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.[1] 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.[1]
Chinese records show that in 188 AD the comet reached apparent magnitude 0.1.[2] In 1862, the comet was as bright as Polaris.[3] The comet made a return appearance in 1992, and became visible with binoculars.[4] In 2126 it will be a bright naked-eye comet reaching about apparent magnitude 0.7.[2]
It is the parent body of the Perseid meteor shower, perhaps the best known shower and among the most reliable in performance.[5]
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%.[6]
Comet Swift–Tuttle has been described as "the single most dangerous object known to humanity".[7] However, the probability of Earth impact per orbit is about a million to one against. It is estimated as 2×10−8 (0.000002%).[6] It is the largest Solar System object that makes repeated close approaches to Earth with a relative velocity of 60 km/s.[7][8]
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.[6]
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