10th Largest Moon of Jupiter - Carme (46th Moon outwards from Jupiter)

Carme is a retrograde irregular satellite of Jupiter. Carme is the second largest retrograde (after Pasiphaë) and third largest irregular satellite after Himalia, Elara and Pasiphaë.

Carme is the 10th largest moon of Jupiter and 67th largest moon in the Solar System currently known.

Discovery

Carme was discovered by Seth Barnes Nicholson with the 100-inch Hooker telescope at the Mount Wilson Observatory in California on 30th July 1938.

Naming

Carme did not receive its present name until 1975. Nicholson himself declined to propose names. Before then, it was simply known as Jupiter XI. It was sometimes called "Pan" between 1955 and 1975 (Pan is now the name of a satellite of Saturn).

Carme the moon is named after the mythological Carme, mother by Zeus of Britomartis, a Cretan goddess.

Carme, the Latinized form of Greek Karmê ("shearer"), was a female Cretan spirit who assisted the grain harvest of Demeter's Cretan predecessor. According to the Olympian mythology, she was the mother, by Zeus, of the virginal huntress Britomartis, also called Diktynna, whom she bore at Kaino.

Stats

Diameter (mean): 46 km
Semi-major axis: 23,197,992 km
Orbital Period: -721.82 days
Group: Carme group

Orbit

Carme gives its name to the Carme group, made up of irregular retrograde moons orbiting Jupiter at a distance ranging between 23 and 24 Gm and at an inclination of about 165°. Carme's orbital elements are as of January 2000 and they are continuously changing due to solar and planetary perturbations.

Carme is the largest member of the Carme group, a family of Jovian moons which have similar orbits and appearance and are therefore thought to have a common origin.

Formation

Carme was probably a D-type asteroid (possibly from the Hilda family or the Jupiter Trojans) that suffered a collision, which broke off a number of pieces, either before or after being captured by Jupiter's gravity. Those pieces became the other 16 moons in the Carme group.

Moons in Carme group all are very similar in color -- light red -- except for Kalyke, which is considerably redder than the others. All of these characteristics support the idea that the Carme moons began as a captured asteroid, rather than forming as part of the original Jupiter system.

Physical Characteristics

None of the Carme moons is massive enough to pull itself into a sphere, so they are probably all irregularly shaped.

20th Largest Asteroid, 375 Ursula

375 Ursula is a large main belt asteroid and is the 20th largest asteroid currently known.

Discovery

Ursula was discovered by Auguste Honoré Charlois on September 18, 1893, in Nice.

Naming

Any reference of its name to a person or an occurrence is unknown.

Stats

Diameter (mean): 216 km
Aphelion: 3.46 AU
Perihelion: 2.79 AU
Semi-major axis: 3.123 AU
Orbital Period: 5.52 years
Rotation period: 16.83 hrs
Date discovered: 1893.9.18
Class: C
Type: Main-belt Asteroid
(data from JPL Small-Body Database)

Star Occultation

A value of 216 +/- 10 km for the mean diameter of Ursula is given, based on photometric measurements and observations of the occultation of AG + 39 deg 303, a K-type star of visual magnitude 8.8, by 375 Ursula on November 15, 1982.

From the occultation diameter and the absolute visual magnitudes presented, the visual geometric albedo of Ursula is determined to be 0.032, similar to the value typical of C-type asteroids.

19th Largest Asteroid, 48 Doris

48 Doris is a large main belt asteroid, with a diameter of around 222 km. Doris is the 19th largest asteroid currently known.

Discovery

Doris was discovered by Hermann Goldschmidt on September 19, 1857 from his balcony in Paris.

Naming

To find a name for the asteroid, Jacques Babinet of the Academy of Sciences created a shortlist and asked the geologist Élie de Beaumont to make the selection. De Beaumont chose Doris, after an Oceanid in Greek mythology.

Doris, an Oceanid, was a sea nymph in Greek mythology, whose name represented the bounty of the sea. She was the daughter of Oceanus and Tethys and the wife of Nereus. She was also aunt to Atlas, the titan who was made to carry the sky upon his shoulders, whose mother Clymene was a sister of Doris.

Stats

Diameter (mean): 221.8 km
Aphelion: 3.343 AU
Perihelion: 2.88 AU
Semi-major axis: 3.114 AU
Orbital Period: 5.49 years
Rotation period: 11.89 hrs
Date discovered: 1857.9.19
Class: C
Type: Main-belt Asteroid
(data from JPL Small-Body Database)

Star Occultation

An occultation (SAO 118832) on March 19, 1981, suggested a diameter of 219±25 km for Doris.

Observations of an occultation (SAO 161849) on October 14, 1999, using four well-placed chords, indicate an ellipsoid of 278×142 km and that Doris is an extremely irregular shaped object.

18th Largest Asteroid, 532 Herculina

532 Herculina is a large main belt asteroid, with a diameter of around 222 km. Herculina is the 18th largest asteroid currently known.

Discovery

Herculina was discovered on April 20, 1904, by Max Wolf in Heidelberg.

Naming

Herculina was initially catalogued as 1904 NY.

The origin of Herculina's name was lost. The asteroid may be named after the mythical Hercules, or after an unknown woman of that name. The bulk of the asteroids discovered by Wolf around this date were named for characters in operas, but if this name was also drawn from such a source, no explanation has been recorded.

Stats

Diameter (mean): 222 km
Aphelion: 3.26 AU
Perihelion: 2.278 AU
Semi-major axis: 2.77 AU
Orbital Period: 4.61 years
Rotation period: 9.405 hrs
Date discovered: 1904.4.20
Class: S
Type: Main-belt Asteroid
(data from JPL Small-Body Database)

Physical characteristics

Herculina has often been noted for its complex lightcurves, which made determination of its shape and rotation somewhat difficult.

Recent modelling in 2002 of photometric data indicates that Herculina is not spherical, but a blocky shape not unlike a battered cuboid - or, as the analysis described it, it "resembles a toaster". This analysis indicates the presence of multiple large craters, but no major variation in albedo. The approximate ratios of the axes were suggested as 1:1.1:1.3, broadly consistent with earlier models if slightly more elongated.

Satellite (S/1978(532)1) ?

Following anomalous observations during an occultation of the star SAO 120774 in 1978, Herculina became the first asteroid to be "confirmed" to have an asteroid moon. A satellite of about 45 km orbiting at a distance of about 1,000 km from Herculina.

However, careful examination in 1993, using the Hubble Space Telescope, failed to locate the moon.

17th Largest Asteroid, 19 Fortuna

19 Fortuna is 225 km in diameter and is one of the largest main-belt asteroids. Fortuna is the 17th largest asteroid currently known.

Discovery

Fortuna was discovered by John Russell Hind on August 22, 1852 in London.

Naming

Fortuna the asteroid was named after Fortuna, the Roman goddess of luck.

Fortuna (equivalent to the Greek goddess Tyche) was the goddess of fortune and personification of luck in Roman religion. She might bring good luck or bad: she could be represented as veiled and blind, as in modern depictions of Justice, and came to represent life's capriciousness.

