When the Earth Had Two Moons: Cannibal Planets, Dreadful Orbits, Icy Giants, Dirty Comets and the Origins of Today's Night Sky

An astonishing exploration of planet formation and the origins of life by one of the world’s most innovative planetary geologists.

In 1959, the Soviet probe Luna 3 took the first photos of the far side of the moon. Even in their poor resolution, the images stunned scientists: the far side is an enormous mountainous expanse, not the vast lava-plains seen from Earth. Subsequent missions have confirmed this in much greater detail.

How could this be, and what might it tell us about our own place in the universe? As it turns out, quite a lot.

Fourteen billion years ago, the universe exploded into being, creating galaxies and stars. Planets formed out of the leftover dust and gas that coalesced into larger and larger bodies orbiting around each star. In a sort of heavenly survival of the fittest, planetary bodies smashed into each other until solar systems emerged. Curiously, instead of being relatively similar in terms of composition, the planets in our solar system, and the comets, asteroids, satellites and rings, are bewitchingly distinct. So, too, the halves of our moon.

In When the Earth Had Two Moons, esteemed planetary geologist Erik Asphaug takes us on an exhilarating tour through the farthest reaches of time and our galaxy to find out why. Beautifully written and provocatively argued, When the Earth Had Two Moons is not only a mind-blowing astronomical tour but a profound inquiry into the nature of life here―and billions of miles from home.

Erik Asphaug is a professor in the Lunar and Planetary Laboratory at the University of Arizona. He studies planet formation and evolution and has been on the science teams of numerous past and upcoming NASA and international space missions. He lives with his family in Arizona.