisy and expensive rocket, which will then be used only for its proper job of deep-space transport. Here’s one possible design for the Orbital Tower.”

The image of the turning earth vanished as the camera swooped down toward the Tower, and passed through the walls to reveal the structure’s cross-section.

“You’ll see that it consists of four identical tubes—two for up traffic, two for down. Think of it as a four-track vertical subway or railroad, from earth to synchronous orbit.

“Capsules for passengers, freight, fuel would ride up and down the tubes, at several thousand kilometers an hour. Fusion power stations at intervals would provide all the energy needed; since ninety percent of it would be recovered, the net cost per passenger would be only a few dollars. As the capsules fall earthward again, their motors will act as magnetic brakes, generating electricity. Unlike re-entering spacecraft, they won’t waste all their energy heating up the atmosphere and making sonic booms; it will be pumped back into the system. You could say that the down trains will power the up ones. So even at the most conservative estimate, the Space Elevator will be a hundred times more efficient than any rocket.

“And there’s virtually no limit to the traffic it could handle,