Stats

Diameter (mean): 225 km
Aphelion: 2.831 AU
Perihelion: 2.057 AU
Semi-major axis: 2.442 AU
Orbital Period: 3.82 years
Rotation period: 7.443 hrs
Date discovered: 1852.8.22
Class: G
Type: Main-belt Asteroid

Physical Characteristics

Fortuna has one of the darkest known geometric albedos for an asteroid over 150 km in diameter. Its albedo has been measured at 0.028 and 0.037.

Fortuna has a composition similar to 1 Ceres: a darkly colored surface that is heavily space-weathered with the composition of primitive organic compounds, including tholins.

16th Largest Asteroid, 451 Patientia

451 Patientia is an asteroid of the main asteroid belt and with a diameter of 225 km is one of the larger asteroids in the belt. Patientia is the 16th largest asteroids currently known.

Discovery

Patientia was discovered by Auguste Honoré Charlois on December 4, 1899 in Nice.

Naming

The asteroid's provisional designation was 1899 EY. It was named after "patience", as Patientia means "patience" in Latin.

Stats

Diameter (mean): 225 km
Aphelion: 3.295 AU
Perihelion: 2.823 AU
Semi-major axis: 3.065 AU
Orbital Period: 5.37 years
Rotation period: 9.727 hrs
Date discovered: 1899.12.4
Class: ?
Type: Main-belt Asteroid
(data from JPL Small-Body Database)

Orbit

Patientia regularly reaches 11th magnitude in brightness. In January 11th, 2013 and December 12th, 2017, when in favorable oppositions, Patientia will be at magnitudes 10.7 and 10.4 respectively. This is very bright for a later-discovered asteroid.

9th Largest Moon of Neptune - Halimede (9th Moon outwards from Neptune)

Halimede is a retrograde irregular satellite of Neptune. Halimede is the 9th overall largest satellite of Neptune and 61th largest moon in the Solar System currently known.

Discovery

Matthew J. Holman, Harvard-Smithsonian Center for Astrophysics (CfA); John J. Kavelaars, National Research Council of Canada; T. Grav, University of Oslo and CfA; and W. Fraser and Dan Milisavljevic, McMaster University, reported the discovery of three satellites of Neptune on CCD images obtained in 14 August 2002 with the 4-m Blanco telescope at Cerro Tololo.

Naming

The moon was given the temporary designation S/2002 N1.

Halimede the moon is named after one of the Nereids, the fifty daughters of Nereus and Doris.

Stats

Diameter (mean): 62 km

Semi-major axis: 16,611,000 km

Orbital Period: 1879.08 days

Rotation Period: ?

Orbit

Halimede has the second most eccentric (0.2646) and third most inclined (112.712°) orbit around Neptune.

Formation

Given the very similar colour of the satellite to that of Nereid together with the high probability of collision in the past lifespan of the Solar System, it has been suggested that the satellite could be a fragment of Nereid.

Physical characteristics

Little is known about Halimede.

Halimede is about 62 kilometers in diameter (assuming an albedo of 0.04) and appears neutral (grey) in the visible light.

9th Largest Moon of Uranus - Juliet (6th Moon outwards from Uranus)

Juliet is the third-largest inner satellite of Uranus after Puck and Portia. Juliet is the 9th largest Moon of Uranus and the 47th largest moon in the Solar System currently known.

Discovery

Juliet 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 U2.

Juliet the moon is named after Juliet, the heroine of William Shakespeare's play Romeo and Juliet.

Romeo and Juliet is a tragedy written early in the career of playwright William Shakespeare about two young star-crossed lovers. Juliet is the daughter of the Capulets, who are sworn enemies to the Montagues. Nevertheless, she falls in love with Romeo, son of the Montagues, and secretly marries him. The feud between the families eventually leads to the deaths of both Romeo and Juliet. But their death ultimately unite their feuding families. It was among Shakespeare's most popular archetypal stories of young, teenage lovers.

Stats

Diameter (mean): 94 km

Semi-major axis: 64,360 km

Orbital Period: 0.49 days

Orbit

Juliet 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.

Juliet's orbit lies inside Uranus' synchronous orbital radius, and is slowly decaying due to tidal deceleration. The moon will one day either break up into a planetary ring or hit Uranus.

Juliet belongs to a group of satellites called the Portia Group, which includes Portia, Bianca, Cressida, Desdemona, Rosalind, Cupid, Belinda and Perdita. These satellites have similar orbits and photometric properties.

Juliet may collide with Desdemona within the next 100 million years.

Physical characteristics

Little is known about Juliet beyond its size of about 94 km, orbit and geometric albedo of about 0.08.

In the Voyager 2 images, Juliet appears as an elongated object whose major axis points towards Uranus. Juliet's surface is grey in color.

Exploration Status

No close-up image of Juliet has been photographed.

No mission is planned in the foreseeable future.

9th Largest Moon of Saturn - Phoebe (26th Moon outwards from Saturn)

Phoebe is a retrograde irregular satellite of Saturn. Phoebe, is the 9th largest moon of Saturn, and the 26th largest moon in the Solar System currently known.

Phoebe belongs to the Norse group: a large group of retrograde irregular satellites of Saturn. Their semi-major axes range between 12 and 24 Gm, their inclinations between 136° and 175° and their eccentricities between 0.13 and 0.77.

Discovery

Phoebe was discovered by William Henry Pickering on 17 March 1899 from photographic plates that had been taken starting on 16 August 1898 at the Boyden Observatory near Arequipa, Peru, by DeLisle Stewart.

Phoebe was the first satellite to be discovered photographically.

Naming

Phoebe the moon is named after Phoebe, a Titan in Greek mythology.

In Greek mythology Phoebe was one of the original Titans, who were one set of sons and daughters of Uranus and Gaia.

Stats

Diameter (mean): 213 km

Semi-major axis: 12,955,759 km

Orbital Period: -550.56 days

Orbit

For more than 100 years, Phoebe was Saturn's outermost known moon, until the discovery of several smaller moons in 2000. Phoebe is almost 4 times more distant from Saturn than its nearest major neighbor, Iapetus, and is substantially larger than any of the other moons orbiting planets at comparable distances.

All of Saturn's moons up to Iapetus orbit very nearly in the plane of Saturn's equator. The outer irregular satellites follow fairly to highly eccentric orbits, and none is expected to rotate synchronously as all the inner moons of Saturn do (except for Hyperion).

Formation

Most of Saturn's inner moons have very bright surfaces, but Phoebe's albedo is very low (0.06), as dark as lampblack.

Phoebe's dark coloring initially led to scientists surmising that it was a captured asteroid, as it resembled the common class of dark carbonaceous asteroids. These are chemically very primitive and are thought to be composed of original solids that condensed out of the solar nebula with little modification since then.

However, images from Cassini indicate that Phoebe's craters show a considerable variation in brightness, which indicate the presence of large quantities of ice below a relatively thin blanket of dark surface deposits some 300 to 500 metres thick. In addition, quantities of carbon dioxide have been detected on the surface, a finding which has never been replicated on an asteroid.

Morevover, it is estimated that Phoebe is about 50% rock, as opposed to the 35% or so that typifies Saturn's inner moons.

For these reasons, scientists are coming to believe that Phoebe is in fact a captured centaur, one of a number of icy planetoids from the Kuiper belt that orbit the Sun between Jupiter and Neptune.

Physical characteristics

Phoebe is roughly spherical and has a diameter of 213 kilometres, which is equal to about one-fifteenth of the diameter of Earth's Moon.

