The moon's formation still remains a mystery in many ways

Half a century after NASA’s Apollo 17 lunar module lifted off from the Moon’s northeastern side quadrant, planetary scientists still don’t completely understand when or how our Moon first formed. They agree that it was a major impactor — an object nicknamed Theia by lunar scientists — that likely hit Earth about 4.51 billion years ago. But Theia’s estimated size now ranges from an object the size of a proto-Mercury up to an object that was about half the size of present-day Earth.