Introduction
The Moon is Earth's only natural satellite. It is the fifth-largest natural satellite in the Solar System and the largest relative to the size of its host planet among the major planetary satellite systems.
The Moon has been a central object of human observation for thousands of years. Modern spacecraft have transformed it from a distant astronomical object into a world that can be mapped, sampled and explored directly.
NASA describes the Moon as a record of Solar System history, preserving evidence in impact craters, ancient volcanic plains, rocks and deposits of water ice.
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Key Facts About the Moon
| Property | Approximate Value |
|---|---|
| Type | Natural satellite |
| Parent planet | Earth |
| Mean distance from Earth | About 385,000 km |
| Diameter | About 3,475 km |
| Radius | About 1,737 km |
| Surface gravity | About 1/6 of Earth's gravity |
| Atmosphere | Extremely thin exosphere |
| Natural satellites | None |
| Orbital period | About 27.3 days relative to the stars |
| Phase cycle | About 29.5 days |
NASA gives the Moon's average distance from Earth as approximately 385,000 km and its diameter as approximately 3,475 km.
```Formation of the Moon
The leading scientific model is that the Moon formed following a giant impact early in Earth's history.
In this model, a large planetary body collided with the young Earth. Material ejected into orbit around Earth subsequently accumulated to form the Moon.
The exact details of lunar formation remain an active area of planetary-science research, including questions concerning the Moon's initial composition, thermal evolution and early orbital history.
Internal Structure
The Moon has a differentiated interior consisting of a crust, mantle and core.
| Region | Description |
|---|---|
| Crust | The outer rocky layer of the Moon. |
| Mantle | A thick silicate region beneath the crust. |
| Core | A relatively small metallic core containing iron and other elements. |
The Moon's interior is much smaller and less massive than Earth's, but seismic measurements and spacecraft observations have revealed important details about its internal structure.
```Gravity on the Moon
The Moon has substantially weaker surface gravity than Earth.
This is roughly one-sixth of Earth's surface gravity.
The reduced gravity allowed Apollo astronauts to move with the characteristic bounding motion seen in lunar surface footage.
```The Lunar Surface
The Moon's surface is dominated by impact craters, rocky highlands, volcanic plains, mountains, valleys and extensive deposits of fine surface material known as regolith.
Highlands
Bright, elevated and heavily cratered regions that form much of the lunar surface.
Maria
Dark basaltic plains formed primarily by ancient volcanic eruptions.
Impact Craters
Structures produced by collisions with asteroids, comets and other bodies.
Regolith
A layer of fragmented rock and dust produced largely by billions of years of impacts.
Lunar Maria
The large dark regions visible from Earth are called maria, the Latin word for "seas".
Early astronomers incorrectly interpreted these dark regions as bodies of water. Spacecraft and geological studies established that they are extensive basaltic plains created by ancient volcanic activity.
| Feature | Description |
|---|---|
| Mare Imbrium | One of the largest lunar maria. |
| Mare Tranquillitatis | Location of the Apollo 11 landing site. |
| Mare Serenitatis | Large basaltic plain on the lunar near side. |
| Mare Crisium | Prominent roughly circular mare near the eastern lunar near side. |
Impact Craters
Because the Moon has no substantial atmosphere and has experienced relatively little surface erosion compared with Earth, impact craters can survive for extremely long periods.
The lunar surface therefore acts as a geological archive of the impact history of the inner Solar System.
Tycho Crater
Tycho is one of the most prominent impact craters visible from Earth.
It is characterised by a relatively fresh appearance and a spectacular system of bright rays extending across the surrounding terrain.
```The Far Side of the Moon
The Moon is tidally locked to Earth. Its rotation period is equal to its orbital period, so approximately the same hemisphere continually faces Earth.
This does not mean that the far side is permanently dark. Every part of the Moon receives sunlight as the Moon moves through its orbit.
Humans first obtained direct images of the lunar far side in 1959 when a Soviet spacecraft flew past the Moon.
Phases of the Moon
The Moon appears to change shape during the month because we see different proportions of its sunlit hemisphere as it orbits Earth.
New Moon
The Moon's illuminated side faces predominantly away from Earth and the Moon is difficult to see from the ground.
First Quarter
Approximately half of the visible lunar disk is illuminated.
Full Moon
The Earth-facing side of the Moon is largely illuminated.
Last Quarter
Approximately half of the visible disk is illuminated, during the waning part of the cycle.
