Inner satellite
Encyclopedia
In astronomy
, an inner moon is a natural satellite
following a prograde, low inclination orbit
inwards of the large satellites of the parent planet. They are generally thought to have been formed in situ at the same time as the coalescence of the original planet. Neptune
's moons are an exception, as they are likely reaggregates of the pieces of the original bodies, which were disrupted after the capture of the large moon Triton
. Inner satellites are distinguished from other regular satellites
by their proximity to the parent planet, their short orbital periods (usually under a day), their low mass, small size, and irregular shapes.
, Saturn
, Uranus
and Neptune
). Because of their small size, and glare from the nearby planet, they can be very difficult to observe from Earth. Some, such as Pan
and Daphnis
at Saturn, and Naiad
at Neptune, have only ever been observed by spacecraft.
The first inner satellite to be observed was Amalthea
, discovered by E. E. Barnard in 1892. Next were the Saturnian moons Epimetheus
and Janus
, observed in 1966. These two moons are co-orbital (that is, they share the same orbit), and the resulting confusion over their status was not resolved until the Voyager 1
flyby in 1980. Most of the remaining inner satellites were discovered by spacecraft Voyager 1
and Voyager 2
during their flybys of Jupiter (1979), Saturn (1980), Uranus (1986) and Neptune (1989).
The most recent discoveries have been two moons of Uranus (Mab
and Cupid
), found using the Hubble Space Telescope
in 2003, and Daphnis
found orbiting Saturn by the Cassini
spacecraft in 2005.
eccentricity is 0.0012, while the most eccentric inner satellite is Thebe
with e=0.0177. Their inclination to their planets' equatorial planes is also very low. All but one have inclinations below one degree, the median being 0.1°. Naiad, Neptune's closest moon, is the exception, being inclined at 4.75° to Neptune's equator.
The innermost satellites orbit within the planetary rings, well within the fluid Roche limit
, and only the internal strength and friction of their materials prevents them from being torn apart by tidal forces. This means that, if a pebble were placed in the part of the satellite furthest away from the planet, the tidal force outward is stronger that the satellite's gravity planetward, so the pebble would fall upward. this is why photos of these satellites show them to be completely clean of pebbles, dust and rocks.
The most extreme cases are Saturn's moon Pan
, which orbits within the rings at only 70% of its fluid Roche limit, as well as Neptune's moon Naiad
. Naiad's density is unknown, so its precise Roche limit is also unknown, but if its density were below 1100 kg/m³ it would lie at an even smaller fraction of its Roche limit than Pan.
Those satellites which have an orbital period shorter than their planet's rotation period experience tidal deceleration, causing a very gradual spiraling in towards the planet. In the distant future these moons will impact the planet or penetrate deeply enough within their Roche limit to be tidally disrupted into fragments. The moons so affected are Metis
and Adrastea
at Jupiter, and the majority of the inner moons of Uranus and Neptune − out to and including Perdita
and Larissa
, respectively. However, none of Saturn's moons experience this effect because Saturn is a relatively very fast rotator.
, that is, their orbit is synchronous
with their rotation so that they only show one face toward their parent planet. Their long axes are typically aligned to point towards their planet.
between 0.06 (Metis
) and 0.10 (Adrastea
). Saturn's satellites, in contrast, have very bright surfaces, with albedos between 0.4 and 0.6. This is thought to be because their surfaces are being coated with fresh ice particles swept up from the ring system within which they orbit. The inner satellites around the other planets may have been darkened by space weathering
. None of the known inner satellites possesses an atmosphere.
are preferentially deflected towards the planet by its gravitational field, so that the frequency of potential impactors passing through a given cross-sectional area close to the planet is much greater than in interplanetary space. As a result, it has been estimated that very small bodies in inner orbits are expected to be disrupted by external impactors on a timescale much shorter than the age of the solar system. This would place a lower limit on the size of the inner moons that remain.
has the smallest set of inner satellites, including only the following four:
's seven inner satellites are closely related to its ring system
, and many of them orbit within the rings, creating gaps or "shepherded" ringlets between them.
