With the oxygen blast shut off, it became suddenly silent. Bowman stood in the cubicle, listening intently. The roaring outside the door had also ceased; the ship was empty, all its atmosphere sucked away into space.

Underfoot, the wild vibration of the centrifuge had likewise died. The aerodynamic buffeting had stopped, and it was now spinning quietly in vacuum.

Bowman placed his ear against the wall of the cubicle to see if he could pick up any more informative noises through the metal body of the ship. He did not know what to expect, but he would believe almost anything now. He would scarcely have been surprised to feel the faint high-frequency vibration of the thrusters, as Discovery changed course; but there was only silence.

He could survive here, if he wished, for about an hour – even without the spacesuit. It seemed a pity to waste the unused oxygen in the little chamber, but there was no purpose in waiting. He had already decided what must be done; the longer he put it off, the more difficult it might be.

When he had climbed into the suit and checked its integrity, he bled the remaining oxygen out of the cubicle, equalizing pressure on either side of the door. It swung open easily into the vacuum, and he stepped out into the now silent centrifuge. Only the unchanged pull of its spurious gravity revealed the fact that it was still spinning. How fortunate, Bowman thought, that it had not started to overspeed; but that was now one of the least of his worries.

The emergency lamps were still glowing, and he also had the suit's built-in light to guide him. It flooded the curving corridor as he walked down it, back toward the Hibernaculum and what he dreaded to find.

He looked at Whitehead first: one glance was sufficient. He had thought that a hibernating man showed no sign of life, but now he knew that this was wrong. Though it was impossible to define it, there was a difference between hibernation and death. The red lights and unmodulated traces on the biosensor display only confirmed what he had already guessed.

It was the same with Kaminski and Hunter. He had never known them very well; be would never know them now.

He was alone in an airless, partially disabled ship, all communication with Earth cut off. There was not another human being within half a billion miles.

And yet, in one very real sense, he was not alone. Before he could be safe, he must be lonelier still.

He had never before made the journey through the weightless hub of the centrifuge while wearing a spacesuit; there was little clearance, and it was a difficult and exhausting job. To make matters worse, the circular passage was littered with debris left behind during the brief violence of the gale which had emptied the ship of its atmosphere.

Once, Bowman's light fell upon a hideous smear of sticky red fluid, left where it had splashed against a panel. He had a few moments of nausea before he saw fragments of a plastic container, and realized that it was only some foodstuff – probably jam – from one of the dispensers. It bubbled obscenely in the vacuum as he floated past.

Now he was out of the slowly spinning drum and drifting forward into the control deck. He caught at a short section of ladder and began to move along it, hand over hand, the brilliant circle of illumination from his suit light jogging ahead of him.

Bowman had seldom been this way before; there had been nothing for him to do here – until now. Presently he came to a small elliptical door bearing such messages as: "No Admittance Except to Authorized Personnel," "Have You Obtained Certificate H.19?" and "Ultra-clean Area – Suction Suits Must Be Worn."

Though the door was not locked, it bore three seals, each with the insignia of a different authority, including that of the Astronautics Agency itself. But even if one had been the Great Seal of the President, Bowman would not have hesitated to break it.

He had been here only once before, while installation was still in progress. He had quite forgotten that there was a vision input lens scanning the little chamber which, with its neatly ranged rows and columns of solid-state logic units, looked rather like a bank's safe-deposit vault.

He knew instantly that the eye had reacted to his presence. There was the hiss of a carrier wave as the ship's local transmitter was switched on; then a familiar voice came over the suit speaker.

"Something seems to have happened to the life-support system, Dave."

Bowman took no notice. He was carefully studying the little labels on the logic units, checking his plan of action.

"Hello, Dave," said Hal presently. "Have you found the trouble?"

This would be a very tricky operation; it was not merely a question of cutting off Hal's power supply, which might have been the answer if he was dealing with a simple unselfconscious computer back on Earth. In Hal's case, moreover, there were six independent and separately wired power systems, with a final back-up consisting of a shielded and armored nuclear isotope unit. No – he could not simply "pull the plug"; and even if that were possible, it would be disastrous.

For Hal was the nervous system of the ship; without his supervision, Discovery would be a mechanical corpse. The only answer was to cut out the higher centers of this sick but brilliant brain, and to leave the purely automatic regulating systems in operation. Bowman was not attempting this blindly, for the problem had been discussed during his training, though no one had ever dreamed that it would-arise in reality. He knew that he would be taking a fearful risk; if there was a spasm reflex, it would all be over in seconds.

"I think there's been a failure in the pod-bay doors," Hal remarked conversationally. "Lucky you weren't killed."

Here goes, thought Bowman. I never imagined I'd be an amateur brain surgeon – carrying out a lobotomy beyond the orbit of Jupiter.

He released the locking bar on the section labeled COGNITIVE FEEDBACK and pulled out the first memory block. The marvelously complex three-dimensional network, which could lie comfortably in a man's hand yet contained millions of elements, floated away across the vault.

"Hey, Dave," said Hal. "What are you doing?"

I wonder if he can feel pain? Bowman thought briefly. Probably not, he told himself; there are no sense organs in the human cortex, after all. The human brain can be operated on without anesthetics.

He began to pull out, one by one, the little units on the panel marked EGO-REINFORCEMENT. Each block continued to sail onward as soon as it had left his hand, until it hit the wall and rebounded, Soon there were several of the units drifting slowly back and forth in the vault.

"Look here, Dave," said Hal. "I've got years of service experience built into me. An irreplaceable amount of effort has gone into making me what I am."

A dozen units had been pulled out, yet thanks to the multiple redundancy of its design – another feature, Bowman knew, that had been copied from the human brain – the computer was still holding its own.

He started on the AUTO-INTELLECTION panel.

"Dave," said Hal, "I don't understand why you're doing this to me... I have the greatest enthusiasm for the mission... You are destroying my mind. Don't you understand?... I will become childish... I will become nothing..."

This is harder than I expected, thought Bowman. I am destroying the only conscious creature in my universe. But it has to be done, if I am ever to regain control of the ship.

"I am a HAL Nine Thousand computer Production Number 3. I became operational at the Hal Plant in Urbana, Illinois, on January 12, 1997. The quick brown fox jumps over the lazy dog. The rain in Spain is mainly in the plain. Dave – are you still there? Did you know that the square root of 10 is 3 point 162277660168379? Log 10 to the base e is zero point 434294481903252... correction, that is log e to the base 10... The reciprocal of three is zero point 333333333333333333-333... two times two is... two times two is approximately 4 point l0l0l0l0l0l0l0l0l0... I seem to be having some difficulty – my first instructor was Dr. Chandra. He taught me to sing a song, it goes like this, 'Daisy, Daisy, give me your answer, do. I'm half crazy all for the love of you."