Moon

The Moon is Earth's only natural satellite — about a quarter Earth's diameter, with surface gravity about one-sixth of Earth's. It formed roughly 4.5 billion years ago, likely from debris of a giant impact when a Mars-sized body (sometimes called Theia) collided with the early Earth, ejecting material that coalesced into the Moon.

The Moon is tidally locked to Earth, meaning one hemisphere — the "near side" — always faces us. The far side, only seen by spacecraft, looks dramatically different: rougher, more heavily cratered, and lacking the large dark plains called maria that dominate the near side. Maria (Latin for "seas") are vast plains of ancient solidified basaltic lava that erupted billions of years ago.

The Moon's gravity drives Earth's ocean tides and stabilizes Earth's axial tilt, preventing the chaotic climate swings that worlds without a large moon experience. Slowly, the Moon is moving away from Earth (about 3.8 cm per year) — measurable thanks to laser-ranging retroreflectors left by the Apollo missions.

Twelve humans walked on the Moon during the Apollo program (1969-1972), and 380 kg of lunar samples were returned to Earth. NASA's Artemis program plans to return humans, including the first woman, to the lunar surface — this time with the goal of establishing a sustainable lunar presence.