Bowman himself had gone out to retrieve the suspect unit, but when it was tested it appeared, to everyone's surprise, to be in perfectly good order. The automatic checking circuits could find nothing wrong with it. Nor could Hal's twin, SAL 9000, back on Earth, when the information was transmitted to Urbana.

But Hal had insisted on the accuracy of his diagnosis, making pointed remarks about 'human error'. He had suggested that the control unit be put back in the antenna until it finally failed, so that the fault could be precisely located. No one could think of any objection, for the unit could be replaced in minutes, even if it did break down.

Bowman and Poole, however, had not been happy; they both felt that something was wrong, though neither could pinpoint it. For months they had accepted Hal as the third member of their tiny world, and knew his every mood. Then the atmosphere aboard the ship had subtly altered; there was a sense of strain in the air.

Feeling rather like traitors – as a distraught Bowman had later reported to Mission Control – the human two-thirds of the crew had discussed what should be done if their colleague was indeed malfunctioning. In the worst possible case, Hal would have to be relieved of all his higher responsibilities. This would involve disconnection – the computer equivalent of death.

Despite their doubts, they had carried out the agreed programme. Poole had flown out of Discovery in one of the little space pods that served as transporters and mobile workshops during extravehicular activities. Since the somewhat tricky job of replacing the antenna unit could not be performed by the pod's own manipulators, Poole had started to do it himself.

What happened then had been missed by the external cameras, which was a suspicious detail in itself. Bowman's first warning of disaster was a cry from Poole – then, silence. A moment later he saw Poole, tumbling over and over, spinning away into space. His own pod had rammed him, and was itself blasting away out of control.