55P/Tempel–Tuttle

55P/Tempel–Tuttle (commonly known as Comet Tempel–Tuttle) is a retrograde periodic comet with an orbital period of 33 years. It fits the classical definition of a Halley-type comet with a period of between 20 and 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; 8.9 LD) from Earth.

Comet Tempel-Tuttle was recovered on March 4, 1997 by Karen Meech, Olivier Hainaut and James "Gerbs" Bauer, at the University of Hawai`i. At the time it was very faint (22.5 mag), but the recovery proved that it was returning on schedule and that its orbit was very well determined.

The retrograde orbit of 55P/Tempel–Tuttle causes meteors to impact Earth at a high speed of 70 km/s. The orbit intersects that of Earth nearly exactly, hence streams of material ejected from the comet during perihelion passages do not have to spread out much 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 past 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, the 1833 meteor storm was created by the previous 1800 perihelion passage. Between 2021–2030, Earth will often pass through the meteoroid stream left behind from the 1733 orbit.

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.