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Comets-Comet_Hyakutake-PIA01288_modest.jpgThe "Hyakutake" Comet52 visiteOriginal caption:"These are 2 images of the inner coma of Comet Hyakutake made on April 3 and 4, 1996, using the NASA HST-WFPC2. The 1st one, shown in red, was taken through a narrow-band red filter that shows only sunlight scattered by dust particles in the inner coma of the comet. The 2nd one, shown in blue, was taken with an ultraviolet "Woods" filter image that shows the distribution of scattered ultraviolet radiation from H atoms in the inner coma (the "coma" is the head - or dusty-gas atmosphere - of a comet). The square field of view is 14.000 Km on a side and the Sun is toward the upper right corner of the image. H atoms represent the most abundant gas in the whole coma of the comet. They are produced when solar ultraviolet light breaks up molecules of water, the major constituent of the nucleus of the comet. The inner yellow region near the center of the red dust image is dominated by the contribution from the dust which shows sunward directed spiral jets to the upper right".
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Comets-Comet_Hyakutake-PIA01290_modest.jpgThe "Hyakutake" Comet52 visiteFULL-FIELD VIEW (Left)--->This image is 3340 Km acroos and shows that most of the dust is being produced on the sunward-facing hemisphere of the comet. Also at upper left are 3 small pieces which have broken off the comet and are forming there own tails. Icy regions on the nucleus are activated as they rotate into sunlight, ejecting large amounts of dust in the jets that are faintly visible in this image. Sunlight striking this dust eventually turns it around and "blows" it into the tailward hemisphere. What might be another jet is emanating from the nightside of the nucleus, but this direction might be misleading due to the angle of the jet relative to our line-of-site.
CLOSE-UP OF NUCLEUS (Bottom Right) This expanded view of the near-nucleus region is only 760 Km across. The nucleus is near the center of the frame, but the brightest area is probably the tip of the strongest dust jet rather than the nucleus itself. Presumably, the nucleus surface lies just below this bright jet.
CLOSE-UP OF COMET FRAGMENTS (Top Right) This image shows pieces of the nucleus that apparently broke off and were first detected during ground-based observations on March 24. The Hubble image shows at least three separate objects that are probably made up of coarse-grained dust. Large fragments of the nucleus would not be accelerated into the tail, which appears to be the case in this image.
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Comets-Comet_Hyakutake.jpgComet Hyakutake54 visiteDa "NASA - Picture of the Day" del giorno 16 Dicembre 2009:"In 1996, an unexpectedly bright comet passed by planet Earth. Discovered less than two months before, Comet C/1996 B2 Hyakutake came within only 1/10th of the Earth-Sun distance from the Earth in late March. At that time, Comet Hyakutake, dubbed the Great Comet of 1996, became the brightest comet to grace the skies of Earth in 20 years. During its previous visit, Comet Hyakutake may well have been seen by the stone age Magdalenian culture, who 17.000 years ago were possibly among the first humans to live in tents as well as caves.
Pictured above near closest approach as it appeared on 1996 March 26, the long Ion and Dust Tails of Comet Hyakutake are visible flowing off to the left in front of a distant star field that includes both the Big and Little Dippers.
On the far left, the blue Ion Tail appears to have recently undergone a magnetic disconnection event. On the far right, the Comet's green-tinted Coma obscures a dense nucleus of melting dirty ice estimated to be about 5 Km across. A few months later, Comet Hyakutake began its long trek back to the outer Solar System.
Because of being gravitationally deflected by massive planets, Comet Hyakutake is not expected back for about 100.000 years...".MareKromium
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Comets-Comet_ISON-4-10-2013-HST.jpgComet ISON (Hubble Space Telescope's View)52 visiteCaption originale:"Comet C/ISON was imaged with the Hubble Space telescope on April 10, 2013, using the Wide Field Camera 3, when the Comet was 394 million miles from Earth. View larger. Image via NASA, ESA, J.-Y. Li (Planetary Science Institute), and the Hubble Comet ISON Imaging Science Team".
