Adrastea, also known as Jupiter XV, is the second by distance, and the smallest of the four inner moons of Jupiter. It was discovered in photographs taken by Voyager 2 in 1979, making it the first natural satellite to be discovered from images taken by an interplanetary spacecraft, rather than through a telescope. It was officially named after the mythological Adrasteia, foster mother of the Greek god Zeus—the equivalent of the Roman god Jupiter.
In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationship is found between a pair of objects. The physical principle behind orbital resonance is similar in concept to pushing a child on a swing, whereby the orbit and the swing both have a natural frequency, and the body doing the "pushing" will act in periodic repetition to have a cumulative effect on the motion. Orbital resonances greatly enhance the mutual gravitational influence of the bodies. In most cases, this results in an unstable interaction, in which the bodies exchange momentum and shift orbits until the resonance no longer exists. Under some circumstances, a resonant system can be self-correcting and thus stable. Examples are the 1:2:4 resonance of Jupiter's moons Ganymede, Europa and Io, and the 2:3 resonance between Pluto and Neptune. Unstable resonances with Saturn's inner moons give rise to gaps in the rings of Saturn. The special case of 1:1 resonance between bodies with similar orbital radii causes large solar system bodies to eject most other bodies sharing their orbits; this is part of the much more extensive process of clearing the neighbourhood, an effect that is used in the current definition of a planet.
A ring system is a disc or ring, orbiting an astronomical object, that is composed of solid material such as dust and moonlets, and is a common component of satellite systems around giant planets. A ring system around a planet is also known as a planetary ring system.
Voyager 2 is a space probe launched by NASA on August 20, 1977, to study the outer planets and interstellar space beyond the Sun's heliosphere. A part of the Voyager program, it was launched 16 days before its twin, Voyager 1, on a trajectory that took longer to reach gas giants Jupiter and Saturn but enabled further encounters with ice giants Uranus and Neptune. It remains the only spacecraft to have visited a combination of either of the gas giants and both ice giant planets. Voyager 2 was the fourth of five spacecraft to achieve the Solar escape velocity, which allowed it to leave the Solar System.
Metis, also known as Jupiter XVI, is the innermost known moon of Jupiter. It was discovered in 1979 in images taken by Voyager 1, and was named in 1983 after the first wife of Zeus, Metis. Additional observations made between early 1996 and September 2003 by the Galileo spacecraft allowed its surface to be imaged.
Amalthea is a moon of Jupiter. It has the third closest orbit around Jupiter among known moons and was the fifth moon of Jupiter to be discovered, so it is also known as Jupiter V. It is also the fifth largest moon of Jupiter, after the four Galilean Moons. Edward Emerson Barnard discovered the moon on 9 September 1892 and named it after Amalthea of Greek mythology. It was the last natural satellite to be discovered by direct visual observation; all later moons were discovered by photographic or digital imaging.
A natural satellite is in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body. Natural satellites are often colloquially referred to as moons, a derivation from the Moon of Earth.
The following is a timeline of Solar System astronomy.
Pan is the innermost named moon of Saturn. It is a small, walnut-shaped moon approximately 35 kilometres across and 23 km wide that orbits within the Encke Gap in Saturn's A Ring. Pan is a ring shepherd and is responsible for keeping the Encke Gap free of ring particles. It is sometimes described as having the appearance of a ravioli.
There are 80 known moons of Jupiter, not counting a number of moonlets likely shed from the inner moons. All together, they form a satellite system which is called the Jovian system. The most massive of the moons are the four Galilean moons: Io, Europa, Ganymede, and Callisto, which were independently discovered in 1610 by Galileo Galilei and Simon Marius and were the first objects found to orbit a body that was neither Earth nor the Sun. Much more recently, beginning in 1892, dozens of far smaller Jovian moons have been detected and have received the names of lovers or daughters of the Roman god Jupiter or his Greek equivalent Zeus. The Galilean moons are by far the largest and most massive objects to orbit Jupiter, with the remaining 76 known moons and the rings together composing just 0.003% of the total orbiting mass.
