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| Piú viste - Mercury |
ZZ-Mercury-Craters-Kertész_Crater-PIA16624-PCF-LXTT-IPF.jpgKertész Crater (Absolute Natural Colors; credits for the additional process. and color.: Dr Paolo C. Fienga/Lunar Explorer Italia/Italian Planetary Foundation)82 visiteThe bright and white-colored Material so clearly visible all over the Floor of Kertész Crater (whose diameter is approx. 31 Km - such as a little more than 19 miles) is not - despite its color and physical appearence - the Water Ice that has been recently confirmed to be present deep inside many Impact Craters located near Mercury's Poles, but it might well be behaving as Ice would on another Planet.
As a matter of fact, Mercury's Daytime Temperatures are so hot at most Latitudes that several kinds of Rocks (---> Minerals) which would be quite stable at other places in the Solar System, may essentially evaporate on Mercury. That is just one (a new one, we believe) theory that could explain the formation of these bright, irregular Features known as "Hollows" and seen here and, as you should already know very well, in many other places on the Planet Mercury (however, these fascinating Surface Features must, somehow - and in our humble opinion, as IPF -, be related to Impact Mechanics since they can mostly - if not exclusively - be found either inside, or on the Rims and/or, often, in close proximities with Impact Craters).
Date acquired: November, 17th, 2012
Image Mission Elapsed Time (MET): 261598284
Image ID: 2969138
Instrument: Narrow Angle Camera (NAC) of the Mercury Dual Imaging System (MDIS)
Center Latitude: 27,38° North
Center Longitude: 145,5° East
Resolution: 46 meters/pixel
Solar Incidence Angle: 42,6° (meaning that the Sun, at the time that the pictures were taken, was about 47,4° above the imaged Local Mercurian Horizon)
Emission Angle: 59,7°
Sun-Mercury-Messenger (or "Phase") Angle: 91,4°
This picture (which is an Original NASA - MESSENGER Spacecraft b/w frame published on the NASA - Planetary Photojournal with the ID n. PIA 16624) has been additionally processed and then colorized in Absolute Natural Colors (such as the colors that a human eye would actually perceive if someone were onboard the NASA - MESSENGER Spacecraft and then looked down, towards the Surface of Mercury), by using an original technique created - and, in time, dramatically improved - by the Lunar Explorer Italia Team. Different colors, as well as different shades of the same color, mean, among other things, the existence of different Elements (Minerals) present on the Surface of Mercury, each having a different Albedo (---> Reflectivity) and Chemical Composition.MareKromium
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ZZ-Mercury-Craters-Poe_Crater-PIA16755-PCF-LXTT-IPF.jpgPoe Crater (Absolute Natural Colors; credits for the additional process. and color.: Dr Paolo C. Fienga/Lunar Explorer Italia/Italian Planetary Foundation)82 visiteThis picture is an image-mosaic made of 3 (three) frames taken by the NASA - MESSENGER Spacecraft in July 2011; it shows the huge and magnificent Mercurian Crater Poe (so named after the American writer, and whose diameter is approx. 77 Km - or about 48 miles), with its almost black colored Rim standing out from the reddish Volcanic Plains that surround it. Even in this case, a number of relatively small Hollows speckles the dark Rim of Poe Crater like stars, in the blackness of night.
