13th Largest Moon of Saturn - Pandora (5th Moon outwards from Saturn)

Pandora is an inner satellite of Saturn.

Pandora is the 13th largest moon of Saturn, and the 52th largest moon in the Solar System currently known.

Discovery

Pandora was discovered in October 1980 from photos taken by the Voyager 1 probe.


Naming

Pandora was provisionally designated S/1980 S26. In late 1985, the moon was officially named after Pandora, from Greek mythology.

In Greek mythology, Pandora was allegedly the first woman, who was made out of clay. As Hesiod related it, each god helped create her by giving her unique gifts. Zeus ordered Hephaestus to mold her out of earth as part of the punishment of mankind for Prometheus' theft of the secret of fire, and all the gods joined in offering her "seductive gifts".

Pandora's Box is an artifact in Greek mythology, taken from the myth of Pandora's creation in Hesiod's Works and Days. The "box" was actually a large jar given to Pandora, which contained all the evils of the world, although the particular evils, aside from plagues and diseases, are not specified in detail by Hesiod. When Pandora opened the jar, all its contents except for one item were released into the world. The one remaining item was Hope. Today, to open Pandora's box means to create evil that cannot be undone.

Stats

Diameter (mean): 81.4 ± 3.0 km

Semi-major axis: 141,720 km

Orbital Period: 0.629 days

Orbit

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

Pandora is the outer shepherd satellite of the F Ring.

The orbit of Pandora appears to be chaotic, as a consequence of a series of four 118:121 mean motion resonances with Prometheus. The most appreciable changes in their orbits occur approximately every 6.2 years, when the periapsis of Pandora lines up with the apoapsis of Prometheus and the moons approach to within about 1,400 kilometres.

Pandora also has a 3:2 mean-motion resonance with Mimas.

Physical Characteristics

Pandora is more heavily cratered than nearby Prometheus, and has at least two large craters 30 kilometres in diameter.

From Pandora's very low density and relatively high albedo, it seems likely that it is a very porous icy body. There is a lot of uncertainty in these values, however, so this remains to be confirmed.

13th Largest Moon of Jupiter - Lysithea (12th Moon outwards from Jupiter)

Lysithea is a prograde irregular satellite of Jupiter.

Lysithea is the 13th largest moon of Jupiter and 75th largest moon in the Solar System currently known.

Discovery

Lysithea was discovered by Seth Barnes Nicholson on July 6, 1938 with the 100-inch (2.5 m) Hooker telescope at the Mount Wilson Observatory.

Naming

Lysithea the moon is named after the mythological Lysithea, daughter of Oceanus and one of Zeus' lovers.

In Greek mythology, Lysithea was a daughter of Oceanus and one of Zeus' lovers.

Lysithea didn't receive its present name until 1975; before then, it was simply known as Jupiter X. Or it was sometimes called "Demeter" from 1955 to 1975.

In Greek mythology, Demeter is the goddess of the harvest, who presided over grains and the fertility of the earth.

A name ending in "a" was chosen for this moon in keeping with the International Astronomical Union's policy for designating Jupiter's outer moons which have prograde orbits (orbiting in the same direction as Jupiter's rotation).

Stats

Diameter (mean): 36 km

Semi-major axis: 11,740,560 km

Orbital Period: 259.2 day

Orbit

Lysithea belongs to the Himalia group, a family of Jovian satellites which have similar orbits and appearance, and are therefore thought to have a common origin. Its orbital elements are as of January 2000. They are continuously changing due to solar and planetary perturbations.

Physical characteristics

Lysithea has an estimated diameter of 36 km (assuming an albedo of 0.04).

Lysithea 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: Leda, Himalia (the largest) and Elara. A fifth moon, called S/2000 J11, only about 2 km in radius, was considered a candidate for this group. However, it was lost before its orbit could be definitively determined.

S/2000 J11 may have crashed into Himalia, reuniting two pieces of the former asteroid, and perhaps creating a faint temporary ring of Jupiter near the orbit of Himalia.

Centaurs (minor planets)

Centaurs are an unstable orbital class of minor planets that behave with characteristics of both asteroids and comets.

The generic definition of a centaur is a small body that orbits the Sun between Jupiter and Neptune and crosses the orbits of one or more of the giant planets.

It has been estimated that there are around 44,000 centaurs in the Solar System with diameters larger than 1 km.

Naming

They are named after the mythological race of beings, centaurs, which were a mixture of horse and human.

