55P/Tempel–Tuttle (commonly known as Comet Tempel–Tuttle) is a periodic comet with an orbital period of 33 years. It fits the classical definition of a Halley-type comet with (20 years < period < 200 years). It was independently discovered by Wilhelm Tempel on December 19, 1865 and by Horace Parnell Tuttle on January 6, 1866.
It is the parent body of the Leonid meteor shower. In 1699, it was observed by Gottfried Kirch but was not recognized as a periodic comet until the discoveries by Tempel and Tuttle during the 1866 perihelion. In 1933, S. Kanda deduced that the comet of 1366 was Tempel–Tuttle, which was confirmed by Joachim Schubart in 1965. On 26 October 1366, the comet passed 0.0229 AU (3,430,000 km; 2,130,000 mi) from Earth.
The orbit of 55P/Tempel–Tuttle intersects that of Earth nearly exactly, hence streams of material ejected from the comet during perihelion passes do not have to spread out over time to encounter Earth. The comet currently has an Earth-MOID of 0.008 AU (1,200,000 km; 740,000 mi). This coincidence means that streams from the comet at perihelion are still dense when they encounter Earth, resulting in the 33-year cycle of Leonid meteor storms. For example, in November 2009, the Earth passed through meteors left behind mainly from the 1466 and 1533 orbit.
In February, 2016, two bolides detected by the NASA All-Sky Fireball Network were calculated to have orbits consistent with those of 55P, although with a node 100 degrees less than 55P. The reason for this is yet to be determined.
55P/Tempel–Tuttle is estimated to have a nucleus of mass 1.2×1013 kg and radius 1.8 km and a stream of mass 5×1012 kg.