MareKromium
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Comets-Comet_ISON-HST-30Apr-V_I-L-800.jpgComet ISON (Hubble Space Telescope's View)141 visiteAdvanced amateur astronomers are already getting good images of Comet ISON, which currently shines at around +12th magnitude in the constellation Cancer. And although NASAs Deep Impact/EPOXI mission is down for the count, plans are afoot for the Curiosity rover and the Mars Reconnaissance Orbiter to attempt imaging the comet when it makes its closest approach to the Red Planet on October 1st at 0.0724 Astronomical Units (A.U.) or 10,830,000 kilometres distant. If MSL is successful, it would be the first time that a comet has been observed from the surface of another world.
Currently, ISON sits about a magnitude below the projected light curve, (see below) but that isnt all that unusual for a comet. Already, theres been increasing talk of ISON being a dud, but as Universe Todays Nancy Atkinson pointed out in a recent post, these assertions are still premature. The big question is what ISON will do leading up to perihelion, and if it will survive its passage 1.1 million kilometres above the surface of the Sun on November 28th to become a fine comet in the dawn skies in the weeks leading up to Christmas.
ISON is already starting to show a short, spikey tail in amateur images. Tsutomu Seki estimated it to be shining at about magnitude +11.1 on September 16th. Keep in mind, a caveat is in order when talking about the magnitudes of comets. Unlike stars, which are essentially a point source, the brightness of a comet is spread out over a large surface area. Thus, a comet may appear visually fainter than the quoted magnitude, much like a diffuse nebula. Although +6th magnitude is usually the limit for naked eye visibility, Ill bet that most folks wont pick up ISON with the unaided eye from typical suburban sites until it breaks +4th magnitude or so.
The forward scattering of light also plays a key role in the predicted brightness of a comet. The November issue of Astronomy Magazine has a great article on this phenomenon. Its interesting to note that ISON stacks up as a 9 on their accumulated point scale, right at the lower threshold of comet greatness, versus a 15 for sungrazing Comet C/1965 S1 Ikeya-Seki. Another famous 9 was Comet C/1996 B2 Hyakutake, which passed 0.1018 A.U. or 15.8 million kilometres from Earth on March 25, 1996.
ISON will pass 0.429 A.U. or 64.2 million kilometres from Earth the day after Christmas. Bruce Willis can stay home for this one.
Here is a blow-by-blow breakdown of some key dates to watch for as ISON makes its plunge into the inner solar system:
-September 25th: ISON crosses the border from the astronomical constellation of Cancer into Leo.
-September 27th: ISON passes 2 degrees north of the planet Mars.
-October 1st: The 12% illuminated waning crescent Moon passes 10 degrees south of Mars & ISON.
-Early October: ISON may break +10th magnitude and become visible with binoculars or a small telescope.
-October 4th: New Moon occurs. The Moon then exits the dawn sky, making for two weeks of prime viewing.
-October 10th: ISON enters view of NASAs STEREO/SECCHI HI-2A CAMERA:
-October 16th: ISON passes just 2 degrees NNE of the bright star Regulus, making a great guidepost to pin it down with binoculars.
-October 18th: The Full Moon occurs, after which the Moon enters the morning sky.
-October 26th: A great photo-op for astro-imagers occurs, as ISON passes within three degrees the Leo galaxy trio of M95, M96, & M105.
-October 30th: The 17% illuminated Moon passes 6 degrees south of ISON.
-Early November: Comet ISON may make its naked eye debut for observers based at dark sky sites.
-November 3rd: A hybrid (annular-total) solar eclipse occurs, spanning the Atlantic and Central Africa. It may just be possible for well placed observers to catch sight of ISON in the daytime during totality, depending on how quickly it brightens up. The Moon reaching New phase also means that the next two weeks will be prime view time for ISON at dawn.
