6th Largest Moon of Neptune - Despina (3rd Moon outwards from Neptune)

Despina as seen by Voyager 2 (smeared horizontally)

Despina is the third closest inner satelliteis, the sixth largest satellite of Neptune and the 38th largest moon in the Solar System currently known.

Discovery

Despina was discovered in late July 1989 from the images taken by the Voyager 2 spacecraft.

Naming

The moon was given the temporary designation S/1989 N3.

Despina the moon was named after Despoina, a nymph who was a daughter of Poseidon and Demeter.

In Greek mythology, Despoina, Despoena or Despoine, was the daughter of Demeter and Poseidon and sister of Arion. She was the goddess of mysteries of Arcadian cults worshipped under the title Despoina, "the mistress" alongside with her mother Demeter, one of the goddesses of the Eleusinian mysteries.

Stats

Diameter (mean): 150 km

Semi-major axis: 52,526 km

Orbital Period: 0.335 days

Orbit

Despina's orbit lies just inside the Le Verrier ring. As it is also 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.

Despina 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

Despina is irregularly shaped and shows no sign of any geological modification.

Little else is known about Despina. Despina 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.

6th Largest Moon of Uranus - Puck (12th Moon outwards from Uranus)

Puck is the largest inner moon of Uranus and the sixth largest of the Uranus moons. Puck is the 35th largest moon in the Solar System currently known.



Discovery

Puck was discovered by Stephen P. Synnott, who is an American astronomer and Voyager scientist, from the images taken by Voyager 2 on 30 December 1985.



Naming

The moon was given the temporary designation S/1985 U1.

It was later named after the Puck who appears in Shakespeare's A Midsummer Night's Dream, a little sprite who travels around the globe at night with the fairies. In Celtic mythology and English folklore, a Puck is a mischievous sprite, imagined as an evil demon by Christians.

Stats

Diameter: 162 km

Semi-major axis: 86,004 km

Orbital Period: 0.76 days

Orbit

Puck 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 orbit of Puck lies between the rings of Uranus and the first of Uranus' large moons, Miranda.

Physical characteristics

Puck is approximately spherical in shape, has a dark heavily cratered surface, and is grey in color.

There are three named craters on the surface of Puck, the largest being about 45 km in diameter. Observations with Hubble Space Telescope and large terrestrial telescopes found water ice absorption features in the spectrum of Puck.

Nothing is known about the internal structure of Puck. It is probably made of a mixture of water ice with the dark material similar to that found in the rings. This dark material is probably made of rocks or radiation processed organics.

The absence of craters with bright rays implies that Puck is not differentiated meaning that ice and non-ice components have not separated from each other forming a core and mantle.

Exploration Status

So far the only close-up images of Puck have been from the Voyager 2 probe, which photographed the moon during its flyby of Uranus in January 1986.

No other spacecraft has ever visited the Uranian system or Puck, and no mission is planned in the foreseeable future.

6th Largest Moon of Saturn - Enceladus (13th Moon outwards from Saturn)

Enceladus, is the sixth largest moon of Saturn, and 17th largest moon in the Solar System.

Enceladus is one of only three outer solar system bodies (along with Jupiter's moon Io and Neptune's moon Triton) where active eruptions have been observed.

In May 2011 NASA scientists reported that Enceladus "is emerging as the most habitable spot beyond Earth in the Solar System for life as we know it".

Discovery

Enceladus was discovered by Fredrick William Herschel on August 28, 1789, during the first use of his new 1.2 m telescope, then the largest in the world. Herschel first observed Enceladus in 1787, but in his smaller, 16.5 cm telescope, the moon was not recognized.

Naming

Enceladus is named after the Giant Enceladus of Greek mythology.

In Greek mythology, Enceladus, was one of the Gigantes, the enormous children of Gaia (Earth) fertilized by the blood of castrated Uranus.

During the battle between the Gigantes and the Olympian gods, Enceladus was disabled by a spear thrown by the goddess Athena. He was buried on the island of Sicily, under Mount Etna. The volcanic fires of Etna were said to be the breath of Enceladus, and its tremors to be caused by him rolling his injured side beneath the mountain.

Stats

Diameter: 504 km

Semi-major axis: 237,950 km

Orbital Period: 1.37 days

Orbit

Enceladus is one of the major inner satellites of Saturn. Enceladus orbits within the densest part of the E Ring, the outermost of Saturn's rings, an extremely wide but very diffuse disk of microscopic icy or dusty material, beginning at the orbit of Mimas and ending somewhere around the orbit of Rhea. As Enceladus is orbiting inside this ring, in a place where it is narrowest but present in its highest density. Therefore, several theories suspected Enceladus to be the main source of particles for the E Ring.

Enceladus is currently in a 2:1 mean motion orbital resonance with Dione, completing two orbits of Saturn for every one orbit completed by Dione. This resonance helps maintain Enceladus's orbital eccentricity (0.0047) and provides a heating source for Enceladus's geologic activity.

Enceladus rotates synchronously with its orbital period, keeping one face pointed toward Saturn. Analysis of the shape of Enceladus suggests that at some point it was in a 1:4 forced secondary spin-orbit libration. This libration, like the resonance with Dione, could have provided Enceladus with an additional heat source.

Interaction with E Ring

There are two distinct mechanisms feeding the ring with particles.

The first, and probably the most important, source of particles comes from the cryovolcanic plume in the South polar region of Enceladus. While a majority of particles fall back to the surface, some of them escape Enceladus's gravity and enter orbit around Saturn, since Enceladus's escape velocity is only 866 km/h.

The second mechanism comes from meteoric bombardment of Enceladus, raising dust particles from the surface. This mechanism is not unique to Enceladus, but is valid for all Saturn's moons orbiting inside the E Ring.

Physical characteristics

The fresh, clean ice that dominates its surface gives Enceladus probably the most reflective surface of any body in the solar system with a visual geometric albedo of 1.38. Because it reflects so much sunlight, the mean surface temperature at noon only reaches −198 °C (somewhat colder than other Saturnian satellites).

Cryovolcanism

Following the Voyager encounters with Enceladus in the early 1980s, scientists postulated that the moon may be geologically active based on its young, reflective surface and location near the core of the E ring.

Based on the connection between Enceladus and the E ring, it was thought that Enceladus was the source of material in the E ring, perhaps through venting of water vapor from Enceladus's interior. However, the Voyagers failed to provide conclusive evidence that Enceladus is active today.

Instruments on the Cassini spacecraft in 2005 discovered cryovolcanism, where water and other volatiles are the materials erupted instead of silicate rock, on Enceladus.

The first Cassini sighting of a plume of icy particles above Enceladus's south pole came from the Imaging Science Subsystem (ISS) images taken in January and February 2005.

Visual confirmation of venting came in November 2005, when ISS imaged geyser-like jets of icy particles rising from the moon's south polar region. The images taken in November 2005 showed the plume's fine structure, revealing numerous jets (perhaps issuing from numerous distinct vents) within a larger, faint component extending out nearly 500 km from the surface, thus making Enceladus the fourth body in the solar system to have confirmed volcanic activity, along with Earth, Neptune's Triton, and Jupiter's Io.

Atmosphere

Data from the magnetometer instrument during the February 17, 2005 encounter found evidence for an atmosphere at Enceladus. The magnetometer observed an increase in the power of ion cyclotron waves near Enceladus. These waves are produced by the interaction of ionized particles and magnetic fields, and the frequency of the waves can be used to identify the composition, in this case ionized water vapor.

During the next two encounters, the magnetometer team determined that gases in Enceladus's atmosphere are concentrated over the south polar region, with atmospheric density away from the pole being much lower.

The Ultraviolet Imaging Spectrograph (UVIS) confirmed this result by observing two stellar occultations during the February 17 and July 14 encounters. Unlike the magnetometer, UVIS failed to detect an atmosphere above Enceladus during the February encounter when it looked for evidence for an atmosphere over the equatorial region, but did detect water vapor during an occultation over the south polar region during the July encounter.

Cassini flew through this gas cloud during the July 14 encounter, allowing instruments like the Ion and Neutral Mass Spectrometer (INMS) and the cosmic dust analyzer (CDA) to directly sample the plume. INMS measured the composition of the gas cloud, detecting mostly water vapor, as well as minor components like molecular nitrogen, methane, and carbon dioxide.

CDA "detected a large increase in the number of particles near Enceladus," confirming the satellite as the primary source for the E ring. Analysis of the CDA and INMS data suggest that the gas cloud Cassini flew through during the July encounter, and observed from a distance with its magnetometer and UVIS, was actually a water-rich cryovolcanic plume, originating from vents near the south pole.

Cassini's flyby on March 12, 2008 revealed additional chemicals in the plume, including simple and complex hydrocarbons such as propane, ethane, and acetylene.

In July 2009 it was announced that ammonia had been discovered during flybys in July and October 2008.

Possible Water Ocean? Life?

In late 2008, scientists observed water vapor spewing from Enceladus's surface. This could indicate the presence of liquid water, which might also make it possible for Enceladus to support life.