A number of localised objects such as S/2004 S 3, S/2004 S 4
, and S/2004 S 6, sometimes surrounded by a dusty halo, have been seen in the vicinity of the F ring, but at present it is not clear whether they are all transient clumps or whether some may include small solid objects (moons).
has by far the most extensive system of inner satellites, containing thirteen known moons:
has six known inner satellites:
It is thought that they are rubble pile
s re-accreted from fragments of Neptune's original satellites (inner or not). These were strongly perturbed by Triton
in the period soon after that moon's capture into a very eccentric initial orbit. The perurbations led to collisions between the satellites, and that portion of the fragments that were not lost re-accreted into the present inner satellites after Triton's orbit became circularised.
and Voyager 2
. The majority have only been seen in images as single pixels, or resolved to only a few pixels across. However fairly detailed surface features have been seen on the following moons:
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
, an inner moon is a natural satellite
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....
following a prograde, low inclination orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
inwards of the large satellites of the parent planet. They are generally thought to have been formed in situ at the same time as the coalescence of the original planet. Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
's moons are an exception, as they are likely reaggregates of the pieces of the original bodies, which were disrupted after the capture of the large moon Triton
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...
. Inner satellites are distinguished from other regular satellites
Regular moon
In astronomy, a regular moon is a natural satellite following a relatively close and generally prograde orbit with little orbital inclination or eccentricity...
by their proximity to the parent planet, their short orbital periods (usually under a day), their low mass, small size, and irregular shapes.
Discovery
Thirty inner satellites are currently known, found orbiting around all four of the giant planets (JupiterJupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
, Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
, Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...
and Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
). Because of their small size, and glare from the nearby planet, they can be very difficult to observe from Earth. Some, such as Pan
Pan (moon)
Pan is the innermost moon of Saturn. It is a walnut-shaped small moon about 35 kilometres across and 23 km high that orbits within the Encke Gap in Saturn's A Ring. Pan acts as a ring shepherd and is responsible for keeping the Encke Gap free of ring particles.It was discovered by Mark R...
and Daphnis
Daphnis (moon)
Daphnis is an inner satellite of Saturn. It is also known as '; its provisional designation was '. Daphnis is about 8 kilometres in diameter, and orbits the planet in the Keeler Gap within the A ring.- Naming :...
at Saturn, and Naiad
Naiad (moon)
Naiad , also known as Neptune III, is the innermost satellite of Neptune, named after the Naiads of Greek legend.Naiad was discovered sometime before mid-September 1989 from the images taken by the Voyager 2 probe. The last moon to be discovered during the flyby, it was designated...
at Neptune, have only ever been observed by spacecraft.
The first inner satellite to be observed was Amalthea
Amalthea (moon)
Amalthea is the third moon of Jupiter in order of distance from the planet. It was discovered on September 9, 1892, by Edward Emerson Barnard and named after Amalthea, a nymph in Greek mythology. It is also known as '....
, discovered by E. E. Barnard in 1892. Next were the Saturnian moons Epimetheus
Epimetheus (moon)
Epimetheus is an inner satellite of Saturn. It is also known as Saturn XI. It is named after the mythological Epimetheus, brother of Prometheus.-Discovery:Epimetheus occupies essentially the same orbit as the moon Janus...
and Janus
Janus (moon)
Janus is an inner satellite of Saturn. It is also known as Saturn X . It is named after the mythological Janus.-Discovery and orbit:Janus occupies practically the same orbit as the moon Epimetheus...
, observed in 1966. These two moons are co-orbital (that is, they share the same orbit), and the resulting confusion over their status was not resolved until the Voyager 1
Voyager 1
The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of...
flyby in 1980. Most of the remaining inner satellites were discovered by spacecraft Voyager 1
Voyager 1
The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of...
and Voyager 2
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...
during their flybys of Jupiter (1979), Saturn (1980), Uranus (1986) and Neptune (1989).
The most recent discoveries have been two moons of Uranus (Mab
Mab (moon)
Mab , or Uranus XXVI , is an inner satellite of Uranus. It was discovered by Mark R. Showalter and Jack J. Lissauer in 2003 using the Hubble Space Telescope...
and Cupid
Cupid (moon)
Cupid is an inner satellite of Uranus. It was discovered by Mark Showalter and Jack J. Lissauer in 2003 using the Hubble Space Telescope. It was named after a character in William Shakespeare's play Timon of Athens....
), found using the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
in 2003, and Daphnis
Daphnis (moon)
Daphnis is an inner satellite of Saturn. It is also known as '; its provisional designation was '. Daphnis is about 8 kilometres in diameter, and orbits the planet in the Keeler Gap within the A ring.- Naming :...
found orbiting Saturn by the Cassini
Cassini-Huygens
Cassini–Huygens is a joint NASA/ESA/ASI spacecraft mission studying the planet Saturn and its many natural satellites since 2004. Launched in 1997 after nearly two decades of gestation, it includes a Saturn orbiter and an atmospheric probe/lander for the moon Titan, although it has also returned...
spacecraft in 2005.
