Natural satellite

• 1. Introduction

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  A natural satellite is an astronomical body that orbits a planet, a dwarf planet, or a small Solar System body, and sometimes even another natural satellite. In everyday speech we call them moons, by analogy with the Moon, Earth's own satellite, classically known as Selene or Luna. The Solar System holds 455 of these bodies with confirmed orbits, spread across six planetary satellite systems. But the word moon hides centuries of argument. Where is the line between a moon and a planet, or between a moon and a mere fleck of rock too small to name? Why does the same side of most moons always face the world they circle? Can a moon have a moon of its own? And how did a launch in 1957 nearly steal the word away entirely? These questions trace a path from a German astronomer's pamphlet to volcanic plumes on distant worlds.

• 2. Naming The Companions

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  Until Copernicus published De revolutionibus orbium coelestium in 1543, the Moon was simply counted as a planet. There was no class of objects to name until 1610, when the Galilean satellites of Jupiter came into view. Galileo called his discoveries Planetæ, or planets, but later observers wanted a word that set these bodies apart from what they orbited. The German astronomer Johannes Kepler supplied it. In his 1610 pamphlet Narratio de Observatis a se quatuor Iouis satellitibus erronibus, he drew on the Latin satelles, meaning guard, attendant, or companion. The image fit, since each satellite accompanied its planet through the heavens. The word satellite then held its ground for centuries, because it avoided the ambiguity of moon. Then came 1957 and the launch of Sputnik. The terms man-made satellite and artificial moon were swiftly dropped for the plainer satellite, and the word became tied to objects flown in space. That shift handed moon back its respectability, and it now stands interchangeably with natural satellite even in scientific writing. By convention, Moon with a capital M is reserved for Earth's body alone.

• 3. Where The Line Falls

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  There is no agreed lower limit on what counts as a moon. Every natural body with an identified orbit around a Solar System planet has qualified, some only a kilometer across. Objects a tenth that size inside Saturn's rings, never directly observed, are called moonlets instead. Small satellites of asteroids, such as Dactyl, also carry that diminutive. The upper end is just as blurry. Two bodies are sometimes called a double planet rather than a primary and a satellite, with the Earth-Moon system offered as the example. Asteroids like 90 Antiope are treated as double asteroids, yet they have never forced a clean definition. Some writers see the Pluto-Charon system as a double dwarf planet, arguing that its barycentre lies above the surface of the larger body. For every planetary moon in the Solar System, by contrast, the barycenter sits within the radius of the host planet.

• 4. Born Or Captured

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  Regular satellites, those on close, uninclined, nearly circular orbits, are thought to have formed from the same collapsing region of the protoplanetary disk that built their primary. Irregular satellites tell a different story. Riding distant, inclined, eccentric, or retrograde orbits, they are believed to be captured asteroids, sometimes further broken up by collisions. Most major moons follow regular orbits, while most small ones do not. The Moon and the moons of Pluto break this pattern among large bodies. They are thought to have come from the collision of two large protoplanetary objects early in the Solar System's history, the giant impact hypothesis, with debris reaccreting into orbiting moons. Triton is another outlier. Large and on a close circular orbit, it nonetheless moves retrograde, and is thought to be a captured dwarf planet. Some moons keep company of their own kind. Telesto and Calypso lead and follow the Saturnian moon Tethys at its Lagrangian points, while Helene and Polydeuces do the same for Dione, sixty degrees ahead and behind. These are the trojan moons.

• 5. Locked In Place

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  Most regular moons are tidally locked to their primaries, so the same side always faces the planet. Tidal forces raised by the planet bleed energy away, slowing the moon's spin until it nearly stops. Saturn's Hyperion defies this, tumbling chaotically under the gravitational pull of Titan, and Pluto's four small circumbinary moons spin chaotically too, under Charon's influence. The irregular outer satellites of the giant planets sit too far out to lock at all. Jupiter's Himalia, Saturn's Phoebe, and Neptune's Nereid turn in roughly ten hours while taking hundreds of days to orbit. Locking also leaves a mark on shape. Tidally locked moons tend toward ovoid, egg-like forms, squat at the poles and stretched toward their planet. Saturn's Mimas has a major axis 9 percent longer than its polar axis and 5 percent longer than its other equatorial axis. Methone, only about 3 km across, is visibly egg-shaped. Neptune's Proteus is the largest irregularly shaped moon, about as big as an icy moon can grow before settling into a sphere.

• 6. Living Worlds

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  Nineteen known moons in the Solar System are large enough to have been pulled into a round shape, and several remain geologically active. Io is the most volcanically active body in the Solar System. Europa, Enceladus, and Triton show signs of ongoing tectonic activity and cryovolcanism. For the first three, the engine is tidal heating from eccentric orbits close to their giant-planet primaries, a mechanism that once worked on Triton before its orbit circularized. Enceladus and Triton both have features resembling geysers, though on Triton solar heating appears to supply the energy. Titan and Triton carry significant atmospheres, and Titan holds hydrocarbon lakes. Subsurface oceans of liquid water are thought to lie beneath Europa, Ganymede, Callisto, and Titan, while smaller Enceladus supports a global subsurface ocean of its own. Other moons, including Earth's Moon, Ganymede, Tethys, and Miranda, bear scars of past activity driven by decaying radioisotopes, former orbital resonances, or the freezing of their interiors.

• 7. Moons Of Moons

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  No subsatellite, a moon orbiting another moon of a planet, is currently known. In most cases the planet's tidal effects would make such a system unstable. Possible exceptions are large moons on wide orbits, including Titan, Iapetus, Callisto, and the Moon, though forces like the mascons on the Moon could strip a submoon away. Calculations after a possible ring system was detected around Saturn's moon Rhea in 2008 suggested that satellites of Rhea could hold stable orbits, and the suspected narrow rings hinted at shepherd moons. Yet targeted images from the Cassini spacecraft found no rings there. Saturn's Iapetus may once have had a satellite, one of several ideas offered to explain its equatorial ridge. Light-curve analysis suggests Saturn's irregular satellite Kiviuq is extremely prolate, likely a contact binary or even a binary moon.

• 8. Counting The Family

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  Mercury and Venus have no moons at all. Earth has one large satellite, and Mars has two tiny ones, Phobos and Deimos. The giant planets carry sprawling systems, including half a dozen bodies comparable to Earth's Moon: the four Galilean moons, Saturn's Titan, and Neptune's Triton. Saturn holds six more mid-sized moons rounded by hydrostatic equilibrium, and Uranus has five. Among the dwarf planets, Ceres has no known satellite, while Pluto has Charon plus the smaller Styx, Nix, Kerberos, and Hydra. The seven largest moons, each bigger than 2,500 km across, are the four Galileans, Titan, Earth's Moon, and Triton. Ganymede and Titan even outsize the planet Mercury, and Callisto roughly matches it. A planet usually carries at least 10,000 times the mass of any moon orbiting it. The Earth-Moon system breaks that rule. At 3,474 kilometres across, the Moon is 0.273 times Earth's diameter. Beyond our own system, planets around other stars are likely to have satellites too, and although many candidates have turned up, not one has yet been confirmed.