Date acquired: July, 3rd, 2011
Images Mission Elapsed Time (MET): 218204194, 218204190, 218204186
Images ID: 458399, 458398, 458397
Instrument: Narrow Angle Camera (NAC) of the Mercury Dual Imaging System (MDIS)
Center Latitude: 42,83° North
Center Longitude: 158,9° East
Resolution: 213 meters/pixel
Solar Incidence Angle: 42,8° (meaning that the Sun, at the time that the pictures were taken, was about 47,2° above the imaged Local Mercurian Horizon)
Emission Angle: 35,9°
Sun-Mercury-Messenger (or "Phase") Angle: 78,7°
This picture (which is an Original NASA - MESSENGER Spacecraft b/w image mosaic published on the NASA - Planetary Photojournal with the ID n. PIA 16770) has been additionally processed and then colorized in Absolute Natural Colors (such as the colors that a human eye would actually perceive if someone were onboard the NASA - MESSENGER Spacecraft and then looked down, towards the Surface of Mercury), by using an original technique created - and, in time, dramatically improved - by the Lunar Explorer Italia Team. Different colors, as well as different shades of the same color, mean, among other things, the existence of different Elements (Minerals) present on the Surface of Mercury, each having a different Albedo (---> Reflectivity) and Chemical Composition.MareKromium
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ZZ-Mercury-Craters-Pahinui_Crater-PIA16903-PCF-LXTT-IPF.jpgPahinui Crater (Absolute Natural Colors - Credits for the additional process. and color.: Dr Paolo C. Fienga/Lunar Explorer Italia/Italian Planetary Foundation)82 visiteOne of 9 (nine) newly named Mercurian Impact Craters, Pahinui, has intriguing Collapse-looking Pits surrounding its Central Peak. Pahinui Crater ha been so named after the Hawaiian musician Gabby Pahinui, a key figure in the 1970s Hawaiian Renaissance and master of the "slack-key guitar", a style that originated in Hawaii.
This image was acquired by the NASA - MESSENGER Spacecraft as part of the Mercury Dual Imaging System (MDIS)'s High-Resolution Surface Morphology Base Map. The Surface Morphology Base Map covers more than 99% of Mercury's Surface with an average resolution of 200 meters/pixel. Images acquired for the Surface Morphology Base Map typically are obtained at off-vertical Sun Angles (i.e.: high Solar Incidence Angles and therefore low Sun shining over the imaged Local Horizon) and have visible Shadows so as to reveal clearly the Topographic Form of many Geologic Features.
Date acquired: July, 31st, 2011
Image Mission Elapsed Time (MET): 220635824
Image ID: 575089
Instrument: Narrow Angle Camera (NAC) of the Mercury Dual Imaging System (MDIS)
Center Latitude: 28,30° South
Center Longitude: 146,90° East
Resolution: 139 meters/pixel
Solar Incidence Angle: 77,3° (meaning that the Sun, at the time that the picture was taken, was about 12,7° above the imaged Local Mercurian Horizon)
Emission Angle: 22,5°
Sun-Mercury-Messenger (or "Phase") Angle: 99,9°
This picture (which is an Original NASA - MESSENGER Spacecraft b/w frame published on the NASA - Planetary Photojournal with the ID n. PIA 16903) has been additionally processed and then colorized in Absolute Natural Colors (such as the colors that a human eye would actually perceive if someone were onboard the NASA - MESSENGER Spacecraft and then looked down, towards the Surface of Mercury), by using an original technique created - and, in time, dramatically improved - by the Lunar Explorer Italia Team. Different colors, as well as different shades of the same color, mean, among other things, the existence of different Elements (Minerals) present on the Surface of Mercury, each having a different Albedo (---> Reflectivity) and Chemical Composition.MareKromium
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ZZ-Mercury-Craters-Unnamed_Craters-CB-PIA17692-PCF-LXTT-IPF-1.jpgOn the Edge of Caloris Basin (EDM - Possible Absolute Natural Colors; credits for the additional process. and color. Dr Paolo C. Fienga/Lunar Explorer Italia/Italian Planetary Foundation)82 visiteThis picture, which is an Extra Detail Magnification (or "EDM" for short) of a crop taken from an image-mosaic made of 3 (three) frames obtained by the NASA - MESSENGER Spacecraft on October, 24, 2013, features, in particular, two Unnamed Impact Craters (being the larger one, visible at the top of the image, approximately 54 Km - such as about 33,53 miles - in diameter) that are located in the proximities of the Eastern Edge of the Giant Caloris Basin. These two large Unnamed Impact Craters have excavated (at the time of their formation, and according to NASA's Scientists) "Low Reflectance Material", and they both show extrenmly bright Hollows (which, in fact, caused an oversaturation of the picture in the areas where thay are present) within their relatively flat Floors.