In Greek mythology, a centaur or hippocentaur is a member of a composite race of creatures, part human and part horse. In early Attic and Boeotian vase-paintings, they are depicted with the hindquarters of a horse attached to human; in later renderings centaurs are given the torso of a human joined at the waist to the horse's withers, where the horse's neck would be.

The centaurs were usually said to have been born of Ixion and Nephele (the cloud made in the image of Hera).

Orbits

Centaurs' orbits are characterised by a wide range of eccentricity, from highly eccentric (Pholus, Asbolus, Amicus, Nessus) to more circular (Chariklo and the Saturn-crossers: Thereus, Okyrhoe).

Since the centaurs cross the orbits of the giant planets and are not protected by orbital resonances, their orbits are unstable within a timescale of 10⁶ – 10⁷ years.

Dynamical studies of their orbits indicate that centaurs are probably an intermediate orbital state of objects transitioning from the Kuiper belt to the Jupiter family of short-period comets. Objects may be perturbed from the Kuiper belt, whereupon they become Neptune-crossing and interact gravitationally with that planet. They then become classed as centaurs, but their orbits are chaotic, evolving relatively rapidly as the centaur makes repeated close approaches to one or more of the outer planets.

Some centaurs will evolve into Jupiter-crossing orbits whereupon their perihelia may become reduced into the inner Solar System and they may be reclassified as active comets in the Jupiter family if they display cometary activity. Any centaur that is perturbed close enough to the Sun is expected to become a comet.

Centaurs will thus ultimately collide with the Sun or a planet or else they may be ejected into interstellar space after a close approach to one of the planets, particularly Jupiter.

Physical characteristics

The relatively small size of centaurs precludes surface observations, but colour indices and spectra can indicate possible surface composition and can provide insight into the origin of the bodies.

Color

Centaurs display a puzzling diversity of colour that challenges any simple model of surface composition.

Centaurs appear to be grouped into two classes:
1. Very red, for example 5145 Pholus
2. Blue (or blue-grey), for example 2060 Chiron

There are numerous theories to explain this colour difference, but they can be divided broadly into two categories:
1. The colour difference results from a difference in the origin and/or composition of the centaur.
2. The colour difference reflects a different level of space-weathering from radiation and/or cometary activity.

Spectra

The interpretation of spectra is often ambiguous, related to particle sizes and other factors. For example, if there is cometary activity, the spectra observed vary depending on the period of the observation. Water ice signature was detected during a period of low activity and disappeared during high activity.

Origin

The study of centaur development is rich in recent developments but still hampered by limited physical data. Different models have been put forward for possible origin of centaurs.

Simulations indicate that the orbit of some Kuiper-belt objects can be perturbed, resulting in the object's expulsion so that it becomes a centaur. Scattered disk objects would be dynamically the best candidates for such expulsions, but their colours do not fit the bicoloured nature of the centaurs. Plutinos are a class of Kuiper-belt object that display a similar bicoloured nature, and there are suggestions that not all plutinos' orbits are as stable as initially thought, due to perturbation by Pluto.

Further developments are expected with more physical data on KBOs.

34th Largest Asteroid, 6 Hebe

6 Hebe is a large main-belt asteroid and the 34th largest asteroid currently known.




Discovery

Hebe was the sixth asteroid to be discovered, on July 1, 1847 by Karl Ludwig Hencke at Driesen. It was the second and final asteroid discovery by Hencke, who had previously found 5 Astraea.

Naming

The name Hebe, goddess of youth, was proposed by Carl Friedrich Gauss, who was a German mathematician and physical scientist who contributed significantly to many fields, including number theory, statistics, analysis, differential geometry, geodesy, geophysics, electrostatics, astronomy and optics.

In Greek mythology, Hēbē is the goddess of youth (Roman equivalent: Juventas). She is the daughter of Zeus and Hera. Hebe was the cupbearer for the gods and goddesses of Mount Olympus, serving their nectar and ambrosia, until she was married to Heracles.

The name Hebe comes from Greek word meaning "youth" or "prime of life". Juventas likewise means "youth", as can be seen in such derivatives as juvenile. In art, Hebe is usually depicted wearing a sleeveless dress. Hebe was also worshipped as a goddess of pardons or forgiveness; freed prisoners would hang their chains in the sacred grove of her sanctuary at Phlius.