The Phoebean surface is extremely heavily scarred, with craters up to 80 kilometres across, one of which has walls 16 kilometres high.

Material displaced from Phoebe's surface by microscopic meteor impacts may be responsible for the dark surfaces of Hyperion. Debris from the biggest impacts may have been the building blocks of the other moons of Phoebe's group — all of which are less than 10 km in diameter.

Phoebe ring

The Phoebe ring is one of the rings of Saturn. This ring is tilted 27 degrees from Saturn's equatorial plane (and the other rings). It extends from at least 128 to 207 times the radius of Saturn.

The diameter of the ring is equivalent to 300 Saturns lined up side to side. The ring is thick too -- it's about 20 times as thick as the diameter of the planet.

Since the ring's particles are presumed to have originated from micrometeoroid impacts on Phoebe, they should share its retrograde orbit, which is opposite to the orbital motion of the next inner moon, Iapetus.

Inwardly migrating ring material would thus strike Iapetus's leading hemisphere, and is suspected to have triggered the processes that led to the two-tone coloration of that moon.

Although very large, the ring is virtually invisible — it was discovered using NASA's infra-red Spitzer Space Telescope.

Why did it take so long to find something so big? The answer is that the ring is very tenuous, made up of a sparse collection of ice and dust particles. If you could transport yourself to the ring, you wouldn't even know you were there because the particles are so far apart. There's not a lot of sunlight out at Saturn, so this small density of particles doesn't reflect much visible light. Spitzer was able to spot the band because it sees infrared light, or heat radiation, from objects. Even though the ring material is very cold, it still gives off heat that can Spitzer can see.

9th Largest Moon of Jupiter - Pasiphaë (56th Moon outwards from Jupiter)

Pasiphaë is a retrograde irregular satellite of Jupiter and with diameter estimated at 58 km, Pasiphaë is the largest retrograde and third largest irregular satellite after Himalia and Elara.

Pasiphaë is the 9th largest moon of Jupiter and 63th largest moon in the Solar System currently known.

Discovery

Pasiphaë was discovered in 1908 by Philibert Jacques Melotte. Pasiphaë was first spotted on a plate taken at the Royal Greenwich Observatory on the night of February 28, 1908. Inspection of previous plates found it as far back as January 27.

Naming

Pasiphaë initially received the provisional designation 1908 CJ, as it was not clear whether it was an asteroid or a moon of Jupiter. The recognition of the latter case came by April 10.

The moon was simply known as Jupiter VIII and sometimes called "Poseidon" between 1955 and 1975.

The moon was officially named Pasiphaë in 1975.

In Greek mythology, Pasiphaë was the daughter of Helios, the Sun, by the eldest of the Oceanids, Perse. She was given in marriage to King Minos of Crete. With Minos, she was the mother of Ariadne, Androgeus, Glaucus, Deucalion, Phaedra, and Catreus. She was also the mother of "starlike" Asterion, called by the Greeks the Minotaur, after a curse from Poseidon caused her to experience lust for and mate with a white bull sent by Poseidon.

Stats

Diameter (mean): 58 km
Apoapsis: 31,209,300 km
Periapsis: 16,980,250 km
Semi-major axis: 23,609,042 km
Orbital Period: -741.09 days
Group: Pasiphaë group

Orbit

Pasiphaë orbits Jupiter on a high eccentricity and high inclination retrograde orbit. It gives its name to the Pasiphaë group, irregular retrograde moons orbiting Jupiter at distances ranging between 22.8 and 24.1 million km, and with inclinations ranging between 144.5° and 158.3°.

The orbital elements are as of January 2000. They are continuously changing due to solar and planetary perturbations.

Pasiphae is also known to be in a secular resonance with Jupiter, which means that Jupiter's gravity tugs at them at regular intervals in a way that can modify their orbits over time.

Formation

All of the Pasiphae moons are retrograde, which means they orbit Jupiter in the opposite direction from the planet's rotation. Their orbits are also eccentric (elliptical rather than circular) and highly inclined with respect to Jupiter's equatorial plane. All of these characteristics support the idea that the Pasiphae satellites began as one or more captured asteroids, rather than forming as part of the original Jupiter system.

Physical characteristics

Spectroscopical measurements in infrared indicate that Pasiphaë is a spectrally featureless object, consistent with the suspected asteroidal origin of the object.

In the visual spectrum the satellite appears grey, similar to C-type asteroids.

15th Largest Asteroid, 324 Bamberga

324 Bamberga is one of the largest main-belt asteroids and the 15th largest asteroids currently known.

Discovery

Bamberga was discovered by Johann Palisa on February 25, 1892 in Vienna.

Bamberga was the last asteroid which is ever easily visible with binoculars to be discovered.

Naming

Bamberga the asteroid was named after Bamberg, a city in Bavaria, Germany, located in Upper Franconia on the river Regnitz close to its confluence with the river Main.

Bamberg was one of the few cities in Germany not destroyed by World War II bombing because of a nearby artillery factory that prevented planes from getting near to it.

Stats

Diameter (mean): 229 km
Aphelion: 3.591 AU
Perihelion: 1.781 AU
Semi-major axis: 2.686 AU
Orbital Period: 4.40 years
Rotation period: 29.43 hrs
Date discovered: 1892.2.25
Class: C
Type: Main-belt Asteroid
(data from JPL Small-Body Database)

Orbit

Bamberga's very high orbital eccentricity means its opposition magnitude varies greatly, at a rare opposition near perihelion Bamberga can reach a magnitude of +8.0, which is as bright as Saturn's moon Titan.

Such near-perihelion oppositions occur on a regular cycle every twenty-two years, with the last occurring in 1991 and the next in 2013, when attaining magnitude 8.1 on Sept. 13th.

Bamberga's brightness at these rare near-perihelion oppositions makes it the brightest C-type asteroid, roughly one magnitude brighter than 10 Hygiea's maximum brightness of around +9.1.

At such an opposition Bamberga can in fact be closer to Earth than any main-belt asteroid with magnitude above +9.5, getting as close as 0.78 AU.

Bamberga has an unusually long rotation period (29.43 hrs) among the large asteroids. Its spectral class is intermediate between the C-type and P-type asteroids.

Star Occultation

An occultation of a star (HIP 59807) by Bamberga was observed on on 20 April 2007. Results from the occultation agrees well with the expected diameter 229 km.

14th Largest Asteroid, 88 Thisbe

88 Thisbe is one of the largest main-belt asteroids and the 14th largest asteroids currently known.

Discovery

Thisbe was discovered by Christian Heinrich Friedrich Peters on June 15, 1866, in Clinton, Oneida County, New York.

Naming

Thisbe the asteroid was named after Thisbe, heroine of a Roman fable.

Pyramus and Thisbe

The tale is told by Ovid in his Metamorphoses.

Pyramus and Thisbe are two characters of Roman mythology, whose love story of ill-fated lovers is also a sentimental romance.

In the Ovidian version, Pyramus and Thisbe is the story of two lovers in the city of Babylon who occupy connected houses/walls, forbidden by their parents to be wed, because of their parents' rivalry.

Through a crack in one of the walls, they whisper their love for each other. They arrange to meet near at Ninus' tomb under a mulberry tree and state their feelings for each other.

