About 4.5 billion years ago, the most momentous event in the history of Earth occurred: a huge celestial body called Theia collided with the young Earth. How the collision unfolded and what exactly happened afterward has not been conclusively clarified. What is certain, however, is that the size, composition, and orbit of Earth changed as a result—and that the impact marked the birth of our constant companion in space, the moon.

What kind of body was it that so dramatically altered the course of our planet’s development? How big was Theia? What was it made of? And from which part of the solar system did it hurtle toward Earth?