Earth's two moons? It's not lunacy, but new theory

This diagram provided by Martin Jutzi and Erik Asphaug, University of California, Santa Cruz via Nature shows a simulation of four stages of a collision between the Moon and a companion moon, four percent of the lunar mass, about 4 billion years ago. Earth once had a second moon, until it made the fatal mistake of smacking its big sister, some astronomers now theorize. For awhile when the Earth was young, it had a big moon, the one you see now, and a smaller "companion moon" orbiting above. Then one day that smaller moon collided into the bigger one in what astronomers are calling the "big splat." (AP Photo/Martin Jutzi and Erik Asphaug, University of California, Santa Cruz via Nature

This artist's rendering provided by Martin Jutzi and Erik Asphaug, University of California, Santa Cruz via Nature shows a simulation of a collision between the Moon and a companion moon, four percent of the lunar mass, about 4 billion years ago. Earth once had a second moon, until it made the fatal mistake of smacking its big sister, some astronomers now theorize. For awhile when the Earth was young, it had a big moon, the one you see now, and a smaller "companion moon" orbiting above. Then one day that smaller moon collided into the bigger one in what astronomers are calling the "big splat." (AP Photo/Martin Jutzi and Erik Asphaug, University of California, Santa Cruz via Nature)

This undated NASA handout image shows a view of the lunar surface taken from the Apollo 8 spacecraft looking southward from high altitude across the Southern Sea. The Moon's highlands, long a mystery, may have been thrown up billions of years ago by a slow-motion collision with a smaller companion moon knocked off its orbit, says a study released Wednesday