Eventually it was discovered that in the E-ring about 6% of particles contain 0.5–2% of sodium salts by mass, which is a significant amount. In the parts of the plume close to Enceladus the fraction of "salty" particles increases to 70% by number and >99% by mass. Such particles presumably are frozen spray from the salty underground ocean.

On the other hand, the small salt-poor particles form by homogenous nucleation directly from the gas phase. The sources of salty particles are uniformly distributed along the tiger stripes, whereas sources of "fresh" particles are closely related to the high-speed gas jets. The "salty" particles move slowly and mostly fall back onto the surface, while the fast "fresh" particles escape to the E-ring, explaining its salt-poor composition.

The "salty" composition of the plume strongly suggests that its source is a subsurface salty ocean or subsurface caverns filled with salty water. The presence of liquid water under the crust means there has to be an internal heat source. Scientists now believe it is a combination of radioactive decay and tidal heating, as tidal heating alone is not enough to explain the heat.

Additionally, Cassini found traces of organic compounds in some dust grains. Enceladus is therefore a candidate for harboring extraterrestrial life.

6th Largest Moon of Jupiter - Amalthea (3rd Moon outwards from Jupiter)

Greyscale Galileo images of Amalthea

Amalthea is the largest of the inner satellites of Jupiter, the sixth largest overall in size. Only the four Galilean moons and Himalia are larger. Amalthea is the 34th largest moon in the Solar System currently known.

Discovery

Amalthea was discovered on September 9, 1892, by Edward Emerson Barnard using the 36 inch (91 cm) refractor telescope at Lick Observatory.

Amalthea was the last planetary satellite to be discovered by direct visual observation (as opposed to photographically) and was the first new satellite of Jupiter since Galileo Galilei's discovery of the Galilean satellites in 1610.

Naming

The satellite is named after the nymph Amalthea from Greek mythology who nursed the infant Zeus (the Greek equivalent of Jupiter) with goat's milk.

Stats

Diameter (mean): 167 km

Semi-major axis: 181,366 km

Orbital Period: 0.498 days

Rotation Period: Synchronous

Orbit

Amalthea is in a close orbit around Jupiter and is within the outer edge of the Amalthea Gossamer Ring, which is formed from dust ejected from its surface.

Amalthea is tidally locked with the planet, the long axis pointing towards Jupiter at all times.

From Amalthea's surface, Jupiter would be an astonishing sight in its sky, roughly 92 times larger than the full moon. Because Amalthea is in synchronous rotation, Jupiter would not appear to move, and would be invisible from one side of Amalthea.

The Sun would disappear behind the planet's bulk for an hour and a half each revolution. Amalthea's short rotation period gives it just under six hours of daylight.

Physical characteristics

The surface of Amalthea is very red. The reddish color may be due to sulfur originating from Io or some other non-ice material.

Bright patches of green appear on the major slopes of Amalthea, but the nature of this color is currently unknown.

The surface of Amalthea is slightly brighter than surfaces of other inner satellites of Jupiter. There is also 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 — presumably ice — from the interior of the moon.

Formation

Amalthea's irregular shape and large size led in the past to a conclusion that it is a fairly strong, rigid body. It was argued that a body composed of ices or other weak materials would have been pulled into a more spherical shape by its own gravity.

However, on November 5, 2002, the Galileo orbiter made a targeted flyby that came within 160 km of Amalthea and the deflection of its orbit was used to compute the moon's mass. In the end, Amalthea's density was found to be as low as 0.86 g/cm³, so it must be either a relatively icy body or very porous "rubble pile" or, something in between.

Recent measurements from the Subaru telescope suggest that the moon is indeed icy, indicating that it cannot have formed in its current position, since the hot primordial Jupiter would have melted it. It is therefore likely to have formed farther from the planet or to be a captured Solar System body.

7th Largest Asteroid, 511 Davida

511 Davida is one of the largest asteroids in the asteroid belt, having a mean diameter of 289 km. It is the 7th largest asteroid currently known.

Discovery

Davida was discovered by R. S. Dugan in May 30, 1903.

Naming

Davida is named after David Peck Todd, an astronomy professor at Amherst College.

Stats

Diameter (mean): 289 km
Semi-major axis: 3.164 AU
Orbital Period: 5.63 years
Rotation period: 5.131 hrs
Date discovered: 1903.5.30
Class: C
Type: Main-belt Asteroid

Physical characteristics

Davida is a C-type asteroid, which means that it is dark in colouring with a carbonate composition.

Davida is one of the few main-belt asteroids whose shape has been determined by ground-based visual observation. From 2002 to 2007, astronomers at the Keck Observatory used the Keck II telescope, which is fitted with adaptive optics, to photograph Davida.

The asteroid is not a dwarf planet: there are at least two promontories and at least one flat facet with 15-km deviations from a best-fit ellipsoid. The facet is presumably a 150-km global-scale crater.

6th Largest Asteroid, 52 Europa

52 Europa is one of the largest asteroids in the asteroid belt, having a mean diameter of 303 km. It is the 6th largest asteroid currently known.

Discovery

Europa was discovered on February 4, 1858, by Hermann Goldschmidt from his balcony in Paris.

Naming

Europa the asteroid was named after Europa, one of Zeus's conquests in Greek mythology, a name it shares with Jupiter's moon Europa.

Stats

Diameter (mean): 303 km
Semi-major axis: 3.097 AU
Orbital Period: 5.45 years
Rotation period: 5.63 hrs
Date discovered: 1858.2.4
Class: C
(data from JPL Small-Body Database)

Orbit

Europa orbits close to the Hygiea asteroid family, but is not a member.

Physical characteristics

Europa is highly porous and classified as a dark C-type asteroid, meaning the surface contains a large amount of carbon. This gives Europa a dark surface that reflects only a small amount of light that falls on it.

Spectroscopic studies have found evidence of olivines and pyroxenes on the surface, and there is some indication that there may be compositional differences between different regions.

Rotation Period

Europa is one of the larger asteroids with spin period not clearly determined.

Lightcurve data for Europa has been particularly tricky to interpret, so much so that for a long time its period of rotation was in dispute (ranging from 5 and a half hours to 11 hours), despite numerous observations.

It has now been determined that Europa is a prograde rotator, but the exact direction in which its pole points remains ambiguous.

3rd Largest Moon of Pluto - Nix (2th Moon outwards from Pluto)

Pluto has four known natural satellites (Charon, Nix, Hydra and S/2011 P1. (provisional name, also known as P4, identified by the Hubble Space Telescope in 2011).

Nix is the third largest known natural satellite of Pluto. It was discovered along with Hydra in June 2005.

Based on estimated size, Nix is the 48th largest moon in the Solar System currently known.


Discovery

Nix was found by the Hubble Space Telescope's Pluto Companion Search Team, which is composed of Hal A. Weaver, Alan Stern, Max J. Mutchler, Andrew J. Steffl, Marc W. Buie, William J. Merline, John R. Spencer, Eliot F. Young, and Leslie A. Young.

The discovery images were taken on May 15, 2005, and May 18, 2005; the moons were independently discovered by Max J. Mutchler on June 15, 2005, and Andrew J. Steffl on August 15, 2005.

The discoveries were announced on October 31, 2005, after confirmation by precoveries from 2002.

Naming

Nix was provisionally designated S/2005 P2. Nix the moon, was named after Nyx the Greek goddess of darkness and night, and mother of Charon.

The initial proposal was to use the classical spelling Nyx, but to avoid confusion with the asteroid 3908 Nyx, the spelling was changed to Nix. The USGS Gazetteer of Planetary Nomenclature states that Nix is the "Egyptian spelling", of the Greek name.

Stats

Diameter (mean): ~91.5 (46 – 137) km

Semi-major axis: 48,708 km

Orbital Period: 24.86 days

Rotation Period: Unknown

Physical properties

Nix's size has not been directly measured. Calculations based on its brightness give it a diameter of between 46 km, if its geometric albedo is similar to Charon's 35 percent, and about 137 km, if it has a reflectivity of 4 percent like the darkest Kuiper belt objects.

Nix is slightly fainter than Hydra, suggesting that it is somewhat smaller in size.

Exploration Status

The arrival of the New Horizons Spacecraft to Plutonian System in July 2015 is highly anticipated. The true size of Nix and other new discoveries will be revealed.

5th Largest Moon of Neptune - Galatea (4th Moon outwards from Neptune)

Galatea inside of a faint ring arc near Neptune

Galatea is the fifth largest satellite of Neptune and the 31th largest moon in the Solar System currently known.

Discovery

Galatea was discovered in late July 1989 from the images taken by the Voyager 2 probe.

Naming

Galatea was given the temporary designation S/1989 N4.

Neptune's irregular satellites are named for the Nereids, daughters of Nereus and Doris and the attendants of Neptune. Galatea the moon was named after Galatea, one of the Nereids of Greek legend.

Stats

Diameter (mean): 176 km

Semi-major axis: 61,953 km

Orbital Period: 0.429 days

Orbit

Galatea'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.

