Astronomers have long suspected that the moon formed after a small, proto-planet, called Theia, crashed into Earth, knocking a chunk of rock into Earth's orbit. New research by scientists at the University of California Los Angeles suggests that Theia didn't merely sideswipe Earth, but instead fused with our planet, forming both modern Earth and the moon.
Believe it or not, moon rocks returned from the old Apollo missions are still being analyzed, and it turns out that when compared with volcanic rock from here on earth, the O2 isotopes are identical. So that chunk of rock that we call the moon isn't Theia-rock, as had been believed, but is instead a combination of Theia and earth that was ejected into orbit as impact fusion got underway. This was a direct hit, not a glancing blow.