The Orange/Reddish Deposits visible in the North/Eastern Quadrant (---> upper right corner) of the scene, exhibit a so-called "Spectral Signature" that is similar to the one usually left by Pyroclastic Deposits, and this circumstance suggests that this Region (just like way many others on Mercury) might have, once, been the Site of Explosive Volcanism.
Date acquired: October, 24th, 2013
Images Mission Elapsed Time (MET): 24470528, 24960452, 24470516
Images ID: 5033545, 5068359, 5033542
Instrument: Wide Angle Camera (WAC) of the Mercury Dual Imaging System (MDIS)
Center Latitude: 23,72° North
Center Longitude: 179,10° East
Solar Incidence Angle: 24,8° (meaning that the Sun, at the time that the picture was taken, was about 65,2° above the imaged Local Mercurian Horizon)
Emission Angle: 8,4°
Sun-Mercury-Messenger (or "Phase") Angle: 33,2°
This frame (which is an an Extra Detail Magnification (or "EDM" for short) of a crop taken from an Original NASA - MESSENGER Spacecraft's False Colors and Map-Projected image-mosaic published on the NASA - Planetary Photojournal with the ID n. PIA 17692) has been additionally processed, contrast enhanced, magnified to aid the visibility of the smaller details, Gamma corrected and then re-colorized in Absolute Natural Colors (such as the colors that a human eye would actually perceive if someone were onboard the NASA - MESSENGER Spacecraft and then looked down, towards the Surface of Mercury), by using an original technique created - and, in time, dramatically improved - by the Lunar Explorer Italia Team. Different colors, as well as different shades of the same color, mean, among other things, the existence of different Elements (Minerals) present on the Surface of Mercury, each having a different Albedo (---> Reflectivity) and Chemical Composition.MareKromium
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ZZ-Mercury-Craters-Eitoku_Crater-PIA16361-PCF-LXTT.jpgEitoku Crater (Absolute Natural Colors; credits for the additional process. and color.: Dr Paolo C. Fienga/Lunar Explorer Italia/Italian Planetary Foundation)81 visiteThe main and really impressive Impact Crater visible in this NASA - MESSENGER Spacecraft image, Eitoku (which has a diameter of approx. 100 Km - such as about 62 miles) , displays several (almost) Central Peaks that appear especially prominent due to the high Solar Incidence Angle (meaning that the Sun was setting onto the imaged Local Horizon at the time that this picture was taken - and therefore the shadows of each and every Mercurian relief resulted way more visible - i.e.: longer - and suggestive) as well as to the fact that this frame has not been Map Projected. Note that Central (and/or almost Central, like in this case) Peaks are a relatively common Feature in Mercury's larger Impact Craters. Note, also, that the smaller Impact Craters visible on the Floor and Rim (which, on its side, appears quite degraded) of Eitoku Crater indicate/suggest that we are actually looking at an old Impact Feature.
