In the far past, about 4-5 Billion years ago, an alien civilization called the Grenar was expanding its empire. When they came across Earth, they found the high temperature variations caused by a lack of a moon very unfavorable. So they towed an asteroid to orbit it, but a pilot fell asleep, knocked his bjanga juice (their coffee) over. This short circuted the ship and caused it to accelerate. The ship and asteroid crashed into the Earth. The asteroid side-glanced Earth, so it created a large debris field from which the moon formed.
The ship was Nuclear powered and is buried under George Bush's home, which explains why he has trouble thinking.