Stats

Diameter (mean): 185 km
Aphelion: 2.914 AU
Perihelion: 1.935 AU
Semi-major axis: 2.425 AU
Orbital Period: 3.78 years
Rotation period: 7.27 hrs
Date discovered: 1847.7.1
Class: S
Type: Main-belt Asteroid
Satellite: 1 ?
(data from JPL Small-Body Database)

Orbit and Brightness

In brightness, Hebe is the fifth brightest object in the asteroid belt after Vesta, Ceres, Iris and Pallas. It has a mean opposition magnitude of +8.3, about equal to the mean brightness of Titan and can reach +7.5 at an opposition near perihelion.

Physical characteristics

Hebe containing around half a percent of the mass of the asteroid belt. Hebe's size/volume does not rank among the top twenty asteroids. But its apparently high bulk density is even greater than that of the Earth's Moon or even Mars. This high bulk density suggests an extremely solid body that has not been impacted by collisions, which is not typical of asteroids of its size, as they tend to be loosely bound rubble piles.

Lightcurve analysis suggests that Hebe has a rather angular shape, which may be due to several large impact craters. Hebe rotates in a prograde direction.

Major meteorite source

6 Hebe is the probable parent body of the H chondrite meteorites and the IIE iron meteorites. Remarkably, this would imply that it is the source of about 40% of all meteorites striking Earth. Evidence for this connection includes:

- The spectrum of Hebe matches a mix of 60% H chondrite and 40% IIE iron meteorite material.

- The IIE type are unusual among the iron meteorites, and probably formed from impact melt, rather than being fragments of the core of a differentiated asteroid.

- The IIE irons and H chondrites likely come from the same parent body, due to similar trace mineral and oxygen isotope ratios.

- Asteroids with spectra similar to the ordinary chondrite meteorites (accounting for 85% of all falls, including the H chondrites) are extremely rare.

- 6 Hebe is extremely well placed to send impact debris to Earth-crossing orbits. Ejecta with even relatively small velocities (~280 m/s) can enter the chaotic regions of the 3:1 Kirkwood gap at 2.50 AU and the nearby secular resonance which determines the high-inclination edge of the asteroid belt at about 16° inclinations hereabouts.

- Of the asteroids in this "well-placed" orbit, Hebe is the largest.

- An analysis of likely contributors to the Earth's meteorite flux places 6 Hebe at the top of the list, due to its position and relatively large size. If Hebe is not the H-chondrite parent body, then where are the meteorites from Hebe?

Satellite

On March 5, 1977 Hebe occulted Kaffaljidhma (γ Ceti), a moderately bright 3rd magnitude star. No other observed occultations by Hebe have been reported.

As a result of that occultation, a small Hebean moon was reported by Paul D. Maley. It was nicknamed "Jebe". However, the discovery has not been confirmed.

33th Largest Asteroid, 16 Psyche

16 Psyche is a large main-belt asteroids and the largest member of the Themistian asteroid family. Psyche is the 33th largest asteroid and is the most massive metallic M-type asteroid currently known.

Discovery

16 Psyche was discovered on 17 March 1852 by Annibale de Gasparis at Naples.

Naming

Psyche the asteroid was named after the Greek mythological figure Psyche.

In Greek mythology, Psyche was the deification of the human soul. She was portrayed in ancient mosaics as a goddess with butterfly wings (because psyche is also the Greek word for 'butterfly').

Psyche was originally the youngest daughter of the king and queen of Sicily, and the most beautiful person on the island. Suitors flocked to ask for her hand.

She eventually boasted that she was more beautiful than Aphrodite (Venus) herself, and Aphrodite sent Eros to transfix her with an arrow of desire, to make her fall in love with the nearest person or thing available. But even Eros (Cupid) fell in love with her, and took her to a secret place, eventually marrying her and having her made a goddess by Zeus (Jupiter).

Stats

Diameter (mean): 186 km
Aphelion: 3.328 AU
Perihelion: 2.521 AU
Semi-major axis: 2.922 AU
Orbital Period: 4.99 years
Rotation period: 4.20 hrs
Date discovered: 1852.3.17
Class: M
Type: Main-belt Asteroid

Physical Characteristics

Radar observations indicate Pysche has a fairly pure iron–nickel composition. Psyche appears to be a genuine case of an exposed metallic core from a larger differentiated parent body. Psyche is the most massive metallic M-type asteroid currently known.

Unlike some other M-type asteroids, Psyche shows no sign of the presence of water or water-bearing minerals on its surface, consistent with its interpretation as a metallic body. Small amounts of pyroxene appear to be present.