Date acquired: July, 3rd, 2012
Image Mission Elapsed Time (MET): 249790754
Image ID: 2129670
Instrument: Narrow Angle Camera (NAC) of the Mercury Dual Imaging System (MDIS)
Center Latitude: 21,77° South
Center Longitude: 202,6° East
Resolution: 120 meters/pixel
Solar Incidence Angle: 83,4° (meaning that the Sun, at the time that the picture was taken, was about 6,6° above the imaged Local Mercurian Horizon)
Emission Angle: 59,9°
Sun-Mercury-Messenger (or "Phase") Angle: 134,5°
This picture (which is an Original NASA - MESSENGER Spacecraft color frame published on the NASA - Planetary Photojournal with the ID n. PIA 16361) has been additionally processed and then colorized in Absolute Natural Colors (such as the colors that a human eye would actually perceive if someone were onboard the NASA - MESSENGER Spacecraft and then looked down, towards the Surface of Mercury), by using an original technique created - and, in time, dramatically improved - by the Lunar Explorer Italia Team. Different colors, as well as different shades of the same color, mean, among other things, the existence of different Elements (Minerals) present on the Surface of Mercury, each having a different Albedo (---> Reflectivity) and Chemical Composition.MareKromium
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ZZ-Mercury-Craters-Unnamed_Crater_with_Hollows-PIA16940-PCF-LXTT-IPF-0.jpgHollowland (Absolute Natural Colors; credits for the additional process. and color.: Dr Paolo C. Fienga/Lunar Explorer Italia/Italian Planetary Foundation)81 visiteThis image (taken by the NASA - Messenger Spacecraft in middle February 2013), shows us an area of ~ 23 Km - such as about 14,28 miles - across that is located within an Unnamed Mercurian Impact Basin that has been litterally "etched" by Hollows. Out of curiosity, this is the only area within the Basin in which Hollows are found. To the bottom left portion of the frame, some part of the Peak Ring of the Basin can also be seen and, most likely, this Peak Ring has been heavily modified by a number of subsequent impacts.
Date acquired: February, 13th, 2013
Image Mission Elapsed Time (MET): 3038823
Image ID: 3509823
Instrument: Narrow Angle Camera (NAC) of the Mercury Dual Imaging System (MDIS)
Center Latitude: 43,52° North
Center Longitude: 291,50° East
Resolution: 21 meters/pixel
Solar Incidence Angle: 74,2° (meaning that the Sun, at the time that the picture was taken, was about 15,8° above the imaged Local Mercurian Horizon)
Emission Angle: 10,4°
Sun-Mercury-Messenger (or "Phase") Angle: 63,8°
This picture (which is an Original NASA - MESSENGER Spacecraft b/w frame published on the NASA - Planetary Photojournal with the ID n. PIA 16940) has been additionally processed and then colorized in Absolute Natural Colors (such as the colors that a human eye would actually perceive if someone were onboard the NASA - MESSENGER Spacecraft and then looked down, towards the Surface of Mercury), by using an original technique created - and, in time, dramatically improved - by the Lunar Explorer Italia Team. Different colors, as well as different shades of the same color, mean, among other things, the existence of different Elements (Minerals) present on the Surface of Mercury, each having a different Albedo (---> Reflectivity) and Chemical Composition.MareKromium
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ZZ-Mercury-Craters-Unnamed_Crater-PIA18246-PCF-LXTT-IPF.jpgIntersecting Craters (Absolute Natural Colors - Credits for the additional process. and color.: Dr Paolo C. Fienga/Lunar Explorer Italia/Italian Planetary Foundation)81 visiteRunning diagonally though the scene (which is approx. 14 Km - such as about 8,69 miles - across), and running from the upper left (Sx) to the lower right (Dx) corner of the frame, you can see the Wall of an ancient Impact Ccrater that encompasses another Impact Crater known as Purcell. Purcell Crater and this other one (both quite large) are both pretty ancient, as evidenced by the numerous small Impact Craters that cover them, including their Walls. Furthermore, this High-Resolution Image taken by the NASA - MESSENGER Spacecraft on April, 11, 2014, also reveals how the small Impact Craters that formed on the Wall display some differences from the ones which formed on the Flat neighboring Surfaces. Frankly speaking, we, as IPF, honestly believe that the differences (mostly in shape) existing among these smaller Impact Craters that NASA is talknig about, may simply (and reasonalbly) be due to different Angles of Impact, rather than their age and/or the size of the Impactors that created them.