Galatea 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.

Shepherd moon

Galatea appears to be a shepherd moon for the Adams ring that is 1000 km outside its orbit. Resonances with Galatea in the ratio 42:43 are also considered the most likely mechanism for confining the unique ring arcs that exist in this ring.

Physical characteristics

Little else is known about Galatea. Galatea 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.

5th Largest Moon of Uranus - Miranda (14th Moon outwards from Uranus)

Miranda is the fifth largest of the Uranus moons, and the 18th largest moon in the Solar System.

Miranda is one of the few bodies in the Solar System in which the equatorial circumference is shorter than the pole-to-pole circumference.

Discovery

Miranda was discovered in telescopic photos of the Uranian system by Gerard P. Kuiper on February 16, 1948 at McDonald Observatory.

Naming

Miranda the moon was named after Miranda from William Shakespeare's play The Tempest by Kuiper in his report of the discovery.

Stats

Diameter: 472 km

Semi-major axis: 129,320 km

Orbital Period: 1.41 days

Formation

Miranda looks like it was pieced together from parts that didn't quite merge properly. At 472 km in diameter, it's only one-seventh as large as Earth's moon, a size that seems unlikely to support much tectonic activity. Yet Miranda sports one of the strangest and most varied landscapes among extraterrestrial bodies.

Scientists disagree about what processes are responsible for Miranda's features. One possibility is that the moon may have been smashed apart in some colossal collision, and the pieces then haphazardly reassembled.

Another, perhaps more likely, scenario is that the coronae are sites of large rocky or metallic meteorite strikes which partially melted the icy subsurface and resulted in episodic periods of slushy water rising to Miranda's surface and refreezing.

Orbit

Miranda 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.

Physical characteristics

Miranda's surface may be mostly water ice, with the low-density body also probably containing silicate rock and organic compounds in its interior.

Miranda's surface has patchwork regions of broken terrain indicating intense geological activity in the moon's past, and is criss-crossed by huge canyons.

Exploration Status

So far the only close-up images of Miranda have been from the Voyager 2 probe, which photographed the moon during its flyby of Uranus in January 1986.

No other spacecraft has ever visited the Uranian system or Miranda, and no mission is planned in the foreseeable future.

5th Largest Moon of Saturn - Tethys (14th Moon outwards from Saturn)

Tethys, is the fifth largest moon of Saturn, and 16th largest moon in the Solar System.

Discovery

Tethys was discovered by Giovanni Domenico Cassini, an Italian/French astronomer, in 21st March 1684. He found Tethys and Dione using a large aerial telescope he set up on the grounds of the Paris Observatory.



Naming

Tethys is named after the titan Tethys of Greek mythology.

In Greek mythology, Tethys, daughter of Uranus and Gaia, was an archaic Titaness and aquatic sea goddess, invoked in classical Greek poetry but not venerated in cult. Tethys was both sister and wife of Oceanus. She was mother of the chief rivers of the world known to the Greeks, such as the Nile, the Alpheus, the Maeander, and about three thousand daughters called the Oceanids.

Stats

Diameter: 1,062 km

Semi-major axis: 294,619 km

Orbital Period: 1.89 days

Orbit

Tethys takes as long to rotate on its axis as it does to make one orbit of Saturn; and therefore always keeps the same hemisphere pointed to Saturn.

The Tethyan orbit lies deep inside the magnetosphere of Saturn, so the plasma co-rotating with the planet strikes the trailing hemipshere of the moon. Tethys is also subject to constant bombardment by the energetic particles (electrons and ions) present in the magnetosphere.

The co-orbital moons Telesto and Calypso are located within Tethys' Lagrangian points L4 and L5, 60 degrees ahead and behind Tethys in its orbit respectively.

Physical characteristics

The density of Tethys is 0.98 g/cm³, indicating that it is composed almost entirely of water-ice. The mass of rocky material cannot exceed 6% of the mass of this moon.

The surface of Tethys is one of the most reflective (at visual wavelengths) in the solar system, with a visual albedo of 1.229.

This very high albedo is the result of the sandblasting of particles from Saturn's E-ring, a faint ring composed of small, water-ice particles generated by Enceladus's south polar geysers.

The high albedo indicates that the surface of Tethys is composed of almost pure water ice with only a small amount of a dark material. The visible spectrum of the moon is flat and featureless. No compound other than crystalline water ice has been unambiguously identified on Tethys. (Possible constituents include organics, ammonia and carbon dioxide.)

The dark material in the ice has the same spectral properties as seen on the surfaces of the dark Saturnian moons — Iapetus and Hyperion. The most probable candidate is nanophase iron or hematite.

Life?

It is not known whether Tethys is differentiated into a rocky core and ice mantle. However, if it is differentiated, the radius of the core is about 145 km. Due to the action of tidal and rotational forces, Tethys has the shape of triaxial ellipsoid. The dimensions of this ellipsoid are consistent with this moon having a homogeneous interior.

The existence of a subsurface ocean — a layer of liquid water in the interior of Tethys — is considered unlikely.

5th Largest Moon of Jupiter - Himalia (11th Moon outwards from Jupiter)

Himalia as seen by Cassini–Huygens

Himalia is the largest irregular satellite of Jupiter, the fifth largest overall in size, and only the four Galilean moons are larger. Himalia is the 32th largest moon in the Solar System currently known.

Discovery

Himalia was discovered by Charles Dillon Perrine at the Lick Observatory on 3 December 1904. Himalia is Jupiter's most easily-observed small satellite.

Naming

Himalia is named after the nymph Himalia who bore three sons of Zeus (the Greek equivalent of Jupiter).

Stats

Diameter (mean): 170 km

Semi-major axis: 11,451,971 km

Orbital Period: 250.37 days

Rotation Period: 7.78 hours

Orbit

Himalia is the largest member of the group that bears its name, the moons orbiting between 11.4 and 13 million kilometers from Jupiter at an inclination of about 27.5°. The orbital elements are as of January 2000. as they are continuously changing due to solar and planetary perturbations.

Physical characteristics

Himalia appears neutral in color (grey), like the other members of its group, similar to a C-type asteroid. Measurements by Cassini confirm a featureless spectrum, with a slight absorption at 3 μm which could indicate the presence of water.

Exploration Status

In November 2000, the Cassini spacecraft, en route to Saturn, made a number of images of Himalia, including photos from a distance as close as 4.4 million km. The moon covers only a few pixels, but seems to be an elongated object with axes 150 ± 20 and 120 ± 20 km, close to the Earth-based estimations.

In February and March 2007, the New Horizons spacecraft en route to Pluto made a series of images of Himalia, culminating in photos from a distance of eight million km. Again, Himalia appears only a few pixels across.

Possible Collision

The small moon S/2000 J 11, 4 kilometres in diameter, has gone missing since its discovery in 2000. One theory is that it has crashed into Himalia, creating a faint ring. This possible ring appears as a faint streak near Himalia in images from NASA's New Horizons mission to Pluto.

This suggests that Jupiter sometimes gains and loses small moons through collisions.

Quasi-satellite of Earth - Asteroid 2003 YN107

Discovery

2003 YN107 is a very small Near-Earth object. It was discovered by the Lincoln Near Earth Asteroid Research (LINEAR) system in orbit around the Sun on December 20, 2003.

Stats

Diameter: 0.01 - 0.03 km

Semi-major axis: 0.997 AU (almost same as Earth)

Rotation: not known

Minor planet category: Aten asteroid

Orbit

2003 YN107 revolves around the Sun on an Earth-like, almost circular, orbit. Its orbital period of 363.846 days also is very close to the Sidereal year. Its most remarkable properties are that it has kept a distance of less than 0.1 AU in the time from 1996 to 2006 and that it slowly orbits the Earth during one year.

Before 1996, the asteroid had been on a so-called horseshoe orbit around the sun, along the Earth's orbit. After 2006, it had regained such an orbit.

However, 2003 YN107 won't be gone forever. In about 60 years it will lap Earth again, resuming its role as a temporary, quasi-satellite of Earth.

5th Largest Asteroid, 704 Interamnia

Interamnia is probably the fifth-most-massive asteroid after Ceres, Vesta, Pallas, and Hygiea. Interamnia is easily the largest of the F-type asteroids.

Discovery

Interamnia was discovered on October 2, 1910 by Vincenzo Cerulli. Despite its size, due to its very dark surface and relatively large distance from the Sun, it appears very dim when observed from Earth. For this reason, although it is the 5th largest asteroid, it was discovered only in 1910.

Naming

Interamnia was named after the Latin name for Teramo, Italy, where Cerulli worked.

Stats

Diameter (mean): 317 km
Semi-major axis: 3.059 AU
Orbital Period: 5.35 years
Rotation period: 8.727 hrs
Date discovered: 1910.10.2
Class: F
Type: Main-belt Asteroid
(data from JPL Small-Body Database)

Orbit

Interamnia's orbit is slightly more eccentric that that of Hygiea (15% versus 12%) but differs from Hygiea's in its much greater inclination and slightly shorter period. Another difference is that Interamnia's perihelion is located on the opposite side from the perihelia of the "big four", so that Interamnia at perihelion is actually closer to the Sun than Ceres and Pallas are at the same longitude.

