Naiad is the innermost satellite of Neptune and also the smallest among the inner moons. Naiad is the 8th overall largest and 59th largest moon in the Solar System currently known.
Discovery
Naiad was discovered sometime before mid-September 1989 from the images taken by the Voyager 2 spacecraft. Naiad was the last moon to be discovered during the Voyager 2 spacecraft flyby.
Naming
The moon was given the temporary designation S/1989 N6.
Naiad the moon was named after the Naiads of Greek legend.
In Greek mythology, the Naiads or Naiades were a type of nymph who presided over fountains, wells, springs, streams, and brooks. They are distinct from river gods, who embodied rivers, and the very ancient spirits that inhabited the still waters of marshes, ponds and lagoon-lakes, such as pre-Mycenaean Lerna in the Argolid.
Stats
Diameter (mean): 66 km
Semi-major axis: 48,227 km
Orbital Period: 0.294 days
Orbit
Naiad orbits about 23,500 km above Neptune's cloud tops. Since this is below the synchronous orbit radius, its orbit is slowly decaying due to tidal deceleration and may eventually impact Neptune's atmosphere, or break up into a planetary ring upon passing its Roche limit due to tidal stretching.
Naiad orbits Neptune well within its fluid Roche limit, and its density is expected to be low enough that it may be very close to its actual Roche limit already.
Naiad takes as long to rotate on its axis as it does to make one orbit of Neptune; and therefore always keeps the same hemisphere pointed to Neptune.
Physical characteristics
Naiad is irregularly shaped and probably has not been modified by any internal geological processes after its formation.
Little else is known about Naiad. Naiad is likely, like the other satellites inward of Triton, a rubble pile re-accreted from fragments of Neptune's original satellites, which were smashed up by perturbations from Triton soon after that moon's capture into a very eccentric initial orbit.
Tuesday, 10 January 2012
8th Largest Moon of Uranus - Portia (7th Moon outwards from Uranus)
Portia is the second-largest inner satellite of Uranus after Puck. Portia is the 8th largest Moon of Uranus and the 42th largest moon in the Solar System currently known.
Discovery
Portia was discovered by Stephen P. Synnott, who is an American astronomer and Voyager scientist, from the images taken by Voyager 2 on 3 January 1986.
Naming
The moon was given the temporary designation S/1986 U1.
Portia the moon is named after Portia, the heroine of William Shakespeare's play The Merchant of Venice.
Portia is the heroine of William Shakespeare's The Merchant of Venice. A rich, beautiful, and intelligent heiress, she is bound by the lottery set forth in her father's will, which gives potential suitors the chance to choose between three caskets composed of gold, silver and lead. If they choose the right casket – the casket containing Portia's portrait – they win Portia's hand in marriage. If they choose the wrong casket, they must leave and never seek another woman in marriage.
Stats
Diameter (mean): 135 km
Semi-major axis: 66,097 km
Orbital Period: 0.51 days
Orbit
Portia takes as long to rotate on its axis as it does to make one orbit of Uranus; and therefore always keeps the same hemisphere pointed to Uranus.
The Portian orbit, which lies inside Uranus' synchronous orbital radius, is slowly decaying due to tidal deceleration. The moon will one day either break up into a planetary ring or hit Uranus.
Portia heads a group of satellites called the Portia Group, which includes Bianca, Cressida, Desdemona, Juliet, Rosalind, Cupid, Belinda and Perdita. These satellites have similar orbits and photometric properties.
Physical characteristics
Little is known about Portia beyond its size of about 135 km, orbit and geometric albedo of about 0.08.
In the Voyager 2 images, Portia appears as an elongated object whose major axis points towards Uranus. Portia's surface is grey in color. Observations with Hubble Space Telescope and large terrestrial telescopes found water ice absorption features in the spectrum of Portia.
Exploration Status
No close-up image of Portia has been photographed.
No mission is planned in the foreseeable future.
Discovery
Portia was discovered by Stephen P. Synnott, who is an American astronomer and Voyager scientist, from the images taken by Voyager 2 on 3 January 1986.
Naming
The moon was given the temporary designation S/1986 U1.
Portia the moon is named after Portia, the heroine of William Shakespeare's play The Merchant of Venice.