Date acquired: April, 11th, 2014
Image Mission Elapsed Time (MET): 39535979
Image ID: 6104625
Instrument: Narrow Angle Camera (NAC) of the Mercury Dual Imaging System (MDIS)
Center Latitude: 81,25° North
Center Longitude: 212,20° East
Solar Incidence Angle: 81,4° (meaning that the Sun, at the time that the picture was taken, was about 8,6° above the imaged Local Mercurian Horizon)
Emission Angle: 36,3°
Sun-Mercury-Messenger (or "Phase") Angle: 82,5
This picture (which is an Original NASA - MESSENGER Spacecraft's b/w and Map-Projected image published on the NASA - Planetary Photojournal with the ID n. PIA 18246) has been additionally processed, contrast enhanced, Gamma corrected, magnified to aid the visibility of the details and then colorized in Absolute Natural Colors (such as the colors that a human eye would actually perceive if someone were onboard the NASA - MESSENGER Spacecraft and then looked outside, towards the Surface of Mercury), by using an original technique created - and, in time, dramatically improved - by the Lunar Explorer Italia Team. Different colors, as well as different shades of the same color, mean, among other things, the existence of different Elements (Minerals) present on the Surface of Mercury, each having a different Albedo (---> Reflectivity) and Chemical Composition.MareKromium
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ZZ-Mercury-Limb-PIA16398-PCF-LXTT-IPF.jpgMercurian Horizon (Absolute Natural Colors; credits for the additional process. and color.: Dr Paolo C. Fienga/Lunar Explorer Italia/Italian Planetary Foundation)80 visiteIn this image, Mercury's Cratered Surface stretches to the Horizon. Limb images with the Wide Angle Camera (WAC) have been a part of MDIS imaging campaigns since the beginning of orbital operations. A few months ago, in June 2012, NAC images of Mercury's Limb were added to the regular weekly observations, resulting in a few images each week with dramatic views of Mercury's Horizon such as seen here.
This image was acquired as part of MDIS's limb imaging campaign. Once per week, MDIS captures images of Mercury's Limb, with an emphasis on imaging the Limb of the Southern Hemisphere. These Limb images provide information about Mercury's shape and complement measurements of topography made by the Mercury Laser Altimeter (MLA) of Mercury's Northern Hemisphere.
Date acquired: July 17, 2012
Image Mission Elapsed Time (MET): 250986760
Image ID: 2215464
Instrument: Narrow Angle Camera (NAC) of the Mercury Dual Imaging System (MDIS)
Center Latitude: 30,3° South
Center Longitude: 59,4° East
Scale: the bottom of the image is approximately 300 Km (about 190 miles) in lengthMareKromium
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ZZ-Mercury-Limb-PIA16299-PCF-LXTT-IPF-1.jpgOn the Limb... (Absolute Natural Colors; credits for the additional process. and color. Dr Paolo C. Fienga/Lunar Explorer Italia/Italian Planetary Foundation)80 visiteEven though quite a few Impact Craters are visible in this Absolute Natural Color view of Mercury's Limb, the Crater Degas (whose diameter is approx. 55 Km) gets easily noticed. Located near the center of the image, the distinctive gray color of the Low-Reflectance (---> Albedo) Material associated with Degas Crater (Material that covers most of the Floor, the Inner and Outer Rim, as well as a little bit of the Surface located in close proximity with the Crater itself) makes a clear contrast with the surrounding Terrain and neighboring Craters.
This image was acquired as a Targeted Observation that occurred simultaneously with a measurement carried out by the Mercury Atmospheric and Surface Composition Spectrometer (MASCS). Targeted Observations that involve both MDIS and MASCS, once the data from both instruments are combined, facilitate a better understanding of the color and reflectance of small-scale Geologic Features which are present on the Surface of Mercury.
