"Go out of doors any clear night," continued Morgan, "and you will see that commonplace wonder of our age – the stars that never rise or 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 kilometres 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. For 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 towards 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's the exact opposite. You have to build upwards and downwards simultaneously from the synchronous satellite, according to a careful programme. The trick is to keep the structure's centre of gravity always balanced at the stationary point; if you don't, it will move into the wrong orbit, and start drifting slowly round 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 kilometres – and the lowest hundred, going down through the atmosphere, may be the most critical part, for 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 shall 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 towards 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 kilometres an hour. Fusion power stations at intervals would provide all the energy needed; as ninety percent of it would be recovered, the net cost per passenger would be only a few dollars. For as the capsules fall earthwards 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 elevator will be a hundred times more efficient than any rocket.

“And there's virtually no limit to the traffic it could handle, for additional tubes could be added as required. If the time ever comes when a million people a day wish to visit Earth or to leave it – the orbital tower could cope with them. After all, the subways of our great cities once did as much…”

Rajasinghe touched a button, silencing Morgan in mid-sentence.

“The rest is rather technical – he goes on to explain how the tower can act as a cosmic sling, and send payloads whipping off to the moon and planets without the use of any rocket power at all. But I think you've seen enough to get the general idea.”

“My mind is suitably boggled,” said Professor Sarath. “But what on earth or off it – has all this to do with me? Or with you, for that matter?”

“Everything in due time, Paul. Any comments, Maxine?”

“Perhaps I may yet forgive you; this could be one of the stories of the decade – or the century. But why the hurry – not to mention the secrecy?”

“There's a lot going on that I don't understand, which is where you can help me. I suspect that Morgan's fighting a battle on several fronts; he's planning an announcement in the very near future, but doesn't want to act until he's quite sure of his ground. He gave me that presentation on the understanding that it wouldn't be sent over public circuits. That's why I had to ask you here.”

“Does he know about this meeting?”

“Of course; indeed, he was quite happy when I said I wanted to talk to you, Maxine. Obviously, he trusts you and would like you as an ally. And as for you, Paul, I assured him that you could keep a secret for up to six days without apoplexy.”

“Only if there's a very good reason for it.”

“I begin to see light,” said Maxine Duval. “Several things have been puzzling me, and now they're starting to make sense. First of all, this is a space project; Morgan is Chief Engineer, Land.”

“So?”

“Yhu should ask, Johan! Think of the bureaucratic in-fighting, when the rocket designers and the aerospace industry get to hear about this! Trillion dollar empires will be at stake, just to start with. If he's not very careful, Morgan will be told 'Thank you very much – now we'll take over. Nice knowing you.'”

“I can appreciate that, but he has a very good case. After all, the Orbital Tower is a building – not a vehicle.”

“Not when the lawyers get hold of it, it won't be. There aren't many buildings whose upper floors are moving at ten kilometres a second, or whatever it is, faster than the basement.”

“You may have a point. Incidentally, when I showed signs of vertigo at the idea of a tower going a good part of the way to the moon, Dr. Morgan said, 'Then don't think of it as a tower going up – think of it as a bridge going out'. I'm still trying, without much success.”

“Oh!” said Maxine Duval suddenly. “That's another piece of your jig-saw puzzle. The Bridge.”

“What do you mean?”

“Did you know that Terran Construction's Chairman, that pompous ass Senator Collins, wanted to get the Gibraltar Bridge named after him?”

“I didn't; that explains several things. But I rather like Collins – the few times we've met, I found him very pleasant, and very bright. Didn't he do some first-rate geothermal engineering in his time?”