Comet Swift–Tuttle is a large periodic comet with a 1995 orbital period of 133 years that is in a 1:11 orbital resonance with Jupiter. 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. Observation was also recorded in 69 BC, and it was probably visible to the naked eye in 322 BC. In the discovery year of 1862, the comet was as bright as Polaris. 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. After the 1862 observations it was thought that the comet would return between 1979 and 1983, but it didn't show up. However, it had been suggested in 1902 that this was the same comet as that observed by Ignatius Kegler on July 3, 1737, and on this basis Brian Marsden calculated that it would return only in 1992, which in fact it did. It is the parent body of the Perseid meteor shower, perhaps the best known shower and among the most reliable in performance. The comet's perihelion is just under that of Earth, while its aphelion is just over that of Pluto. An unusual aspect of its orbit is that it was recently captured into a 1:11 orbital resonance with Jupiter; it completes one orbit for every 11 of Jupiter. It was the first comet in a retrograde orbit to be found in a resonance. In principle this would mean that its proper long-term average period would be 130.48 years, as it librates about the resonance. Over the short term, between epochs 1737–2126 the orbital period varies between 128 and 136 years. However, it only entered this resonance about 1000 years ago, and will probably exit the resonance in several thousand years.
Threat to Earth
The comet is on an orbit that makes repeated close approaches to the Earth-Moon system, and has an Earth-MOID of. Upon its September 1992 rediscovery, the comet's date of perihelion passage was off from the 1973 prediction by 17 days. It was then noticed that if its next perihelion passage was also off by another 15 days, the comet could impact the Earth on August 14, 2126. 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 Marsden and added to the list of perihelion passages at the Minor Planet Center. This information and subsequent observations have led to recalculation of its orbit, which indicates the comet's orbit is sufficiently stable 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. and within from Earth on August 24, 2261. A close encounter with Earth is predicted for the comet's return to the inner Solar System in the year 3044, with the closest approach estimated to be. Another close encounter is predicted for the year 4479, around Sept. 15; the close approach is estimated to be less than 0.05 AU, with a probability of impact of 1 in a million. 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. Comet Swift–Tuttle is by far the largest near-Earth object to cross Earth's orbit and make 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. The comet has been described as "the single most dangerous object known to humanity". In 1996, the long-term possibility of Comet Swift–Tuttle impacting Earth was compared to 433 Eros, and they were compared to about 3000 other kilometer sized objects as objects of concern.
