Comet Encke or Encke's Comet (official designation: 2P/Encke) is a periodic comet that completes an orbit of the Sun once every 3.3 years. (This is the shortest period of a reasonably bright comet; the faint main-belt comet 311P/PANSTARRS has a period of 3.2 years.) Encke was first recorded by Pierre Méchain in 1786, but it was not recognized as a periodic comet until 1819 when its orbit was computed by Johann Franz Encke; like Halley's Comet, it is unusual in being named after the calculator of its orbit rather than its discoverer. Like most comets, it has a very low albedo, reflecting only 4.6% of the light it receives. The diameter of the nucleus of Encke's Comet is 4.8 km.
Johann Franz Encke (recognition of periodicity)
|Discovery date||17 January 1786|
1786 I; 1795; 1805;|
1819 I; 1822 II; 1825 III;
1829; 1832 I; 1835 II;
1838; 1842 I; 1845 IV
|Orbital characteristics A|
|Epoch||September 22, 2006 (JD 2454000.5)|
|Semi-major axis||2.2178 AU|
|Orbital period||3.30 a|
|Last perihelion||10 March 2017|
|Next perihelion||~25-27 June 2020|
As its official designation implies, Encke's Comet was the first periodic comet discovered after Halley's Comet (designated 1P/Halley). It was independently observed by several astronomers, the second being Caroline Herschel in 1795  and the third Jean-Louis Pons in 1818. Its orbit was calculated by Johann Franz Encke, who through laborious calculations was able to link observations of comets in 1786 (designated 2P/1786 B1), 1795 (2P/1795 V1), 1805 (2P/1805 U1) and 1818 (2P/1818 W1) to the same object. In 1819 he published his conclusions in the journal Correspondance astronomique, and predicted correctly its return in 1822 (2P/1822 L1). It was recovered by Carl Ludwig Christian Rümker at Parramatta Observatory on 2 June 1822.
Comets are in unstable orbits that evolve over time due to perturbations and outgassing. Given Encke's low orbital inclination near the ecliptic and brief orbital period of 3 years, the orbit of Encke is frequently perturbed by the inner planets.
Encke's orbit gets as close as 0.17309 AU (25,894,000 km; 16,090,000 mi) to Earth (minimum orbit intersection distance). On 4 July 1997, Encke passed 0.19 AU from Earth, and on June 29, 2172 it will make a close approach of roughly 0.1735 AU. On 18 November 2013, it passed 0.02496 AU (3,734,000 km; 2,320,000 mi) from Mercury. Close approaches to Earth usually occur every 33 years.
On April 20, 2007, STEREO-A observed the tail of Comet Encke to be temporarily torn off by magnetic field disturbances caused by a coronal mass ejection (a blast of solar particles from the Sun). The tail grew back due to the continuous shedding of dust and gas by the comet.
Comet Encke is believed to be the originator of several related meteor showers known as the Taurids (which are encountered as the Northern and Southern Taurids across November, and the Beta Taurids in late June and early July). A shower has similarly been reported affecting Mercury.
Measurements on board the NASA satellite MESSENGER have revealed Encke may contribute to seasonal meteor showers on Mercury. The Mercury Atmospheric and Surface Composition Spectrometer (MASCS) instrument discovered seasonal surges of calcium since the probe began orbiting the planet in March 2011. The spikes in calcium levels are thought to originate from small dust particles hitting the planet and knocking calcium-bearing molecules into the atmosphere in a process called impact vaporization. However, the general background of interplanetary dust in the inner Solar System cannot, by itself, account for the periodic spikes in calcium. This suggests a periodic source of additional dust, for example, a cometary debris field.
Effects on EarthEdit
More than one theory has associated Encke's Comet with impacts of cometary material on Earth, and with cultural significance.
A theory holds that the ancient symbol of the swastika appeared in a variety of cultures across the world at a similar time, and could have been inspired by the appearance of a comet from head on, as the curved jets would be reminiscent of the swastika shape (see Comets and the swastika motif). Comet Encke has sometimes been identified as the comet in question. In their 1982 book Cosmic Serpent (page 155) Victor Clube and Bill Napier reproduce an ancient Chinese catalogue of cometary shapes from the Mawangdui Silk Texts, which includes a swastika-shaped comet, and suggest that some comet drawings were related to the breakup of the progenitor of Encke and the Taurid meteoroid stream. Fred Whipple in his The Mystery of Comets (1985, page 163) points out that Comet Encke's polar axis is only 5 degrees from its orbital plane: such an orientation is ideal to have presented a pinwheel like aspect to our ancestors when Encke was more active.
Importance in the scientific history of luminiferous aetherEdit
Comet Encke (and Biela's Comet) had a role in scientific history in the generally discredited concept of luminiferous aether. As its orbit was perturbed and shortened, the shortening could only be ascribed to the drag of an "ether" through which it orbited in outer space. One reference reads:
- Encke's comet is found to lose about two days in each successive period of 1200 days. Biela's comet, with twice that length of period, loses about one day. That is, the successive returns of these bodies is found to be accelerated by this amount. No other cause for this irregularity has been found but the agency of the supposed ether.
