Natural satellite

A naturally occurring celestial body that orbits a planet, dwarf planet, or other larger body. Natural satellites range from small captured rocks to worlds with atmospheres, oceans, and geological activity.

Orbital mechanics: The study of the motion of objects under gravity and other forces. It describes the gravitational paths that keep a satellite orbiting its primary.
Earth's Moon: Earth's only permanent natural satellite, formed early in the Solar System's history. It is the nearest and most familiar example of a natural satellite.
Giant-impact hypothesis: The hypothesis that the Moon formed from debris ejected by a collision between early Earth and a planetary body. It is the leading explanation for Earth's unusually large moon relative to its planet.
Planetary science: The study of planets, moons, planetary systems, and the processes that shape them. Satellites are major worlds whose geology and histories inform planetary science.
Artificial satellite: A human-made object placed in orbit around a celestial body. Unlike a natural satellite, it is constructed and launched by humans.
Hill sphere: The region around an astronomical body where its gravity dominates satellite motion relative to a more massive body's pull. A satellite generally must orbit within its primary's Hill sphere to remain bound.
Europa: A moon of Jupiter with an icy surface and strong evidence for a subsurface ocean. Its hidden ocean makes it a leading case for potentially habitable environments beyond Earth.
Circumplanetary disk: A disk of gas and solids orbiting a forming planet, from which regular satellites can accrete. Large satellite systems can assemble from material orbiting a young giant planet.
Planetary habitability: The capacity of an environment to support conditions suitable for life as understood from Earth-based biology. Subsurface oceans on icy satellites expand the range of environments considered potentially habitable.
Planet: A celestial body that orbits a star, is nearly round from its own gravity, and has cleared its orbital neighborhood. A planet orbits a star directly; a natural satellite orbits another body.

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