|Discovered by||Lewis Swift
Horace Parnell Tuttle
|Discovery date||July 16, 1862|
|1737 N1; 1737 II; 1862 O1;
1862 III; 1992 S2; 1992 XXVIII
|Orbital characteristics A|
|Epoch||October 10, 1995
|Semi-major axis||26.092 AU|
|Orbital period||133.28 yr|
|Last perihelion||December 11, 1992|
|Next perihelion||July 12, 2126|
Comet Swift–Tuttle (formally designated 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. It was independently discovered by Lewis Swift on July 16, 1862 and by Horace Parnell Tuttle on July 19, 1862. It has a well determined orbit and has a comet nucleus 26 km in diameter.
The comet made a return appearance in 1992, when it was rediscovered by Japanese astronomer Tsuruhiko Kiuchi and became visible with binoculars. Its solid nucleus is about 26 kilometers (16 mi) across, considerably larger than the 10 km object hypothesized to have wiped out the dinosaurs in the Cretaceous–Paleogene extinction event.
Potential threat to Earth 
The comet is on an orbit which puts it close to the Earth and the Moon. Upon its 1992 rediscovery, the comet's date of perihelion passage was off from the then-current prediction by 17 days. It was then noticed that, if its next perihelion passage (August 14, 2126) was also off by another 15 days, the comet would very likely strike the Earth or Moon. 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 in 69 BC and AD 188, which was quickly confirmed by Brian G. Marsden. This information and subsequent observations have led to recalculation of its orbit, which indicates the comet's orbit is very stable, and that there is absolutely no threat over the next two thousand years. Astronomers believe that in the 2126 pass it will likely be a great naked-eye comet like Hale–Bopp.
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. 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. As the largest Solar System object that makes repeated close passes of Earth, and which does so at a relative velocity of 60 km/s, leading to an estimated impact energy of ≈27 times that of the Cretaceous–Paleogene impactor, Comet Swift–Tuttle has been described as "the single most dangerous object known to humanity".
- "JPL Small-Body Database Browser: 109P/Swift–Tuttle". Jet Propulsion Laboratory. 1995-03-29 last obs. Retrieved 2011-05-09.
- Syuichi Nakano (1999-11-18). "109P/Swift–Tuttle (NK 798)". OAA Computing and Minor Planet Sections. Retrieved 2011-05-09.
- Britt, Robert (2005-08-11). "Top 10 Perseid Meteor Shower Facts". Space.com. Retrieved 2009-08-10.
- Weissman, Paul R. (2007). The cometary impactor flux at the Earth. In Milani, A.; Valsecchi, G.B.; Vokrouhlicky, D. "Near Earth Objects, our Celestial Neighbors: Opportunity and Risk; Proceedings IAU Symposium No. 236, 2006". Proceedings of the International Astronomical Union 2 (Cambridge University Press). pp. 441–450. doi:10.1017/S1743921307003559. Archived from the original on 2009-08-15. Retrieved 2009-08-13
- Bedient, John. "AMS Meteor Showers page", American Meteor Society, 20 June 2010. Retrieved 2010-7-31.
- Chambers, J. E. (1995). "The long-term dynamical evolution of Comet Swift–Tuttle". Icarus (Academic Press) 114 (2): 372–386. Bibcode:1995Icar..114..372C. doi:10.1006/icar.1995.1069.
- Chandler, David L (2005-06-01). "Comet put on list of potential Earth impactors". New Scientist. Retrieved 2008-08-25.
- Kronk, Gary; Marsden, Brian G. (1992). "Periodic Comet Swift–Tuttle (1992t)". Central Bureau for Astronomical Telegrams. Retrieved 2009-06-19.
- Stephens, Sally (1993). "on Swift–Tuttle's possible collision". Astronomical Society of the Pacific. Retrieved 2008-08-25.
- Verschuur, Gerrit L. (1997). Impact!: the threat of comets and asteroids. Oxford University Press. pp. 256 (see p. 116). ISBN 978-0-19-511919-0.
- 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 6.2×1018 g. An encounter velocity of 60 km/s yields an impact velocity of 61 km/s, giving an impact energy of 1.15×1032 ergs, or 2.75×109 megatons, about 27.5 times the estimated energy of the K–T impact event.
- 109P/Swift-Tuttle at the Minor Planet Center's Database
- 109P/Swift–Tuttle at NASA's Jet Propulsion Laboratory's Small-Body Database
- Periodic Comet Swift–Tuttle from NASA's Astronomy Picture of the Day
|Periodic comets (by number)|