The moon may be a harsh mistress, if you believe Robert Heinlein, but it's also lonely. Of all the planets in our solar system that have satellites, Earth is only one with just one. But that may not always have been the case.
According to an article in Time magazine, two astronomers from the University of California at Santa Cruz, Martin Jutzi and Erik Asphaug, argue that Earth once had a second, much smaller moon ... Moon Jr.
Jutzi and Asphaug were trying to understand why the moon is slightly lopsided (the far side is rocky and mountainous, while the near side is smooth) when they came up with their new theory.
When the moon was formed by Earth colliding with a Mars-sized planet, the resulting debris solidified and formed our moon. Another, smaller lunar body could have been formed at the same time and fallen into the same orbit.
So what happened to Moon Jr.? After several million years, Moon Jr.'s orbit decayed, and it gently impacted with the moon. Jutzi and Asphaug said this could explain both the moon's uneven surface and why the near side is covered in smooth lava.
Sounds like a lot of guesswork, and there's no way to prove any of it without more lunar samples, something not likely to happen in these days of funding cuts and NASA penny-pinching. But it's something to think about. How cool would it be if Moon Jr. were still in orbit? How would it have affected life on Earth? How would it have affected the space race?