A half century after NASA's Apollo 17 lunar module lifted off the moon's northeastern near side quadrant, planetary scientists still don't completely understand when or how our moon first formed. They do agree that it involved a major impactor—an object dubbed Theia by lunar scientists—that likely struck Earth some 4.51 billion years ago. But the estimated size of Theia now ranges from a proto-Mercury-sized object all the way up to an object that was about half the size of present-day Earth.
