'I'm sorry,' said the suddenly contrite Chang. 'I shouldn't have kept you up.'

'Oh no – it's extremely interesting. Please go on.'

'Definitely not. Maybe some other time. Goodnight, Rosie – and thanks for the coffee.'

'Goodnight, Sir.'

Steward Third Class Rose McCullen glided (not too skilfully) towards the still open door. Chang did not bother to look back when he heard it close.

It was thus a considerable shock when, a few seconds later, he was addressed by a completely unfamiliar female voice.

'Mr Chang – don't bother to touch the alarm button – it's disconnected. Here are the landing coordinates. Take the ship down.'

Slowly, wondering if he had somehow dozed off and was having a nightmare, Chang rotated his chair.

The person who had been Rose McCullen was floating beside the oval hatchway, steadying herself by holding on to the locking lever of the door. Everything about her seemed to have changed; in a moment of time, their roles had been reversed. The shy steward – who had never before looked at him directly – was now regarding Chang with a cold, merciless stare that made him feel like a rabbit hypnotized by a snake. The small but deadly-looking gun nestling in her free hand seemed an unnecessary adornment; Chang had not the slightest doubt that she could very efficiently kill him without it.

Nevertheless, both his self-respect and his professional honour demanded that he should not surrender without some sort of a struggle. At the very least, he might be able to gain time.

'Rose,' he said – and now his lips had difficulty in forming a name which had become suddenly inappropriate – 'this is perfectly ridiculous. What I told you just now – it's simply not true. I couldn't possibly land the ship by myself. It would take hours to compute the correct orbit, and I'd need someone to help me. A co-pilot, at least.'

The gun did not waver.

'I'm not a fool, Mr Chang. This ship isn't energy-limited, like the old chemical rockets. The escape velocity of Europa is only three kilometres a second. Part of your training is an emergency landing with the main computer down. Now you can put it into practice: the window for an optimum touchdown at the coordinates I will give you opens in five minutes.'

'That type of abort,' said Chang, now beginning to sweat profusely, 'has an estimated twenty-five per cent failure rate' – the true figure was ten per cent, but in the circumstances he felt that a little exaggeration was justified – 'and it's years since I checked out on it.'

'In that case,' answered Rose McCullen, 'I'll have to eliminate you and ask the Captain to send me someone more qualified. Annoying, because we'll miss this window and have to wait a couple of hours for the next one. Four minutes left.'

Second Officer Chang knew when he was beaten; but at least he had tried.

'Let me have those coordinates,' he said.

Captain Laplace woke instantly at the first gentle tapping, like a distant woodpecker, of the attitude control jets. For a moment he wondered if he was dreaming: no, the ship was definitely turning in space.

Perhaps it was getting too hot on one side and the thermal control system was making some minor adjustments. That did happen occasionally, and was a black mark for the officer on duty, who should have noticed that the temperature envelope was being approached.

He reached for the intercom button to call – who was it? – Mr Chang on the bridge. His hand never completed the movement.

After days of weightlessness, even a tenth of a gravity is a shock. To the Captain it seemed like minutes, though it must have been only a few seconds, before he could unbuckle his restraining harness and struggle out of his bunk. This time, he found the button and jabbed it viciously. There was no reply.

He tried to ignore the thuds and bumps of inadequately secured objects that had been taken unawares by the onset of gravity. Things seemed to go on falling for a long time, but presently the only abnormal sound was the muffled, far-off scream of the drive at full blast.

He tore open the curtain of the cabin's little window, and looked out at the stars. He knew roughly where the ship's axis should have been pointing; even if he could only judge it to within thirty or forty degrees, that would allow him to distinguish between the two possible alternatives.

Galaxy could be vectored either to gain, or to lose, orbital velocity. It was losing it – and therefore preparing to fall towards Europa.

There was an insistent banging on the door, and the Captain realized that little more than a minute could really have passed. Second Officer Floyd and two other crew members were crowded in the narrow passageway.

'The bridge is locked, Sir,' Floyd reported breathlessly. 'We can't get in – and Chang doesn't answer. We don't know what's happened.'

'I'm afraid I do,' Captain Laplace answered, climbing into his shorts. 'Some madman was bound to try it sooner or later. We've been hijacked, and I know where. But I'm damned if I know why.'

He glanced at his watch, and did a quick mental calculation.

'At this thrust level, we'll have deorbited within fifteen minutes – make it ten for safety. Any way we can cut the drive without endangering the ship?'

Second Officer Yu, Engineering, looked very unhappy, but volunteered a reluctant reply.

'We could pull the circuit breakers in the pump motor lines, and cut off the propellant supply.'

'Can we get at them?'

'Yes – they're on Deck Three.'

'Then let's go.'

'Er – then the independent backup system would take over. For safety, that's behind a sealed bulkhead on Deck Five – we'd have to get a cutter – no, it couldn't be done in time.'

Captain Laplace had been afraid of that. The men of genius who had designed Galaxy had tried to protect the ship from all plausible accidents. There was no way they could have safeguarded it against human malevolence.

'Any alternatives?'

'Not in the time available, I'm afraid.'

'Then let's get to the bridge and see if we can talk to Chang – and whoever is with him.'

And who could that be? he wondered. He refused to believe that it could be one of his regular crew. That left – of course, there was the answer! He could see it all. Monomaniac researcher tries to prove theory – experiments frustrated – decides that the quest for knowledge takes precedence over everything else.

It was uncomfortably like one of those cheap 'mad scientist' melodramas, but it fitted the facts perfectly. He wondered if Dr Anderson had decided that this was the only road to a Nobel Prize.

That theory was swiftly demolished when the breathless and dishevelled geologist arrived gasping:

'For God's sake, Captain – what's happening? We're under full thrust! Are we going up – or down?'

'Down,' answered Captain Laplace. 'In about ten minutes, we'll be in an orbit that will hit Europa. I can only hope that whoever's at the controls knows what he's doing.'

Now they were at the bridge, facing the closed door. Not a sound came from the far side.

Laplate rapped as loudly as he possibly could without bruising his knuckles.

'This is the Captain! Let us in!'

He felt rather foolish at giving an order which would certainly be ignored, but he hoped for at least some reaction. To his surprise, he got one.

The external speaker hissed into life, and a voice said: 'Don't attempt anything foolish, Captain. I have a gun, and Mr Chang is obeying my orders.'

'Who was that?' whispered one of the officers. 'It sounds like a woman!'

'You're right,' said the Captain grimly. That certainly cut down the alternatives, but didn't help matters in any way.

'What do you hope to do? You know you can't possibly get away with it!' he shouted, trying to sound masterful rather than plaintive.