Comet Swift–Tuttle#Historic observations

Chinese records indicate that, in 188, the comet reached apparent magnitude 0.1.{{r|Yau_1994}} Observation was also recorded in 69 BCE, and it was probably visible to the naked eye in 322 BCE.{{r|Chambers_1995}}

In the discovery year of 1862, the comet was as bright as Polaris.{{cite book|last=Levy|first=David H.|title=David Levy's Guide to Observing Meteor Showers|url=https://books.google.com/books?id=jAr7HMnlPGUC&pg=PA12|year=2008|publisher=Cambridge University Press|isbn=978-0-521-69691-3|page=12}}

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 3 July 1737{{r|Kronk_1999}} and on this basis Brian Marsden calculated correctly that it would return in 1992.{{r|Chambers_1995}}

{{stack|File:Animation of 109P／Swift–Tuttle orbit.gif}}

{{stack|File:Animation of Comet Swift–Tuttle in 2126.gif}}

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.{{r|Chambers_1995}} It was the first comet in a retrograde orbit to be found in a resonance.{{r|Chambers_1995}} 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.{{r|MPC}} However, it only entered this resonance about 1000 years ago, and will probably exit the resonance in several thousand years.{{r|Chambers_1995}}

The comet is on an orbit that makes repeated close approaches to the Earth–Moon system,{{r|Chambers_1995}} and has an Earth-MOID (Minimum orbit intersection distance) of {{convert|0.0009|AU|km mi|abbr=on|lk=on}}.<

{{r|jpldata}} Upon its September 1992 rediscovery, the comet's date of perihelion passage was off from the 1973 prediction by 17 days. 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 14 August 2126 (IAUC 5636: 1992t).

File:Comet_Swift-Tuttle_distance_vs_year.svg

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; these two sightings were quickly confirmed by Brian Marsden and added to the list of perihelion passages at the Minor Planet Center.{{r|MPC}} Around 25 July 188 CE the comet passed about {{convert|0.129|AU|e6km|abbr=unit}} from Earth.{{r|Yau_1994}}

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. It is now known that the comet will pass {{convert|0.153|AU|e6km e6mi|abbr=unit}} from Earth on August 5, 2126.{{r|jpldata}}{{efn|name=uncertainty}} and within {{convert|0.147|AU|e6km e6mi|abbr=unit}} from Earth on August 24, 2261.{{r|Yau_1994}}

File:Swift-Tuttle Comet Particles.jpg, originating from Comet Swift–Tuttle, from the ISS]]

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 {{convert|0.011|AU|e6mi e6km|abbr=unit}}.{{cite web| url = https://astrosociety.org/edu/publications/tnl/23/23.html| url-status = dead| archive-url = https://web.archive.org/web/20121219103151/http://astrosociety.org/edu/publications/tnl/23/23.html| archive-date = 2012-12-19| title = ASP: Cosmic Collisions}} 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.{{r|Chambers_1995}} 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{{e|−8}} (0.000002%).{{r|Chambers_1995}}

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.{{r|jpldata}} With a relative velocity of 60 km/s,{{r|Weissman_2006|Verschuur_1997}} an Earth impact would have an estimated energy of ~27 times that of the Cretaceous–Paleogene impactor.{{efn|name=K-T_impact}} The comet has been described as "the single most dangerous object known to humanity".{{r|Verschuur_1997}} 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.{{r|NYT-19960425}}

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{{Notelist|refs=

{{efn|name=speed|1={{nowrap|1=v = 42.1219 {{radic|1/r − 0.5/a}}}}, where r is the distance from the Sun, and a is the semimajor axis.}}

{{efn|name=uncertainty|1=The 3-sigma uncertainty in the comet's closest approach to Earth on 5 August 2126 is about [https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%27DES%3D109P%3BCAP%27&START_TIME=%272126-08-05%2015:50%27&STOP_TIME=%272126-08-06%27&STEP_SIZE=%271%20day%27&QUANTITIES=%2720,39%27 ±10 thousand km.]}}

{{efn|name="K-T_impact"|1=This calculation can be carried out in the manner given by Weissman for Comet Hale–Bopp, as follows: A radius of 13.5 km and an estimated density of 0.6 g/cm³ gives a cometary mass of 6.2{{e|18}} g. An encounter velocity of 60 km/s yields an impact velocity of 61 km/s, giving an impact energy of 1.15{{e|32}} ergs, or 2.75{{e|9}} megatons, about 27.5 times the estimated energy of the K–T impact event.}}

}}

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• {{cite web |url=https://www.forbes.com/sites/startswithabang/2016/08/11/the-most-dangerous-object-known-to-humanity |title=The Most Dangerous Object Known To Humanity |first=Ethan |last=Siegel |website=Forbes |date=2016-08-11 }}

• [http://www.minorplanetcenter.net/db_search/show_object?object_id=109P 109P/Swift-Tuttle] at the Minor Planet Center's Database
• {{JPL Small Body|id=1000140}}
• {{APOD |date=19 February 1996 |title=Periodic Comet Swift–Tuttle}}

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