Portia is the heroine of William Shakespeare's The Merchant of Venice. A rich, beautiful, and intelligent heiress, she is bound by the lottery set forth in her father's will, which gives potential suitors the chance to choose between three caskets composed of gold, silver and lead. If they choose the right casket – the casket containing Portia's portrait – they win Portia's hand in marriage. If they choose the wrong casket, they must leave and never seek another woman in marriage.
Stats
Diameter (mean): 135 km
Semi-major axis: 66,097 km
Orbital Period: 0.51 days
Orbit
Portia takes as long to rotate on its axis as it does to make one orbit of Uranus; and therefore always keeps the same hemisphere pointed to Uranus.
The Portian orbit, which lies inside Uranus' synchronous orbital radius, is slowly decaying due to tidal deceleration. The moon will one day either break up into a planetary ring or hit Uranus.
Portia heads a group of satellites called the Portia Group, which includes Bianca, Cressida, Desdemona, Juliet, Rosalind, Cupid, Belinda and Perdita. These satellites have similar orbits and photometric properties.
Physical characteristics
Little is known about Portia beyond its size of about 135 km, orbit and geometric albedo of about 0.08.
In the Voyager 2 images, Portia appears as an elongated object whose major axis points towards Uranus. Portia's surface is grey in color. Observations with Hubble Space Telescope and large terrestrial telescopes found water ice absorption features in the spectrum of Portia.
Exploration Status
No close-up image of Portia has been photographed.
No mission is planned in the foreseeable future.
Monday, 9 January 2012
8th Largest Moon of Saturn - Hyperion (22th Moon outwards from Saturn)
Hyperion is second largest bodies known to be highly irregularly shaped (non-ellipsoidal; i.e., not in hydrostatic equilibrium) in the solar system. The only larger moon known to be irregular in shape is Neptune's moon Proteus.
Discovery
William Lassel discovered Hyperion in 1848. The same year William Cranch Bond, with his son George Phillips Bond, independently discovered the moon, and all three men are jointly credited with the discovery.
Hyperion was the first non-round moon to be discovered.
Naming
Hyperion the moon is named after Hyperion, the Titan god of watchfulness and observation – the elder brother of Cronus, the Greek equivalent of Saturn.
In Greek Mythology, Hyperion was one of the twelve Titans of Ancient Greece, the sons and daughters of Gaia (the physical incarnation of Earth) and Ouranos (literally meaning 'the Sky'). He was the brother of Cronus. He was also the lord of light, and the Titan of the east.
Stats
Diameter (mean): 270 km
Semi-major axis: 1,481,010 km
Orbital Period: 21.28 days
Formation
Hyperion is distinguished by its irregular shape, its chaotic rotation, and its unexplained sponge-like appearance.
The largest crater on Hyperion is approximately 121.57 km in diameter and 10.2 km deep. A possible explanation for Hyperion's irregular shape is that it is a fragment of a larger body that was broken by a large impact in the distant past.
Composition
Hyperion's low density indicates that it is composed largely of water ice with only a small amount of rock. It is thought that Hyperion may be similar to a loosely accreted pile of rubble in its physical composition.
However, unlike most of Saturn's moons, Hyperion has a low albedo (0.2–0.3), indicating that it is covered by at least a thin layer of dark material. This may be material from Phoebe (which is much darker) that got past Iapetus. Hyperion is redder than Phoebe and closely matches the color of the dark material on Iapetus.
Surface features
Hyperion's surface is covered with deep, sharp-edged craters that give it the appearance of a giant sponge. Dark material fills the bottom of each crater. The reddish substance contains long chains of carbon and hydrogen and appears very similar to material found on other Saturnian satellites, most notably Iapetus.
The latest analyses of data obtained by NASA's Cassini spacecraft during its flybys of Hyperion in 2005 and 2006 show that about 40 percent of the moon is empty space. It was suggested in July 2007 that this porosity allows craters to remain nearly unchanged over the eons.
The new analyses also confirmed that Hyperion is composed mostly of water ice with very little rock.
Rotation
The Voyager 2 images and subsequent ground based photometry indicate that Hyperion's rotation is chaotic, that is, its axis of rotation wobbles so much that its orientation in space is unpredictable.
