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| Ultimi arrivi - Saturn: the "Ringed Beauty" and His Moons |
Dione-EB-MF-LXTT-IPF-2.jpgDione (Enhanced Natural Colors; credits for the additional process. and color.: Elisabetta Bonora and Marco Faccin)105 visitenessun commentoMareKromiumMag 20, 2012
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Helene-PCF-LXTT-IPF.jpgHelene (Absolute Natural Colors; credits for the additional process. and color.: Dr Paolo C. Fienga - Lunexit Team) 123 visiteHelene (one of the many moons of the Giant Gas-Planet Saturn) was discovered by the French Astronomers Pierre Laques and Jean Lecacheux in 1980, from ground-based observations carried out at the "Pic du Midi" Observatory; at first, it was designated S/1980 S 6. Afterwards, in 1988, it was officially named Helene (after Helen of Troy, who was the granddaughter of Cronus - Saturn -, according to Greek mythology).
This small moon (we are talking about a Celestial Body, mostly made of Ice and Rock, which is approx. 30 Km across) is also designated as Saturn XII, a number which it received in 1982, under the designation of Dione B (since Helene is co-orbital with Dione and located in its Leading Lagrangian Point - L4). Helene is one of 4 (four) known so-called "Trojan Moons" of the Saturnian System.
For sake of clarity, please remember that:
A) the Saturnian System contains two sets of Trojan Moons. Both Tethys and Dione have, in fact, two Trojan Moons: Telesto and Calypso, which are located in Tethys' L4 and L5 Lagrangian Points, respectively, and Helene with Polydeuces, which are located in Dione's L4 and L5 Lagrangian Points, respectively;
B) in Astronomy, a co-orbital configuration refers to 2 (two) or more Celestial Objects - such as Asteroids, Moons, or even Planets - that orbit at the same, or very similar, distance from their Parent Object as well as from each other. In other words, they are in a 1:1 (one-to-one) mean Motion Resonance;
C) there are several Classes of co-orbital Objects, depending on their Point of Libration. The most common and best-known Class, is the Trojan, which librate around one of the two stable Lagrangian Points (also known as "Trojan Points"), L4 and L5, 60° ahead of, and behind, the larger Parent Body, respectively. Another class is the so-called "Horseshoe Orbit", in which the Celestial co-Orbital Objects librate around 180° from the larger Parent Body. Objects librating around 0° are called, instead, "Quasi-Satellites".
When two co-Orbital Objects are of similar masses (and thus they exert a non-negligible - i.e.: meaningful - Gravitational Influence on each other) they can even arrive to exchange their orbits. For instance, talking about Janus and Epimetheus, we know that the Timing and Magnitude of their Momenta exchange in such a way that the two moons actually "trade" their orbits, while never getting closer (---> approaching each other) than about 10.000 Km. The exchange takes place about once every 4 years; the last close approaches occurred on January 21, 2006, and in 2010, when Janus' Orbital Radius increased by ~20 Km, while Epimetheus' decreased by ~80 Km. However, Janus' orbit is less affected by the swop, because Janus itself is about 4 times more massive than Epimetheus. As far as it is currently known, this "arrangement" is unique in the Solar System.
As we already said hereabove, Helene was initially observed from Earth in 1980 but, when the NASA - Voyager Probes passed through the Saturnian System, they allowed us to get much closer views of it. Afterwards, the NASA - Cassini Spacecraft, which went into orbit around Saturn in 2004, provided still better views and allowed more in-depth analysis of this moon, including views of its Surface under different lighting conditions. Although conventional Impact Craters and Hills do appear, this image also shows Terrain that looks unusually smooth and streaked. Some of the closest images of Helene to date are from the Cassini Spacecraft's 1800 Km Fly-By that occurred on March 3, 2010, and another very successful imaging sequence which was obtained in June 2011.
There have been many other approaches over the course of the Cassini mission, and future Fly-Bys may yield additional data.This frame has been colorized in Natural Colors (such as the colors that a perfect human eye would actually perceive if someone were onboard the NASA - Cassini Spacecraft and then looked outside, towards the Saturnian moon Helene), by using an original technique created - and, in time, dramatically emproved - by the Lunar Explorer Italia Team.MareKromiumMag 04, 2012
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Rhea-PIA14605-PCF-LXTT-IPF.jpgRhea (Absolute Natural Colors; credits for the additional process. and color.: Elisabetta Bonora and Marco Faccin - Lunar Explorer Italia)182 visiteCaption NASA:"Cassini looks over the heavily Cratered Surface of Rhea during the Spacecraft's Fly-By of this Saturnian moon that occurred on March 10, 2012. This view is centered on Terrain located at about 58° North Latitude and 84° West Longitude.
