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.
Theia and Earth were neighbors, new research suggests