Hyperion is the only moon in the Solar System known to rotate chaotically. It is also the only regular natural satellite in the Solar System not to be tidally locked.
The fact that Hyperion's rotation is not locked probably accounts for the relative uniformity of it's surface, in contrast to many of Saturn's other moons which have contrasting trailing and leading hemispheres.
4:3 resonance with Titan
Hyperion is unique among the large moons in that it is very irregularly shaped, has a fairly eccentric orbit, and is near a much larger moon, Titan.
These factors combine to restrict the set of conditions under which a stable rotation is possible. The 3:4 orbital resonance between Titan and Hyperion may also make a chaotic rotation more likely.
Sunday, 8 January 2012
8th Largest Moon of Jupiter - Elara (13th Moon outwards from Jupiter)
Elara is a prograde irregular satellite of Jupiter and the second biggest satellite in the Himalia group.
Elara is the 8th largest moon of Jupiter and 50th largest moon in the Solar System currently known.
Discovery
Elara was discovered on 5 January 1905 by Charles Dillon Perrine in photographs taken with the Crossley 36-inch reflector at Lick Observatory.
Naming
Elara the moon is named for one of the lovers of Zeus, the Greek equivalent of the Roman god Jupiter.
In Greek mythology, Elara was the daughter of King Orchomenus and mother of Tityos. She was one of Zeus' lovers and he hid her from his wife, Hera, by placing her deep beneath the earth. This was where she gave birth to Tityos, a giant who is sometimes said to be the son of Gaia, the earth goddess, for this reason. It is further added that Elara died in labour because of the enormous size of her baby.
Stats
Diameter (mean): 86 km
Semi-major axis: 11,741,000 km
Orbital Period: 259.64 days
Rotation Period: 12 hours
Orbit
Elara belongs to the Himalia group, a family of Jovian satellites which have similar orbits and appearance, with orbital elements continuously changing due to solar and planetary perturbations.
Formation
Elara may be a chunk of an asteroid (a C or D class asteroid, judging by the fact that it reflects only about 4% of the light it receives), which was broken apart in a collision either before or after being captured by Jupiter's gravity. In this scenario, the other pieces became the other moons in the Himalia group.
Elara is the 8th largest moon of Jupiter and 50th largest moon in the Solar System currently known.
Discovery
Elara was discovered on 5 January 1905 by Charles Dillon Perrine in photographs taken with the Crossley 36-inch reflector at Lick Observatory.
Naming
Elara the moon is named for one of the lovers of Zeus, the Greek equivalent of the Roman god Jupiter.
In Greek mythology, Elara was the daughter of King Orchomenus and mother of Tityos. She was one of Zeus' lovers and he hid her from his wife, Hera, by placing her deep beneath the earth. This was where she gave birth to Tityos, a giant who is sometimes said to be the son of Gaia, the earth goddess, for this reason. It is further added that Elara died in labour because of the enormous size of her baby.
Stats
Diameter (mean): 86 km
Semi-major axis: 11,741,000 km
Orbital Period: 259.64 days
Rotation Period: 12 hours
Orbit
Elara belongs to the Himalia group, a family of Jovian satellites which have similar orbits and appearance, with orbital elements continuously changing due to solar and planetary perturbations.
Formation
Elara may be a chunk of an asteroid (a C or D class asteroid, judging by the fact that it reflects only about 4% of the light it receives), which was broken apart in a collision either before or after being captured by Jupiter's gravity. In this scenario, the other pieces became the other moons in the Himalia group.
Thursday, 5 January 2012
Trans-Neptunian object
A trans-Neptunian object (TNO) is any minor planet in the Solar System that orbits the Sun at a greater average distance (semi-major axis) than Neptune.
The first trans-Neptunian object to be discovered was Pluto in 1930, and in 1978 Pluto's moon Charon. Due to the vast distance, it took more than 60 years to discover the second trans-Neptunian object, (15760) 1992 QB1, in 1992.
Now over 1200 trans-Neptunian objects appear on the Minor Planet Center's List Of Transneptunian Objects. As of November 2009, two hundred of these have their orbits well-enough determined that they have been given a permanent minor planet designation.
Distribution and classification
The Kuiper belt, scattered disk, and Oort cloud are three conventional divisions of this volume of space, though treatments vary and a few objects such as Sedna do not fit easily into any division.