-November 5th: ISON crosses the border from the astronomical constellation of Leo into Virgo.
-November 7th: ISON passes less than a degree from the +3.6 magnitude star Zavijava (Beta Virginis).
-November 8th: ISON passes through the equinoctial point in Virgo around 16:00 EDT/20:00 UT, passing into the southern celestial hemisphere and south of the ecliptic.
-November 14th: ISON passes less than a degree from the 10th magnitude galaxy NGC 4697.
-November 17th: The Moon reaches Full, passing into the morning sky.
-November 18th: ISON passes just 0.38 degrees north of the bright star Spica.
-November 22nd: ISON crosses into the astronomical constellation of Libra.
-November 23rd: ISON sits 4.7 degrees SSW of the planet Mercury and 4.9 SSW of Saturn, respectively.
-November 25th: ISON pays a visit to another famous comet, passing just 1.2 degrees south of short period comet 2P/Encke which may shine at +8th magnitude.
-November 27th: ISON enters the field of view of SOHOs LASCO C3 coronagraph.
-November 28th: ISON reaches perihelion at ~18:00 PM EST/ 23:00 UT.
After that, all bets are off. The days leading up to perihelion will be tense ones, as ISON then rounds the Sun on a date with astronomical destiny. Will it join the ranks of the great comets of the past? Will it stay intact, or shatter in a spectacular fashion? Watch this space for ISON updates well be back in late November with our post-perihelion guide!
Be sure to also enjoy recently discovered Comet C/2013 R1 Lovejoy later the year.
(source: http://www.universetoday.com/104818/comet-ison-a-viewing-guide-from-now-to-perihelion/#ixzz2fpLTKJwD)
Read more: http://www.universetoday.com/104818/comet-ison-a-viewing-guide-from-now-to-perihelion/#ixzz2fpLG75usMareKromium
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Comets-Comet_ISON-Nucleus.jpgComet ISON (Tian Lian 1 - Satellite's View - Controversial: possible fake)79 visiteEd ecco il Nucleo di ISON: come vedete, assomiglia TANTISSIMO a quello di Tempel-1 e Wild-2. Mia opinione? Immagine genuina. Con tutto quanto ne segue e consegue... Il punto che, a quanto leggo (ma NON sul Sito dell'Agenzia Spaziale Cinese, sul quale non posso capirci nulla), il Satellite Geostazionario Tian-Lian 1 NON ha fotocamere! E se questo vero (ma non so se vero), allora evidente che siamo davanti all'ennesima bufala...MareKromium
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Comets-Comet_ISON-Nucleus_Crop_Chumack.jpgComet ISON89 visite"Comet ISON is probably at least twice as big as Comet Lovejoy and will pass a bit farther from the suns surface" notes Knight. This would seem to favor Comet ISON surviving and ultimately putting on a good show.
One of the most exciting possibilities would be a partial break-up. "If Comet ISON splits, it might appear as a 'string of pearls' when viewed through a telescope," speculates Battams. It might even resemble the famous Comet Shoemaker-Levy 9 that hit Jupiter in 1994.
A break-up would pose no threat to Earth, assures Yeomans. "Comet ISON is not on a collision course. If it breaks up, the fragments would continue along the same safe trajectory as the original comet."
Whatever happens, northern sky watchers will get a good view. For months after it swings by the sun, Comet ISON will be well placed for observers in the northern hemisphere. It will pass almost directly over the North Pole, making it a circumpolar object visible all night long.
