If it did not explode, both Zoya and Volodya would probably be purged, sent to labour camps in Siberia or just shot. Volodya had already talked to his parents, and they had promised to take care of Kotya and Galina.

As they would if Volodya and Zoya were killed by the test.

In the strengthening light Volodya saw, at various distances around the tower, an odd variety of buildings: houses of brick and wood, a bridge over nothing, and the entrance to some kind of underground structure. Presumably the army wanted to measure the effect of the blast. Looking more carefully he saw trucks, tanks, and obsolete aircraft, placed for the same purpose, he imagined. The scientists were also going to assess the impact of the bomb on living creatures: there were horses, cattle, sheep, and dogs in kennels.

The confab on the platform ended with a decision. The three scientists nodded and resumed their work.

A few minutes later Zoya came down and greeted her husband.

‘Is everything all right?’ he said.

‘We think so,’ Zoya replied.

‘You think so?’

She shrugged. ‘We’ve never done this before, obviously.’

They got into the truck and drove, across country that was already a wasteland, to the distant control bunker.

The other scientists were close behind.

At the bunker they all put on welders’ goggles as the countdown ticked away.

At sixty seconds, Zoya held Volodya’s hand.

At ten seconds, he smiled at her and said: ‘I love you.’

At one second, he held his breath.

Then it was as if the sun had suddenly risen. A light stronger than noon flooded the desert. In the direction of the bomb tower, a ball of fire grew impossibly high, reaching for the moon. Volodya was startled by the lurid colours in the fireball: green, purple, orange and violet.

The ball turned into a mushroom whose umbrella kept rising. At last the sound arrived, a bang as if the largest artillery piece in the Red Army had been fired a foot away, followed by rolling thunder that reminded Volodya of the terrible bombardment of the Seelow Heights.

At last the cloud began to disperse and the noise faded.

There was a long moment of stunned silence.

Someone said: ‘My God, I didn’t expect that.’

Volodya embraced his wife. ‘You did it,’ he said.

She looked solemn. ‘I know,’ she said. ‘But what did we do?’

‘You saved Communism,’ said Volodya.

(ii)

‘The Russian bomb was based on Fat Man, the one we dropped on Nagasaki,’ said Special Agent Bill Bicks. ‘Someone gave them the plans.’

‘How do you know?’ Greg asked him.

‘From a defector.’

They were sitting in Bicks’s carpeted office in the Washington headquarters of the FBI at nine o’clock in the morning. Bicks had his jacket off. His shirt was stained in the armpits with sweat, though the building was comfortably air-conditioned.

‘According to this guy,’ Bicks went on, ‘a Red Army intelligence colonel got the plans from one of the scientists on the Manhattan Project team.’