Date acquired: October, 6th, 2012
Images Mission Elapsed Time (MET): 258052701, 258052721, 258052705
Images ID: 2717216, 2717221, 2717217
Instrument: Wide Angle Camera (WAC) of the Mercury Dual Imaging System (MDIS)
Center Latitude: 35,98° North
Center Longitude: 231,2° East
Center Resolution: 533 meters/pixel
This picture (which is an Original NASA - MESSENGER Spacecraft color frame published on the NASA - Planetary Photojournal with the ID n. PIA 16299) has been additionally processed and then re-colorized in Absolute Natural Colors (such as the colors that a human eye would actually perceive if someone were onboard the NASA - MESSENGER Spacecraft and then looked down, towards the Surface of Mercury), by using an original technique created - and, in time, dramatically improved - by the Lunar Explorer Italia Team. Different colors, as well as different shades of the same color, mean, among other things, the existence of different Elements (Minerals) present on the Surface of Mercury, each having a different Albedo (---> Reflectivity) and Chemical Composition.MareKromium
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ZZ-Mercury-Craters-Neruda_Crater-PIA17515-PCF-LXTT-IPF-1.jpgNeruda Crater (EDM - Absolute Natural Colors; credits for the additional process. and color.: Dr Paolo C. Fienga/Lunar Explorer Italia/Italian Planetary Foundation)80 visiteThis frame is an Extra Detail Magnification (or "EDM", for short) featuring the Central Peaks' System of the Mercurian Impact Crater named "Neruda", after the Chilean poet Pablo Neruda (born on July, 12, 1904 in Parral - Chile - and died on September, 23, 1973 in the Chilean State's Capital, Santiago). As you can see, this Impact Crater (whose diameter is approx. 112 Km - such as about 69,55 miles) exhibits several Central Peaks punctuated by a few more recent, small Craters, resulting in a rugged Surface mostly made of ups and downs (and now think how difficult it would be if one hypothetical Astronaut decided - or just needed - to traverse the whole Crater's highly irregular Floor...). Similarly, the Crater's namesake Pablo Neruda experienced a number of ups and downs in his life, from success as a poet, through poverty, war and ultimately alleged poisoning.
Date acquired: July, 24th, 2012
Image Mission Elapsed Time (MET): 251577944
Image ID: 2256593
Instrument: Narrow Angle Camera (NAC) of the Mercury Dual Imaging System (MDIS)
Center Latitude: 52,20° South
Center Longitude: 125,30° East
Solar Incidence Angle: 80,4° (meaning that the Sun, at the time that the picture was taken, was about 9,6° above the imaged Local Mercurian Horizon)
Emission Angle: 2,4°
Sun-Mercury-Messenger (or "Phase") Angle: 78,0°
This picture (which is a crop taken from an Original NASA - MESSENGER Spacecraft's b/w and Map-Projected image published on the NASA - Planetary Photojournal with the ID n. PIA 17515) has been additionally processed, contrast enhanced, magnified to aid the visibility of the details and then colorized in Absolute Natural Colors (such as the colors that a human eye would actually perceive if someone were onboard the NASA - MESSENGER Spacecraft and then looked down, towards the Surface of Mercury), by using an original technique created - and, in time, dramatically improved - by the Lunar Explorer Italia Team. Different colors, as well as different shades of the same color, mean, among other things, the existence of different Elements (Minerals) present on the Surface of Mercury, each having a different Albedo (---> Reflectivity) and Chemical Composition.MareKromium
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ZM-O-TheSwingby-GIF.gifThe Swing-by (or "Gravity Assist"): deceleration (GIF-Movie)79 visitePer ottenere l'Effetto Fionda, il veicolo spaziale deve effettuare un ravvicinato Fly-By (sorvolo) del pianeta.
Consideriamo, a titolo di esempio, una sonda diretta verso un Pianeta Gigante: per esempio, Giove. All'avvicinarsi della sonda a Giove, la gravità di quest'ultimo attrarrà la sonda a sè, così aumentandone la velocità.
Ma dopo aver passato Giove, la gravità del Pianeta continuerà ad attrarre il veicolo, rallentandolo.
L'effetto sulla Velocità, relativamente al pianeta, è nullo (come deve essere in virtù di quanto previsto dal Principio di Conservazione dell'Energia), ma la direzione del veicolo risulterà cambiata.