According to their distance from the Sun and their orbit parameters, TNOs are classified in two large groups:
1. The (classical) Kuiper belt -
Contains objects with an average distance to the Sun of 30 to about 55 AU, usually having close-to-circular orbits with a small inclination from the ecliptic. Kuiper belt objects are further classified into the following two groups:
a. Resonant objects are locked in an orbital resonance with Neptune
Objects with a 1:2 resonance are also called twotinos, and objects with a 2:3 resonance are called plutinos, after their most prominent member, Pluto.
b. Classical Kuiper belt objects (also called cubewanos)
These have no such resonance, moving on almost circular orbits, unperturbed by Neptune. Examples are 1992 QB1, 50000 Quaoar and Makemake.
2. The scattered disk -
Contains objects further from the Sun, usually with very irregular orbits (i.e. very elliptical and having a strong inclination from the ecliptic). A typical example is the most massive known TNO, Eris.
The first trans-Neptunian object to be discovered was Pluto in 1930, and in 1978 Pluto's moon Charon. Due to the vast distance, it took more than 60 years to discover the second trans-Neptunian object, (15760) 1992 QB1, in 1992.
Now over 1200 trans-Neptunian objects appear on the Minor Planet Center's List Of Transneptunian Objects. As of November 2009, two hundred of these have their orbits well-enough determined that they have been given a permanent minor planet designation.
Distribution and classification
The Kuiper belt, scattered disk, and Oort cloud are three conventional divisions of this volume of space, though treatments vary and a few objects such as Sedna do not fit easily into any division.
According to their distance from the Sun and their orbit parameters, TNOs are classified in two large groups:
1. The (classical) Kuiper belt -
Contains objects with an average distance to the Sun of 30 to about 55 AU, usually having close-to-circular orbits with a small inclination from the ecliptic. Kuiper belt objects are further classified into the following two groups:
a. Resonant objects are locked in an orbital resonance with Neptune
Objects with a 1:2 resonance are also called twotinos, and objects with a 2:3 resonance are called plutinos, after their most prominent member, Pluto.
b. Classical Kuiper belt objects (also called cubewanos)
These have no such resonance, moving on almost circular orbits, unperturbed by Neptune. Examples are 1992 QB1, 50000 Quaoar and Makemake.
2. The scattered disk -
Contains objects further from the Sun, usually with very irregular orbits (i.e. very elliptical and having a strong inclination from the ecliptic). A typical example is the most massive known TNO, Eris.
7th Largest Moon of Neptune - Thalassa (2nd Moon outwards from Neptune)
Thalassa is the second innermost satellite, the seventh largest satellite of Neptune and the 51th largest moon in the Solar System currently known.
Discovery
Thalassa was discovered sometime before mid-September 1989 from the images taken by the Voyager 2 spacecraft.
Naming
The moon was given the temporary designation S/1989 N5.
Thalassa the moon was named after the sea goddesss Thalassa, a daughter of Aether and Hemera from Greek mythology. "Thalassa" is also the Greek word for "sea".
Stats
Diameter (mean): 82 km
Semi-major axis: 50,075 km
Orbital Period: 0.311 days
Orbit
Thalassa's orbit lies below Neptune's synchronous orbit radius, so it is slowly spiralling inward due to tidal deceleration and may eventually impact Neptune's atmosphere, or break up into a planetary ring upon passing its Roche limit due to tidal stretching.
Relatively soon after, the spreading debris may impinge upon the third moon Despina's orbit.
Thalassa takes as long to rotate on its axis as it does to make one orbit of Neptune; and therefore always keeps the same hemisphere pointed to Neptune.
Physical characteristics
Thalassa is irregularly shaped and shows no sign of any geological modification.
Unusually for irregular bodies, Thalassa appears to be roughly disk-shaped.
Little else is known about Thalassa. Thalassa is likely, like the other satellites inward of Triton, a rubble pile re-accreted from fragments of Neptune's original satellites, which were smashed up by perturbations from Triton soon after that moon's capture into a very eccentric initial orbit.
7th Largest Moon of Uranus - Sycorax (23rd Moon outwards from Uranus)
Uranus is known to have nine irregular moons, which circle the planet at a distance much greater than that of Oberon, the furthest of the large moons.
