The Fountains of Paradise

One could easily imagine that the finger of God was reaching down from heaven, tracing a furrow through the clouds. Even Rajasinghe, who understood the basics of weather control, had no idea that such precision was now possible; but he could take a modest pride in the fact that, almost forty years ago, he had played his part in its achievement.

It had not been easy to persuade the surviving superpowers to relinquish their orbital fortresses and hand them over to the Global Weather Authority, in what was – if the metaphor could be stretched that far – the last and most dramatic example of beating swords into ploughshares. Now the lasers that had once threatened mankind directed their beams into carefully selected portions of the atmosphere, or onto heat-absorbing target areas in remote regions of the earth. The energy they contained was trifling, compared to that of the smallest storm; but so is the energy of the falling stone that triggers an avalanche, or the single neutron that starts a chain reaction.

Beyond that, Rajasinghe knew nothing of the technical details, except that they involved networks of monitoring satellites, and computers that held within their electronic brains a complete model of the earth's atmosphere, land surfaces and seas. He felt rather like an awestruck savage, gaping at the wonders of some advanced technology, as he watched the little cyclone move purposefully into the west, until it disappeared below the graceful line of palms just inside the ramparts of the Pleasure Gardens.

Then he glanced up at the invisible engineers and scientists, racing round the world in their man-made heavens.

"Very impressive," he said. "But I hope you know exactly what you're doing."

25. Orbital Roulette

"I should have guessed," said the banker ruefully, "that it would have been in one of those technical appendices that I never looked at. And now you've seen the whole report, I'd like to know the answer. You've had me worrying, ever since you raised the problem."

"It's brilliantly obvious," Morgan answered, "and I should have thought of it myself."

And I would have done – eventually – he told himself, with a fair degree of confidence. In his mind's eye he saw again those computer simulations of the whole immense structure, twanging like a cosmic violin string, as the hours-long vibrations raced from earth to orbit and were reflected back again. And superimposed on that he replayed from memory, for the hundredth time, the scratched movie of the dancing bridge. There were all the clues he needed.

"Phobos sweeps past the tower every eleven hours and ten minutes, but luckily it isn't moving in exactly the same plane – or we'd have a collision every time it went round. It misses on most revolutions and the danger times are exactly predictable – to a thousandth of a second, if desired. Now the elevator, like any piece of engineering, isn't a completely rigid structure. It has natural vibration periods, which can be calculated almost as accurately as planetary orbits. So what your engineers propose to do is to tune the elevator, so that its normal oscillations – which can't be avoided anyway – always keep it clear of Phobos. Every time the satellite passes by the structure, it isn't there – it's sidestepped the danger zone by a few kilometres."

There was a long pause from the other end of the circuit.

"I shouldn't say this," said the Martian at last, "but my hair is standing on end."

Morgan laughed. "Put as bluntly as this, it does sound like – what was it called – Russian Roulette. But remember, we're dealing with exactly predictable movements. We always know where Phobos will be, and we can control the displacement of the tower, simply by the way we schedule traffic along it."

"Simply," thought Morgan, was hardly the right word, but anyone could see that it was possible. And then an analogy flashed into his mind that was so perfect, yet so incongruous, that he almost burst into laughter. No – it would not be a good idea to use it on the banker.

Once again, he was back at the Tacoma Narrows Bridge, but this time in a world of fantasy. There was a ship that had to sail beneath it, on a perfectly regular schedule. Unfortunately, the mast was a metre too tall.

No problem. Just before it was due to arrive, a few heavy trucks would be sent racing across the bridge, at intervals carefully calculated to match its resonant frequency. A gentle wave would sweep along the roadway from pier to pier, the crest timed to coincide with the arrival of the ship. And so the mast-head would glide beneath, with whole centimetres to spare… On a scale thousands of times larger, this was how Phobos would miss the structure towering out into space from Mons Pavonis.

"I'm glad to have your assurance," said the banker, "but I think I'd do a private check on the position of Phobos before I take a trip."