The moons of Saturn are numerous and diverse, ranging from tiny moonlets only tens of meters across to enormous Titan, which is larger than the planet Mercury. Saturn has 83 moons with confirmed orbits that are not embedded in its rings – of which only 13 have diameters greater than 50 kilometers – as well as dense rings that contain millions of embedded moonlets and innumerable smaller ring particles. Seven Saturnian moons are large enough to have collapsed into a relaxed, ellipsoidal shape, though only one or two of those, Titan and possibly Rhea, are currently in hydrostatic equilibrium. Particularly notable among Saturn's moons are Titan, the second-largest moon in the Solar System, with a nitrogen-rich Earth-like atmosphere and a landscape featuring dry river networks and hydrocarbon lakes, Enceladus, which emits jets of gas and dust from its south-polar region, and Iapetus, with its contrasting black and white hemispheres.
The planet Neptune has 14 known moons, which are named for minor water deities in Greek mythology. By far the largest of them is Triton, discovered by William Lassell on October 10, 1846, 17 days after the discovery of Neptune itself; over a century passed before the discovery of the second natural satellite, Nereid. Neptune's outermost moon Neso, which has an orbital period of about 26 Julian years, orbits farther from its planet than any other moon in the Solar System.
The rings of Saturn are the most extensive ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometers to meters, that orbit around Saturn. The ring particles are made almost entirely of water ice, with a trace component of rocky material. There is still no consensus as to their mechanism of formation. Although theoretical models indicated that the rings were likely to have formed early in the Solar System's history, newer data from Cassini suggested they formed relatively late.
The planet Jupiter has a system of faint planetary rings. The Jovian rings were the third ring system to be discovered in the Solar System, after those of Saturn and Uranus. The main ring was discovered in 1979 by the Voyager 1 space probe and the system was more thoroughly investigated in the 1990s by the Galileo orbiter. The main ring has also been observed by the Hubble Space Telescope and from Earth for several years. Ground-based observation of the rings requires the largest available telescopes.
The rings of Uranus are intermediate in complexity between the more extensive set around Saturn and the simpler systems around Jupiter and Neptune. The rings of Uranus were discovered on March 10, 1977, by James L. Elliot, Edward W. Dunham, and Jessica Mink. William Herschel had also reported observing rings in 1789; modern astronomers are divided on whether he could have seen them, as they are very dark and faint.
The rings of Neptune consist primarily of five principal rings and were first discovered on 22 July 1984 by Patrice Bouchet, Reinhold Häfner and Jean Manfroid at La Silla Observatory (ESO) in Chile during an observing program proposed by André Brahic and Bruno Sicardy from Paris Observatory, and at Cerro Tololo Interamerican Observatory by F. Vilas and L.-R. Elicer for a program led by William Hubbard. They were eventually imaged in 1989 by the Voyager 2 spacecraft. At their densest, they are comparable to the less dense portions of Saturn's main rings such as the C ring and the Cassini Division, but much of Neptune's ring system is quite tenuous, faint and dusty, more closely resembling the rings of Jupiter. Neptune's rings are named after astronomers who contributed important work on the planet: Galle, Le Verrier, Lassell, Arago, and Adams. Neptune also has a faint unnamed ring coincident with the orbit of the moon Galatea. Three other moons orbit between the rings: Naiad, Thalassa and Despina.
Daphnis is an inner satellite of Saturn. It is also known as Saturn XXXV; its provisional designation was S/2005 S 1. Daphnis is about 8 kilometres in diameter, and orbits the planet in the Keeler Gap within the A ring.
A shepherd moon is a small natural satellite that clears a gap in planetary-ring material or keeps particles within a ring contained. The name is a result of the fact they limit the "herd" of the ring particles as a shepherd.
Neptune has been directly explored by one space probe, Voyager 2, in 1989. As of January 2022, there are no confirmed future missions to visit the Neptunian system, although a tentative Chinese mission has been planned for launch in 2024. NASA, ESA, and independent academic groups have proposed future scientific missions to visit Neptune. Some mission plans are still active, while others have been abandoned or put on hold.
In astronomy, a regular moon is a natural satellite following a relatively close and prograde orbit with little orbital inclination or eccentricity. They are believed to have formed in orbit about their primary, as opposed to irregular moons, which were captured.