Sycorax is the largest retrograde irregular satellite of Uranus and seventh overall of the Uranus' moons. Sycorax is the 39th largest moon in the Solar System currently known.
Discovery
Sycorax was discovered on 6 September 1997 by Brett J. Gladman, Philip D. Nicholson, Joseph A. Burns and John J. Kavelaars using the 200-inch Hale telescope, together with Caliban, the second largest retrograde irregular moon of Uranus.
Naming
The moon was given the temporary designation S/1997 U1.
Sycorax the moon was named after Sycorax, Caliban's mother in William Shakespeare's play The Tempest.
Sycorax, an unseen character in William Shakespeare's play The Tempest (1611), is a powerful witch and the mother of Caliban. Sycorax has died before the action of the play begins. She had been banished to the same island in the Mediterranean to which Prospero was later banished and had been responsible for the unjust imprisonment and enslavement of the sprite Ariel, later freed by Prospero.
Stats
Diameter: ~150 km
Semi-major axis: 12,179,000 km
Orbital Period: -1288.28 days
Rotation Period: 3.6 hours
Orbit
Sycorax follows a distant orbit, more than 20 times further from Uranus than the furthest regular moon Oberon. Its orbit is retrograde, moderately inclined and eccentric.
The orbital parameters suggest that it may belong, together with Setebos and Prospero, to the same dynamic cluster, suggesting common origin.
Origin
All the irregular moons are probably captured objects that were trapped by Uranus soon after its formation.
Sycorax is hypothesized to be a captured object: it did not form in the accretionary disk, which existed around Uranus just after its formation.
The exact capture mechanism is not known, but capturing a moon requires the dissipation of energy. The possible capture processes include: gas drag in the protoplanetary disk, many-body interactions and the capture during the fast growth of the Uranus' mass (so called pull-down).
Physical characteristics
Sycorax's light red color differs from the gray of the other irregular moons, implying a different origin.
Nothing is known about the internal structure of Sycorax.
Exploration Status
No close-up image of Sycorax has been photographed.
No mission is planned in the foreseeable future.
Sycorax is the largest retrograde irregular satellite of Uranus and seventh overall of the Uranus' moons. Sycorax is the 39th largest moon in the Solar System currently known.
Discovery
Sycorax was discovered on 6 September 1997 by Brett J. Gladman, Philip D. Nicholson, Joseph A. Burns and John J. Kavelaars using the 200-inch Hale telescope, together with Caliban, the second largest retrograde irregular moon of Uranus.
Naming
The moon was given the temporary designation S/1997 U1.
Sycorax the moon was named after Sycorax, Caliban's mother in William Shakespeare's play The Tempest.
Sycorax, an unseen character in William Shakespeare's play The Tempest (1611), is a powerful witch and the mother of Caliban. Sycorax has died before the action of the play begins. She had been banished to the same island in the Mediterranean to which Prospero was later banished and had been responsible for the unjust imprisonment and enslavement of the sprite Ariel, later freed by Prospero.
Stats
Diameter: ~150 km
Semi-major axis: 12,179,000 km
Orbital Period: -1288.28 days
Rotation Period: 3.6 hours
Orbit
Sycorax follows a distant orbit, more than 20 times further from Uranus than the furthest regular moon Oberon. Its orbit is retrograde, moderately inclined and eccentric.
The orbital parameters suggest that it may belong, together with Setebos and Prospero, to the same dynamic cluster, suggesting common origin.
Origin
All the irregular moons are probably captured objects that were trapped by Uranus soon after its formation.
Sycorax is hypothesized to be a captured object: it did not form in the accretionary disk, which existed around Uranus just after its formation.
The exact capture mechanism is not known, but capturing a moon requires the dissipation of energy. The possible capture processes include: gas drag in the protoplanetary disk, many-body interactions and the capture during the fast growth of the Uranus' mass (so called pull-down).
Physical characteristics
Sycorax's light red color differs from the gray of the other irregular moons, implying a different origin.
Nothing is known about the internal structure of Sycorax.
Exploration Status
No close-up image of Sycorax has been photographed.
No mission is planned in the foreseeable future.
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