He had looked long and hard at the northern face of Rama, choosing the point of touchdown. After much thought, he had decided to avoid the obvious spot—the exact centre, on the axis itself. A clearly marked circular disc, a hundred metres in diameter, was centred on the Pole, and Norton had a strong suspicion that this must be the outer seal of an enormous airlock. The creatures who had built this hollow world must have had some way of taking their ships inside. This was the logical place for the main entrance, and Norton thought it might be unwise to block the front door with his own vessel.

But this decision generated other problems. If Endeavour touched down even a few metres from the axis, Rama’s rapid spin would start her sliding away from the pole. At first, the centrifugal force would be very weak, but it would be continuous and inexorable. Commander Norton did not relish the thought of his ship slithering across the polar plain, gaining speed minute by minute until it was slung off into space at a thousand kilometres an hour when it reached the edge of the disc.

It was possible that Rama’s minute gravitational field—about one thousandth of Earth’s—might prevent this from happening. It would hold Endeavour against the plain with a force of several tons, and if the surface was sufficiently rough the ship might stay near the Pole. But Commander Norton had no intention of balancing an unknown frictional force against a quite certain centrifugal one.

Fortunately, Rama’s designers had provided an answer. Equally spaced around the polar axis were three low, pillbox-shaped structures, about ten metres in diameter. If Endeavour touched down between any two of these, the centrifugal drift would fetch her up against them and she would be held firmly in place, like a ship glued against a quayside by the incoming waves.

“Contact in fifteen seconds,” said Joe. As he tensed himself above the duplicate controls, which he hoped he would not have to touch, Commander Norton became acutely aware of all that had come to focus on this instant of time. This, surely, was the most momentous landing since the first touchdown on the Moon, a century and a half ago.