Mat’s right that it’s a good thing we’re here after things have sorted themselves out. Early on in Earth’s history, about 4 billion years ago, there was a period of “late heavy bombardment” (http://simple.wikipedia.org/wiki/Late_Heavy_Bombardment) when all the inner planets, including Earth, were being hit by nasty large asteroids all the time. Earth would not have been a nice place to be.
The moon is alredy falling. It is called orbit. The moon is falling towards earth but because the earth, the moon, the sun and all things gravity it is always justmissing. Themoon is not realy moving in a perfect circle more like many straight lines as it plunges towards earth, misses and tries again. Earth does it to the sun and the moondoes it to the earth.