Thisbe arrives first, but upon seeing a lioness with a mouth bloody from a recent kill, she flees, leaving behind her veil. The lioness drinks from a nearby fountain, then by chance mutilates the veil Thisbe had left behind.

When Pyramus arrives, he is horrified at the sight of Thisbe's veil, assuming that a fierce beast had killed her. Pyramus kills himself, falling on his sword in proper Roman fashion, and in turn splashing blood on the white mulberry leaves. Pyramus' blood stains the white mulberry fruits, turning them dark.

Thisbe returns, eager to tell Pyramus what had happened to her, but she finds Pyramus' dead body under the shade of the mulberry tree. Thisbe, after a brief period of mourning, stabs herself with the same sword.

In the end, the gods listen to Thisbe's lament, and forever change the colour of the mulberry fruits into the stained colour to honour the forbidden love.

Stats

Diameter (mean): 232 km
Semi-major axis: 2.767 AU
Orbital Period: 4.60 years
Rotation period: 6.042 hrs
Date discovered: 1866.6.15
Class: B
Type: Main-belt Asteroid
(data from JPL Small-Body Database)

Star Occultation

An occultation of a star (SAO 187124) by Thisbe was observed on October 7, 1981. Results from the occultation indicate a larger than expected diameter of 232 km.

13th Largest Asteroid, 3 Juno

Juno seen at four wavelengths. A large crater appears dark at 934 nm

3 Juno is the second-most-massive S-type asteroid after 15 Eunomia, and the 13th largest asteroid overall currently known.


Discovery

Juno was discovered on September 1, 1804, by German astronomer Karl Ludwig Harding.

Juno is unusually reflective and can reach +7.5, which is brighter than Neptune or Titan, at a favourable opposition. This is the reason for Juno being discovered before the larger asteroids.

Juno was originally considered a planet, along with 1 Ceres, 2 Pallas, and 4 Vesta. In 1811, Schröter estimated Juno to be as large as 2290 km in diameter. All four were re-classified as asteroids as additional asteroids were discovered. Juno's small size and irregular shape preclude it from being designated a dwarf planet.

Naming

Juno the asteroid was named after the mythological figure Juno, the highest Roman goddess.

Juno is an ancient Roman goddess, the protector and special counselor of the state. She is a daughter of Saturn and sister (but also the wife) of the chief god Jupiter and the mother of Mars and Vulcan. Juno also looked after the women of Rome.

Stats

Diameter (mean): 234 km
Semi-major axis: 2.671 AU
Orbital Period: 4.37 years
Rotation period: 7.21 hrs
Date discovered: 1804.9.1
Class: S
Type: Main-belt Asteroid
Group: Juno group
(data from JPL Small-Body Database)

Orbit

Juno orbits at a slightly closer mean distance to the Sun than Ceres or Pallas. Its orbit is moderately inclined at around 12° to the ecliptic, but has an extreme eccentricity, greater than that of Pluto.

This high eccentricity brings Juno closer to the Sun at perihelion than Vesta and further out at aphelion than Ceres. Juno had the most eccentric orbit of any known body until 33 Polyhymnia was discovered in 1854, and of asteroids over 200 km in diameter only 324 Bamberga has a more eccentric orbit.

Juno's orbit appears to have changed slightly around 1839, very likely due to perturbations from a passing asteroid, whose identity has not been determined. An alternate but less likely explanation is an impact by a sizeable body.

Physical Characteristics

Amongst S-type asteroids, Juno is unusually reflective, which may be indicative of distinct surface properties. This high albedo explains its relatively high apparent magnitude for a small object not near the inner edge of the asteroid belt.

Spectroscopic studies of the Junonian surface permit the conclusion that Juno could be the progenitor of chondrites, a common type of stony meteorite composed of iron-bearing silicates such as olivine and pyroxene.

Infrared images reveal that Juno possesses an approximately 100 km-wide crater or ejecta feature, the result of a geologically young impact.

12th Largest Asteroid, 15 Eunomia

15 Eunomia is a very large asteroid in the inner asteroid belt. It is the largest of the stony (S-type) asteroids, and the 12th largest asteroid overall currently known.


Discovery

Eunomia was discovered by Annibale de Gasparis on July 29, 1851.



Naming

Eunomia the asteroid, was named after Eunomia - a minor Greek goddess of law and legislation, and one of the daughters of Themis and Zeus.

Stats

Diameter (mean): 255 km
Semi-major axis: 2.643 AU
Orbital Period: 4.30 years
Rotation period: 6.083 hrs
Date discovered: 1851.7.29
Class: S
Type: Main-belt Asteroid
Group: Eunomia group
(data from JPL Small-Body Database)

Physical Characteristics

Eunomia appears to be an elongated but fairly regularly shaped body, with what appear to be four sides of differing curvature and noticeably different average compositions. Its elongation led to the suggestion that Eunomia may be a binary object, but this has been refuted.

Eunomia's surface is composed of silicates and some nickel-iron, and is quite bright. Calcium-rich pyroxenes and olivine, along with nickel-iron metal, have also been detected.

Spectroscopic studies suggest that Eunomia has regions with differing compositions: A larger region dominated by olivine, which is pyroxene-poor and metal-rich, and another somewhat smaller region on one hemisphere (the less pointed end) that is noticeably richer in pyroxene, and has a generally basaltic composition.

Formation

This composition indicates that the parent body was likely subject to magmatic processes, and became at least partially differentiated under the influence of internal heating in the early period of the Solar System. The range of compositions of the remaining Eunomian asteroids, formed by a collision of the common parent body, is large enough to encompass all the surface variations on Eunomia itself. Interestingly, the majority of smaller Eunomian asteroids are more pyroxene rich than Eunomia's surface, and contain very few metallic (M-type) bodies.

Altogether, these lines of evidence suggest that Eunomia is the central remnant of the parent body of the Eunomia family, which was stripped of most of its crustal material by the disrupting impact, but was perhaps not disrupted itself. However, there is uncertainty over Eunomia's internal structure and relationship to the parent body. Computer simulations of the collision are more consistent with Eunomia being a re-accumulation of most of the fragments of a completely shattered parent body. Yet Eunomia's quite high density would indicate that it is not a rubble pile after all. Whatever the case in this respect, it appears that any metallic core region, if present, has not been exposed.

An older explanation of the compositional differences, that Eunomia is a mantle fragment of a far larger parent body (with a bit of crust on one end, and a bit of core on the other), appears to be ruled out by studies of the mass distribution of the entire Eunomia family. These indicate that the largest fragment (that is, Eunomia) has about 70% of the mass of the parent body, which is consistent with Eunomia being a central remnant, with the crust and part of the mantle stripped off.

11th Largest Asteroid, 31 Euphrosyne

31 Euphrosyne is one of the largest main-belt asteroids and the 11th largest overall currently know. Euphrosyne was the first asteroid found from North America.

Discovery

Euphrosyne was discovered by James Ferguson on September 1, 1854.

Naming

Euphrosyne the asteroid is named after Euphrosyne, one of the Charites in Greek mythology.

In Greek mythology, Euphrosyne was one of the Charites, known in English also as the "Three Graces". She is also the Goddess of Joy, a daughter of Zeus and Eurynome, and the incarnation of grace and beauty. Also known as the goddess of Mirth.