Physical characteristics

Although Interamnia is the largest asteroid after the "big four", it is a very little-studied body. There exist very few details of its internal composition or shape, and no lightcurve analysis has yet been done to Interamnia.

Its apparently high bulk density (though subject to much error) suggests an extremely solid body entirely without internal porosity or traces of water.

This also strongly suggests that Interamnia is large enough to have fully withstood all the collisions that have occurred in the asteroid belt since the Solar System was formed.

4th Largest Asteroid, 10 Hygiea

10 Hygiea is the fourth largest asteroid by volume and mass. It is the largest of the class of dark C-type asteroids with a carbonaceous surface.

Discovery

Hygiea was discovered by Annibale de Gasparis on April 12, 1849, in Naples, Italy. It was the first of his nine asteroid discoveries.

Despite its size, due to its dark surface and larger-than-average distance from the Sun, it appears very dim when observed from Earth. For this reason several smaller asteroids were observed before Hygiea.

Naming

Hygiea, the asteroid, was named after Hygieia, the Greek goddess of health, daughter of Asclepius (God of Medicine and Healing in ancient Greek religion).

Stats

Diameter (mean): 407 km
Semi-major axis: 3.138 AU
Orbital Period: 5.56 years
Rotation period: 27.623 hrs
Date discovered: 1849.4.12
Class: C
Type: Main-belt Asteroid
(data from JPL Small-Body Database)

Physical characteristics

Hygiea's surface is composed of primitive carbonaceous material similar to the chondrite meteorites. Aqueous alteration products have been detected on its surface, which could indicate the presence of water ice in the past which was heated sufficiently to melt. The primitive surface composition would indicate that Hygiea had not been melted during the early period of Solar system formation, in contrast to other large planetesimals like 4 Vesta.

It is the largest of the class of dark C-type asteroids with a carbonaceous surface that are dominant in the outer asteroid belt — which lie beyond the Kirkwood gap at 2.82 AU.

Hygiea appears to have a noticeably oblate spheroid shape, with an average diameter of 444 ± 35 km and a semimajor axis ratio of 1.11. This is much more than for the other objects in the "big four" — the dwarf planet Ceres and the asteroids 2 Pallas and 4 Vesta.

Generally Hygiea's properties are the most poorly known out of the "big four" objects in the asteroid belt.

It is an unusually slow rotator, taking 27 hours and 37 minutes for a revolution, whereas 6 to 12 hours are more typical for large asteroids.

Its direction of rotation is not certain at present, due to a twofold ambiguity in lightcurve data that is exacerbated by its long rotation period — which makes single-night telescope observations span at best only a fraction of a full rotation — but it is believed to be retrograde.

2nd Largest Moon of Pluto - Hydra (4th Moon outwards from Pluto)

Pluto has four known natural satellites (Charon, Nix, Hydra and S/2011 P1. (provisional name, also known as P4, identified by the Hubble Space Telescope in 2011).

Hydra is the second largest known natural satellite of Pluto. It was discovered along with Nix in June 2005.

Based on estimated size, Hydra is the 45th largest moon in the Solar System currently known.

Discovery

Hydra was found by the Hubble Space Telescope's Pluto Companion Search Team, which is composed of Hal A. Weaver, Alan Stern, Max J. Mutchler, Andrew J. Steffl, Marc W. Buie, William J. Merline, John R. Spencer, Eliot F. Young, and Leslie A. Young.

The discovery images were taken on May 15, 2005, and May 18, 2005; the moons were independently discovered by Max J. Mutchler on June 15, 2005, and Andrew J. Steffl on August 15, 2005.

The discoveries were announced on October 31, 2005, after confirmation by precoveries from 2002.

Naming

Hydra was provisionally designated S/2005 P 1. Hydra the moon, was named after Hydra, the nine-headed serpent who battled Hercules in Greco-Roman Mythology.

Stats

Diameter (mean): ~ 114 (61 – 167) km

Semi-major axis: 64,749 km

Orbital Period: 38.21 days

Rotation Period: Unknown

Physical properties

Hydra's size has not been directly measured. Calculations based on its brightness give it a diameter of between 61 km, if its geometric albedo is similar to Charon's 35 percent, and about 167 km, if it has a reflectivity of 4 percent like the darkest Kuiper belt objects.

Exploration Status

The arrival of the New Horizons Spacecraft to Plutonian System in July 2015 is highly anticipated. The true size of Hydra and other new discoveries will be revealed.

4th Largest Moon of Neptune - Larissa (5th Moon outwards from Neptune)

Two views of Larissa by Voyager 2

Larissa is the fourth largest satellite of Neptune and the 28th largest moon in the Solar System currently known.

Larissa is irregular (non-spherical) in shape and appears to be heavily cratered, with no sign of any geological modification.

Discovery

Larissa was first discovered by Harold J. Reitsema, William B. Hubbard, Larry A. Lebofsky and David J. Tholen, based on fortuitous ground-based stellar occultation observations on May 24, 1981.

Larissa was given the temporary designation S/1981 N 1 and announced on 29 May 1981. The moon was recovered and confirmed to be the only object in its orbit during the Voyager 2 flyby in 1989 after which it received the additional designation S/1989 N 2 on August 2, 1989.

Naming

Larissa the moon is named after Larissa, a lover of Poseidon (Neptune) in Greek mythology and eponymous nymph of the city in Thessaly.

Stats

Diameter (mean): 194 km

Semi-major axis: 73,548 km

Orbital Period: 0.555 days

Orbit

Larissa's orbit is circular but not perfect and 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.

Larissa 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

Little else is known about Larissa. Larissa 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.

4th Largest Moon of Uranus - Ariel (15th Moon outwards from Uranus)

Ariel is the fourth largest of the Uranus moons, and the 14th largest moon in the Solar System.

The surface shows signs of more recent geological activity than other Uranian moons, most likely due to tidal heating.

Discovery

Ariel, along with another Uranian satellite, Umbriel, was discovered by William Lassell on October 24, 1851.

Naming

Ariel is named after the leading sylph in The Rape of the Lock. It is also the name of the spirit who serves Prospero in Shakespeare's The Tempest.

Stats

Diameter: 1,158 km

Semi-major axis: 191,020 km

Orbital Period: 2.52 days

Formation

Ariel is thought to have formed from an accretion disc or subnebula; a disc of gas and dust that either existed around Uranus for some time after its formation or was created by the giant impact that most likely gave Uranus its large obliquity.

Orbit

Ariel's orbital period is around 2.52 Earth days, coincident with its rotational period. In other words, Ariel is a synchronous or tidally locked satellite, with one face always pointing toward its parent planet.

Ariel's orbit lies completely inside the Uranian magnetosphere. This is important, because the trailing hemispheres of airless satellites orbiting inside a magnetosphere (like Ariel) are struck by magnetospheric plasma, which co-rotates with the planet.

This bombardment may lead to the darkening of the trailing hemispheres, which is actually observed for all Uranian moons except Oberon.

Composition

Ariel consists of roughly equal parts water ice and a dense non-ice component. The latter could be made of rock and carbonaceous material including heavy organic compounds known as tholins.

The presence of water ice is supported by infrared spectroscopic observations, which have revealed crystalline water ice on the surface of the moon. Water ice absorption bands are stronger on Ariel's leading hemisphere than on the trailing hemisphere.

The cause of this asymmetry is not known, but it may be related to the bombardment by charged particles from the magnetosphere of Uranus, which is stronger on the trailing hemisphere. The energetic particles tend to sputter water ice, decompose methane trapped in ice as clathrate hydrate and darken other organics, leaving a dark, carbon-rich residue behind.

Except for water, the only other compound identified on the surface of Ariel by infrared spectroscopy is carbon dioxide, which is concentrated mainly on its trailing hemisphere. Ariel shows the strongest spectroscopic evidence for CO2 of any Uranian satellite, and was the first Uranian satellite on which this compound was discovered.

The origin of the carbon dioxide is not completely clear. It might be produced locally from carbonates or organic materials under the influence of the energetic charged particles coming from Uranus's magnetosphere or solar ultraviolet radiation.

This hypothesis would explain the asymmetry in its distribution, as the trailing hemisphere is subject to a more intense magnetospheric influence than the leading hemisphere.

Another possible source is the outgassing of primordial CO2 trapped by water ice in Ariel's interior. The escape of CO2 from the interior may be related to past geological activity on this moon.

Life

Ariel is likely differentiated, with an inner core of rock surrounded by a mantle of ice. The current state of Ariel's icy mantle is unclear, although the existence of a subsurface ocean is considered unlikely.

Exploration Status

So far the only close-up images of Ariel have been from the Voyager 2 probe, which photographed the moon during its flyby of Uranus in January 1986.

