They had some good arguments, of course – including the results of a secret Department of Defense study, Project BARSOOM, which had been carried out by Harvard's School of Psychology in 1989. In this experiment in controlled sociology, various sample populations had been assured that the human race had made contact with extraterrestrials. Many of the subjects tested were – with the help of drugs, hypnosis, and visual effects – under the impression that they had actually met creatures from other planets, so their reactions were regarded as authentic.

Some of these reactions had been quite violent; there was, it seemed, a deep vein of xenophobia in many otherwise normal human beings. In view of mankind's record of lynchings, pogroms, and similar pleasantries, this should have surprised no one; nevertheless, the organizers of the study had been deeply disturbed, and the results had never been released. The five separate panics caused in the twentieth century by radio broadcasts of H.G. Wells's War of the Worlds also reinforced the study's conclusions.

Despite these arguments, Bowman sometimes wondered if the cultural shock danger was the only explanation for the mission's extreme secrecy. Some hints that had been dropped during his briefings suggested that the U.S.-U.S.S.R. bloc hoped to derive advantage by being the first to contact intelligent extraterrestrials.

From his present viewpoint, looking back on Earth as a dim star almost lost in the Sun, such considerations now seemed ludicrously parochial.

He was rather more interested – even though this was now very much water under the bridge – in the theory put forward to account for Hal's behavior. No one would ever be sure of the truth, but the fact that one of the Mission Control 9000s had been driven into an identical psychosis, and was now under deep therapy, suggested that the explanation was the correct one. The same mistake would not be made again; and the fact that Hal's builders had failed fully to understand the psychology of their own creation showed how difficult it might be to establish communication with truly alien beings.

Bowman could easily believe Dr. Simonson's theory that unconscious feelings of guilt, caused by his program conflicts, had made Hal attempt to break the circuit with Earth. And he liked to think – though this again was something that could never be proved – that Hal had no intention of killing Poole. He had merely tried to destroy the evidence; for once the AE-35 unit reported as burned out was proved to be operational, his lie would be revealed. After that, like any clumsy criminal caught in a thickening web of deception, he had panicked.

And panic was something that Bowman understood better than he had any wish to, for he had known it twice during his life. The first time was as a boy, when he bad been caught in a line of surf and nearly drowned; the second was as a spaceman under training, when a faulty gauge had convinced him that his oxygen would be exhausted before he could reach safety.

On both occasions, he had almost lost control of all his higher logical processes; he had been within seconds of becoming a frenzied bundle of random impulses. Both times he had won through, but he knew well enough that any man, in the right circumstances, could be dehumanized by panic.

If it could happen to a man, then it could happen to Hal; and with that knowledge the bitterness and the sense of betrayal he felt toward the computer began to fade. Now, in any event, it belonged to a past that was wholly overshadowed by the threat, and the promise, of the unknown future.

Apart from hasty meals back in the carrousel – luckily the main food dispensers had not been damaged – Bowman practically lived on the control deck. He catnapped in his seat, and so could spot any trouble as soon as the first signs of it appeared on the display. Under instructions from Mission Control, he had jury-rigged several emergency systems, which were working tolerably well. It even seemed possible that he would survive until the Discovery reached Saturn – which, of course, she would do whether he was alive or not.

Though he had little enough time for sightseeing, and the sky of space was no novelty to him, the knowledge of what now lay out there beyond the observation ports sometimes made it difficult for him to concentrate even on the problem of survival. Dead ahead, as the ship was now oriented, sprawled the Milky Way, with its clouds of stars so tightly packed that they numbed the mind. There were the fiery mists of Sagittarius, those seething swarms of suns that forever hid the heart of the galaxy from human vision. There was the ominous black shadow of the Coal Sack, that hole in space where no stars shone. And there was Alpha Centauri, nearest of all alien suns – the first stop beyond the Solar System.

Although outshone by Sirius and Canopus, it was Alpha Centauri that drew Bowman's eyes and mind whenever he looked out into space. For that unwavering point of brightness, whose rays had taken four years to reach him, had come to symbolize the secret debates that now raged on Earth, and whose echoes came to him from time to time.

No one doubted that there must be some connection between TMA-1 and the Saturnian system, but hardly any scientists would admit that the creatures who had erected the monolith could possibly have originated there. As an abode of life, Saturn was even more hostile than Jupiter, and its many moons were frozen in an eternal winter three hundred degrees below zero. Only one of them – Titan – possessed an atmosphere; and that was a thin envelope of poisonous methane.

So perhaps the creatures who had visited Earth's Moon so long ago were not merely extraterrestrial, but extrasolar – visitors from the stars, who had established their bases wherever it suited them. And this at once raised another problem: could any technology, no matter how advanced, bridge the awful gulf that lay between the Solar System and the nearest alien sun?

Many scientists flatly denied the possibility. They pointed out that Discovery, the fastest ship ever designed, would take twenty thousand years to reach Alpha Centauri – and millions of years to travel any appreciable distance across the galaxy. Even if, during the centuries to come, propulsion systems improved out of all recognition, in the end they would meet the impassable barrier of the speed of light, which no material object could exceed. Therefore, the builders of TMA-1 must have shared the same sun as man; and since they had made no appearance in historic times, they were probably extinct.

A vocal minority refused to agree. Even if it took centuries to travel from star to star, they contended, this might be no obstacle to sufficiently determined explorers. The technique of hibernation, used on Discovery herself, was one possible answer. Another was the self-contained artificial world, embarking on voyages that might last for many generations.

In any event, why should one assume that all intelligent species were as short-lived as Man? There might be creatures in the universe to whom a thousand-year voyage would present nothing worse than slight boredom...

These arguments, theoretical though they were, concerned a matter of the utmost practical importance; they involved the concept of "reaction time." If TMA-1 had indeed sent a signal to the stars – perhaps with the help of some further device near Saturn – then it would not reach its destination for years. Even if the response was immediate, therefore, humanity would have a breathing space which could certainly be measured in decades – more probably in centuries. To many people, this was a reassuring thought.

But not to all. A few scientists – most of them beachcombers on the wilder shores of theoretical physics – asked the disturbing question: "Are we certain that the speed of light is an unbreakable barrier?" It was true that the Special Theory of Relativity had proved to be remarkably durable, and would soon be approaching its first centenary; but it had begun to show a few cracks. And even if Einstein could not be defied, he might be evaded.