t could so easily have turned out differently. About 4.5bn years ago, the Earth is believed to have collided with another planet, Theia, resulting in the formation of the moon. A more glancing blow might have resulted in a “hit-and-run” and a moon-less Earth; while more of a head-on collision may have blasted away much of Earth’s mantle, leaving no atmosphere. Instead, it seems to have been something in between, which destroyed 10-60% of Earth’s atmosphere, but also left us with the moon. Jacob Kegerreis, from Durham University, and colleagues used a supercomputer to explore the possible outcomes when two rocky planets bash into each other. From high-angle slow impacts to head-on high-speed collisions, the scientists simulated over 300 different kinds of impact. Their findings, published in the Astrophysical Journal Letters, show that there are a wide range of impact scenarios that could have formed the moon, ranging from a Mars-sized planet having a grazing (45°) and relatively slow collision with early Earth, to a faster and more direct collision between two planets of more equal mass. “We hope this will help us understand the history of Earth’s atmosphere and narrow down the different ways the moon might have formed,” says Kegerreis.