I moved away from him, sick of the sound of his voice. ‘You’ll get used to stuff like this, Jerry,’ he called softly from behind me. ‘We all did. Even the old you managed, in the end.’

I needed to put some space between us, but there wasn’t really anywhere to go on that rooftop, particularly given how crowded it was.

‘Here it comes,’ I heard someone shout.

It was as if the sun had dropped from out of the sky, trailing flames. The asteroid was now much too bright to see directly, and I instinctively averted my gaze. I looked instead over towards Mayer and his entourage, all of whom had put on dark glasses, so they could follow the asteroid’s plummeting descent with relative ease.

I looked down at the concrete beneath my feet, seeing my shadow sweep in an arc as the asteroid roared through the atmosphere at tens of thousands of kilometres an hour. We were close enough to the impact zone that I could actually feel the heat on my face, even at a distance of a few hundred kilometres. The land directly beneath its path must surely be in flames.

And then, almost as soon as it had appeared, the asteroid was gone from sight. It was, I knew, busy burying itself deep in the Earth’s crust, somewhere just off the East Coast of the USA. A wall of burning air would soon come rushing towards us at thousands of kilometres an hour, flattening everything in its path, and right behind that would be earthquakes of a kind never experienced by human beings in this alternate.

And then, finally, the ocean would come sweeping inland in a series of kilometre-high tsunamis, drowning the entire continent and delivering the final blow to anything left standing by the first wave of destruction.

I looked up again in time to see a black wall rising from beyond the horizon, rushing towards us at phenomenal speed. I felt a kind of primal terror I had never experienced, even when confronted by the night patrol. A gust swept across the roof, catching at people’s hair; one of the children giggled nervously, and the building beneath our feet began to sway, gently.

My ears popped in the same instant that windows shattered all across Philadelphia. The building’s swaying grew in intensity, and I heard a stifled scream.

‘Everybody in place,’ Mayer shouted, ushering the tourists back towards the stairwell. ‘We’ve got a couple of minutes, tops, before the shockwave reaches Philly.’

They’re cutting it too close, I thought. Much too close.

We followed after them, crowding down the narrow stairs to the lower floor, all of us clustering together at the centre of the transfer stage. There was barely enough room to fit all of us inside.