Encke's pole tumbles in an 81-year period, therefore it will accelerate for half that time, and decelerate for the other half of the time, (since the orientation of the comets rotation to solar heating determines how its orbit changes due to outgassing forward or aft of the comets course). The authors of this 1860 textbook of course could not know that the pole of the comet would tumble as it does over such a long period of time, or that outgassing would induce a thrust to change its course.
Comet Encke as rendered in Celestia using data acquired from Earth-based observations.
- "JPL Small-Body Database Browser: 2P/Encke" (2017-03-04 last obs). Retrieved 2017-03-13.
- Ley, Willy (September 1968). "Mission to a Comet". For Your Information. Galaxy Science Fiction. pp. 101–110.
- 2P/Encke past, present and future orbits by Kazuo Kinoshita
- Herschel, Caroline Lucretia (1876). Herschel, Mrs. John, ed. Memoir and Correspondence of Caroline Herschel. London: John Murray, Albemarle Street.
- Biographical Encyclopedia of Astronomers. p. 924.
- Kronk, Gary. "2P/Encke". Gary W. Kronk's Cometography. Retrieved 5 July 2014.
- "JPL Close-Approach Data: 2P/Encke". 2013-08-18. Retrieved 2013-10-06.
- "The Sun Rips Off a Comet's Tail". Science@NASA. 2007-10-01. Retrieved 2009-10-20.
- Nemiroff, R.; Bonnell, J., eds. (3 October 2007). "Comet Encke's Tail Ripped Off". Astronomy Picture of the Day. NASA.
- Whipple, 1940; Klačka, 1999).
- Rosemary M. Killen; Joseph M. Hahn (December 10, 2014). "Impact Vaporization as a Possible Source of Mercury's Calcium Exosphere". Icarus. Bibcode:2015Icar..250..230K. doi:10.1016/j.icarus.2014.11.035.
- Williams, V.; Kornoš, L.; Williams, I.P. (2006). "The Taurid complex meteor showers and asteroids". Contributions of the Astronomical Observatory Skalnaté Pleso. 36: 103–117. arXiv:0905.1639. Bibcode:2006CoSka..36..103P.
- M. Killen & Joseph M. Hahn (17 December 2014). "Mercury Experiences Seasonal Meteor Showers, Say NASA Scientists". Web Article. Sci-News.com. Retrieved 29 December 2014.
- Kresak, L'. (1978). "The Tunguska object - A fragment of Comet Encke". Astronomical Institutes of Czechoslovakia. Astronomical Institutes of Czechoslovakia. 29: 129. Bibcode:1978BAICz..29..129K.
- "Archived copy". Archived from the original on 2013-12-05. Retrieved 2014-03-28.
- Klačka, Jozef (1999). "Meteor Streams of Comet Encke. Taurid Meteor Complex". Abstract
- Whipple, F.L. (1940). "Photographic meteor studies. III. The Taurid shower." Proc. Amer. Phil. Soc., 83, 711-745.
- Master, S. and Woldai, T. (2004) The UMM Al Binni structure in the Mesopotamian marshlands of Southern Iraq, as a postulated late holocene meteorite impact crater : geological setting and new LANDSAT ETM + and Aster satellite imagery. Johannesburg, University of Witwatersrand, Economic Geology Research Institute (EGRI), 2004. EGRI - HALL : information circular 382, p. 21
http://www.itc.nl/library/Papers_2004/tech_rep/woldai_umm.pdf (1.56 MB) ×Professor Nayr,Hahs(2002)Geological Researcher at University of Oxford 
- Master, S. and Woldai, T. (2004) Umm al Binni structure, southern Iraq, as a postulated late holocene meteorite impact crater : new satellite imagery and proposals for future research. Presented at the ICSU workshop : comet - asteroid impacts and human society, Santa Cruz de Tenerife, Canary Islands, Spain, November 27- December 2, 2004. p. 20
- Hamacher, D. W. (2005) "The Umm Al Binni Structure and Bronze Age Catastrophes", The Artifact: Publications of the El Paso Archaeological Society, Vol. 43
- Hamacher, D. W. (2006) "Umm al Binni lake: Effects of a possible Holocene bolide impact", Astronomical Society of Australia Meeting 40, #15
|Wikimedia Commons has media related to Comet Encke.|
This article's use of external links may not follow Wikipedia's policies or guidelines. (May 2015) (Learn how and when to remove this template message)
- Orbital simulation from JPL (Java) / Ephemeris
- 2P/Encke at the Minor Planet Center's Database
- Gary W. Kronk's Cometography page for 2P
- 2P/Encke at CometBase database
- 2P/Encke light curve by Seiichi Yoshida
- Comets and the Bronze Age Collapse
- Solar Storm Tears Tail From Comet Encke
- NASA Astronomy Picture of the Day: Animation of Encke's tail being torn away (3 October 2007)
- HI-1A movie showing Comet Encke (April 2007)
- Importance in history of luminiferous aether, search for Encke's comet
- Study of the April 20, 2007 CME-Comet Interaction Event with an [sic] MHD Model (arxiv 0903.4942)