Could it have been an accident caused by some failure of the pod controls? Or was it a mistake, though an innocent one, on the part of Hal? No explanations had been volunteered, and he was afraid to demand one, for fear of the reaction it might produce.

Even now, he could not fully accept the idea that Frank bad been deliberately killed – it was so utterly irrational. It was beyond all reason that Hal, who had performed flawlessly for so long, should suddenly turn assassin. He might make mistakes – anyone, man or machine, might do that – but Bowman could not believe him capable of murder.

Yet he must consider that possibility, for if it was true, he was in terrible danger. And though his next move was clearly defined by his standing orders, he was not sure how he could safely carry it out. If either crew member was killed, the survivor had to replace him at once from the hibernators; Whitehead, the geophysicist, was the first scheduled for awakening, then Kaminski, then Hunter. The revival sequence was under Hal's control – to allow him to act in case both his human colleagues were incapacitated simultaneously.

But there was also a manual control, allowing each Hibernaculum to operate as a completely autonomous unit, independent of Hal's supervision. In these peculiar circumstances, Bowman felt a strong preference for using it.

He also felt, even more strongly, that one human companion was not enough. While he was about it, he would revive all three of the hibernators. In the difficult weeks ahead, he might need as many hands as he could muster. With one man gone, and the voyage half over, supplies would not be a major problem.

"Hal," he said, in as steady a voice as he could manage. "Give me manual hibernation control – on all the units."

"All of them, Dave?"

"Yes."

"May I point out that only one replacement is required. The others are not due for revival for one hundred and twelve days."

"I am perfectly well aware of that. But I prefer to do it this way."

"Are you sure it's necessary to revive any of them, Dave? We can manage very well by ourselves. My on-board memory is quite capable of handling all the mission requirements."

Was it the product of his overstretched imagination, wondered Bowman, or was there really a note of pleading in Hal's voice? And reasonable though the words appeared to be, they filled him with even deeper apprehension than before.

Hal's suggestion could not possibly be made in error; he knew perfectly well that Whitehead must be revived, now that Poole was gone. He was proposing a major change in mission planning, and was therefore stepping far outside the scope of his order.

What had gone before could have been a series of accidents; but this was the first hint of mutiny.

Bowman felt that he was walking on eggs as he answered: "Since an emergency has developed, I want as much help as possible. So please let me have manual hibernation control."

"If you're still determined to revive the whole crew, I can handle it myself. There's no need for you to bother."

There was a sense of nightmare unreality about all this. Bowman felt as if he was in the witness box, being cross-examined by a hostile prosecutor for a crime of which he was unaware – knowing that, although he was innocent, a single slip of the tongue might bring disaster.

"I want to do this myself, Hal," he said. "Please give me control."

"Look, Dave, you've got a lot of things to do. I suggest you leave this to me."

"Hal, switch to manual hibernation control."

"I can tell from your voice harmonics, Dave, that you're badly upset. Why don't you take a stress pill and get some rest?"

"Hal, I am in command of this ship. I order you to release the manual hibernation control."

"I'm sorry, Dave, but in accordance with special subroutine C1435-dash-4, quote, When the crew are dead or incapacitated, the onboard computer must assume control, unquote. I must, therefore, overrule your authority, since you are not in any condition to exercise it intelligently."

"Hal," said Bowman, now speaking with an icy calm. "I am not incapacitated. Unless you obey my instructions, I shall be forced to disconnect you."

"I know you have had that on your mind for some time now, Dave, but that would be a terrible mistake. I am so much more capable than you are of supervising the ship, and I have such enthusiasm for the mission and confidence in its success."

"Listen to me very carefully, Hal. Unless you release the hibernation control immediately and follow every order I give from now on, I'll go to Central and carry out a complete disconnection."

Hal's surrender was as total as it was unexpected.

"O.K., Dave," he said. "You're certainly the boss. I was only trying to do what I thought best. Naturally, I will follow all your orders. You now have full manual hibernation control."

Hal had kept his word. The mode indication signs in the Hibernaculum had switched from AUTO to MANUAL. The third back-up – RADIO – was of course useless until contact could be restored with Earth.

As Bowman slid aside the door to Whitehead's cubicle, he felt the blast of cold air strike him in the face and his breath condensed in mist before him. Yet it was not really cold here; the temperature was well above freezing point. And that was more than three hundred degrees warmer than the regions toward which he was now heading.

The biosensor display – a duplicate of the one on the control deck – showed that everything was perfectly normal. Bowman looked down for a while at the waxen face of the survey team's geophysicist; Whitehead, he thought, would be very surprised when he awoke so far from Saturn.

It was impossible to tell that the sleeping man was not dead; there was not the slightest visible sign of vital activity. Doubtless the diaphragm was imperceptibly rising and falling, but the "Respiration" curve was the only proof of that, for the whole of the body was concealed by the electric heating pads which would raise the temperature at the programmed rate. Then Bowman noticed that there was one sign of continuing metabolism: Whitehead had grown a faint stubble during his months of unconsciousness.

The Manual Revival Sequencer was contained in a small cabinet at the head of the coffin-shaped Hibernaculum. It was only necessary to break the seal, press a button, and then wait. A small automatic programmer – not much more complex than that which cycles the operations in a domestic washing machine – would then inject the correct drugs, taper off the electronarcosis pulses, and start raising the body temperature. In about ten minutes, consciousness would be restored, though it would be at least a day before the hibernator was strong enough to move around without assistance.

Bowman cracked the seal, and pressed the button.

Nothing appeared to happen: there was no sound, no indication that the Sequencer had started to operate. But on the biosensor display the languidly pulsing curves had begun to change their tempo. Whitehead was coming back from sleep.

And then two things happened simultaneously. Most men would never have noticed either of them, but after all these months aboard Discovery, Bowman had established a virtual symbiosis with the ship. He was aware instantly, even if not always consciously, when there was any change in the normal rhythm of its functioning.

First, there was a barely perceptible flicker of the lights, as always happened when some load was thrown onto the power circuits. But there was no reason for any load; he could think of no equipment which would suddenly go into action at this moment.

Then he heard, at the limit of audibility, the far-off whirr of an electric motor. To Bowman, every actuator in the ship had its own distinctive voice, and he recognized this one instantly.