Comet Swift–Tuttle (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 and Horace Parnell Tuttle in July 1862. It has a well-known orbit and has a comet nucleus 26 km in diameter.
Chinese records show that in 188 AD the comet reached apparent magnitude 0.1. In 1862, the comet was as bright as Polaris. The comet made a return appearance in 1992, and became visible with binoculars. In 2126 it will be a bright naked-eye comet reaching about apparent magnitude 0.7.
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 1×10−6, or 0.0001%.
Comet Swift–Tuttle has been described as "the single most dangerous object known to humanity". However, the probability of Earth impact per orbit is about a million to one against. It is estimated as 2×10−8 (0.000002%). It is the largest Solar System object that makes repeated close approaches to Earth with a relative velocity of 60 km/s.
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