No other spacecraft has ever visited the Uranian system or Ariel, and no mission is planned in the foreseeable future.

4th Largest Moon of Saturn - Dione (17th Moon outwards from Saturn)

Dione, is the fourth-largest moon of Saturn, and 15th largest moon in the Solar System.

Discovery

Dione was discovered by Giovanni Domenico Cassini, an Italian/French astronomer, in 21st March 1684. He found Dione using a large aerial telescope he set up on the grounds of the Paris Observatory.




Naming

It is named after the titan Dione of Greek mythology.

Dione, a female Titan, a daughter of Oceanus and Tethys, and, according to others, of Uranus and Ge, or of Aether and Ge. She was beloved by Zeus, by whom she became the mother of Aphrodite. When Aphrodite was wounded by Diomedes, Dione received her daughter in Olympus, and pronounced the threat respecting the punishment of Diomedes. Dione was present, with other divinities, at the birth of Apollo and Artemis in Delos.

Stats

Diameter: 1,123 km

Semi-major axis: 377,396 km

Orbital Period: 2.74 days

Orbit

Dione takes as long to rotate on its axis as it does to make one orbit of Saturn; and therefore always keeps the same hemisphere pointed to Saturn.

Physical characteristics

Dione is the densest of Saturn's moons with the exception of Titan. It is composed mainly of water ice, but must contain a larger amount of rocky material than Saturn's other ice moons, Tethys and Rhea.

Though somewhat smaller and denser, Dione is otherwise very similar to Rhea. They both have similar albedo features and varied terrain, and both have dissimilar leading and trailing hemispheres.

Dione's leading hemisphere is heavily cratered and is uniformly bright. Its trailing hemisphere, meanwhile, contains an unusual and distinctive surface feature: a network of bright ice cliffs.

The ice cliffs

When the Voyager space probe photographed Dione in 1980, it showed what appeared to be wispy features covering its trailing hemisphere. The origin of these features was mysterious, as all that was known was that the material has a high albedo and is thin enough that it does not obscure the surface features underneath.

One hypothesis was that shortly after its formation Dione was geologically active, and some process such as ice volcanism resurfaced much of its surface, with the streaks forming from eruptions along cracks in Dione's surface that fell back to the surface as snow or ash. Later, after the internal activity and resurfacing ceased, cratering continued primarily on the leading hemisphere and wiped out the streak patterns there.

This theory was proven wrong by the Cassini probe flyby of December 13, 2004, which produced close-up images. These revealed that the 'wisps' were in fact not ice deposits at all, but rather bright ice cliffs created by tectonic fractures (chasmata); Dione has been revealed as a world riven by enormous fractures on its trailing hemisphere.

Atmosphere

NASA's Cassini spacecraft revealed that Dione has a thin atmosphere during a recent close flyby of Dione.

Dione lacks a strong gravitational fields to prevent atmospheric particles from escaping into space. Dione's thin atmosphere exists only because it's constantly being recharged.

Saturn is surrounded by a belt of highly energetic particles, akin to the Van Allen belts around Earth. Dione is located in this belt, and the reason it possesses an atmosphere is that these hot and very fast particles continuously splatter on the moon's surface.

When the particles hit Dione, they cause the moon's surface ice to break apart chemically, releasing molecules that become the moon's atmosphere.

Life?

Unknown but unlikely.

4th Largest Moon of Jupiter - Europa (6th Moon outwards from Jupiter)

Europa is the smallest of the four Galilean moons, but still one of the largest bodies in the Solar system. Europa is the fourth largest moon of Jupiter and the sixth-largest moon in the Solar System.

Discovery

Europa's discovery is credited to Galileo Galilei, who was the first to observe it on January 7, 1610.

Together with Ganymede, Callisto and Io, they are collectively known as Galilean satellites after the discover.

Naming

Europa is named after a Phoenician noblewoman in Greek mythology, Europa, who was courted by Zeus and became the queen of Crete.

Stats

Diameter: 3,122 km

Semi-major axis: 671,034 km

Orbital Period: 3.55 days

Orbit

Europa is in a 1:2 mean-motion orbital resonance with Io, completing one orbit of Jupiter for every two orbits completed by Io.

Like its fellow Galilean satellites, Europa is tidally locked to Jupiter, with one hemisphere of the satellite constantly facing the planet.

Subsurface ocean

Most planetary scientists believe that a layer of liquid water exists beneath Europa's surface, kept warm by tidally generated heat.

Europa's surface temperature averages about 110 K (−160 °C) at the equator and only 50 K (−220 °C) at the poles, keeping Europa's icy crust as hard as granite.

The first hints of a subsurface ocean came from theoretical considerations of tidal heating (a consequence of Europa's slightly eccentric orbit and orbital resonance with the other Galilean moons).

Galileo imaging team members argue for the existence of a subsurface ocean from analysis of Voyager and Galileo images. The most dramatic example is "chaos terrain", a common feature on Europa's surface that some interpret as a region where the subsurface ocean has melted through the icy crust.

Thick-ice Model

The best evidence for the thick-ice model is a study of Europa's large craters. The largest impact structures are surrounded by concentric rings and appear to be filled with relatively flat, fresh ice.

Based on this and on the calculated amount of heat generated by Europan tides, it is predicted that the outer crust of solid ice is approximately 10–30 km thick, including a ductile "warm ice" layer, which could mean that the liquid ocean underneath may be about 100 km deep. This leads to a volume of Europa's oceans of 3 × 1018 m3, slightly more than two times the volume of Earth's oceans.

Thin-ice Model

The thin-ice model suggests that Europa's ice shell may be only a few kilometers thick. However, most planetary scientists conclude that this model considers only those topmost layers of Europa's crust which behave elastically when affected by Jupiter's tides.

One example is flexure analysis, in which the moon's crust is modeled as a plane or sphere weighted and flexed by a heavy load. Models such as this suggest the outer elastic portion of the ice crust could be as thin as 200 metres.

If the ice shell of Europa is really only a few kilometers thick, this "thin ice" model would mean that regular contact of the liquid interior with the surface could occur through open ridges, causing the formation of areas of chaotic terrain.

Atmosphere

Observations with the Goddard High Resolution Spectrograph of the Hubble Space Telescope revealed that Europa has a tenuous atmosphere composed mostly of molecular oxygen (O2).

Unlike the oxygen in Earth's atmosphere, Europa's is not of biological origin. The surface-bounded atmosphere forms through radiolysis, the dissociation of molecules through radiation.

Solar ultraviolet radiation and charged particles (ions and electrons) from the Jovian magnetospheric environment collide with Europa's icy surface, splitting water into oxygen and hydrogen constituents. These chemical components are then adsorbed and "sputtered" into the atmosphere.

Molecular oxygen is the densest component of the atmosphere because it has a long lifetime; after returning to the surface, it does not freeze like water or hydrogen peroxide molecule but rather desorbs from the surface and starts another ballistic arc. Molecular hydrogen never reaches the surface, as it is light enough to escape Europa's surface gravity.

Life

Europa has emerged as one of the top Solar System locations in terms of potential habitability and possibly, hosting extraterrestrial life.

Life could exist in its under-ice ocean, perhaps subsisting in an environment similar to Earth's deep-ocean hydrothermal vents. Life in such an ocean could possibly be similar to microbial life on Earth in the deep ocean.

Quasi-satellite of Earth - Asteroid 2002 AA29

Discovery

2002 AA29 is a small near-Earth asteroid that was discovered on January 9, 2002 by the LINEAR (Lincoln Near Earth Asteroid Research) automatic sky survey.

2002 AA29's mean orbital period about the Sun is exactly one sidereal year. This means that it is locked into a relationship with the Earth, since such an orbit is only stable under particular conditions. 2002 AA29 follows a so-called horseshoe orbit along the path of the Earth.

Only 5 asteroids of this sort are known, locked into a 1:1 resonance with the Earth. The first was 3753 Cruithne, discovered in 1986.

Stats

Diameter: ~0.06 km

Semi-major axis: 0.993 AU (same as Earth)

Rotation: 33 minutes

Minor planet category: Aten asteroid

(data from JPL Small-Body Database)

Orbit

2002 AA29's orbit lies for the most part inside the Earth’s orbit. From orbital disturbances by the gas giant planets, mainly Jupiter, and the Yarkovsky effect (force due to asymmetrical absorption and emission of infra-red radiation), asteroids are diverted into the inner Solar System, where their orbits further influenced by close approaches with the inner planets.

2002 AA29 has probably been brought in the same way from the outer Solar System into the Earth’s influence. However, it is also suggested that the asteroid has always been on a near-Earth orbit and thus that it or a precursor body was formed near Earth’s orbit. In this case one possibility is that it could be a fragment from a collision of a middle-sized asteroid with the Earth or the Moon.

The orbit of the asteroid is almost circular, with an eccentricity of 0.012 which is even lower than that of the Earth at 0.0167. At the time of its discovery the orbit of 2002 AA29 was unique, because of which the asteroid is often called the first true co-orbital companion of the Earth, since the paths of previously discovered asteroids are not very similar to the Earth’s orbit.

