‘Great idea,’ cried Rocky Rocket, back from his lunar pleasure-trip, and circled the half-finished tower, which immediately collapsed in on itself. Tsiolkovsky trembled, paled and went back to join his ancestors.
‘Yeah.’ Julian shrugged regretfully. ‘That was the weak point in Tsiolkovsky’s plan. No material in the world seemed stable enough for such a construction. The tower would inevitably collapse under its own weight, or be torn apart by the forces exerted upon it. It was only in the fifties that the idea regained popularity, except that now people were thinking about firing a satellite into geostationary orbit and lowering a cable from there to the Earth—’
‘Erm – excuse me,’ Rocky Rocket cleared his throat.
‘Yes? What is it?’
‘This is embarrassing, boss, but—’ The little rocket blushed and awkwardly scraped its stubby fins. ‘What does geostationary mean exactly?’
Julian laughed. ‘No problem, Rocky, Sir Isaac, an apple, please.’
‘Got it,’ said Newton, and slung another apple in the air. This time the fruit sped straight into the air, showing no signs of falling back again.
‘If we imagine that the Earth and similar bodies aren’t there, no gravity is exerted on the apple. According to the impulse that accelerated its mass when thrown by Sir Isaac Newton’s muscles, it will fly and fly without ever coming to a standstill. We know this effect as centrifugal force. Let’s put the Earth back where it was, and now gravitation, which we’ve already mentioned, comes into play, to some extent counteracting centrifugal force. If the apple is far enough away from the Earth, the Earth’s field of gravity has become too weak to bring it back, and it will disappear into space. If it’s too close, the Earth’s gravity will pull it back. Now, geostationary orbit, GEO in short, is found at the exact point where the Earth’s force of attraction and centrifugal force balance one another out perfectly, at an altitude of 35,786 kilometres. From there, the apple can neither escape nor fall back down. Instead, it remains for ever in GEO, as long as it circles the Earth synchronously with its rotational velocity, which is why a geostationary object always seems to stand above the same point.’
The Earth spun before their eyes. Newton’s apple seemed to stand motionlessly above the equator, fixed to an island in the Pacific. It wasn’t really standing still, of course, it was circling the planet at a speed of 11,070 kilometres per hour, while the Earth rotated below it at 1674 kilometres per hour, measured at the equator. The effect was startling. Just as the valve of a bicycle tyre always stands above the same point on the hub when the wheel is turned, the satellite stayed in place, as if nailed up above the island.
‘Geostationary orbit is ideal for a space elevator. First for the stable installation of the top floor in a stable position, secondly because of the fixed position of that floor. So once it was clear that you would just need to lower a cable 35,786 kilometres long from that point and anchor it to the ground, the question arose of what loads such a cable would have to support. The greatest tension would arise at the centre of gravity, in the GEO itself, which meant that a cable would have to become either broader or more resilient towards the top.’
Immediately just such a cable stretched between the island and the satellite, into which the apple had suddenly transmuted. Small cabins travelled up and down it.
‘In this context a further consideration arose. Why not extend the cable beyond the centre of gravity? To recap: in geostationary orbit gravity and centrifugal force balance one another out. Beyond it, the relationship between the two forces alters in favour of centrifugal force. A vehicle climbing the cable from the Earth needs to use only a tiny fraction of the energy that would be required to catapult it upwards on a rocket. With increasing altitude the influence of gravity declines in favour of centrifugal force, which means that less and less energy is required until hardly any at all is needed in geostationary orbit. Now, if we imagine the cable being extended to an altitude of 143,800 kilometres, the vehicle could go charging beyond the geostationary orbit: it would be continuously accelerated and would actually gain in energy. A perfect springboard for interstellar travel, to Mars or anywhere else!’
The cabins were now transporting construction components into orbit, to be assembled into a space station. Rocky Rocket loaded up the cabins and started visibly sweating.
‘One way or another the advantages of a space elevator were quite obvious. To carry a kilo of cargo load to an altitude of almost 36,000 kilometres, you no longer needed 50,000 dollars, just 200, and you could also use the lift 365 days a year around the clock. Suddenly the idea of building gigantic space stations and adequately armoured spaceships no longer seemed like a problem. The colonisation of space became a tangible possibility, and inspired the British science-fiction author Arthur C. Clarke to write his novel The Fountains of Paradise, in which he describes the construction of space elevators like this.’
‘But why does the thing have to be built at the equator, of all places?’ asked Rocky Rocket, wiping the sweat from his tip. ‘Why not at the North Pole or the South Pole, where it’s nice and cool? And why in the middle of the stupid sea and not, for example in’ – his eyes gleamed, he took a few dance steps and clicked his fingers – ‘Las Vegas?’
‘I’m not sure if you seriously want to set off for space surrounded by penguins,’ Julian replied sceptically. ‘But it wouldn’t work anyway. It’s only at the equator that you can exploit the Earth’s rotation to achieve a maximum of centrifugal force. It’s only there that geostationary objects are possible.’ He thought for a moment. Then he said, ‘Listen, I want to explain something to you. Imagine you’re a hammer-thrower.’
The little rocket seemed to like the idea. He threw out his chest and tensed his muscles.
‘Where’s the hammer?’ he crowed. ‘Bring it here!’
‘It’s not a real hammer these days, idiot, that’s just its name. These days the hammer is a metal ball on a steel cable.’ Julian conjured the object out of nowhere and pressed the handle firmly into both of Rocky’s hands. ‘Now you have to spin on your axis with your arms outstretched.’
‘Why?’
‘To speed up the hammer. Let it spin.’
‘Heavy, isn’t it?’ Rocky groaned and pulled on the steel cable. He started to spin around, faster and faster. The cable tightened, the sphere lifted from the ground and reached a horizontal position. ‘Can I throw it now?’ he panted.
‘In a minute. For now you’ve just got to imagine you’re not Rocky, you’re the planet Earth. Your head is the North Pole, your feet are the South Pole. In between them is the axis that you’re spinning around. If that’s the case, what’s the middle of your body?’
‘Huh? What? The equator, obviously.’
‘Well done.’
‘Can I throw it now?’
‘Wait. From the middle of your body, the equator, the hammer swings out, pulled tight by centrifugal force, just as the cable of the space elevator must be pulled tight.’
‘I get it. Can I do it?’
‘Just one moment! Your hands are, in a sense, our Pacific islands, the metal sphere is the satellite or the space station in geostationary orbit. That clear?’
‘It’s clear.’
‘Okay. Now raise your hands. Go on spinning, but lift them high above your head.’
Rocky followed the instructions. The steel cable immediately lost its tension and the ball came crashing down on the little rocket. He rolled his eyes, staggered and fell to the ground.