Tuttavia, nelle nostre riflessioni, dovremo tener conto che i pianeti non sono fermi nello Spazio, ma si muovono lungo le loro orbite (attorno al Sole). La velocità dell'astronave/sonda non è cambiata allorchè misurata in riferimento a Giove, MA se la misureremo in riferimento al Sole potremo notare che essa è differente.
In base alla traiettoria scelta, l'astronave potrà guadagnare fino a due volte la Velocità Orbitale del pianeta.
Nel caso di Giove, questa è di oltre 13 Km/s. In questo modo, la gravità di Giove avrà prestato al nostro veicolo una quantità di Momento Angolare supplementare necessaria affinché esso possa giungere sino a Saturno usando poco (o zero) combustibile in più rispetto a quello usato per raggiungere Giove stesso.
Un Trasferimento alla Hohmann per Saturno richiederebbe un "Delta V" totale di 15,7 Km/s, il quale è al di fuori delle capacità degli attuali razzi. Usare più di una Fionda Gravitazionale, poi, potrà anche comportare degli allungamenti del percorso ma diminuirà in modo considerevole il "Delta V", permettendoci di mandare in orbita veicoli spaziali più grandi (e, quindi, necessitanti di maggior propellente).
Questa strategia è stata utilizzata dalla sonda Cassini-Huygens, la quale è effettuato due Swing-By con Venere, uno con la Terra, ed infine uno con a Giove, nel momento finale (ed allorchè diretta verso Saturno).
Rispetto al Trasferimento alla Hohmann, questo sistema ha ridotto il "Delta V" a 2 Km/s, e così la grande e pesante sonda Cassini-Huygens ha potuto raggiungere Saturno usando propulsori (a razzo) relativamente piccoli.
Questo tipo di missioni richiede un'attenta sincronizzazione e per questo motivo l'individuazione della Finestra di Lancio OTTIMALE è una parte cruciale per la corretta riuscita della missione.
Durante l'avvicinamento della nave spaziale ad un pianeta, l'efficacia della propulsione del razzo aumenta e quindi piccole spinte effettuate in prossimità del pianeta che fornirà il Gravity-Assist, inevitabilmente produrranno dei grandi cambiamenti nella velocità finale della nostra astronave.
Un buon metodo per ottenere più energia da un Gravity-Assist è quello di utilizzare i motori una volta in prossimità dell'afelio.
La spinta di un razzo comporta sempre lo stesso cambiamento della velocità, ma il cambiamento dell'energia cinetica è proporzionale alla velocità del veicolo al momento dell'accensione del razzo.
Come sempre, per ottenere il massimo dell'energia dal razzo, si dovrà accenderlo quando il veicolo si trova alla sua velocità massima, e cioè all'afelio.
Per esempio, un trasferimento alla Hohmann dalla Terra a Giove porterà l'astronave ad un fly-by con Giove con una velocità all'afelio pari a 60 Km/s ed una velocità finale di 5,6 Km/s, e cioè 10,7 volte inferiore.
Non va dimenticato, tuttavia, che il principale limite pratico dell'uso di una Fionda Gravitazionale è dato dalla quantità di massa disponibile per ricevere la spinta. E non solo: un altro limite è quello determinato dall'atmosfera del pianeta che si intende usare: in teoria, infatti, più si sarà vicini al pianeta che deve fornire l'assist e maggiore sarà la spinta che si potrà ottenere.
Ma se una sonda viene "collocata", durante il fly-by, in posizione troppo prossima all'atmosfera del Corpo Celeste che deve fornire l'assist, l'energia persa a causa dell'attrito con la sua atmosfera (o meglio: i suoi strati superiori) potrebbe essere superiore a quella guadagnata dall'Effetto Fionda.