The very low orbital eccentricity of 2002 AA29 is also an indication that it must always have been on a near-Earth orbit, or the Yarkovsky effect must have comparatively strongly caused it to spiral into the inner Solar System over billions of years, since as a rule asteroids which have been steered by planets have orbits with higher eccentricity.

Physical nature

Relatively little is known about 2002 AA29 itself. With a size of about 50 to 110 metres it is very small, on account of which it is seen from the Earth as a small point even with large telescopes, and can only be observed using highly sensitive CCD cameras. At the time of its closest approach on 8 January 2003 it had an apparent brightness in the visible region of magnitude 20.4.

So far nothing concrete is known about the composition of 2002 AA29. Because of its nearness to the Sun, it cannot however consist of volatile substances such as water ice, since these would evaporate or sublime.

Presumably it will have a dark, carbon-bearing or somewhat lighter silicate-rich surface; in the former case the albedo would be around 0.05, in the latter somewhat higher at 0.15 to 0.25. It is due to this uncertainty that the figures for its diameter cover such a wide range.

3rd Largest Asteroid, 4 Vesta

Vesta is thought to be a remnant protoplanet with a differentiated interior. It lost some 1% of its mass less than a billion years ago in a collision that left an enormous crater occupying much of its southern hemisphere. Debris from this event has fallen to Earth as Howardite–Eucrite–Diogenite (HED) meteorites, a rich source of information about the asteroid.

Discovery

Heinrich Wilhelm Matthäus Olbers discovered Pallas in 1802, the year after the discovery of Ceres. He proposed that the two objects were the remnants of a destroyed planet.

He sent a letter with his proposal to the English astronomer William Herschel, suggesting that a search near the locations where the orbits of Ceres and Pallas intersected might reveal more fragments. These orbital intersections were located in the constellations of Cetus and Virgo.

Olbers commenced his search in 1802, and on March 29, 1807 he discovered Vesta in the constellation Virgo — a coincidence, as Ceres, Pallas, and Vesta are not fragments of a larger body.

As the asteroid Juno had been discovered in 1804, this made Vesta the fourth object to be identified in the region that is now known as the asteroid belt.

Naming

As Olbers already had credit for discovering a planet, Pallas, he gave the honor of naming his new discovery to German mathematician Carl Friedrich Gauss. Gauss's orbital calculations had enabled astronomers to confirm the existence of Ceres, the first asteroid, and who had computed the orbit of the new planet in the remarkably short time of 10 hours.

Gauss decided on the Roman virgin goddess of home and hearth, Vesta.

Stats

Diameter: 530 km
Semi-major axis: 2.362 AU
Orbital Period: 3.63 years
Rotation period: 5.342 hrs
Date discovered: 1807.3.29
Class: V
Family: Vesta
Type: Main-belt Asteroid
(data from JPL Small-Body Database)

Planetary Status

When Vesta was discovered by Olbers, Ceres, Pallas, and Juno were classified as planets. Vesta was likewise classified as a planet, and the Solar System was thought to have eleven planets.

After the discovery of Vesta, no further objects were discovered for 38 years. However, in 1845 new asteroids started being discovered at a rapid pace. By 1851 there were fifteen asteroids, in addition to the seven major planets. That year Benjamin Apthorp Gould suggested numbering asteroids in their order of discovery.

Orbit

Vesta orbits in the inner asteroid belt interior to the Kirkwood gap at 2.50 AU. Vesta's orbit lies entirely within the Cererian orbit.

Physical characteristics

Vesta is the brightest asteroid visible from Earth.

Vesta's eastern and western hemispheres show markedly different terrains. From preliminary spectral analyses of the Hubble Space Telescope images, the eastern hemisphere appears to be some kind of high-albedo, heavily cratered "highland" terrain with aged regolith, and craters probing into deeper plutonic layers of the crust.

On the other hand, large regions of the western hemisphere are taken up by dark geologic units thought to be surface basalts, perhaps analogous to the lunar maria.

Exploration

NASA's Dawn spacecraft entered orbit around Vesta on July 16, 2011 for a planned one-year exploration, and what is known about Vesta will be refined and extended as data from Dawn is received, analyzed and published.

Future Status

Vesta's shape is relatively close to a gravitationally relaxed oblate spheroid, but the large concavity and protrusion at the pole combined with a mass less than 5×1020 kg precluded Vesta from automatically being considered a dwarf planet under International Astronomical Union (IAU) Resolution XXVI 5.

Vesta may be listed as a dwarf planet in the future, if it is convincingly determined that its shape, other than the large impact basin at the southern pole, is due to hydrostatic equilibrium, as currently believed.

2nd Largest Asteroid, 2 Pallas

An ultraviolet image of Pallas showing flattened shape taken by the Hubble Telescope.






Discovery

Pallas, formally designated 2 Pallas, is the second asteroid to have been discovered (after Ceres), and one of the largest.

On March 28, 1802, astronomer Heinrich Wilhelm Matthäus Olbers was attempting to locate Ceres when he noticed another moving object in the vicinity. This was the asteroid Pallas, coincidentally passing near Ceres at the time.

When Pallas was discovered, it was counted as a planet. The discovery of many more asteroids after 1845 eventually led to their re-classification.

Naming

2 Pallas is named after Pallas Athena, an alternate name for the goddess Athena. In some mythologies Athena killed Pallas, then adopted her friend's name out of mourning. (There are several male characters of the same name in Greek mythology, but the first asteroids were invariably given female names).

Stats

Diameter: 545 km
Semi-major axis: 2.772 AU
Orbital Period: 4.62 years
Rotation period: 7.813 hrs
Date discovered: 1802.3.28
Class: B
Type: Main-belt Asteroid
(data from JPL Small-Body Database)

Orbit

Pallas's orbit is highly inclined and somewhat eccentric, despite being at the same distance from the Sun as the central part of the asteroid belt.

Pallas's axial tilt is very high, about 78±13°, based on data from the Hubble Space Telescope obtained in 2007, as well as the observations by the Keck telescope in 2003–2005. This means that, every Palladian summer and winter, large parts of the surface are in constant sunlight or constant darkness for a time of the order of an Earth year.

Surface Characteristics

Based on spectroscopic observations, the primary component of the Palladian surface material is a silicate that is low in iron and water.

Pallas is believed to have undergone at least some degree of thermal alteration and partial differentiation, which suggests that it was a protoplanet.

During the planetary formation stage of the Solar System, objects grew in size through an accretion process to approximately this size. Many of these objects were incorporated into larger bodies, which became the planets, while others were destroyed in collisions with other protoplanets. Pallas is likely one of the survivors from this early stage of planetary formation.

3rd Largest Moon of Neptune - Nereid (8th Moon outwards from Neptune)

Nereid is the third largest Neptunian moon, with a highly eccentric orbit. Nereid is the 21st largest moon in the Solar System.

Discovery

Nereid was discovered on May 1, 1949, by Gerard P. Kuiper, on photographic plates taken with the 82-inch telescope at the McDonald Observatory.


Naming

The moon is named after the Nereids, sea-nymphs of Greek mythology and attendants of the god Neptune.

In Greek mythology, the Nereids are sea nymphs, the fifty daughters of Nereus and Doris, sisters to Nerites. They often accompany Poseidon and can be friendly and helpful to sailors fighting perilous storms.

Stats

Diameter (mean): 340 km

Semi-major axis: 5,513,818 km

Orbital Period: 360.14 days

Rotation Period: 11 hrs 31 minutes

Orbit

Nereid orbits Neptune in the prograde direction at an average distance of 5,513,818 km, but its high eccentricity of 0.7507 takes it as close as 1,372,000 km and as far as 9,655,000 km from the planet.

The unusual orbit suggests that it may be either a captured asteroid or Kuiper belt object, or that it was an inner moon in the past and was perturbed during the capture of Neptune's largest moon Triton.

Physical characteristics

Nereid is rather large for an irregular satellite. The shape of Nereid is not known.

Spectrally Nereid appears neutral in colour and water ice has been detected on its surface. Its spectrum appears to be intermediate between Uranus's moons Titania and Umbriel, which suggests that Nereid's surface is composed of a mixture of water ice and some spectrally neutral material.

Nereid's spectrum is markedly different from the outer-Solar-System minor planets, centaurs Pholus, Chiron and Chariklo, suggesting that Nereid possibly formed around Neptune rather than being a captured body.

Exploration

The only spacecraft to visit Nereid is Voyager 2, which passed Nereid at a distance of 4,700,000 km between April 20 and August 19, 1989.

Voyager 2 obtained 83 images of the moon with observation accuracies of 70 km to 800 km.

Not much details is known about Nereid.

3rd Largest Moon of Uranus - Umbriel (16th Moon outwards from Uranus)

Umbriel is the third largest of the Uranus moons, and the 13th largest moon in the Solar System.

Discovery

Umbriel, along with another Uranian satellite, Ariel, was discovered by William Lassell on October 24, 1851.

