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Moon Origin

February 7, 2006

When Apollo astronauts went to the moon, they wanted to find out where it came from. Now one of our listeners has the same question.


Is the moon made of earth? I’m Bob Hirshon and this is Science Update.

Listener Chuck LaRusso of Mooresville, North Carolina, heard that the moon is a chunk of the Earth that broke off, and he want to know if it’s true. Well, Chuck, we turned to astronomer Antonin Bouchez of Caltech. He says what you heard is essentially right—and the evidence is that moon rocks look a lot like Earth rocks that have been cooked in a huge collision.

So the leading theory is now that an object about the size of Mars hit the Earth sort of a glancing blow, and sprayed off a bunch of the Earth into a disk, which orbited the Earth for several hundred years and then solidified into the moon.

If you have a loony science question, call us at 1-800-why-isit. If we use use it on the air, you’ll win a Science Update mug. I’m Bob Hirshon, for AAAS, the science society.