Orbits
All inner satellites follow nearly circular, prograde orbits. The medianMedian
In probability theory and statistics, a median is described as the numerical value separating the higher half of a sample, a population, or a probability distribution, from the lower half. The median of a finite list of numbers can be found by arranging all the observations from lowest value to...
eccentricity is 0.0012, while the most eccentric inner satellite is Thebe
Thebe (moon)
Thebe also known as ', is the fourth of Jupiter's moons by distance from the planet. It was discovered by Stephen P. Synnott in images from the Voyager 1 space probe taken on March 5, 1979, while orbiting around Jupiter...
with e=0.0177. Their inclination to their planets' equatorial planes is also very low. All but one have inclinations below one degree, the median being 0.1°. Naiad, Neptune's closest moon, is the exception, being inclined at 4.75° to Neptune's equator.
The innermost satellites orbit within the planetary rings, well within the fluid Roche limit
Roche limit
The Roche limit , sometimes referred to as the Roche radius, is the distance within which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction...
, and only the internal strength and friction of their materials prevents them from being torn apart by tidal forces. This means that, if a pebble were placed in the part of the satellite furthest away from the planet, the tidal force outward is stronger that the satellite's gravity planetward, so the pebble would fall upward. this is why photos of these satellites show them to be completely clean of pebbles, dust and rocks.
The most extreme cases are Saturn's moon Pan
Pan (moon)
Pan is the innermost moon of Saturn. It is a walnut-shaped small moon about 35 kilometres across and 23 km high that orbits within the Encke Gap in Saturn's A Ring. Pan acts as a ring shepherd and is responsible for keeping the Encke Gap free of ring particles.It was discovered by Mark R...
, which orbits within the rings at only 70% of its fluid Roche limit, as well as Neptune's moon Naiad
Naiad (moon)
Naiad , also known as Neptune III, is the innermost satellite of Neptune, named after the Naiads of Greek legend.Naiad was discovered sometime before mid-September 1989 from the images taken by the Voyager 2 probe. The last moon to be discovered during the flyby, it was designated...
. Naiad's density is unknown, so its precise Roche limit is also unknown, but if its density were below 1100 kg/m³ it would lie at an even smaller fraction of its Roche limit than Pan.
Those satellites which have an orbital period shorter than their planet's rotation period experience tidal deceleration, causing a very gradual spiraling in towards the planet. In the distant future these moons will impact the planet or penetrate deeply enough within their Roche limit to be tidally disrupted into fragments. The moons so affected are Metis
Metis (moon)
Metis , also known as ', is the innermost 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...
and Adrastea
Adrastea (moon)
Adrastea , also known as ', is the second by distance, and the smallest of the four inner moons of Jupiter. It was discovered in Voyager 2 probe photographs taken in 1979, making it the first natural satellite to be discovered from images taken by an interplanetary spacecraft, rather than...
at Jupiter, and the majority of the inner moons of Uranus and Neptune − out to and including Perdita
Perdita (moon)
Perdita is an inner satellite of Uranus. Perdita's discovery was complicated. The first photographs of Perdita were taken by the Voyager 2 spacecraft in 1986, but it was not recognized from the photographs for more than a decade. In 1999, the moon was noticed by Erich Karkoschka and reported...
and Larissa
Larissa (moon)
Larissa , also known as Neptune VII, is the fifth-closest inner satellite of Neptune. It is named after Larissa, a lover of Poseidon in Greek mythology and eponymous nymph of the city in Thessaly.- Discovery :...
, respectively. However, none of Saturn's moons experience this effect because Saturn is a relatively very fast rotator.
