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Comet Swift–Tuttle (formally designated 109P/Swift–Tuttle) is a periodic comet with an orbital period of 133 years. It fits the classical definition of a Halley-type comet with a period between 20 and 200 years.〔 It was independently discovered by Lewis Swift on July 16, 1862 and by Horace Parnell Tuttle on July 19, 1862. It has a well determined orbit and has a comet nucleus 26 km in diameter. Chinese records indicate that, in 188, the comet reached apparent magnitude 0.1.〔 In the discovery year of 1862, the comet was as bright as Polaris.〔(David Levy's Guide to Observing Meteor Showers ) (pg12)〕 The comet made a return appearance in 1992, when it was rediscovered by Japanese astronomer Tsuruhiko Kiuchi and became visible with binoculars. In 2126 it will be a bright naked-eye comet reaching about apparent magnitude 0.7. It is the parent body of the Perseid meteor shower, perhaps the best known shower and among the most reliable in performance.〔Bedient, John. ("AMS Meteor Showers page" ), ''American Meteor Society'', 20 June 2010. Retrieved 2010-7-31.〕 An unusual aspect of its orbit is that it is captured into a 1:11 orbital resonance with Jupiter; it completes one orbit for every 11 of Jupiter.〔 ==Potential threat to Earth== The comet is on an orbit that makes repeated close approaches to the Earth-Moon system.〔 Upon its 1992 rediscovery, the comet's date of perihelion passage was off from the then-current prediction by 17 days. It was then noticed that, if its next perihelion passage (July 11, 2126) was also off by another 15 days (occurred on July 26), the comet would pass perilously close to Earth or the Moon on August 14, 2126 (IAUC 5636: 1992t). Given the size of the nucleus of Swift–Tuttle, this was of some concern. This prompted amateur astronomer and writer Gary W. Kronk to search for previous apparitions of this comet. He found the comet was most likely observed by the Chinese in 69 BC and AD 188, which was quickly confirmed by Brian G. Marsden. This information and subsequent observations have led to recalculation of its orbit, which indicates the comet's orbit is very stable, and that there is absolutely no threat over the next two thousand years. It is now known that the comet will pass from Earth on August 5, 2126.〔 A close encounter with Earth is predicted for the comet's return to the inner Solar System in the year 4479, around Sept. 15; the closest approach is estimated to be 0.03–0.05 AU, with a probability of impact of 1, or 0.0001%. Subsequent to 4479, the orbital evolution of the comet is more difficult to predict; the probability of Earth impact per orbit is estimated as 2 (0.000002%).〔 It is the largest Solar System object that makes repeated close approaches to Earth with a relative velocity of 60 km/s.〔 An Earth impact would have an estimated energy of ≈27 times that of the Cretaceous–Paleogene impactor.〔This calculation can be carried out in the manner given by Weissman for Comet Hale–Bopp, as follows: A radius of 13.5 km and an estimated density of 0.6 g/cm3 gives a cometary mass of 6.2 g. An encounter velocity of 60 km/s yields an impact velocity of 61 km/s, giving an impact energy of 1.15 ergs, or 2.75 megatons, about 27.5 times the estimated energy of the K–T impact event.〕 Comet Swift–Tuttle has been described as "the single most dangerous object known to humanity". 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Comet Swift–Tuttle」の詳細全文を読む スポンサード リンク
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