Not A Trouble-free Theory

Nevertheless, the theory faced the same stumbling block that early theories did. The impact of a body three times the mass of Mars would have set the Earth spinning so rapidly that, even today, it would be turning faster than it is. Cameron and others proposed variations of the theory to get around this problem. One possibility they explored was that another large object might have struck the Earth from the opposite direction after the moon was formed, slowing the planet's spin.

Despite the one remaining difficulty, astronomers in 1999 considered the giant-impact theory the best explanation for the origin of the moon. They think they can now sketch fairly accurately how the moon was born and how it developed over time into the body that today illuminates our night sky.