Dimensions
The inner satellites are small in comparison with the major moons of their respective planets. All are too small to have attained a gravitationally-collapsed spheroidal shape. Many are highly elongated, such as for example, Amalthea, which is twice as long as wide. By far the largest of the inner satellites is Proteus, which is about 440 km across in its longest dimension and close to spherical, but not spherical enough to be considered a gravitationally collapsed shape. Most known inner satellites are 50 to 200 km across, while the smallest confirmed is Daphnis at 6 to 8 km in size. Unconfirmed bodies orbiting close to Saturn's F ring, such as S/2004 S 6, may be somewhat smaller moons, if they are not transient clumps of dust instead. The Cassini spacecraft has recently found indications (small dusty rings) that even smaller moonlets may be orbiting in the Cassini Division. The size of the smallest known inner moons around the outer planets increases with distance from the Sun, but this trend is thought to be due to increasingly difficult lighting and observing conditions rather than any physical trend. Smaller inner moons may eventually be discovered.Rotation
Inner satellites are all tidally lockedTidal locking
Tidal locking occurs when the gravitational gradient makes one side of an astronomical body always face another; for example, the same side of the Earth's Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner...
, that is, their orbit is synchronous
Synchronous orbit
A synchronous orbit is an orbit in which an orbiting body has a period equal to the average rotational period of the body being orbited , and in the same direction of rotation as that body.-Properties:...
with their rotation so that they only show one face toward their parent planet. Their long axes are typically aligned to point towards their planet.
Surfaces
All the inner satellites of Jupiter, Uranus, and Neptune have very dark surfaces with an albedoAlbedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
between 0.06 (Metis
Metis (moon)
Metis , also known as ', is the innermost 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...
) and 0.10 (Adrastea
Adrastea (moon)
Adrastea , also known as ', is the second by distance, and the smallest of the four inner moons of Jupiter. It was discovered in Voyager 2 probe photographs taken in 1979, making it the first natural satellite to be discovered from images taken by an interplanetary spacecraft, rather than...
). Saturn's satellites, in contrast, have very bright surfaces, with albedos between 0.4 and 0.6. This is thought to be because their surfaces are being coated with fresh ice particles swept up from the ring system within which they orbit. The inner satellites around the other planets may have been darkened by space weathering
Space weathering
Space weathering is a blanket term used for a number of processes that act on any body exposed to the harsh space environment. Airless bodies incur many weathering processes:* collisions of galactic cosmic rays and solar cosmic rays,* irradiation, implantation, and sputtering from solar wind...
. None of the known inner satellites possesses an atmosphere.
Cratering
The inner satellites that have been imaged show heavily cratered surfaces. The cratering rate for bodies orbiting close to Giant planets is greater than for the main and outer moons because of gravitational focusing: Sun-orbiting bodies which pass into a large region in the vicinity of a giant planetGas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...
are preferentially deflected towards the planet by its gravitational field, so that the frequency of potential impactors passing through a given cross-sectional area close to the planet is much greater than in interplanetary space. As a result, it has been estimated that very small bodies in inner orbits are expected to be disrupted by external impactors on a timescale much shorter than the age of the solar system. This would place a lower limit on the size of the inner moons that remain.
Accumulation of ring material
At least two of Saturn's inner moons (Atlas and Prometheus) have equatorial ridges. The ridge on Atlas is particularly prominent. Also, Pandora is mantled by some kind of fine deposit. It has been suggested that these features are due to the accretion of ring material onto these moons. Further evidence for such a process may include the low density of these bodies (due, perhaps, to the looseness of material so accumulated), and their high albedo. Prometheus has been seen to attract diffuse material from the F ring during periodic close approaches.Inner satellites of Jupiter
JupiterJupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
has the smallest set of inner satellites, including only the following four:
- MetisMetis (moon)Metis , also known as ', is the innermost 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...
- AdrasteaAdrastea (moon)Adrastea , also known as ', is the second by distance, and the smallest of the four inner moons of Jupiter. It was discovered in Voyager 2 probe photographs taken in 1979, making it the first natural satellite to be discovered from images taken by an interplanetary spacecraft, rather than...
- AmaltheaAmalthea (moon)Amalthea is the third moon of Jupiter in order of distance from the planet. It was discovered on September 9, 1892, by Edward Emerson Barnard and named after Amalthea, a nymph in Greek mythology. It is also known as '....
- ThebeThebe (moon)Thebe also known as ', is the fourth of Jupiter's moons by distance from the planet. It was discovered by Stephen P. Synnott in images from the Voyager 1 space probe taken on March 5, 1979, while orbiting around Jupiter...
Inner satellites of Saturn
SaturnSaturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
's seven inner satellites are closely related to its ring system
Rings of Saturn
The rings of Saturn are the most extensive planetary ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometres to metres, that form clumps that in turn orbit about Saturn...
, and many of them orbit within the rings, creating gaps or "shepherded" ringlets between them.