It is possible that at least some examples of enstatite chondrite meteorites originated from this asteroid, based on similar spectral analysis results.

Asteroid family ?

If Psyche is the core remnant of a larger parent body, we might expect other asteroids on similar orbits. However, we do not find any asteroid family belonging to Psyche.

One hypothesis is that the collision occurred very early in the Solar System's history, and all the other remnants have since been ground into fragments by subsequent collisions or had their orbits perturbed beyond recognition.

Star occultation

16 Psyche on August 21, 2010 occulted the star HIP 22112 in Taurus over a long path that passed from the central Baja peninsula to the central Atlantic coast. Maximum duration was predicted to be 10.3 seconds. For this event, 12 observers at 17 sites recorded 14 chords across the profile of the asteroid. Fifteen sites used video to record the event while one station used visual techniques and one station used drift scan.

The resulting chords and least squares ellipse produce a smooth ellipse with dimensions of 235.4 ± 3.9 x 230.4 ± 2.4 km. The maximum occultation duration of 10.12 seconds occurred at station nine and is just 2% shorter than predicted. The observed path was just 32 km north of the prediction.

Only two other occultations by Psyche have been observed: a single-chord event on 2002 March 22 and a five-chord occultation on 2004 May 16 (TYC 5783-01228-1). The four useable chords for the 2004 event were spaced well across the asteroid, being fit by an ellipse with axes of 214 ± 6 km by 181 ± 7 km.

32th Largest Asteroid, 128 Nemesis

128 Nemesis is a large asteroid in the main asteroid belt. It is the 32th largest asteroid currently known.

Discovery

Nemesis was discovered by James Craig Watson on November 25, 1872 from Ann Arbor, Michigan, USA.

Naming

Nemesis the asteroid was named after Nemesis, the goddess of retribution in Greek mythology.

In Greek mythology, Nemesis, also called Rhamnousia/Rhamnusia ("the goddess of Rhamnous") at her sanctuary at Rhamnous, north of Marathon, was the spirit of divine retribution against those who succumb to hubris (arrogance before the gods). The Greeks personified vengeful fate as a remorseless goddess: the goddess of revenge.

Stats

Diameter (mean): 188 km
Aphelion: 3.099 AU
Perihelion: 2.408 AU
Semi-major axis: 2.751 AU
Orbital Period: 4.56 years
Rotation period: 39 hrs
Date discovered: 1872.11.25
Class: C
Group: Nemesis group
Type: Main-belt Asteroid
Satellite: 0
(data from JPL Small-Body Database)

Orbit

Nemesis is the largest member of the Nemesian asteroid family bearing its name.

Physical Characteristics

Nemesis is a very dark main-belt asteroid, of carbonaceous composition.

Rotation

Nemesis rotates very slowly, taking about one and half Earth days (39 hours) to complete one revolution.

Extensive photoelectric lightcurves of Nemesis show that it has extremely long rotational period, the longest known currently for asteroids.

31th Largest Asteroid, 372 Palma

372 Palma is a large asteroid in the main asteroid belt. It is the 31th largest asteroid currently known.

Discovery

Palma was discovered by Auguste Honoré Charlois on August 19, 1893 from Nice.

Naming

Palma the asteroid is named for the capital city of Majorca, an island located in the Mediterranean Sea, one of the Balearic Islands (Spain), and south of France.

The capital of Majorca, Palma, was founded as a Roman camp called Palmaria upon the remains of a Talaiotic settlement. The turbulent history of the city saw it subject to several Vandal sackings during the fall of the Roman Empire. It was later reconquered by the Byzantines, colonised by the Moors (who called it Medina Mayurqa), and finally established by James I of Aragon. In 1983, Palma became the capital of the autonomous region of the Balearic Islands.

Palma the asteroid is one of seven of Charlois's discoveries that was expressly named by the Astromomisches Rechen-Institut (Astronomical Calculation Institute).

Stats

Diameter (mean): 189 km
Aphelion: 3.971 AU
Perihelion: 2.327 AU
Semi-major axis: 3.149 AU
Orbital Period: 5.59 years
Rotation period: 8.57 hrs
Date discovered: 1893.8.19
Class: B
Type: Main-belt Asteroid
(data from JPL Small-Body Database)

Star Occultation

On the morning of January 26, 2007 the occultation of the star 32 Lyncis by 372 Palma was observed.

The observations can be fit to an ellipse with dimensions 207 kilometers by 184 kilometers, to an accuracy of +/-2 km.