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Ison is Here


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Earth Distance: 215,121,721 miles (2.314236 AU) Constellation: Leo Visual Magnitude: 11.38 There are now just 65 days until Comet ISON hurtles around our Sun to emerge, hopefully, as one of the brightest comets in recent history Comet C/2012 S1 (ISON) is on its way to skirt around the Sun in November 2013. Will Comet ISON achieve Great Comet status? I summarize what we know about this promising comet and what it might become So, why are astronomers so excited about sungrazing comets? Or, even any comet? Comets are frozen left-overs from the formation of our Solar System. While comets have been in a deep freeze for the past 4 billion years, planets and asteroids have changed a lot from their original compositions. Better understanding of their ices, dust, and organic matter, and how they have changed over the past billions of years, tell us about the origins of our Sun, the planets, and, possibly, life on Earth. To astronomers, every bright comet is an opportunity to learn more about our Solar System. The orbit of Comet ISON has an interesting similarity with another bright sungrazing comet, the Great Comet of 1680. The two comets don't have the same orbits, but the similarity between these two bright sungrazers is remarkable. One thing, however, is for sure: they are not the same comet. Comet 1680 is currently 250 AU from the Sun, and isn't due back for 9000 years. But the similarity suggests they have a common origin. Perhaps they are two fragments of a much larger comet Comet ISON's absolute magnitude, that is, its intrinsic brightness, seems to be between those of two spectacular comets: Comet C/1965 S1 (Ikeya-Seki) and Comet C/2006 P1 (McNaught). Both of these comets made very close passes by the Sun, and were possibly the two brightest since the 1930s. Specifically, their absolute magnitudes are 3.9 (Ikeya-Seki), 6.0 (ISON), and 9.5 (McNaught). The magnitude system is ordered such that fainter objects have higher values. It is also on a logarithmic scale, where a difference of 2.5 magnitudes corresponds to a factor of 10 in apparent brightness; a 5 magnitude difference is a factor of 100. Given the same distance from the Sun and distance from the Earth, Comet ISON would be roughly 25 times brighter than Comet McNaught, and one-seventh as bright as Comet Ikeya-Seki. Thus, Comet ISON is in good company, and we may expect a show similar to these two comets, which makes for a very exciting prediction Earth and Mars Close Approaches Comet ISON makes two interesting close approaches. The first, in October 2013, we will watch the comet pass by Mars at the small distance of only 0.07 AU. This distance is small on the grand scale of the Solar System, but it is still 10 million kilometers. The close encounter may make comet ISON observable to NASA and ESA's spacecraft orbiting Mars. Maybe we can even hope to see a picture of ISON from NASA's Curiosity rover? Although there is no possibility of such a close approach between the Earth and comet, the second close approach I want to bring up will be between the comet's orbit and the Earth. In January 2014, the Earth will swing past a part of space that Comet ISON already traveled through, at a small distance of only 0.03 AU. This encounter brings up the possibility of a meteor shower on Earth. Meteor showers from Oort Cloud comets are rare events indeed. Stay tuned while astronomers consider this possibility Will Comet ISON survive perihelion? Most sungrazing comets do not survive perihelion; they are disrupted by the Sun's intense radiation and gravity. Yet most sungrazing comets are quite small, only tens of meters across [4]. Comet ISON, being so bright, is more like Comet Ikeya-Seki in size, which did survive its encounter with the Sun. Their larger sizes insulate their interiors from the Sun's energy. If Comet ISON does survive, it will zoom back out to the outer-solar system, and on it's way give people in the Northern Hemisphere one last opportunity to see it, only this time at night. Anything can happen Comet C/2011 W3 (Lovejoy) Comet C/2011 W3 (Lovejoy), post-disintegration. (credit: Dan Burbank/NASA) Comets are notoriously unpredictable, and anything can happen. Well, maybe not anything, but they can certainly be surprising. Some possibilities include: •The comet could fizzle. Comets are made of ices and dust, and the relative proportions of each material, as well as the structure of the nucleus, determine its activity levels. Currently



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