- PanPan (moon)Pan is the innermost moon of Saturn. It is a walnut-shaped small moon about 35 kilometres across and 23 km high that orbits within the Encke Gap in Saturn's A Ring. Pan acts as a ring shepherd and is responsible for keeping the Encke Gap free of ring particles.It was discovered by Mark R...
– shepherd satellite in the Encke division of Saturn's A Ring - DaphnisDaphnis (moon)Daphnis is an inner satellite of Saturn. It is also known as '; its provisional designation was '. Daphnis is about 8 kilometres in diameter, and orbits the planet in the Keeler Gap within the A ring.- Naming :...
– shepherd satellite in the Keeler Gap of Saturn's A Ring - AtlasAtlas (moon)Atlas is an inner satellite of Saturn.Atlas was discovered by Richard Terrile in 1980 from Voyager photos and was designated '. In 1983 it was officially named after Atlas of Greek mythology, because it "holds the rings on its shoulders" like the Titan Atlas held the sky up above the Earth...
– shepherd satellite of the outer edge of Saturn's A Ring - PrometheusPrometheus (moon)-Animations:-External links:* at ** anaglyph view of Prometheus...
and PandoraPandora (moon)Pandora is an inner satellite of Saturn. It was discovered in 1980 from photos taken by the Voyager 1 probe, and was provisionally designated '. In late 1985 it was officially named after Pandora from Greek mythology. It is also designated as '.Pandora is the outer shepherd satellite of the F Ring...
– inner and outer shepherd satellites, respectively, of Saturn's narrow F Ring - EpimetheusEpimetheus (moon)Epimetheus is an inner satellite of Saturn. It is also known as Saturn XI. It is named after the mythological Epimetheus, brother of Prometheus.-Discovery:Epimetheus occupies essentially the same orbit as the moon Janus...
and JanusJanus (moon)Janus is an inner satellite of Saturn. It is also known as Saturn X . It is named after the mythological Janus.-Discovery and orbit:Janus occupies practically the same orbit as the moon Epimetheus...
– co-orbital satellites that periodically exchange orbital energy with each other
A number of localised objects such as S/2004 S 3, S/2004 S 4
S/2004 S 4
' is the provisional designation of an unconfirmed object seen orbiting Saturn within the inner strand of the F ring on June 21, 2004. It was spotted while J. N. Spitale was trying to confirm the orbit of another provisional object, that was seen 5 hours earlier just exterior to the F ring...
, and S/2004 S 6, sometimes surrounded by a dusty halo, have been seen in the vicinity of the F ring, but at present it is not clear whether they are all transient clumps or whether some may include small solid objects (moons).
Inner satellites of Uranus
UranusUranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...
has by far the most extensive system of inner satellites, containing thirteen known moons:
- CordeliaCordelia (moon)Cordelia is the innermost moon of Uranus. It was discovered from the images taken by Voyager 2 on January 20, 1986, and was given the temporary designation S/1986 U 7. It was not detected again until the Hubble Space Telescope observed it in 1997...
and OpheliaOphelia (moon)Ophelia is a moon of Uranus. It was discovered from the images taken by Voyager 2 on January 20, 1986, and was given the temporary designation S/1986 U 8. It was not seen until the Hubble Space Telescope recovered it in 2003. Ophelia was named after the daughter of Polonius, Ophelia, in...
– inner and outer shepherd satellites, respectively, for Uranus' brightest ring, the narrow Epsilon ringRings of UranusThe planet Uranus has a system of rings 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 Douglas J. Mink... - BiancaBianca (moon)Bianca is an inner satellite of Uranus. It was discovered from the images taken by Voyager 2 on January 23, 1986, and was given the temporary designation S/1986 U 9. It was named after the sister of Katherine in Shakespeare's play The Taming of the Shrew...
- CressidaCressida (moon)Cressida is an inner satellite of Uranus. It was discovered from the images taken by Voyager 2 on 9 January 1986, and was given the temporary designation S/1986 U 3. It was named after the Trojan daughter of Calchas, a tragic heroine who appears in William Shakespeare's play Troilus and...
- DesdemonaDesdemona (moon)Desdemona is an inner satellite of Uranus. It was discovered from the images taken by Voyager 2 on 13 January 1986, and was given the temporary designation S/1986 U 6. Desdemona is named after the wife of Othello in William Shakespeare's play Othello...
