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

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

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



Discovery

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


Naming

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

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

Stats

Diameter: 396 km

Semi-major axis: 185,404 km

Orbital Period: 0.942 days

Orbit

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

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

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

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

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

Physical characteristics

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

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

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

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

Curious Resemblances

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

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

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

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

Discovery

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

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

Naming

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

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

Stats

Diameter (mean): 99 km

Semi-major axis: 221,889 km

Orbital Period: 0.675 days

Rotation Period: Synchronous

Orbit

Thebe is the outermost of the inner Jovian moons.

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

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

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

Physical characteristics

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

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

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

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

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

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

Stats

Diameter: 0.16 - 0.35 km

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

Rotation: not known

Orbit

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

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

9th Largest Asteroid, 65 Cybele

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

Discovery

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

Naming

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

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

Stats

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

Orbit

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

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

Physical characteristics

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

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

Life?

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

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

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

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

Discovery

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

Naming

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

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

Stats

Diameter (mean): 7 km

Semi-major axis: 706 ± 5 km

Orbital Period: 1.38 days

Rotation period: ?

Orbit

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

Physical characteristics

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

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

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

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

Discovery

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

Naming

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

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

Stats

Diameter (mean): 18 km

Semi-major axis: 1356 ± 5 km

Orbital Period: 3.65 days

Rotation period: ?

Orbit

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

Physical characteristics

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

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

8th Largest Asteroid, 87 Sylvia

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










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

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

Discovery

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

Naming

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

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

Stats

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

Orbit

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

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

Physical characteristics

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

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

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