The Moon's far side, although not lacking for light, remained
dark in the sense of hidden or obscured until the space race
between the US and USSR took aim at the Moon. The Soviets' Luna
3 probe returned the first images of the far side in 1959, and
the results were a bit of a surprise. The near side is covered
with large, dark, basaltic flows that are called maria; these
are rare on the far side, which is dominated by the rugged lunar
highlands. A number of explanations have been offered for this
difference, but today's issue of Nature contains what is
certainly the most dramatic one yet: it suggests that the
highlands are the remains of the Earth's missing moon, plastered
across the far side of the one remaining Moon. A consensus has
formed around the theory that the Moon originated from a
collision early in the history of the solar system, when a near-
Mars sized body smacked into the Earth. The resulting debris
coalesced into two bodies. Models of this process nicely account
for some of the difference betw