Earth's only natural satellite
A short field guide to the Moon: what it is made of, how it moves, and how people and machines have explored it since 1959.
Where it came from
The leading explanation is the giant-impact hypothesis. About 4.5 billion years ago a Mars-sized body collided with the young Earth, and debris from both bodies collected in orbit and formed the Moon. Rock samples returned by Apollo support this: their oxygen isotopes are almost identical to Earth's, and the Moon has only a small iron core.
What we see
The dark patches are maria, wide plains of basalt from ancient lava flows. The brighter areas are older, heavily cratered highlands. Because the Moon has almost no atmosphere, craters stay sharp for billions of years, which makes its surface a record of the early Solar System.
Why we keep going back
Orbiters have found water ice in permanently shadowed craters near the poles. It could one day supply drinking water, oxygen and rocket propellant. That is why the south polar region is the target of most current missions.