Then the camera caught, tantalizingly blurred by distance, a glimpse of something very strange. Many miles away, the golden landscape reared itself into a curiously symmetrical cone, like a volcanic mountain. Around the summit of that cone was a halo of small, puffy clouds – all about the same size, all quite distinct and isolated. There was something disturbing and unnatural about them – if, indeed, the word "natural" could ever be applied to this awesome panorama.

Then, caught by some turbulence in the rapidly thickening atmosphere, the probe twisted around to another quarter of the horizon, and for a few seconds the screen showed nothing but a golden blur. Presently it stabilized; the "sea" was much closer, but as enigmatic as ever. One could now observe that it was interrupted here and there with patches of darkness, which might have been holes or gaps leading to still deeper layers of the atmosphere.

The probe was destined never to reach them. Every mile, the density of the gas around it had been doubling, the pressure mounting as it sank deeper and deeper toward the hidden surface of the planet. It was still high above that mysterious sea when the picture gave one premonitory flicker, then vanished, as the first explorer from Earth crumpled beneath the weight of the miles of atmosphere above it.

It had given, in its brief life, a glimpse of perhaps one millionth of Jupiter, and had barely approached the planet's surface, hundreds of miles down in the deepening mists. When the picture faded from the screen, Bowman and Poole could only sit in silence, turning the same thought over in their minds.

The ancients had, indeed, done better than they knew when they named this world after the lord of all the gods. If there was life down there, how long would it take even to locate it? And after that, how many centuries before men could follow this first pioneer – in what kind of ship?

But these matters were now no concern of Discovery and her crew. Their goal was a still stranger world, almost twice as far from the Sun – across another half billion miles of comet-haunted emptiness.

The familiar strains of "Happy Birthday," hurled across seven hundred million miles of space at the velocity of light, died away among the vision screens and instrumentation of the control deck. The Poole family, grouped rather self-consciously round the birthday cake on Earth, lapsed into a sudden silence.

Then Mr. Poole, Senior, said gruffly: "Well, Frank, can't think of anything else to say at the moment, except that our thoughts are with you, and we're wishing you the happiest of birthdays."

"Take care, darling," Mrs. Poole interjected tearfully. "God bless you."

There was a chorus of "good-byes," and the vision screen went blank. How strange to think, Poole told himself, that all this had happened more than an hour ago; by now his family would have dispersed again and its members would be miles from home. But in a way that time lag, though it could be frustrating, was also a blessing in disguise. Like every man of his age, Poole took it for granted that he could talk instantly, to anyone on Earth, whenever he pleased. Now that this was no longer true, the psychological impact was profound. He had moved into a new dimension of remoteness, and almost all emotional links had been stretched beyond the yield point.

"Sorry to interrupt the festivities," said Hal, "but we have a problem."

"What is it?" Bowman and Poole asked simultaneously.

"I am having difficulty in maintaining contact with Earth. The trouble is in the AE-35 unit. My Fault Prediction Center reports that it may fail within seventy-two hours."

"We'll take care of it," Bowman replied. "Let's see the optical alignment."

"Here it is, Dave. It's still O.K. at the moment."

On the display screen appeared a perfect half-moon, very brilliant against a background almost free of stars. It was covered with clouds, and showed not one geographical feature that could be recognized. Indeed, at first glance it could be easily mistaken for Venus.

But not at a second one, for there beside it was the real Moon which Venus did not possess – a quarter the size of Earth, and in exactly the same phase. It was easy to imagine that the two bodies were mother and child, as many astronomers had believed, before the evidence of the lunar rocks had proved beyond doubt that the Moon had never been part of Earth.

Poole and Bowman studied the screen in silence for half a minute. This image was coming to them from the long-focus TV camera mounted on the rim of the big radio dish; the cross-wires at its center showed the exact orientation of the antenna. Unless the narrow pencil beam was pointed precisely at Earth, they could neither receive nor transmit. Messages in both directions would miss their target and would shoot, unheard and unseen, out through the Solar System and into the emptiness beyond. If they were ever received, it would not be for centuries – and not by men.

"Do you know where the trouble is?" asked Bowman.

"It's intermittent and I can't localize it. But it appears to be in the AE-35 unit."

"What procedure do you suggest?"

"The best thing would be to replace the unit with a spare, so that we can check it over."

"O.K. – let us have the hard copy."

The information flashed on the display screen; simultaneously, a sheet of paper slid out of the slot immediately beneath it. Despite all the electronic read-outs, there were times when good old-fashioned printed material was the most convenient form of record.

Bowman studied the diagrams for a moment, then whistled.

"You might have told us," he said. "This means going outside the ship."

"I'm sorry," Hal replied. "I assumed you knew that the AE-35 unit was on the antenna mounting."

"I probably did, a year ago. But there are eight thousand subsystems aboard. Anyway, it looks a straightforward job. We only have to unlock a panel and put in a new unit."

"That suits me fine," said Poole, who was the crew member designated for routine extravehicular activity. "I could do with a change of scenery. Nothing personal, of course."

"Let's see if Mission Control agrees," said Bowman. He sat still for a few seconds, marshaling his thoughts, then started to dictate a message.

"Mission Control, this is X-ray-Delta-One. At two-zero-four-five, on-board fault prediction center in our niner-triple-zero computer showed Alpha Echo three five unit as probable failure within seventy-two hours. Request check your telemetry monitoring and suggest you review unit in your ship systems simulator. Also, confirm your approval our plan to go EVA and replace Alpha Echo three five unit prior to failure. Mission Control, this is X-ray-Delta-One, two-one-zero-three transmission concluded."

Through years of practice, Bowman could switch at a moment's notice to this jargon – which someone had once christened "Technish" – and back again to normal speech, without clashing his mental gears. Now there was nothing to do but to wait for the confirmation, which would take at least two hours as the signals made the round trip past the orbits of Jupiter and Mars.

It came while Bowman was trying, without much success, to beat Hal at one of the geometrical pattern games stored in his memory.

"X-ray-Delta-One, this is Mission Control, acknowledging your two-one-zero-three. We are reviewing telemetric information on our mission simulator and will advise.

"Roger your plan to go EVA and replace Alpha-Echo three-five unit prior to possible failure. We are working on test procedures for you to apply to faulty unit."

The serious business having been completed, the Mission Controller reverted to normal English.