Naming

Umbriel is named after the 'dusky melancholy sprite' in Alexander Pope's poem The Rape of the Lock, and the name suggests the Latin umbra, meaning shadow.

Stats

Diameter: 1,169 km

Semi-major axis: 266,300 km

Orbital Period: 4.14 days

Formation

Umbriel is thought to have formed from an accretion disc or subnebula; a disc of gas and dust that either existed around Uranus for some time after its formation or was created by the giant impact that most likely gave Uranus its large obliquity.

Orbit

Umbriel's orbital period is around 4.1 Earth days, coincident with its rotational period. In other words, Umbriel is a synchronous or tidally locked satellite, with one face always pointing toward its parent planet.

Umbriel's orbit lies completely inside the Uranian magnetosphere. This is important, because the trailing hemispheres of airless satellites orbiting inside a magnetosphere (like Umbriel) are struck by magnetospheric plasma, which co-rotates with the planet.

This bombardment may lead to the darkening of the trailing hemispheres, which is actually observed for all Uranian moons except Oberon.

Composition

Umbriel consists of water ice, while a dense non-ice component constitutes around 40% of its mass. The latter could be made of rock and carbonaceous material including heavy organic compounds known as tholins.

The presence of water ice is supported by infrared spectroscopic observations, which have revealed crystalline water ice on the surface of the moon. Water ice absorption bands are stronger on Umbriel's leading hemisphere than on the trailing hemisphere.

The cause of this asymmetry is not known, but it may be related to the bombardment by charged particles from the magnetosphere of Uranus, which is stronger on the trailing hemisphere. The energetic particles tend to sputter water ice, decompose methane trapped in ice as clathrate hydrate and darken other organics, leaving a dark, carbon-rich residue behind.

Life

The current state of Umbriel's icy mantle is unclear, although the existence of a subsurface ocean is considered unlikely.

Exploration Status

So far the only close-up images of Umbriel have been from the Voyager 2 probe, which photographed the moon during its flyby of Uranus in January 1986.

No other spacecraft has ever visited the Uranian system or Umbriel, and no mission is planned in the foreseeable future.

3rd Largest Moon of Saturn - Iapetus (23rd Moon outwards from Saturn)

(left) Cassini mosaic of Iapetus, showing the bright trailing hemisphere with part of the dark area appearing on the right; the large crater Engelier is near the bottom.















Iapetus, is the third-largest moon of Saturn, and eleventh largest moon in the Solar System.

Discovery

Iapetus was discovered by Giovanni Domenico Cassini, an Italian/French astronomer, in 25th October 1671.

He had discovered the moon on the western side of Saturn and tried viewing it on the eastern side some months later, but was unsuccessful. The pattern continued the following year as he was able to observe it on the western side, but not the eastern side.

Cassini finally observed Iapetus on the eastern side in 1705 with the help of an improved telescope, finding it two magnitudes dimmer on that side.

Cassini correctly surmised that Iapetus has a bright hemisphere and a dark hemisphere, and that it is tidally locked, always keeping the same face towards Saturn. This means that the bright hemisphere is visible from Earth when Iapetus is on the western side of Saturn, and that the dark hemisphere is visible when Iapetus is on the eastern side. The dark hemisphere was later named Cassini Regio in his honour.

Naming

Iapetus is named after the Titan Iapetus from Greek mythology.

In Greek mythology, Iapetus, also Iapetos or Japetus, was a Titan, the son of Uranus and Gaia, and father (by an Oceanid named Clymene or Asia) of Atlas, Prometheus, Epimetheus, and Menoetius and through Prometheus, Epimetheus and Atlas an ancestor of the human race. He was the Titan of Mortal Life, while his son, Prometheus, was the creator of mankind.

Stats

Diameter: 1,469 km

Semi-major axis: 3,560,820 km

Orbital Period: 79.32 days

Orbit

The orbit of Iapetus is somewhat unusual. Although it is Saturn's third-largest moon, it orbits much farther from Saturn than the next closest major moon, Titan.

It has also the most inclined orbital plane of the regular satellites; only the irregular outer satellites like Phoebe have more inclined orbits. The cause of this is unknown.

Because of this distant, inclined orbit, Iapetus is the only large moon from which the rings of Saturn would be clearly visible; from the other inner moons, the rings would be edge-on and difficult to see.

Physical characteristics

Unlike most moons, its overall shape is neither spherical nor ellipsoid, but has a bulging waistline and squashed poles. Also, its unique equatorial ridge is so high that it visibly distorts the moon's shape even when viewed from a distance. These features often lead it to be characterized as walnut-shaped.

Equatorial Ridge

A mystery of Iapetus is the equatorial ridge that runs along the center of Cassini Regio, about 1,300 km long, 20 km wide, 13 km high. It was discovered when the Cassini spacecraft imaged Iapetus on December 31, 2004.

Peaks in the ridge rise more than 20 km above the surrounding plains, making them some of the tallest mountains in the Solar System. The ridge forms a complex system including isolated peaks, segments of more than 200 km and sections with three near parallel ridges.

The ridge system is heavily cratered, indicating that it is ancient. The prominent equatorial bulge gives the moon a walnut-like appearance.

Two-tone coloration

The difference in colouring between the two Iapetian hemispheres is striking. The leading hemisphere and sides are dark (albedo 0.03–0.05) with a slight reddish-brown coloring, while most of the trailing hemisphere and poles are bright (albedo 0.5-0.6, almost as bright as Europa).

Thus, the apparent magnitude of the trailing hemisphere is around 10.2, whereas that of the leading hemisphere is around 11.9.

The pattern of coloration is analogous to a spherical yin-yang symbol or the two sections of a tennis ball.

The dark region is named Cassini Regio, and the bright region is divided into Roncevaux Terra north of the equator, and Saragossa Terra south of it.

NASA scientists now believe that the dark material is lag (residue) from the sublimation (evaporation) of water ice on the surface of Iapetus, possibly darkened further upon exposure to sunlight.

Because of Iapetus's slow rotation of 79 days (equal to its revolution and the longest in the Saturnian system), the moon would have had the warmest daytime surface temperature and coldest nighttime temperature in the Saturnian system.

Even before the development of the color contrast; near the equator, heat absorption by the dark material results in a daytime temperatures of 129 K in the dark Cassini Regio compared to 113 K in the bright regions.

The difference in temperature means that ice preferentially sublimates from Cassini Regio, and deposits in the bright areas and especially at the even colder poles. Over geologic time scales, this would further darken Cassini Regio and brighten the rest of Iapetus, creating a positive feedback thermal runaway process of ever greater contrast in albedo, ending with all exposed ice being lost from Cassini Regio.

However, a separate process of color segregation would be required to get the thermal feedback started. The initial dark material is thought to have been debris blasted by meteors off small outer moons in retrograde orbits and swept up by the leading hemisphere of Iapetus.

3rd Largest Moon of Jupiter - Io (5th Moon outwards from Jupiter)

Io, the innermost of the four Galilean moons, is the third largest moon of Jupiter and the fourth-largest moon in the Solar System.

Discovery

Io's discovery is credited to Galileo Galilei, who was the first to observe it on January 7, 1610.

Together with Ganymede, Callisto and Europa, they are collectively known as Galilean satellites after the discover.


Naming

Io was named after the Greek mythological figure Io. Io, in Greek mythology, was a priestess of Hera in Argos, a nymph who was seduced by Zeus, who changed her into a heifer to escape detection.

Stats

Diameter: 4,821 km

Semi-major axis: 421,700 km

Orbital Period: 1.77 days

Orbit

Io is in a 2:1 mean-motion orbital resonance with Europa and a 4:1 mean-motion orbital resonance with Ganymede, completing two orbits of Jupiter for every one orbit completed by Europa, and four orbits for every one completed by Ganymede.

This resonance helps maintain Io's orbital eccentricity (0.0041), which in turn provides the primary heating source for its geologic activity.

Io takes as long to rotate on its axis as it does to make one orbit of Jupiter; and therefore always keeps the same hemisphere pointed to Jupiter.

Geological Activity

With over 400 active volcanoes, Io is the most geologically active object in the Solar System. This extreme geologic activity is the result of tidal heating from friction generated within Io's interior as it is pulled between Jupiter and the other Galilean satellites — Europa, Ganymede and Callisto.

Several volcanoes produce plumes of sulfur and sulfur dioxide that climb as high as 500 km above the surface. Io's surface is also dotted with more than 100 mountains that have been uplifted by extensive compression at the base of the moon's silicate crust. Some of these peaks are taller than Earth's Mount Everest.

Composition

Unlike most satellites in the outer Solar System, which are mostly composed of water ice, Io is primarily composed of silicate rock surrounding a molten iron or iron sulfide core. Most of Io's surface is characterized by extensive plains coated with sulfur and sulfur dioxide frost.

Io's volcanism is responsible for many of the satellite's unique features. Its volcanic plumes and lava flows produce large surface changes and paint the surface in various shades of yellow, red, white, black, and green, largely due to allotropes and compounds of sulfur.