Stats

Diameter (mean): 256 km
Semi-major axis: 3.155 AU
Orbital Period: 5.60 years
Rotation period: 5.53 hrs
Date discovered: 1854.9.1
Class: C
Type: Outer Main-belt Asteroid
(data from JPL Small-Body Database)

Orbit

Euphrosyne orbits near the asteroid belt's outer edge. Consequently Euphrosyne is never visible with binoculars, having a maximum magnitude at the best possible opposition of around +10.2, which is actually fainter than any of the thirty asteroids discovered before it.

Euphrosyne's orbit is also quite unusual and bears a considerable resemblance to that of 2 Pallas in its high inclination and eccentricity.

Whereas Pallas and Eris - the only larger bodies with comparably tilted orbits - have nodes near perihelion and aphelion, Euphrosyne's perihelion lies at the northernmost point of its orbit.

Physical Characteristics

Euphrosyne is a very little-studied body despite being one of the largest asteroids. It is a C-type asteroid with a primitive surface and has fairly dark body.

The mass estimate of Euphrosyne in Baer (2011) makes it apparently the 5th-most-massive asteroid, coming after only the big four. It also has the highest estimated density, indicating that it is a solid body like the other largest asteroids.

Moon of asteroid 107 Camilla -- (S/2001 (107) I)

Hubble Space Telescope image of asteroid 107 Camilla and its satellite (arrow)

The moon of 107 Camilla is the 3rd largest moon of the asteroids currently known.

Discovery

On 1 March 2001, a satellite of Camilla was found by A. Storrs, F. Vilas, R. Landis, E. Wells, C. Woods, B. Zellner and M. Gaffey using the Hubble Space Telescope.

Naming

The moon was given the temporary designation S/2001 (107)1, but has not yet received an official name.

Stats

Diameter (mean): 11 km

Semi-major axis: 1235 ± 16 km

Orbital Period: 3.71 days

Rotation period: ?

Physical characteristics

The satellite is estimated to measure about 11 km in diameter. Assuming a similar density to the primary, this would give it an approximate mass of ~1.5×1015 kg.

It has a similar colour to 107 Camilla.

10th Largest Asteroid, 107 Camilla

107 Camilla is a very large asteroid in the outer main asteroid belt. It is the 10th largest asteroid currently known.

Discovery

Camilla was discovered by Norman Robert Pogson on Nov 17, 1868 from Madras (Chennai), India.

Naming

Camilla the asteroid was named after Camilla, Queen of the Volsci in Roman mythology.

In Roman mythology, Camilla of the Volsci was the daughter of King Metabus and Casmilla. Driven from his throne, Metabus was chased into the wilderness by armed Volsci, his infant daughter in his hands. The river Amasenus blocked his path, and, fearing for the child's welfare, Metabus bound her to a spear. He promised Diana that Camilla would be her servant, a warrior virgin. He then safely threw her to the other side, and swam across to retrieve her.

Stats

Diameter (mean): 259 km
Semi-major axis: 3.50 AU
Orbital Period: 6.55 years
Rotation period: 4.844 hrs
Date discovered: 1868.11.17
Class: C
Group: Cybele group
Type: Outer Main-belt Asteroid
Satellite: 1

Orbit

Camilla is a member of the Cybele group located beyond the core of the asteroid belt.

Cybele asteroids are a group of asteroids in the outer main belt with a semi-major axis between 3.27 AU and 3.7 AU, an eccentricity less than 0.3, and an inclination less than 25°. The group is named for the asteroid 65 Cybele. As of 2010, the group is thought to have been formed by the breakup of larger object in the distant past.

Physical characteristics

Camilla has a very dark surface and primitive carbonaceous composition.

Trans-Neptunian Objects: (5) Detached objects

Detached objects are a dynamical class of bodies in the outer Solar System beyond the orbit of Neptune. These objects have orbits whose points of closest approach to the Sun (perihelion) are sufficiently distant from the gravitational influence of Neptune that they are essentially unaffected by Neptune and the other planets: this makes them appear to be "detached" from the Solar System.

In this way, they differ substantially from the majority of the known trans-Neptunian objects (TNOs), which form a loosely defined set of populations that have been perturbed to varying degrees onto their current orbit by gravitational encounters with the gas giants, predominantly Neptune. Detached objects have larger perihelia than those other TNO populations.

At least nine such bodies have been securely identified, of which the largest, most distant, and best known is Sedna.

Orbits

Detached objects have perihelia much larger than Neptune's aphelion. They often have highly elliptical, very large orbits with semi-major axes of up to a few hundred astronomical units (AU).

Such orbits cannot have been created by gravitational scattering by the gas giants (in particular, Neptune). Instead, a number of explanations have been put forward, including an encounter with a passing star or a distant planet-sized object.

Classification

Detached objects generally have a perihelion distance greater than 40 AU, deterring strong interactions with Neptune, which has an approximately circular orbit 30 AU from the Sun.

However, there are no clear boundaries between the scattered and detached regions, since both can coexist as TNOs in an intermediate region with perihelion distance between 37 and 40 AU.

The discovery of 90377 Sedna together with a few other objects such as 2000 CR105 and 2004 XR190 (also known as "Buffy") has motivated discussion of a category of distant objects that may also be inner Oort cloud objects or (more likely) transitional objects between the scattered disc and the inner Oort cloud.

As they are so far away, one of the problems with Detached Object classification is that weak resonances with Neptune may exist and would be difficult to prove due to chaotic planetary perturbations and the current lack of accuracy in the orbits of these distant objects.

These objects have orbital periods of more than 300 years and most have only been observed over a short observation arc of a couple years. Due to their great distance and slow movement against background stars, it may be decades before most of these distant orbits are determined well enough to confidently confirm or rule out a resonance.

Trans-Neptunian Objects: (4) Scattered Disc Objects (SDO)

The scattered disc (or scattered disk) is a distant region of the Solar System that is sparsely populated by icy minor planets. It is a subset of the broader family of trans-Neptunian objects.

The scattered-disc objects (SDOs) have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater than 30 astronomical units.

These extreme orbits are believed to be the result of gravitational "scattering" by the gas giants, and the objects continue to be subject to perturbation by the planet Neptune.

While the nearest distance to the Sun approached by scattered objects is about 30–35 AU, their orbits can extend well beyond 100 AU. This makes scattered objects "among the most distant and cold objects in the Solar System".

The innermost portion of the scattered disc overlaps with a torus-shaped region of orbiting objects traditionally called the Kuiper belt, but its outer limits reach much farther away from the Sun and farther above and below the ecliptic than the belt proper.

As of 2011, over 200 SDOs have been identified.

Orbits

The scattered disc is a very dynamic environment. Because they are still capable of being perturbed by Neptune, SDOs' orbits are always in danger of disruption; either of being sent outward to the Oort cloud or inward into the centaur population and ultimately the Jupiter family of comets.

SDOs are typically characterized by orbits with medium and high eccentricities with a semi-major axis greater than 50 AU, but their perihelia bring them within influence of Neptune.

Having a perihelion of roughly 30 AU is one of the defining characteristics of scattered objects, as it allows Neptune to exert its gravitational influence.

