There was a cloud, of gas and dust, slowly spinning. Much of it was the hydrogen and helium which had emerged from the Big Bang itself, but it was tainted by crystals of ice — ammonia, water and methane — and dust motes rich in iron, magnesium and silica, even some grains of pure metal. These were flotsam from older stars, stars already dead.

…And now another star died, a giant, in the conclusive spasm of supernova. A flood of energy and matter hammered into the cloud.

The cloud lost its stability, and began to collapse, to a spinning disc. The central mass shone cherry red, then gradually brightened to white, until — after a hundred million years — it burst into fusion life.

It was the protostar which would become the sun.

Within the disc, solid panicles began to crystallize. There were grains of rock — silicate minerals called olivines and pyroxenes — and minerals of iron and nickel, kamacite and taenite. The particles, stuck together by melting ice, formed planetesimals, muddy lumps which swarmed on looping, irregular orbits around the sun.

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?”

Of course I know that. “Yes, Dad.”