After he had reported to the jubilant listeners on Earth and Midway, he sat for a moment recovering his breath. Strange to think that this was his second visit, but he could remember little of that first one, twelve years ago and thirty-six thousand kilometres away. During what had, for want of a better term, been called the foundation laying, there had been a small party in the Basement, and numerous zero-gee toasts had been squirted. For this was not only the very first section of the Tower to be built; it would also be the first to make contact with Earth, at the end of its long descent from orbit. Some kind of ceremony therefore seemed in order, and Morgan now recalled that even his old enemy, Senator Collins, had been gracious enough to attend and to wish him luck with a barbed but good-humoured speech. There was even better cause for celebration now.

Already Morgan could hear a faint tattoo of welcoming raps from the far side of the airlock. He undid his safety belt, climbed awkwardly on to the seat, and started to ascend the ladder. The overhead hatch gave a token resistance, as if the powers marshalled against him were making one last feeble gesture, and air hissed briefly while pressure was equalised. Then the circular plate swung open and downwards, and eager hands helped him up into the Tower. As Morgan took his first breath of the fetid air he wondered how anyone could have survived here; if his mission had been aborted, he felt quite certain that a second attempt would have been too late.

The bare, bleak cell was lit only by the solar-fluorescent panels which had been patiently trapping and releasing sunlight for more than a decade, against the emergency that had arrived at last. Their illumination revealed a scene that might have come from some old war; here were homeless and dishevelled refugees from a devastated city, huddling in a bomb shelter with the few possessions they had been able to save. Not many such refugees, however, would have carried bags labelled PROJECTION, LUNAR HOTEL CORPORATION, PROPERTY OF THE FEDERAL REPUBLIC OF MARS, or the ubiquitous MAY/NOT/BE STOWED IN VACUUM. Nor would they have been so cheerful; even those who were lying down to conserve oxygen managed a smile and a languid wave. Morgan had just returned the salute when his legs buckled beneath him, and everything blacked out. Never before in his life had he fainted, and when the blast of cold oxygen revived him his first emotion was one of acute embarrassment. His eyes came slowly into focus, and be saw masked shapes hovering over him. For a moment he wondered if he was in hospital; then brain and vision returned to normal. While he was still unconscious, his precious cargo must have been unloaded.

Those masks were the molecular sieves he had carried up to the Tower; worn over nose and mouth, they would block the CO2 but allow oxygen to pass. Simple yet technologically sophisticated, they would enable men to survive in an atmosphere which would otherwise cause instant suffocation. It required a little extra effort to breathe through them, but Nature never gives something for nothing – and this was a very small price to pay.

Rather groggily, but refusing any help, Morgan got to his feet and was belatedly introduced to the men and women he had saved. One matter still worried him: while he was unconscious, had CORA delivered any of her set speeches? He did not wish to raise the subject, but he wondered.

“On behalf of all of us,” said Professor Sessui, with sincerity yet with the obvious awkwardness of a man who was seldom polite to anyone, “I want to thank you for what you've done. We owe our lives to you.”

Any logical or coherent reply to this would have smacked of false modesty, so Morgan used the excuse of adjusting his mask to mumble something unintelligible. He was about to start checking that all the equipment had been unloaded when Professor Sessui added, rather anxiously; “I'm sorry we can't offer you a chair – this is the best we can do.” He pointed to a couple of instrument boxes, one on top of the other. “You really should take it easy.”

The phrase was familiar; so CORA had spoken. There was a slightly embarrassed pause while Morgan registered this fact, and the others admitted that they knew, and he showed that he knew they knew – all without a word being uttered, in the kind of psychological infinite regress that occurs when a group of people share completely a secret which nobody will ever mention again.

He took a few deep breaths – it was amazing how quickly one got used to the masks – and then sat down on the proffered seat. I'm not going to faint again, he told himself with grim determination. I must deliver the goods, and get out of here as quickly as possible – hopefully, before there are any more pronouncements from CORA.

“That can of sealant,” he said, pointing to the smallest of the containers he had brought, “should take care of your leak. Spray it round the gasket of the airlock; it sets hard in a few seconds. Use the oxygen only when you have to; you may need it to sleep. There's a CO2 mask for everyone, and a couple of spares. And here's food and water for three days – that should be plenty. The transporter from 10K should be here tomorrow. As for the Medikit – I hope you won't need that at all.”

He paused for breath; it was not easy to talk while wearing a CO2 filter, and he felt an increasing need to conserve his strength. Sessui's people could now take care of themselves, but he still had one further job to do – and the sooner the better.

Morgan turned to Driver Chang and said quietly: “Please help me to suit up again. I want to inspect the track.”

“That's only a thirty-minute suit you're wearing!”

“I'll need ten minutes – fifteen at the most.”

“Dr. Morgan – I'm a space-qualified operator – you're not. No-one's allowed to go out in a thirty-minute suit without a spare pack, or an umbilical. Except in an emergency, of course.”

Morgan gave a tired smile. Chang was right, and the excuse of immediate danger no longer applied. But an emergency was whatever the Chief Engineer said it was.

“I want to look at the damage,” he answered, “and examine the tracks. It would be a pity if the people from 10K can't reach you, because they weren't warned of some obstacle.”

Chang was clearly not too happy about the situation (what had that gossiping CORA jabbered while he was unconscious?), but raised no further arguments as he followed Morgan into the north lock.

Just before he closed the visor Morgan asked, “Any more trouble with the Professor?”

Chang shook his head. “I think the CO2 has slowed him down. And if he starts up again – well, we outnumber him six to one, though I'm not sure if we can count on his students. Some of them are just as crazy as he is; look at that girl who spends all her time scribbling in the corner. She's convinced that the sun's going out, or blowing up – I'm not sure which – and wants to warn the world before she dies. Much good that would do. I'd prefer not to know.”

Though Morgan could not help smiling, he felt quite sure that none of the professor's students would be crazy. Eccentric, perhaps – but also brilliant; they would not be working with Sessui otherwise. One day he must find out more about the men and women whose lives he had saved; but that would have to wait until they had all returned to earth, by their separate ways.

“I'm going to take a quick walk around the Tower,” said Morgan, “and I'll describe any damage so that you can report to Midway. It won't take more than ten minutes. And if it does – well, don't try to get me back.”

Driver Chang's reply, as he closed the inner door of the airlock, was very practical and very brief. “How the hell could I?” he asked.