In tal caso, la nostra Sonda, post fly-by, rallenterà e questo effetto può essere utile se l'obiettivo, nel caso concreto, non è quello di accelerare, bensì di perdere energia (esattamente ciò che la Sonda Messenger sta facendo con Mercurio).
(Nota Lunexit: questa seconda Tecnica, ripresa anche nel romanzo di Arthur C. Clarke "2010, l'Anno del Contatto" è nota come "Aerofrenata" (o "Aerobraking"). Nel romanzo - di cui suggerisco caldamente la lettura -, l'Aerofrenata veniva impiegata per rallentare l'astronave Sovietica Leonov che, una volta arrivata nei pressi di Giove, doveva poi raggiungere, con velocità ottimale - e cioè non solo con una velocità largamente inferiore rispetto a quella mantenuta durante la "crociera", ma anche con uno scarso e/o nullo consumo di propellente - ovviamente preziosissimo nelle operazioni effettuate nello Spazio Profondo (ossìa lo spazio che si trova oltre la Fascia degli Asteroidi) - il piccolo satellite "Io" e quindi posizionarsi stabilmente nel Punto di Lagrange esistente fra Io e Giove, per effettuare un rendez-vous con l'altra astronave Discovery)
I gravity-assist che vorrebbero usare il Sole in sé, invece, sono (al momento) impossibili in quanto esso non è in grado di cedere Momento Angolare.
Tuttavia, l'idea di accendere i motori quando si è giunti in prossimità del Sole, se posta in essere, avrà comunque l'effetto di aumentare il guadagno effettivo di velocità ottenibile dai soli razzi.
C'è comunque da considerare, a tal proposito, il limite dovuto alla resistenza al calore della sonda e quindi, logica alla mano, non si potrà arrivare mai troppo vicini al Sole senza (rischiare di) subire danni irreparabili.
(da Wikipedia - Voce "Fionda Gravitazionale" - note, correzioni e commenti ulteriori: Dr Paolo C. Fienga)MareKromium
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ZZ-Mercury-Rupes-Terror_Rupes-PIA17881-PCF-LXTT-IPF-01.jpgTerror Rupes (EDM - Absolute Natural Colors; credits for the additional process. and color.: Dr Paolo C. Fienga/Lunar Explorer Italia/Italian Planetary Foundation)78 visiteIn this picture, we can better see the Southern Mercurian Relief known as "Terror Rupes": it is the long, Cliff-like Landform visible right in the middle of the frame. Terror Rupes is one of Mercury's most prominent Lobate Scarps, and it was so named after the HMS (---> Her Majesty's Ship) Terror: an eighteenth-century warship that, later, participated in Scientific Polar Explorations.
Date acquired: February, 4th, 2012
Image Mission Elapsed Time (MET): 236853865
Image ID: 1353252
Instrument: Wide Angle Camera (WAC) of the Mercury Dual Imaging System (MDIS)
Center Latitude: 69,30° South
Center Longitude: 96,00° East
Solar Incidence Angle: 80,3° (meaning that the Sun, at the time that the picture was taken, was about 9,7° above the imaged Local Mercurian Horizon)
Emission Angle: 50,9°
Sun-Mercury-Messenger (or "Phase") Angle: 114,5°
This picture (which is a crop taken from an Original NASA - MESSENGER Spacecraft's b/w and NON-Map-Projected image published on the NASA - Planetary Photojournal with the ID n. PIA 17881) has been additionally processed, contrast enhanced, Gamma corrected, magnified to aid the visibility of the details and then colorized in Absolute Natural Colors (such as the colors that a human eye would actually perceive if someone were onboard the NASA - MESSENGER Spacecraft and then looked outside, towards the Surface of Mercury), by using an original technique created - and, in time, dramatically improved - by the Lunar Explorer Italia Team. Different colors, as well as different shades of the same color, mean, among other things, the existence of different Elements (Minerals) present on the Surface of Mercury, each having a different Albedo (---> Reflectivity) and Chemical Composition.MareKromium
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