This image was taken in visible light with the Cassini spacecraft narrow-angle camera. The view was obtained at a distance of approximately 27.000 miles (such as about 43.000 Km) from Rhea and at a Sun-Rhea-Spacecraft, or Phase, Angle of 67°. Image scale is 827 feet (252 meters) per pixel".MareKromiumApr 26, 2012
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Saturn_and_Friends-EB-MF-LXTT.jpgSaturn and some "Friends" (Natural Colors; credits for the additional process. and color.: Elisabetta Bonora and Marco Faccin - Lunexit Team)160 visitenessun commentoMareKromiumApr 23, 2012
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Janus-MF-EB-LXTT.jpgJanus (RAW Natural Colors; credits for the additional process. and color.: Elisabetta Bonora and Marco Faccin - Lunexit Team)146 visiteJanus - apparently, and according to our Readers, one of the most interesting Celestial Bodies that we have shown and talked about on our APOD in the past - is one of the so-called "Inner Natural Satellites" of the Giant Gas-Planet Saturn; it is also called and known, sometimes, as Saturn X, and it occupies (better yet: shares) the same orbit as the Saturnian Inner Minor moon Epimetheus. This (now we know NOT infrequent) circumstance, however caused, in the past, some confusion to many astronomers which, at the time of Janus' discovery - and according to their different observations - assumed that there should have been only one Celestial Body in the orbit where Janus was spotted.
For a long time, in fact, the Astronomical Community struggled to figure out what was going on, until they realized that what they were trying to solve, was (let us use this expression) an "impossible equation": in other words, they were trying to reconcile (so-called "reductio ad unum") different observations of different and distinct Objects, on the wrong assumption that what they all were looking at, was just a single Celestial Body.
The discovery of Janus is attributed to its first observer, such as the French Astronomer Audouin Dollfus, who spotted Janus on December 15, 1966. The newly discovered Object was given the temporary designation of "S/1966 S 2". Previously, though, another Astronomer (Jean Texereau) had also photographed the very same Celestial Body (to be exact, on October 29, 1966), but without realizing the discovery! On December 18 of the same year, then, Richard Walker made a similar observation which is now credited as the discovery of Epimetheus.
Twelve years later, in October 1978, Stephen M. Larson and John W. Fountain finally realized that the 1966 observations would have been better explained by assuming that they were relative NOT to one, BUT to two distinct Objects (as we said, Janus and Epimetheus), sharing very similar (or perhaps the same, as we know now) orbits. Finally, in the AD 1980, the NASA - Voyager 1 Spacecraft confirmed that this "controversial moon" was, in fact, a couple of co-orbital Celestial Bodies.
Afterwards, Janus was observed on other occasions and given different provisional designations. It was observed, for instance, by the NASA - Pioneer 11 Probe when it passed near Saturn on September 1st, 1979. Janus was also observed by Dan Pascu on February 19, 1980 (and then provisionally designed as "S/1980 S 1") and then by John W. Fountain (once again), Stephen M. Larson (again), Harold J. Reitsema and Bradford A. Smith on the 23rd (and, this time, the observed object was, always provisionally, recorded as "S/1980 S 2").
All these Scientists, in a way and in the end, should therefore share, to various degrees, the title of Discoverer of Janus, but this "fair dispute", so far (and as far as we know), has not been solved.
As far as the physical characteristics of Janus are concerned, we can say that this relatively small moon (whose dimensions are approx. 203 × 185 × 153 Km) is extensively cratered, and a few of these craters are more than 30 Km across. The Janian Surface, as we already wrote in the past, appears to be older than Prometheus' one, but younger than Pandora's. Furthermore, if you consideri its oval (or, if you prefer, its "non-spherical") shape, its (very) low Density and relatively high Albedo, it seems logic to conclude, for the time being, that Janus should just be a porous and icy "rubble pile" (such as the final result of a cloud of Cosmic Debris that have coalesced under the influence of gravity).
The so-called Rubble Piles-type Celestial Bodies have a low density because there are, we believe, large cavities between the various "chunks" - such as fragments - of rocky material that made them up; Rubble Piles may also form when an Asteroid or a moon (which may originally be monolithic - such as "one single piece of rock") is smashed by an impact, and the shattered pieces left over by the impact, in time, fall back together, primarily due to self-gravitation (---> mutual attraction). In other words: a Celestial Body like Janus is formed, in our opinion, by a mass of different materials that cohered over time (we repeat: something like a "cloud" of Cosmic Debris - rocks, boulders, dust etc. - which, in time, due to a mutual gravitational attraction, got together until they became one - yet extremely fragile - Celestial Body).