Although motions in the scattered disc are random, they do tend to follow similar directions, which means that SDOs can become trapped in temporary resonances with Neptune. Examples of resonant orbits within the scattered disc include 1:3, 2:7, 3:11, 5:22 and 4:79.

The Minor Planet Center (MPC) makes a clear distinction between the Kuiper belt and the scattered disc; separating those objects in stable orbits (the Kuiper belt) from those in scattered orbits (the scattered disc and the centaurs). However, the difference between the Kuiper belt and the scattered disc is not clearcut, and many astronomers see the scattered disc not as a separate population but as an outward region of the Kuiper belt.

Composition

Scattered objects, like other trans-Neptunian objects, have low densities and are composed largely of frozen volatiles such as water and methane.

Spectral analysis of selected Kuiper belt and scattered objects has revealed signatures of similar compounds. Both Pluto and Eris, for instance, show signatures for methane.

Most SDOs have a white or greyish appearance. One explanation is the exposure of whiter subsurface layers by impacts; another is that the scattered objects' greater distance from the Sun creates a composition gradient, analogous to the composition gradient of the terrestrial and gas giant planets.

Mike Brown, discoverer of the scattered object Eris, suggests that its paler colour could be because, at its current distance from the Sun, its atmosphere of methane is frozen over its entire surface, creating an inches-thick layer of bright white ice.

Trans-Neptunian Objects: (3) Cubewanos (Classical KBOs)

Cubewanos, or Classical Kuiper belt objects, are low-eccentricity (below 0.15) Kuiper belt objects (KBO) that orbit beyond Neptune and are not controlled by an orbital resonance with Neptune.

Cubewanos have orbits with semi-major axes in the 40–50 AU range and, unlike Pluto, do not cross Neptune’s orbit. That is, they have low-eccentricity and sometimes low-inclination orbits like the classical planets.

There are 301 cubewanos identified, as of 2011.

Naming

The name "cubewano" derives from the first trans-Neptunian object (TNO) found after Pluto and Charon, (15760) 1992 QB1. Similar objects found later were often called "QB1-o's", or "cubewanos", after this object. Though the term "classical" is much more frequently used in the scientific literature.

Orbit

Most cubewanos are found between the plutinos (2:3 orbital resonance with Neptune) and the twotinos (1:2 orbital resonance with Neptune).

The majority of objects (the so-called 'cold population'), have low inclinations and near-circular orbits. A smaller population (the 'hot population') is characterised by highly inclined, more eccentric orbits.

The Deep Ecliptic Survey reports the distributions of the two populations; one with the inclination centered at 4.6° (named Core) and another with inclinations extending beyond 30° (Halo).

When the orbital eccentricities of cubewanos and plutinos are compared, it can be seen that the cubewanos form a clear 'belt' outside Neptune's orbit, whereas the plutinos approach, or even cross Neptune's orbit.

When orbital inclinations are compared, 'hot' cubewanos can be easily distinguished by their higher inclinations, as the plutinos typically keep orbits below 20°. (No clear explanation currently exists for the inclinations of 'hot' cubewanos).

Physical Characteristics

The difference in colour between the red cold population and more heterogeneous hot population was observed as early as in 2002. Recent studies, based on a larger data set, indicate the cut-off inclination of 12° between the cold and hot populations while confirming the distinction between the homogenous red cold population and the bluish hot population.

Another difference between the low-inclination (cold) and high-inclination (hot) classical objects is the observed number of binary objects.

Binaries are quite common on low-inclination orbits and are typically similar-brightness systems.

Binaries are less common on high-inclination orbits and their components typically differ in brightness.

This correlation, together with the differences in colour, support further the suggestion that the currently observed classical objects belong to at least two different overlapping populations, with different physical properties and orbital history.

Trans-Neptunian Objects: (2) Other resonances with Neptune

In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers.

Besides the most dominant 2:3 resonance of the plutinos, other resonances exist (information at 2011):

1. 1:1 resonance (Neptune trojans, 30 AU, period ~ 165 years)
A population of 8 objects currently known.

2. 4:5 resonance (34.9 AU, period ~ 205 years)
A population of 5 objects currently known.

3. 3:4 resonance (36.4 AU, period ~ 220 years)
A population of 9 objects currently known.

4. 3:5 resonance (42.3 AU, period ~275 years)
A population of 10 objects currently known.

5. 4:7 resonance (43.7 AU, period ~290 years)
A population of 24 objects currently known.

6. 5:9 resonance (44.5 AU, period ~295 years)
A population of 3 objects currently known.

7. 1:2 resonance ("twotinos", 47.7 AU, period ~330 years)
A population of 15 objects currently known.

8. 4:9 resonance (52 AU, period ~370 years)
A population of 2 objects currently known.

9. 3:7 resonance (52.9 AU, period ~385 years)
A population of 5 objects currently known.

10. 5:12 resonance (55 AU, period ~395 years)
A population of 2 objects currently known.

11. 2:5 resonance (55.4 AU, period ~410 years)
A population of 15 objects currently known.

12. 3:8 resonance (57 AU, period ~440 years)
1 object currently known.

13. 1:3 resonance ("threetinos", 62.6 AU, period ~495 years)
A population of 2 objects currently known.

14. 2:7 resonance (70 AU, period ~580 years)
1 object currently known.

15. 1:4 resonance ("fourtinos", 75.8 AU, period ~660 years)
1 object currently known.

16. 1:5 resonance (87.9 AU, period ~820 years)
1 object currently known.

Twotinos

The resonance at 47.8 AU is often considered as the outer "edge" of the Kuiper belt and the objects in this resonance are sometimes referred to as twotinos.

There are far fewer objects in this resonance (a total of 15 current known) than plutinos. Long-term orbital integration shows that the 1:2 resonance is less stable than 2:3 resonance; only 15% of the objects in 1:2 resonance were found to survive 4 Gyr as compared with 28% of the plutinos.

Consequently it might be that twotinos were originally as numerous as plutinos, but their population has dropped significantly below that of plutinos since.

Coincidental vs true resonances

One of the concerns is that weak resonances may exist and would be difficult to prove due to the current lack of accuracy in the orbits of these distant objects. Many objects have orbital periods of more than 300 years and most have only been observed over a short observation arc of a couple years.

Due to their great distance and slow movement against background stars, it may be decades before many of these distant orbits are determined well enough to confidently confirm whether a resonance is true or merely coincidental.

Trans-Neptunian Objects: (1) 2:3 resonance - Plutinos

2:3 resonance ("plutinos", period ~250 years)

A plutino is a trans-Neptunian object in 2:3 mean motion resonance with Neptune. For every 2 orbits that a plutino makes, Neptune orbits 3 times.

Plutinos form the inner part of the Kuiper belt and represent about a quarter of the known Kuiper belt objects (KBOs). Plutinos are the largest class of the resonant trans-Neptunian objects (i.e. bodies in orbital resonances with Neptune).

Origin

It is thought that objects that are currently in mean orbital resonances with Neptune initially followed independent heliocentric paths. As Neptune migrated outward early in the Solar System's history, the bodies it approached would have been scattered. During this process, some of them would have been captured into resonances.

The 2:3 resonance is the strongest and most stable among all resonances. This is the main reason it contains the largest number of bodies.

