The regular daily and monthly rhythms of Earth’s only natural satellite, the Moon, have guided timekeepers for thousands of years. Its influence on Earth’s cycles, notably tides, has been charted by many cultures in many ages.
The Moon moderates Earth’s wobble on its axis, leading to a relatively stable climate over billions of years. From Earth, we always see the same face of the Moon because the Moon is spinning on its axis at the same speed that it is going around Earth (that is, it is in synchronous rotation with Earth).
The light areas of the Moon are known as the highlands. The dark features, called maria (Latin for seas), are impact basins that were filled with lava between 4.2 and 1.2 billion years ago. These light and dark areas represent rocks of different composition and ages, which provide evidence for how the early crust may have crystallized from a lunar magma ocean. The craters themselves, which have been preserved for billions of years, provide an impact history for the Moon and other bodies in the inner solar system.
The leading theory of the Moon’s origin is that a Mars-sized body collided with Earth approximately 4.5 billion years ago, and the resulting debris from both Earth and the impactor accumulated to form our natural satellite. The newly formed Moon was in a molten state. Within about 100 million years, most of the global “magma ocean” had crystallized, with less-dense rocks floating upward and eventually forming the lunar crust.
The early Moon may have developed an internal dynamo, the mechanism for global magnetic fields for terrestrial planets. Since the ancient time of volcanism, the arid, lifeless Moon has remained nearly unchanged. With too sparse an atmosphere to impede impacts, a steady rain of asteroids, meteoroids, and comets strikes the surface. Over billions of years, the surface has been ground up into fragments ranging from huge boulders to powder.
Nearly the entire Moon is covered by a rubble pile of charcoal-gray, powdery dust and rocky debris called the lunar regolith. Beneath is a region of fractured bedrock referred to as the megaregolith.
The Moon was first visited by the Soviet Union’s uncrewed Luna 1 and 2 in 1959, and, as of April 2019, seven nations have followed. The U.S. sent three classes of robotic missions to prepare the way for human exploration: the Rangers (1961–1965) were impact probes, the Lunar Orbiters (1966–1967) mapped the surface to find landing sites, and the Surveyors (1966–1968) were soft landers.
The first human landing on the Moon was on July 20, 1969. During the Apollo missions of 1969–1972, 12 American astronauts walked on the Moon and used a Lunar Roving Vehicle to travel on the surface and extend their studies of soil mechanics, meteoroids, lunar ranging, magnetic fields, and solar wind. The Apollo astronauts brought back 382 kilograms (842 pounds) of rock and soil to Earth for study.
After a long hiatus, lunar exploration resumed in the 1990s with the U.S. robotic missions Clementine and Lunar Prospector. Results from both missions suggested that water ice might be present at the lunar poles, but a controlled impact of the Prospector spacecraft produced no observable water. The U.S. began a new series of robotic lunar missions with the joint launch of the Lunar Reconnaissance Orbiter (LRO) and Lunar Crater Observation and Sensing Satellite (LCROSS) in 2009. In 2011, a pair of repurposed spacecraft began the ARTEMIS (Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon’s Interaction with the Sun) mission. In 2012, the Gravity Recovery and Interior Laboratory (GRAIL) twin spacecraft studied the Moon’s gravity field and produced the highest-resolution gravity field map of any celestial body.
In March 2019, NASA Administrator JIm Bridenstine announced plans to send U.S. astronauts back to the surface of the Moon by 2024.
The European Space Agency, Japan, China and India all have sent missions to explore the Moon. China has landed two rovers on the surface, including the first-ever landing on Moon's far side in 2019. In another first, a private company from Israel sent a spacecraft to land on the Moon in April 2019. Israel's Beresheet successfully orbited the Moon, but was lost during a landing attempt.