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109P/Swift–Tuttle
	Perseid meteor, originating from Comet Swift–Tuttle, from the ISS
Discovery
Discovered by	Lewis Swift; Horace Parnell Tuttle
Discovery date	July 16, 1862
Designations
Alternative designations	1737 N1; 1737 II; 1862 O1;; 1862 III; 1992 S2; 1992 XXVIII
Orbital characteristics
Epoch	October 10, 1995; (JD 2450000.5)
Observation arc	257 years
Aphelion	51.225 AU
Perihelion	0.9595 AU
Semi-major axis	26.092 AU
Eccentricity	0.9632
Orbital period	133.28 yr; 133y 7m (perihelion to perihelion)
Max. orbital speed	42.6 km/s (26.5 mi/s)
Min. orbital speed	0.8 km/s (0.50 mi/s) (2059-Dec-12)
Inclination	113.45°
Last perihelion	December 12, 1992
Next perihelion	July 12, 2126
Earth MOID	0.0009 AU (130,000 km; 84,000 mi)
Physical characteristics
Dimensions	26 km (16 mi)

Perihelion date	Orbital period (years)
Orbital period at different passages^[1]
2392-09-16	132.7
2261-08-10	134.6
2126-07-12	136.2
1992-12-12	135.0
1862-08-23	131.7
1737-06-15	127.8
1610-02-06	130.5
1479-10-18	133.4
1348-05-02	135.0

Comet Swift–Tuttle (formally designated 109P/Swift–Tuttle) is a large periodic comet with a 1995 (osculating) 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, which has an orbital period between 20 and 200 years.^[4] The comet was independently discovered by Lewis Swift on July 16, 1862 and by Horace Parnell Tuttle on July 19, 1862.

Its nucleus is 26 km (16 mi) in diameter.^[4] Swift–Tuttle is the parent body of the Perseid meteor shower, perhaps the best known shower and among the most reliable in performance.^[5]

The comet made a return appearance in 1992, when it was rediscovered by Japanese astronomer Tsuruhiko Kiuchi and became visible with binoculars.^[6] It was last observed in April 1995 when it was 8.6 AU (1.3 billion km) from the Sun.^[2] In 2126, it will be a bright naked-eye comet reaching an apparent magnitude of about 0.7.^[7]

Historic observations

CE/BCE	Date
Older passages^[1]
188 CE	188-07-10
69 BCE	–68-08-27
322 BCE	–321-09-27

Chinese records indicate that, in 188, the comet reached apparent magnitude 0.1.^[7] Observation was also recorded in 69 BCE, and it was probably visible to the naked eye in 322 BCE.^[8]

In the discovery year of 1862, the comet was as bright as Polaris.^[9]

After the 1862 observations, it was incorrectly theorized that the comet would return between 1979 and 1983. However, it had been suggested in 1902 that this was the same comet as that observed by Ignatius Kegler on July 3, 1737,^[10] and on this basis Brian Marsden calculated correctly that it would return in 1992.^[8]

Orbit

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.^[8] It was the first comet in a retrograde orbit to be found in a resonance.^[8] 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 and 2126 the orbital period varies between 128 and 136 years.^[2] However, it only entered this resonance about 1000 years ago, and will probably exit the resonance in several thousand years.^[8]

Threat to Earth

The comet is on an orbit that makes repeated close approaches to the Earth–Moon system,^[8] and has an Earth-MOID (Minimum orbit intersection distance) of 0.0009 AU (130,000 km; 84,000 mi).^[4] Upon its September 1992 rediscovery, the comet's date of perihelion passage was off from the 1973 prediction by 17 days.^[11] It was then noticed that if its next perihelion passage (July 2126) was also off by another 15 days (July 26), the comet could impact the Earth or the Moon on August 14, 2126 (IAUC 5636: 1992t).^[12]

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 at least twice, first in 69 BCE and later in 188 CE;^[13] these two sightings were quickly confirmed by Brian Marsden and added to the list of perihelion passages at the Minor Planet Center.^[2] Around 25 July 188 CE the comet passed about 0.129 AU (19.3 million km) from Earth.^[14]

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.^[11] It is now known that the comet will pass 0.153 AU (22.9 million km; 14.2 million mi) from Earth on August 5, 2126.^[4]^[b] and within 0.147 AU (22.0 million km; 13.7 million mi) from Earth on August 24, 2261.^[14]

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 one million miles (1,600,000 km; 0.011 AU).^[15] 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.^[8] Subsequent to 4479, the orbital evolution of the comet is more difficult to predict; the probability of Earth impact per orbit is estimated as 2×10⁻⁸ (0.000002%).^[8]

Comet Swift–Tuttle is by far the largest near-Earth object (Apollo or Aten asteroid or short-period comet) to cross Earth's orbit and make repeated close approaches to Earth.^[16] With a relative velocity of 60 km/s,^[17]^[18] an Earth impact would have an estimated energy of ~27 times that of the Cretaceous–Paleogene impactor.^[19] The comet has been described as "the single most dangerous object known to humanity".^[18] In 1996, the long-term possibility of Comet Swift–Tuttle impacting Earth was compared to 433 Eros and about 3000 other kilometer-sized objects of concern.^[20]

Comet Swift–Tuttle Closest Earth Approach on 2126-Aug-05 15:50 UT^[4]
Date & time of closest approach	Earth distance (AU)	Sun distance (AU)	Velocity wrt Earth (km/s)	Velocity wrt Sun (km/s)	Uncertainty region (3-sigma)	Reference
2126-08-05 15:50	0.15337 AU (22.944 million km; 14.257 million mi; 59.69 LD)	1.04 AU (156 million km; 97 million mi; 400 LD)	58.3	40.8	± 11000 km	Horizons

Notes

^ v = 42.1219 √1/r − 0.5/a, where r is the distance from the Sun, and a is the semimajor axis.
^ The 3-sigma uncertainty in the comet's closest approach to Earth on 5 August 2126 is about ±10 thousand km.