The planetesimals collided.

Where an impact was head-on, the worldlets could be shattered. But where the collisions were gentle, the worldlets could nudge into each other, stick together, merge. Soon, some aggregations were large enough to draw in their smaller companions.

Thus, young Earth: a chaotic mixture of silicates, metals and trapped gases, cruising like a hungry shark in a thinning ring of worldlets.

Earth’s bulk was warm, for the heat of accumulation and of supernova radioactive decay was trapped inside. The metals, heavier than the silicates, sank to the centre, and around the new, hot core, a rocky mantle gathered. Gases trapped in the core were driven out, and formed Earth’s first atmosphere: a massive layer of hydrogen, helium, methane, water, nitrogen and other gases, amounting to ten per cent of Earth’s total mass.

Earth’s evolution continued, busily, logically.

But something massive was approaching.

“Look up, Tracy. Look at the Moon. You know, we take that damn thing for granted. But if it suddenly appeared in the sky, if it was Mercury hauled up here from the centre of the Solar System, my gosh, it would be the story of the century…”

It was 1973.

Her father, Jays, had been back from the Moon only a couple of months. Tracy Malone, ten years old, thought he’d come back… different.

“Look up,” he said again, and she obeyed, turning from his face to the Moon.

The face of the Man in the Moon glared down at Tracy. It was a composition of grey and white, flat and unchanging, hanging like a lantern in the muggy Houston sky.

“The Moon looks like a disc,” said her father, in his stiff schoolteacher way. “But it isn’t. That’s an optical illusion. It’s a rocky world, a ball. You know that, don’t you, sweet pea?”