NASA explains that the Moon always has half of its surface illuminated by the Sun; its changing phases arise from our changing viewing geometry as it orbits Earth.
```The Lunar Month
There are two important lunar periods.
| Period | Approximate Length | Meaning |
|---|---|---|
| Sidereal Month | 27.3 days | Time required for the Moon to orbit Earth relative to the distant stars. |
| Synodic Month | 29.5 days | Time between successive equivalent lunar phases, such as full Moon to full Moon. |
Solar and Lunar Eclipses
Eclipses occur when the Sun, Earth and Moon become aligned closely enough for one body to pass through another body's shadow.
Solar Eclipse
The Moon passes between Earth and the Sun, blocking some or all of the Sun's disk.
Lunar Eclipse
Earth passes between the Sun and Moon and Earth's shadow falls across the Moon.
Total Eclipse
The relevant disk is completely covered from a particular viewing location.
Partial Eclipse
Only part of the relevant disk enters the shadow or is obscured.
The Lunar Atmosphere
The Moon does not possess a substantial atmosphere like Earth's. Instead, it has an extremely tenuous environment called an exosphere.
NASA states that the lunar exosphere is composed primarily of extremely sparse helium, neon and argon, together with trace amounts of other species.
Water on the Moon
Although the Moon appears dry, scientists have detected water and water ice in several lunar environments.
Permanently shadowed regions near the lunar poles are especially important because some of these extremely cold locations can preserve water ice.
Lunar water is scientifically important and could potentially become a resource for future exploration. ESA identifies the investigation and possible utilisation of lunar resources, including water, as an important aspect of future lunar exploration.
```The Moon and Earth's Tides
The Moon's gravitational field produces an important component of Earth's ocean tides.
The Sun also contributes to tides, but the Moon's proximity to Earth makes its tidal influence particularly important.
The strong dependence on distance explains why the nearby Moon can exert a substantial tidal influence despite the Sun having much greater mass.
```Temperature on the Moon
The Moon experiences extreme temperature variations because it has no substantial atmosphere to redistribute heat efficiently.
Daytime
Sunlit surfaces can become extremely hot.
Night-time
Surfaces can become extremely cold after prolonged exposure to darkness.
Polar Shadows
Permanently shadowed regions can remain exceptionally cold.
Lunar Dust
The Moon's surface is covered by regolith produced by impacts and the continual fragmentation of rocks.
Lunar dust is extremely fine and abrasive. It became an important engineering and human-factors issue during the Apollo missions and remains a major consideration for future lunar operations.
ESA highlights lunar dust as one of the important environmental challenges for future explorers.
```Exploration of the Moon
The Moon is the only extraterrestrial world on which humans have directly walked.
Robotic exploration began decades before the Apollo landings and continues today through orbiters, landers, rovers and sample-return missions.
Luna Programme
Soviet robotic missions that achieved major milestones including the first soft landing and robotic sample return.
Surveyor
NASA robotic landers that investigated the lunar surface before Apollo.
Lunar Reconnaissance Orbiter
NASA spacecraft providing detailed observations and mapping of the lunar surface.
Chang'e Programme
China's lunar exploration programme involving orbiters, landers, rovers and sample-return missions.
The Apollo Programme
NASA's Apollo programme conducted the first crewed missions to the lunar surface.
Apollo 11
Apollo 11 landed in the Mare Tranquillitatis on 20 July 1969.
Neil Armstrong and Edwin "Buzz" Aldrin became the first humans to walk on another world, while Michael Collins remained in lunar orbit in the command module.
The mission demonstrated that humans could travel to another planetary body, operate on its surface and return safely to Earth.
```Scientific Instruments on the Moon
Apollo astronauts deployed scientific instruments that continued to provide measurements after the astronauts returned to Earth.
| Instrument | Purpose |
|---|---|
| Seismometers | Detect lunar seismic activity and provide information about the Moon's interior. |
| Laser Retroreflectors | Allow extremely precise measurements of the Earth–Moon distance. |
| Heat Flow Experiments | Investigate heat escaping from the lunar interior. |
| Magnetometers | Measure local magnetic fields. |
The Artemis Era
Modern lunar exploration is increasingly international and includes both robotic and crewed programmes.
NASA's Artemis programme is intended to establish a sustained human and scientific presence around and on the Moon while developing technologies and operational experience relevant to future deep-space exploration.