Will Comet ISON fizzle ... or sizzle? Stay tuned to Science@NASA for updates.MareKromium
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Comets-Comet_Ikeya_Zhang_2.jpgComet Ikeya-Zhang (from Colorado)59 visiteThis lovely early evening view of the comet in Rocky Mountain skies looks northwest over ridges and low clouds. The time exposure was recorded on March 31st from an 8.000 foot elevation near Yampa, Colorado, USA. Sporting a sweeping yellowish dust tail and blue ion tail eight to ten degrees long, Ikeya-Zhang is nestled near the horizon in the northern constellation of Andromeda. To the comet's left is the bright star Mirach or Beta Andromedae while the stretched celestial fuzzball to the comet's right is M 31 or the Andromeda galaxy, the nearest bright spiral galaxy to our own Milky Way. As the days pass, Comet Ikeya-Zhang's apparent motion through the sky is towards the right in this image. Tonight, comet-watchers blessed with clear skies should find Ikeya-Zhang posing perfectly for binoculars and cameras just above M 31, less than two degrees from the center of the bright galaxy.
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Comets-Comet_Ikeya_Zhang_3.jpgComet Ikeya-Zhang - the "ion tail"65 visiteThis composite color image from March 3rd, 2002, captured with a wide-field telescope, shows this active comet's bright, condensed coma and a delightful array of subtle structures in its developing tail. The bluish tail stretches for 5 degrees or so against a background of stars in the constellation Pisces. Cataloged as comet C/2002 C1, improved orbit determinations now make it seem very likely that Comet Ikeya-Zhang has been around here before. Refined calculations indicate this comet's last trip through the inner Solar System was 344 years ago, in 1661, when it was recorded as a bright comet.
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Comets-Comet_Ikeya_Zhang_4.jpgComet Ikeya-Zhang - the "ion tail"63 visiteAs Comet Ikeya-Zhang approached the Sun, it developed a complex blue "ion tail". The "ion tail" is composed of ions that boiled off the nucleus and were pushed away from the Sun by the out-flowing fast-moving particles of the solar wind. Complexity in the tail is created by comet nucleus rotation, variability in the comet surface evaporation rate and variability of the Sun's magnetic field and solar wind.
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Comets-Comet_Lulin-00.jpgComet Lulin is approaching...54 visiteDa "NASA - Picture of the Day", del giorno 2 Febbraio 2009:"How bright will Comet Lulin become? No one knows for sure. Although it is notoriously difficult to accurately predict the brightness of newly discovered comets, Comet Lulin could well become visible to the unaided eye later this month (such as February 2009).
As Comet Lulin moves into the Northern Sky in mid February to rise around midnight, it should at least be spotted by comet watchers with binoculars and a good sky chart. Tracking observations indicate that the comet officially designated C/2007 N3 (Lulin) has now swung by the Sun and is approaching Earth on a trajectory that will bring it within half the Earth-Sun distance in late February.
Comet Lulin's orbit indicates that this is likely the comet's first trip into the Inner Solar System. The comet was discovered by Quanzhi Ye of Sun Yat-sen University, on images obtained by Chi-Sheng Lin at the Lu-Lin Observatory of National Central University.
In this picture, taken from Italy last Friday, are Comet Lulin's coma and tails, one tail pointing away from the Sun, and an anti-tail - dust that trails the comet in its orbit and may appear to point toward the Sun".MareKromium
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Comets-Comet_Lulin-01.jpgStill Comet Lulin52 visiteDa "NASA - Picture of the Day" del giorno 7 Febbraio 2009:"Sweeping through the Inner Solar System, Comet Lulin is easily visible in both Northern and Southern Hemispheres with binoculars or a small telescope. Recent changes in Lulin's lovely greenish Coma and Tails are featured in this two panel comparison of images taken on January 31st (top) and February 4th, 2009.
Taken from dark New Mexico Skies, the images span over 2. In both views the comet sports an apparent Anti-Tail at the left - the comet's Dust Tail appearing almost edge on from an earth-based perspective as it trails behind in Lulin's orbit.
Extending to the right of the coma, away from the Sun, is the beautiful Ion Tail. Remarkably, as captured in the bottom panel, Comet Lulin's Ion Tail became disconnected on February 4, likely buffeted and torn away by magnetic fields in the Solar Wind.
In 2007 NASA satellites recorded a similar disconnection event for Comet Encke. Don't worry, though: comet tails can grow back".MareKromium
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