SEP 12, 2011 - Mystery of the Missing Moon is especially timely this week, what with the full moon today. Or should we make that, full moons? Aye, there's the rub! Most astronomers agree that the familiar Moon we see in the sky was created when a Mars-size planet crashed into Earth. The resulting debris that was blasted off eventually coalesced into our Moon, according to this theory. And, say some astronomers, there was an intermediate stage; in which there were two moons orbiting around our home planet. Eventually those two moons crashed into each other and merged to form the Moon we know and love. If that sounds crazy, consider: it may be the explanation for the Moon's dark side being so different from the side that faces Earth. And there's a way to test the theory, with precise instrumentation aboard satellites in orbit around the Moon. NASA launched those satellites last week, the so-called GRAIL mission. The pair of satellites will be months getting into the proper orbits. When they do, their data should reveal whether there are significant differences in the distribution of mass between the Moon's dark side and the side we see. If significant differences exist, they may be taken as evidence of the two-moon theory - although further missions (including surface rock sampling from the dark side) will be required to settle the issue. Ironically, there are myths about Earth's second moon (called Lilith) . . .