Orbital characteristics

The 2:3 resonance at 39.4 AU is by far the dominant category among the resonant objects, with 92 confirmed and 104 possible member bodies. The objects following orbits in this resonance are named plutinos after Pluto, the first such body discovered.

The orbital periods of plutinos cluster around 247.3 years (1.5 × Neptune's orbital period), varying by at most a few years from this value.

The name refers only to the orbital resonance and does not imply common physical characteristics; it was invented to describe those bodies smaller than Pluto (hence the diminutive) following similar orbits. The class includes Pluto itself and its moons.

Aside from Pluto itself and Charon, the first plutino, 1993 RO, was discovered on September 16, 1993.

Long-term stability

The orbits of Pluto and the plutinos are stable, despite crossing that of much larger Neptune, because they are in a 2:3 resonance with it. The resonance ensures that, when they approach perihelion and Neptune's orbit, Neptune is consistently distant (averaging a quarter of its orbit away)

The gravitational influence of Pluto is usually neglected given its small mass. However, the resonance width (the range of semi-axes compatible with the resonance) is very narrow and only a few times larger than Pluto’s Hill sphere (gravitational influence). Consequently, depending on the original eccentricity, some plutinos will be driven out of the resonance by interactions with Pluto.

8th Largest Moon of Neptune - Naiad (1st Moon outwards from Neptune)

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.

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.

8th Largest Moon of Saturn - Hyperion (22th Moon outwards from Saturn)

Hyperion, is the 8th largest moon of Saturn, and 24th largest moon in the Solar System.

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.

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.

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.

7th Largest Moon of Neptune - Thalassa (2nd Moon outwards from Neptune)

A simulated view of Thalassa orbiting 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.

7th Largest Moon of Saturn - Mimas (9th Moon outwards from Saturn)

Mimas, is the seventh largest moon of Saturn, and 20th largest moon in the Solar System.

Mimas is the smallest known astronomical body that is rounded in shape due to self-gravitation.



Discovery

Mimas was discovered by the astronomer Fredrick William Herschel on 17 September 1789.


Naming

Mimas the moon is named after one of the Titans in Greek mythology, Mimas.

Mimas was one of the Gigantes of Greek mythology. Like the other giant sons of Gaia, Mimas had serpents for legs and was born fully armoured. Mimas was slain by Hephaestus during the war against the Olympians by a volley of molten iron.

Stats

Diameter: 396 km

Semi-major axis: 185,404 km

Orbital Period: 0.942 days

Orbit

A number of features in Saturn's rings are related to resonances with Mimas. Mimas is responsible for clearing the material from the Cassini Division, the gap between Saturn's two widest rings, the A ring and B ring.

Particles in the Huygens Gap at the inner edge of the Cassini division are in a 2:1 resonance with Mimas. They orbit twice for each orbit of Mimas. The repeated pulls by Mimas on the Cassini division particles, always in the same direction in space, force them into new orbits outside the gap.

The boundary between the C and B ring is in a 3:1 resonance with Mimas. Recently, the G ring was found to be in a 7:6 co-rotation eccentricity resonance with Mimas; the ring's inner edge is about 15,000 kilometres inside Mimas' orbit.

Mimas is also in a 2:1 mean motion resonance with the larger moon Tethys, and in a 2:3 resonance with the outer F ring shepherd moonlet, Pandora.

Mimas rotates synchronously with its orbital period, keeping one face pointed toward Saturn.

Physical characteristics

The low density of Mimas, 1.15 g/cm3, indicates that it is composed mostly of water ice with only a small amount of rock.

Mimas' most distinctive feature is a colossal impact crater 130 kilometres across, named Herschel after the moon's discoverer. Herschel's diameter is almost a third of the moon's own diameter; its walls are approximately 5 kilometres high, parts of its floor measure 10 kilometres deep, and its central peak rises 6 kilometres above the crater floor.

The impact that made this crater must have nearly shattered Mimas: fractures can be seen on the opposite side of Mimas that may have been created by shock waves from the impact travelling through the moon's body.

The Mimantean surface is saturated with smaller impact craters, but no others are anywhere near the size of Herschel. Although Mimas is heavily cratered, the cratering is not uniform. Most of the surface is covered with craters greater than 40 kilometres in diameter, but in the south polar region, craters greater than 20 kilometres are generally lacking.

Curious Resemblances

When seen from certain angles, Mimas closely resembles the Death Star, a fictional space station known from the film Star Wars Episode IV: A New Hope, which is said to be roughly 140 kilometres in diameter. This resemblance stems from the fact that Herschel can appear in Mimas' northern hemisphere, much like the concave disc of the Death Star's "superlaser".

This is purely coincidental, as the first film was made three years before the first close-up photographs of Mimas were taken.

7th Largest Moon of Jupiter - Thebe (4th Moon outwards from Jupiter)

Thebe is the second largest of the inner satellites of Jupiter and the seventh largest moon overall in size. Thebe is the 46th largest moon in the Solar System currently known.

Discovery

Thebe was discovered by Stephen P. Synnott in images from the Voyager 1 space probe taken on March 5, 1979, while orbiting around Jupiter.

After its discovery by Voyager 1, Thebe was photographed by Voyager 2 space probe in 1980. And later, in more detail, by the Galileo orbiter in the 1990s.

Naming

Thebe was initially given the provisional designation S/1979 J2.

In 1983 the moon was officially named after the mythological nymph Thebe who was a lover of Zeus — the Greek equivalent of Jupiter.

Stats

Diameter (mean): 99 km

Semi-major axis: 221,889 km

Orbital Period: 0.675 days

Rotation Period: Synchronous

Orbit

Thebe is the outermost of the inner Jovian moons.

The orbit of Thebe lies near the outer edge of the Thebe Gossamer Ring, which is composed of the dust ejected from the satellite. After ejection the dust drifts in the direction of the planet under the action of Poynting-Robertson drag forming a ring inward of the moon.

Similarly to all inner satellites of Jupiter, Thebe rotates synchronously with its orbital motion, thus keeping one face always looking toward Jupiter.

Thebe's orientation is such that the long axis always points to Jupiter. At the surface points closest to and furthest from Jupiter, the surface is thought to be near the edge of the Roche lobe, where Thebe's gravity is only slightly larger than the centrifugal force. As a result, the escape velocity in these two points is very small, thus allowing dust to escape easily after meteorite impacts, and ejecting it into the Thebe Gossamer Ring.

Physical characteristics

Thebe is irregularly shaped, with the closest ellipsoidal approximation being 116×98×84 km.

The surface of Thebe is dark and appears to be reddish in color. There is a substantial asymmetry between leading and trailing hemispheres: the leading hemisphere is 1.3 times brighter than the trailing one. The asymmetry is probably caused by the higher velocity and frequency of impacts on the leading hemisphere, which excavate a bright material (probably ice) from the interior of the moon.

The surface of Thebe is heavily cratered and it appears that there are at least three or four impact craters that are very large, each being roughly comparable in size to Thebe itself.

The largest (diameter about 40 km) crater is situated on the side that faces away from Jupiter, and is called Zethus (the only surface feature on Thebe to have received a name).

Quasi-satellite of Earth - (164207) Asteroid 2004 GU9

Asteroid 2004 GU9 was discovered was discovered by the Lincoln Near Earth Asteroid Research (LINEAR) system in New Mexico, in orbit around the Sun on 13th April 2004.