- JulietJuliet (moon)Juliet is an inner satellite of Uranus. It was discovered from the images taken by Voyager 2 on 3 January 1986, and was given the temporary designation S/1986 U 2. It is named after the heroine of William Shakespeare's play Romeo and Juliet...
- PortiaPortia (moon)Portia is an inner satellite of Uranus. It was discovered from the images taken by Voyager 2 on 3 January 1986, and was given the temporary designation S/1986 U 1. The moon is named after Portia, the heroine of William Shakespeare's play The Merchant of Venice...
– Associated with the ring R/2003 U 2 - RosalindRosalind (moon)Rosalind is an inner satellite of Uranus. It was discovered from the images taken by Voyager 2 on 13 January 1986, and was given the temporary designation S/1986 U 4. It was named after the daughter of the banished Duke in William Shakespeare's play As You Like It...
- CupidCupid (moon)Cupid is an inner satellite of Uranus. It was discovered by Mark Showalter and Jack J. Lissauer in 2003 using the Hubble Space Telescope. It was named after a character in William Shakespeare's play Timon of Athens....
- BelindaBelinda (moon)- External links :* by *...
- PerditaPerdita (moon)Perdita is an inner satellite of Uranus. Perdita's discovery was complicated. The first photographs of Perdita were taken by the Voyager 2 spacecraft in 1986, but it was not recognized from the photographs for more than a decade. In 1999, the moon was noticed by Erich Karkoschka and reported...
- PuckPuck (moon)Puck is an inner moon of Uranus. It was discovered in December 1985 by the Voyager 2 spacecraft. The name Puck comes from Celtic mythology and English folklore. The orbit of Puck lies between the rings of Uranus and the first of Uranus' large moons, Miranda. Puck is approximately spherical in...
- MabMab (moon)Mab , or Uranus XXVI , is an inner satellite of Uranus. It was discovered by Mark R. Showalter and Jack J. Lissauer in 2003 using the Hubble Space Telescope...
– Associated with the ring R/2003 U 1
Inner satellites of Neptune
NeptuneNeptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
has six known inner satellites:
- NaiadNaiad (moon)Naiad , also known as Neptune III, is the innermost satellite of Neptune, named after the Naiads of Greek legend.Naiad was discovered sometime before mid-September 1989 from the images taken by the Voyager 2 probe. The last moon to be discovered during the flyby, it was designated...
- ThalassaThalassa (moon)Thalassa , also known as Neptune IV, is the second innermost satellite of Neptune. Thalassa was named after sea goddesss Thalassa, a daughter of Aether and Hemera from Greek mythology. "Thalassa" is also the Greek word for "sea"....
- DespinaDespina (moon)Despina , also known as Neptune V, is the third closest inner satellite of Neptune. It is named after Despoina, a nymph who was a daughter of Poseidon and Demeter....
- GalateaGalatea (moon)Galatea , also known as Neptune VI, is the fourth closest inner satellite of Neptune. It is named after Galatea, one of the Nereids of Greek legend.Galatea was discovered in late July 1989 from the images taken by the Voyager 2 probe...
− inner shepherd satellite of the very narrow Adams ringRings of NeptuneThe rings of Neptune consist primarily of five principal rings predicted in 1984 by André Brahic and imaged in 1989 by the Voyager 2 spacecraft...
, which is broken into arcs - LarissaLarissa (moon)Larissa , also known as Neptune VII, is the fifth-closest inner satellite of Neptune. It is named after Larissa, a lover of Poseidon in Greek mythology and eponymous nymph of the city in Thessaly.- Discovery :...
- ProteusProteus (moon)Proteus , also known as Neptune VIII, is the second largest Neptunian moon, and Neptune's largest inner satellite. Discovered by Voyager 2 spacecraft in 1989, it is named after Proteus, the shape-changing sea god of Greek mythology...
It is thought that they are rubble pile
Rubble pile
In astronomy, rubble pile is the informal name for an object that is not a monolith, consisting instead of numerous pieces of rock that have coalesced under the influence of gravity...
s re-accreted from fragments of Neptune's original satellites (inner or not). These were strongly perturbed by Triton
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...
in the period soon after that moon's capture into a very eccentric initial orbit. The perurbations led to collisions between the satellites, and that portion of the fragments that were not lost re-accreted into the present inner satellites after Triton's orbit became circularised.
Exploration
Most of the inner satellites have been imaged by the spacecraft Voyager 1Voyager 1
The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of...
and Voyager 2
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...