In the light of the above considerations and assumptions, we, as IPF, believe that Janus can reasonably be assimilated, for instance (and among others), to Asteroid 25143 Itokawa or 433 Eros.MareKromiumApr 23, 2012
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Enceladus-N00185224_to_N00185282-EB-MF-LXTT-IPF.gifFountains of Light (a GIF-Movie by Elisabetta Bonora and Marco Faccin - Lunexit Team)96 visitenessun commentoMareKromiumMar 30, 2012
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Enceladus-N00185279-82_-_cl1cl2,bl1,red,ir1,ir3_filter-EB-MF-LXTT-IPF.jpgFountains of Light (Natural Colors; credits for the additional process. and color.: Elisabetta Bonora and Marco Faccin - Lunexit Team)88 visitenessun commentoMareKromiumMar 30, 2012
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Enceladus-PIA14588-PCF-LXTT.jpgCrescent Enceladus (Absolute Natural Colors; credits for the additional process. and color.: Dr Paolo C. Fienga - Lunexit Team)217 visiteCaption NASA:"The Cassini Spacecraft looks at a brightly illuminated Enceladus and examines the Surface of its Leading Hemisphere. North on Enceladus is up and rotated 21° to the right.
The image was taken in Visible Light with the Cassini Spacecraft narrow-angle camera on Nov. 6, 2011. The view was obtained at a distance of approx. 67.700 miles (such as about 109.000 Km) from Enceladus and at a Phase Angle of 21°. Image scale is 2130 feet (649 meters) per pixel".MareKromiumDic 30, 2011
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Hyperion-PIA14583-PCF-LXTT.jpgHyperion (Absolute Natural Colors; credits for the additional process. and color.: Dr Paolo C. Fienga - Lunexit Team)211 visiteCaption NASA:The Cassini Spacecraft looks at Saturn's highly irregular moon Hyperion in this view taken from the Spacecraft's during the Fly-By that occurred on August, 25th, 2011. Hyperion (approx. 168 miles, or about 270 Km across) has an irregular shape, and it tumbles through its orbit: that is, it does not spin at a constant rate or in a constant orientation. (A standard reference latitude-longitude system has not yet been devised for this moon)
Images such as this one extend previous coverage and allow a better inventory of the Surface Features, the satellite's shape and changes in its spin.
The image was taken with the Cassini Spacecraft narrow-angle camera using a combination of spectral filters sensitive to wavelengths of Polarized Green Light centered at 617 and 568 nanometers. The view was obtained at a distance of approx. 36.000 miles (such as about 58.000 Km) from Hyperion and at a Phase Angle of 43° degrees. Image scale is 1145 feet (349 meters) per pixel".
MareKromiumNov 22, 2011
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Enceladus-PIA14578-PCF-LXTT.jpgEnceladus' Southern Regions (Absolute Natural Colors; credits for the additonal process. and color.: Dr Paolo C. Fienga - Lunar Explorer Italia)189 visiteCaption NASA:"This image was taken in Visible Light with the Cassini Spacecraft narrow-angle camera on Sept. 13, 2011. The view was acquired at a distance of approx. 26.000 miles (such as about 41.842,84 Km) from Enceladus and at a Phase Angle of 52°. Image scale is roughly 830 feet (253 meters) per pixel".
MareKromiumNov 18, 2011
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Tethys-PIA14581-PCF-LXTT-1.jpgTethys (Sx: False b/w - credits for the additional process.: Dr Paolo C. Fienga - Lunexit Team - Dx: Original NASA b/w RAW Frame)250 visitenessun commentoMareKromiumNov 15, 2011
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Tethys-PIA14581-PCF-LXTT-0.jpgTethys (False b/w - credits for the additional process.: Dr Paolo C. Fienga - Lunexit Team)196 visiteCaption NASA:"Saturn's moon Tethys shows off its tortured Surface in this Cassini Spacecraft image. On the top left of the image there is huge Odysseus Crater. On the bottom right, instead, there is Ithaca Chasma: a series of Scarps that run North-to-South across the moon for more than 620 miles (approx. 1000 Km). North on Tethys is up and rotated 25° to the right. This view looks toward the area between the Leading Hemisphere and Saturn-facing side of Tethys.
The image was taken in Visible Green Light with the Cassini Spacecraft narrow-angle camera on Sept. 14, 2011. The view was acquired at a distance of approximately 178.000 miles (approx. 287.000 Km) from Tethys and at a Sun-Tethys-Spacecraft, or Phase, Angle of 11°. Image scale is about 1 mile (1,6 Km) per pixel". MareKromiumNov 15, 2011
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