How did the Moon come to be? Prevailing wisdom has it that 4.5 billion years ago, a collision between the Earth and an object larger than Mars tossed immense quantities of vapor and debris into orbit around our planet. Eventually, the gas and rock formed a disk of dust which cooled and clumped together to form the moon. That theory received a major boost Thursday thanks to a study published in the journal Nature. Using computer simulations, University of Colorado scientists showed how a single moon can grow in this fashion. The researchers conducted 27 simulations which tracked up to 2,700 objects in orbit around the Earth. Two-thirds of the time, the computer produced one moon. What is odd is that other simulations showed two moons forming. So how come we aren't staring at two lunar discs today? If an extra one was created, say the researchers, it must have been pulled into Earth after drifting too close. But perhaps the most intriguing result was the time frame in which the moon formed — about one year from impact to finished product. Not bad for such an awe-inspiring orb.