. The majority have only been seen in images as single pixels, or resolved to only a few pixels across. However fairly detailed surface features have been seen on the following moons:
| Planet | Spacecraft | |||
| Voyager 1 Voyager 1 The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of... |
Voyager 2 Voyager 2 The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space... |
Galileo Galileo spacecraft Galileo was an unmanned spacecraft sent by NASA to study the planet Jupiter and its moons. Named after the astronomer and Renaissance pioneer Galileo Galilei, it was launched on October 18, 1989 by the Space Shuttle Atlantis on the STS-34 mission... |
Cassini | |
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| Jupiter Jupiter Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,... |
Amalthea Amalthea (moon) Amalthea is the third moon of Jupiter in order of distance from the planet. It was discovered on September 9, 1892, by Edward Emerson Barnard and named after Amalthea, a nymph in Greek mythology. It is also known as '.... |
Amalthea Amalthea (moon) Amalthea is the third moon of Jupiter in order of distance from the planet. It was discovered on September 9, 1892, by Edward Emerson Barnard and named after Amalthea, a nymph in Greek mythology. It is also known as '.... Thebe Thebe (moon) Thebe also known as ', is the fourth of Jupiter's moons by distance from the planet. It was discovered by Stephen P. Synnott in images from the Voyager 1 space probe taken on March 5, 1979, while orbiting around Jupiter... |
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| Saturn Saturn Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,... |
Pandora Pandora (moon) Pandora is an inner satellite of Saturn. It was discovered in 1980 from photos taken by the Voyager 1 probe, and was provisionally designated '. In late 1985 it was officially named after Pandora from Greek mythology. It is also designated as '.Pandora is the outer shepherd satellite of the F Ring... Prometheus Prometheus (moon) -Animations:-External links:* at ** anaglyph view of Prometheus... Janus Janus (moon) Janus is an inner satellite of Saturn. It is also known as Saturn X . It is named after the mythological Janus.-Discovery and orbit:Janus occupies practically the same orbit as the moon Epimetheus... Epimetheus Epimetheus (moon) Epimetheus is an inner satellite of Saturn. It is also known as Saturn XI. It is named after the mythological Epimetheus, brother of Prometheus.-Discovery:Epimetheus occupies essentially the same orbit as the moon Janus... |
Atlas Atlas (moon) Atlas is an inner satellite of Saturn.Atlas was discovered by Richard Terrile in 1980 from Voyager photos and was designated '. In 1983 it was officially named after Atlas of Greek mythology, because it "holds the rings on its shoulders" like the Titan Atlas held the sky up above the Earth... Pandora Pandora (moon) Pandora is an inner satellite of Saturn. It was discovered in 1980 from photos taken by the Voyager 1 probe, and was provisionally designated '. In late 1985 it was officially named after Pandora from Greek mythology. It is also designated as '.Pandora is the outer shepherd satellite of the F Ring... Prometheus Prometheus (moon) -Animations:-External links:* at ** anaglyph view of Prometheus... Janus Janus (moon) Janus is an inner satellite of Saturn. It is also known as Saturn X . It is named after the mythological Janus.-Discovery and orbit:Janus occupies practically the same orbit as the moon Epimetheus... Epimetheus Epimetheus (moon) Epimetheus is an inner satellite of Saturn. It is also known as Saturn XI. It is named after the mythological Epimetheus, brother of Prometheus.-Discovery:Epimetheus occupies essentially the same orbit as the moon Janus... |
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| Uranus Uranus Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus... |
Puck Puck (moon) Puck is an inner moon of Uranus. It was discovered in December 1985 by the Voyager 2 spacecraft. The name Puck comes from Celtic mythology and English folklore. The orbit of Puck lies between the rings of Uranus and the first of Uranus' large moons, Miranda. Puck is approximately spherical in... |
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| Neptune Neptune Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times... |
Larissa Larissa (moon) Larissa , also known as Neptune VII, is the fifth-closest inner satellite of Neptune. It is named after Larissa, a lover of Poseidon in Greek mythology and eponymous nymph of the city in Thessaly.- Discovery :... Proteus Proteus (moon) Proteus , also known as Neptune VIII, is the second largest Neptunian moon, and Neptune's largest inner satellite. Discovered by Voyager 2 spacecraft in 1989, it is named after Proteus, the shape-changing sea god of Greek mythology... |
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