Stats

Diameter: 0.16 - 0.35 km

Semi-major axis: 1.001 AU (almost same as Earth)

Rotation: not known

Orbit

What makes 2004 GU9, of great interest to astronomers is the remarkable stability of its orbit. It has remained a quasi-satellite of Earth for at least 600 years.

Before this phase, 2004 GU9 traveled in a horseshoe-shaped orbit that astronomers have tracked back at least 50,000 years. According to Martin Connors of Athabasca University in Alberta, Canada, 2004 GU9 is the first object in its class to maintain a long stretch of orbital stability.

9th Largest Asteroid, 65 Cybele

65 Cybele is one of the largest asteroids in the Solar System and is located in the outer asteroid belt. It is the 9th largest asteroid currently known.

Discovery

Cybele was discovered on March 8, 1861, by Ernst Wilhelm Tempel from the Marseilles Observatory.

Naming

A minor controversy arose from its naming process. Tempel had awarded the honour of naming the asteroid to Carl August von Steinheil in recognition of his achievements in telescope production. Von Steinheil elected to name it "Maximiliana" after the reigning monarch Maximilian II of Bavaria. At the time, asteroids were conventionally given classical names, and a number of astronomers protested this contemporary appellation.

The name Cybele was chosen instead. Cybele was the Phrygian goddess of the earth.

Stats

Diameter (mean): 273 km
Semi-major axis: 3.433 AU
Orbital Period: 6.36 years
Rotation period: 4.04 hrs
Date discovered: 1861.3.8
Class: C
Group: Cybele group
Type: Outer Main-belt Asteroid
Satellite: 1 ?

Orbit

Cybele gives its name to the family of asteroids located beyond the core of the asteroid belt.

Cybele asteroids are a group of asteroids in the outer main belt with a semi-major axis between 3.27 AU and 3.7 AU, an eccentricity less than 0.3, and an inclination less than 25°. As of 2010, the group is thought to have been formed by the breakup of larger object in the distant past.

Physical characteristics

Cybele is a C-type asteroid. It is dark in color, carbonaceous in composition and appeared to have an irregular shape.

In the 1979 Cybelian stellar occultation, a hint of a possible 11 km wide satellite was detected.

Life?

Researchers have revealed that after 24 Themis, 65 Cybele also contains traces of water and complex, long-chained organic molecules.

Those traits had been associated with comets, which spring from colder, more distant reservoirs in the outer solar system, but not their asteroidal cousins. The finding supports the notion that asteroids could have provided early Earth with water for its oceans as well as some of the prebiotic compounds that allowed life to develop.

Moons of asteroid 87 Sylvia -- (87) Sylvia II Remus

Sylvia is orbited by two small moons. They have been named Romulus and Remus [(87) Sylvia I Romulus and (87) Sylvia II Remus].

Discovery

Remus was discovered several years after Romulus on images taken starting on August 9, 2004, and announced on August 10, 2005. It was discovered by Franck Marchis of UC Berkeley, and Pascal Descamps, Daniel Hestroffer, and Jérôme Berthier of the Observatoire de Paris, France, using the Yepun telescope of the European Southern Observatory (ESO) in Chile.

Naming

The moon was given the temporary designation S/2004 (87) 1.

In 2005, the moon is named after Remus, the mythological founder of Rome, one of the twins of Rhea Silvia raised by a wolf.

Stats

Diameter (mean): 7 km

Semi-major axis: 706 ± 5 km

Orbital Period: 1.38 days

Rotation period: ?

Orbit

Remus's orbit is expected to be quite stable − it lies far inside Sylvia's Hill sphere (about 1/100 of Sylvia's Hill radius), but also far outside the synchronous orbit.

Physical characteristics

87 Sylvia has a low density, which indicates that it is probably a rubble pile formed when debris from a collision between its parent body and another asteroid re-accreted gravitationally.

Therefore it is likely that Remus, Sylvia's moon, is smaller rubble pile which accreted in orbit around the main body from debris of the same collision. In this case the albedo and density are expected to be similar to Sylvia's.

Moons of asteroid 87 Sylvia -- (87) Sylvia I Romulus

Sylvia is orbited by two small moons. They have been named Romulus and Remus [(87) Sylvia I Romulus and (87) Sylvia II Remus].

Discovery

Romulus was discovered in 18 February 2001 from the Keck II telescope by Michael E. Brown and Jean-Luc Margot.

Naming

The moon was given the temporary designation S/2001 (87) 1.

In 2005, the moon is named after Romulus, the mythological founder of Rome, one of the twins of Rhea Silvia raised by a wolf.

Stats

Diameter (mean): 18 km

Semi-major axis: 1356 ± 5 km

Orbital Period: 3.65 days

Rotation period: ?

Orbit

Romulus' orbit is expected to be quite stable − it lies far inside Sylvia's Hill sphere (about 1/50 of Sylvia's Hill radius), but also far outside the synchronous orbit.

Physical characteristics

87 Sylvia has a low density, which indicates that it is probably a rubble pile formed when debris from a collision between its parent body and another asteroid re-accreted gravitationally.

Therefore it is likely that Romulus, Sylvia's moon, is smaller rubble pile which accreted in orbit around the main body from debris of the same collision. In this case the albedo and density are expected to be similar to Sylvia's.

8th Largest Asteroid, 87 Sylvia

Left: Adaptive Optics observations of (87) Sylvia, showing its two moonlet satellites, Remus and Romulus










87 Sylvia is one of the largest asteroids in the outer-main asteroid belt, having a mean diameter of 286 km. It is the 8th largest asteroid currently known.

Sylvia is also remarkable for being the first asteroid known to possess more than one moon. Sylvia has 2 known satellites.

Discovery

Sylvia was discovered by Norman Robert Pogson on May 16, 1866 from Madras (Chennai), India.

Naming

In the article announcing the discovery of the asteroid, Pogson explained that he selected the name for the asteroid in reference to Rhea Silvia, mother of Romulus and Remus.

Rhea Silvia (also written as Rea Silvia), and also known as Ilia, was the mythical mother of the twins Romulus and Remus, who founded the city of Rome.

Stats

Diameter (mean): 286 km
Semi-major axis: 3.499 AU
Orbital Period: 6.55 years
Rotation period: 5.184 hrs
Date discovered: 1866.5.16
Class: X
Group: Cybele group
Type: Outer Main-belt Asteroid
Satellite: 2

Orbit

Sylvia is a member of the Cybele group located beyond the core of the asteroid belt.

Cybele asteroids are a group of asteroids in the outer main belt with a semi-major axis between 3.27 AU and 3.7 AU, an eccentricity less than 0.3, and an inclination less than 25°. The group is named for the asteroid 65 Cybele. As of 2010, the group is thought to have been formed by the breakup of larger object in the distant past.

Physical characteristics

Sylvia's shape is strongly elongated and is very dark in color and probably has a very primitive composition. The discovery of its moons made possible an accurate measurement of the asteroid's mass and density.

Sylvia's density was found to be very low (around 1.2 times the density of water), indicating that the asteroid is porous to very porous; from 25% to as much as 60% of it may be empty space, depending on the details of its composition.

However, the mineralogy of the X-type asteroids is not known well enough to constrain this further. Either way, this suggests a loose rubble pile structure.