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. 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.