He pointed towards the distant thread reaching up from the eastern horizon.

'That must be another one.'

'Yes – the Asian Tower. We must look exactly the same to them.'

'How many are there?'

'Just four, equally spaced around the Equator. Africa, Asia, America, Pacifica. The last one's almost empty – only a few hundred levels completed. Nothing to see except water...'

Poole was still absorbing this stupendous concept when a disturbing thought occurred to him.

'There were already thousands of satellites, at all sorts of altitudes, in my time. How do you avoid collisions?'

Indra looked slightly embarrassed.

'You know – I never thought about that – it's not my field.' She paused for a moment, clearly searching her memory. Then her face brightened.

'I believe there was a big clean-up operation, centuries ago. There just aren't any satellites, below the stationary orbit.'

That made sense, Poole told himself. They wouldn't be needed – the four gigantic towers could provide all the facilities once provided by thousands of satellites and space-stations.

'And there have never been any accidents – any collisions with spaceships leaving earth, or re-entering the atmosphere?'

Indra looked at him with surprise.

'But they don't, any more,' She pointed to the ceiling. 'All the spaceports are where they should be – up there, on the outer ring. I believe it's four hundred years since the last rocket lifted off from the surface of the Earth.'

Poole was still digesting this when a trivial anomaly caught his attention. His training as an astronaut had made him alert to anything out of the ordinary: in space, that might be a matter of life or death.

The Sun was out of view, high overhead, but its rays streaming down through the great window painted a brilliant band of light on the floor underfoot. Cutting across that band at an angle was another, much fainter one, so that the frame of the window threw a double shadow.