Numerous extensive lava flows, several more than 500 km in length, also mark the surface.

Atmosphere

The materials produced by volcanism provide material for Io's thin, patchy atmosphere and Jupiter's extensive magnetosphere.

Io has an extremely thin atmosphere consisting mainly of sulfur dioxide (SO2), with minor constituents including sulfur monoxide (SO), sodium chloride (NaCl), and atomic sulfur and oxygen.

The atmosphere has significant variations in density and temperature with time of day, latitude, volcanic activity, and surface frost abundance.

Life

Life is thought to has less chance at Io, then at the other Galilean moons.

Quasi-satellite of Earth - Asteroid 3753 Cruithne

A quasi-satellite is an object in a 1:1 orbital resonance with its planet that stays close to the planet over many orbital periods.

A quasi-satellite's orbit around the Sun takes exactly the same time as the planet's, but has a different eccentricity (usually greater). When viewed from the perspective of the planet, the quasi-satellite will appear to travel in an oblong retrograde loop around the planet.

In contrast to true satellites, quasi-satellite orbits lie outside the planet's Hill sphere, and are unstable. Over time they tend to evolve to other types of resonant motion, where they no longer remain in the planet's neighbourhood, then possibly later move back to a quasi-satellite orbit, etc.

3753 Cruithne is an asteroid in orbit around the Sun in approximate 1:1 orbital resonance with the Earth. It is a near-earth asteroid orbiting the Sun in an apparent horseshoe orbit. It has been incorrectly called "Earth's second moon".

Discovery

Cruithne was discovered on October 10, 1986, by Duncan Waldron on a photographic plate taken with the UK Schmidt Telescope at Siding Spring Observatory, Australia. It was not until 1997 that its unusual orbit was determined by Paul Wiegert and Kimmo Innanen, working at York University in Toronto, and Seppo Mikkola, working at the University of Turku in Finland.

Naming

The asteroid is named after the Cruithne or Cruthin, a people of early medieval Ireland mentioned in the Annals of Ulster.

Stats

Diameter: ~5 km

Semi-major axis: 0.998 AU (same as Earth)

Rotation: 27.44 hours

Minor planet category: Aten asteroid

Orbit

Cruithne is in a normal elliptic orbit around the Sun. Its period of revolution around the Sun, approximately 364 days at present, is almost equal to that of the Earth. Because of this, Cruithne and Earth appear to "follow" each other in their paths around the Sun. This is why Cruithne is sometimes called "Earth's second moon".

Cruithne's closest approach to Earth is approximately thirty times the separation between Earth and the Moon. From 1994 through 2015, Cruithne makes its annual closest approach to Earth every November.

Although Cruithne's orbit is not thought to be stable over the long term, calculations by Wiegert and Innanen showed that it has probably been synchronized with Earth's orbit for a long time.

There is no danger of a collision with Earth for millions of years, if ever. Its orbital path and Earth's do not cross.

Cruithne, having a maximum near-Earth magnitude of +15.8, is fainter than Pluto and would require at least a 12.5-inch reflecting telescope to be seen.

Others Quasi-satellite of Earth

As of 2011, Earth has four other known quasi-satellites: 2002 AA29, 2003 YN107, 2004 GU9 and 2010 SO16.

Kuiper Belt

The Kuiper Belt, sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from the Sun.

It is similar to the asteroid belt, although it is far larger — 20 times as wide and 20 to 200 times as massive. Like the asteroid belt, it consists mainly of small bodies, or remnants from the Solar System's formation.

While the asteroid belt is composed primarily of rock, ices, and metal, the Kuiper objects are composed largely of frozen volatiles (termed "ices"), such as methane, ammonia and water.

The classical (low-eccentricity) belt is home to at least three dwarf planets – Pluto, Haumea, and Makemake. Some of the Solar System's moons, such as Neptune's Triton and Saturn's Phoebe, are also believed to have originated in the region.

Hypotheses

Since the discovery of Pluto, many have speculated that it might not be alone. The region now called the Kuiper belt had been hypothesized in various forms for decades. The number and variety of prior speculations on the nature of the Kuiper belt have led to continued uncertainty as to who deserves credit for first proposing it.

In 1943, Kenneth Edgeworth hypothesized that, in the region beyond Neptune, the material within the primordial solar nebula was too widely spaced to condense into planets, and so rather condensed into a myriad of smaller bodies. From this he concluded that "the outer region of the solar system, beyond the orbits of the planets, is occupied by a very large number of comparatively small bodies" and that, from time to time, one of their number "wanders from its own sphere and appears as an occasional visitor to the inner solar system", becoming a comet.

In 1951, Gerard Kuiper speculated on a similar disc having formed early in the Solar System's evolution; however, he did not believe that such a belt still existed today. Kuiper was operating on the assumption common in his time, that Pluto was the size of the Earth, and had therefore scattered these bodies out toward the Oort cloud or out of the Solar System. Were Kuiper's hypothesis correct, there would not be a Kuiper belt where we now see it.

Discovery

In 1987, astronomer David Jewitt, then at MIT, became increasingly puzzled by "the apparent emptiness of the outer Solar System". He encouraged then-graduate student Jane Luu to aid him in his endeavour to locate another object beyond Pluto's orbit.

Using telescopes at the Kitt Peak National Observatory in Arizona and the Cerro Tololo Inter-American Observatory in Chile, Jewitt and Luu conducted their search in much the same way as Clyde Tombaugh and Charles Kowal had, with a blink comparator. The process was sped up with the arrival of electronic charge-coupled devices or CCDs.

Finally, after five years of searching, on August 30, 1992, Jewitt and Luu announced the "Discovery of the candidate Kuiper belt object" (15760) 1992 QB1. Six months later, they discovered a second object in the region, (181708) 1993 FW.

Exploration

After the New Horizons Spacecraft visit to the Pluto-Charon system in 2015, circumstances permitting, the spacecraft will continue on to study another as-yet undetermined Kuiper Belt Object. Who know what new discoveries await us?

Largest Moon of Pluto - Charon (1st Moon outwards from Pluto)

1994 image of Pluto and Charon (right)




Pluto has four known natural satellites (Charon, Nix, Hydra and S/2011 P1. (provisional name, also known as P4, identified by the Hubble Space Telescope in 2011).

The Pluto–Charon system is noteworthy for being the largest of the Solar System's few binary systems, defined as those whose barycentre lies above the primary's surface. This and the large size of Charon relative to Pluto has led some astronomers to call it a dwarf double planet.

Charon is the 12th largest moon in the Solar System.

The system is also unusual among planetary systems in that each is tidally locked to the other. Pluto and Charon are gravitationally locked, so each keeps the same face towards the other.

Because of this, the rotation period of each is equal to the time it takes the entire system to rotate around its common centre of gravity. Just as Pluto revolves on its side relative to the orbital plane, so the Pluto–Charon system does also.

Discovery

Charon was discovered by astronomer James Christy on June 22, 1978, when he was examining highly magnified images of Pluto on photographic plates taken a couple of months earlier. Christy noticed that a slight elongation appeared periodically.

Subsequent observations of Pluto determined that the bulge was due to a smaller accompanying body. The periodicity of the bulge corresponded to Pluto's rotation period, which was previously known from Pluto's light curve. This indicated a synchronous orbit, which strongly suggested that the bulge effect was real and not spurious.

Naming

Charon was originally known by the temporary designation S/1978 P1, according to the then recently instituted convention.

On June 24, 1978, Christy first suggested the name Charon as a scientific-sounding version of his wife Charlene's nickname, "Char."

Although colleagues at the Naval Observatory proposed Persephone, Christy stuck with Charon after discovering it coincidentally refers to a Greek mythological figure: Charon is the ferryman of the dead, closely associated in myth with the god Hades, whom the Romans identified with their god Pluto.

Stats

Diameter: 1205 km

Semi-major axis: 17,530 km

Orbital Period: 6.39 days

Origin

Simulation work published in 2005 by Robin Canup suggested that Charon could have been formed by a giant impact around 4.5 billion years ago, much like the Earth and Moon. In this model a large Kuiper belt object struck Pluto at high velocity, destroying itself and blasting off much of Pluto's outer mantle, and Charon coalesced from the debris.

However, such an impact should result in an icier Charon and rockier Pluto than scientists have found. It is now thought that Pluto and Charon may have been two bodies that collided before going into orbit about each other. The collision would have been violent enough to boil off volatile ices like methane but not violent enough to have destroyed either body.

Physical characteristics

Unlike Pluto, which is covered with nitrogen and methane ices, the Charonian surface appears to be dominated by less volatile water ice, and also appears to have no atmosphere.

In 2007, observations by the Gemini Observatory of patches of ammonia hydrates and water crystals on the surface of Charon suggested the presence of active cryo-geysers.

Exploration Status

The arrival of the New Horizons Spacecraft to Pluto-Charon in 2015 is highly anticipated. Who know what new discoveries await us?