ESA is contributing major technologies and hardware to lunar exploration, including the European Service Module for NASA's Orion spacecraft. ESA also describes lunar exploration as an important pathway toward future human exploration beyond the Moon.
Science
Investigate lunar geology, water, volatiles and the history of the Solar System.
Technology
Develop systems for operating in deep-space and on planetary surfaces.
Human Exploration
Develop experience and infrastructure for sustained human operations beyond low Earth orbit.
Resources
Investigate the potential use of local lunar resources, including water and oxygen-bearing materials.
The Moon as a Scientific Laboratory
The Moon provides an unusually valuable laboratory for studying planetary processes.
- Impact cratering
- Volcanic activity
- Planetary differentiation
- Regolith formation
- Seismology
- Space weathering
- Planetary magnetism
- Water and volatile compounds
- Early Solar System history
The Moon and Earth
The Earth–Moon system is one of the most important relationships in planetary science.
Tides
The Moon is the dominant astronomical contributor to Earth's ocean tides.
Eclipses
The Moon produces solar and lunar eclipses through geometric alignment with Earth and the Sun.
Orbital Evolution
Tidal interactions cause the Moon to slowly move farther from Earth.
Planetary Science
Studying the Moon provides evidence about Earth's own early history.
The Moon's Orbit
The Moon orbits Earth in an approximately elliptical orbit.
Its orbital plane is inclined relative to Earth's orbital plane around the Sun. This is why solar and lunar eclipses do not occur at every new and full Moon.
Lunar Recession
Tidal interactions between Earth and the Moon transfer angular momentum between the two bodies.
As a consequence, the Moon is gradually moving away from Earth.
Precise laser-ranging experiments using reflectors placed on the lunar surface have allowed scientists to measure this evolution with extraordinary accuracy.
```Moon and Space Navigation
The Moon is also becoming an important destination for developing navigation, communication and operational technologies for deep-space missions.
Future lunar infrastructure could support scientific instruments, robotic exploration, communications and navigation systems.
ESA is studying technologies for lunar communications and navigation as part of its broader lunar exploration activities.
```Future Lunar Exploration
Future missions are expected to investigate areas of the Moon that have received comparatively little direct exploration, including polar regions and permanently shadowed craters.
Lunar South Pole
A major target because of permanently shadowed regions and possible water-ice deposits.
Sample Return
Returning carefully selected lunar material to Earth allows scientists to perform highly detailed laboratory analyses.
Robotic Rovers
Rovers can investigate geological environments across difficult terrain.
Human Operations
Astronauts can perform complex geological and engineering tasks while operating scientific instruments.
The Moon and the Future of Space Exploration
The Moon is likely to remain a major focus of planetary exploration during the coming decades.
Its proximity to Earth makes it an ideal location for testing technologies required for more distant missions.
Possible future activities include:
- Long-duration scientific stations
- Lunar rovers and autonomous systems
- Deep drilling and geological investigations
- Polar resource studies
- Advanced communications and navigation
- Construction and habitat technologies
- Preparation for human missions to Mars
Historical Timeline
Major Questions in Lunar Science
- Exactly how did the Moon form?
- What was the Moon's early thermal and volcanic history?
- Why are the lunar near side and far side so different?
- How much water ice exists at the lunar poles?
- How did the lunar magnetic field develop and disappear?
- How does the lunar interior behave today?
- What does the lunar surface reveal about the early Solar System?
- How can humans operate safely on the lunar surface for long periods?
- Can local lunar resources be used effectively in future missions?
The Moon in Astronomy
The Moon has played an important role in the development of astronomy and physics.
Observations of lunar motion helped ancient astronomers develop mathematical models of the heavens. Later, telescopic observations provided evidence that celestial bodies were not perfect, unchanging spheres.
The Moon subsequently became the first extraterrestrial world explored directly by spacecraft and humans.
```Conclusion
The Moon is Earth's only natural satellite and one of the most scientifically important worlds in our immediate cosmic neighbourhood.
Its cratered surface records billions of years of Solar System history. Its ancient volcanic plains reveal a period when the Moon was much more geologically active, while its interior, exosphere and polar environments continue to provide important scientific questions.
The Moon also has a profound relationship with Earth. Its gravity contributes to ocean tides, its orbital motion produces the familiar cycle of lunar phases, and its alignment with the Sun produces spectacular eclipses.
Human exploration has transformed the Moon from a distant object of observation into a destination for scientific and technological exploration. NASA, ESA and other international partners are now developing the next generation of lunar missions.