At its birth, Earth had no Moon. Then something perhaps the size of Mars slammed into the young planet, flinging molten debris into orbit that became the companion world now pulling our tides, steadyi

Earth began without a Moon. The young planet, barely formed, had no companion in orbit, and how it came by one is a question that took most of a century, and a set of returned Moon rocks, to answer with any confidence. The leading explanation is a collision. Around 4.5 billion years ago, the account […]