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comet
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{{About|the astronomical objects}}{{pp-pc1}}{{short description|Icy small Solar System body}}{{good article}}{{multiple image |perrow=2 |total_width=350
|image1=Deep Impact HRI.jpeg |alt1=Comet Tempel collides with Deep Impact's impactor |image2=Comet 67P on 19 September 2014 NavCam mosaic.jpg |alt2=Comet 67P/Churyumov–Gerasimenko orbited by Rosetta
|image3=17pHolmes 071104 eder vga.jpg |alt3=Comet 17P/Holmes and its blue ionized tail |image4=Comet Wild2.jpg |alt4=Comet Wild 2 visited by Stardust probe
|image5=Comet-Hale-Bopp-29-03-1997 hires adj.jpg |alt5=Hale–Bopp seen from Croatia in 1997 |image6=Iss030e015472 Edit.jpg |alt6=Comet Lovejoy seen from orbit
|footer=Comets – nucleus, coma and tail:
}}A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of solar radiation and the solar wind acting upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice, dust, and small rocky particles. The coma may be up to 15 times the Earth's diameter, while the tail may stretch one astronomical unit. If sufficiently bright, a comet may be seen from the Earth without the aid of a telescope and may subtend an arc of 30° (60 Moons) across the sky. Comets have been observed and recorded since ancient times by many cultures.Comets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star.BOOK, Dark Matter and the Dinosaurs: The Astounding Interconnectedness of the Universe, Ecco/HarperCollins Publishers, New York, Lisa, Randall, 104–105, 2015, 978-0-06-232847-2, Long-period comets are set in motion towards the Sun from the Oort cloud by gravitational perturbations caused by passing stars and the galactic tide. Hyperbolic comets may pass once through the inner Solar System before being flung to interstellar space. The appearance of a comet is called an apparition.Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma (the central part immediately surrounding the nucleus) and the tail (a typically linear section consisting of dust or gas blown out from the coma by the Sun's light pressure or outstreaming solar wind plasma). However, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids.WEB, What is the difference between asteroids and comets,weblink Rosetta's Frequently Asked Questions, European Space Agency, 30 July 2013, Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System.WEB, What Are Asteroids And Comets,weblink Near Earth Object Program FAQ, NASA, 30 July 2013, JOURNAL, 10.1126/science.1150683, Comparison of Comet 81P/Wild 2 Dust with Interplanetary Dust from Comets, 2008, Ishii, H. A., Bradley, J. P., Dai, Z. R., Chi, M., Kearsley, A. T., Burchell, M. J., Browning, N. D., Molster, F., Science, 319, 5862, 447–50, 18218892, 2008Sci...319..447I, The discovery of main-belt comets and active centaur minor planets has blurred the distinction between asteroids and comets. In the early 21st century, the discovery of some minor bodies with long-period comet orbits, but characteristics of inner solar system asteroids, were called Manx comets. They are still classified as comets, such as C/2014 S3 (PANSTARRS).WEB,weblink JPL Small-Body Database Browser C/2014 S3 (PANSTARRS), 27 Manx comets were found from 2013 to 2017.JOURNAL, 2017DPS....4942002S, Chasing Manxes: Long-Period Comets Without Tails, AAA/Division for Planetary Sciences Meeting Abstracts #49, 420.02, Stephens, Haynes, Meech, Karen Jean, Kleyna, Jan, Keane, Jacqueline, Hainaut, Olivier, Yang, Bin, Wainscoat, Richard J., Micheli, Marco, Bhatt, Bhuwan, Sahu, Devendra, 2017, {{As of|2018|07}} there are 6,339 known comets,WEB, Johnston, Robert, Known populations of solar system objects,weblink 2 August 2014, 19 January 2015, a number that is steadily increasing as they are discovered. However, this represents only a tiny fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System (in the Oort cloud) is estimated to be one trillion.BOOK,weblink Asteroids, Comets, and Meteorites: Cosmic Invaders of the Earth, The Living Earth, Infobase, New York, Jon, Erickson, 123, 2003, 978-0-8160-4873-1, BOOK,weblink The Planets: The Definitive Guide to Our Solar System, Dorling Kindersley, London, Heather, Couper, Robert, Dinwiddie, John, Farndon, Nigel, Henbest, David W., Hughes, Giles, Sparrow, Carole, Stott, Colin, Stuart, 222, 2014, 978-1-4654-3573-6, Roughly one comet per year is visible to the naked eye, though many of those are faint and unspectacular.JOURNAL, 10.1006/icar.1998.6048, The Rate of Naked-Eye Comets from 101 BC to 1970 AD, 1999, Licht, A, Icarus, 137, 2, 355–356, 1999Icar..137..355L, Particularly bright examples are called "great comets". Comets have been visited by unmanned probes such as the European Space Agency's Rosetta, which became the first ever to land a robotic spacecraft on a comet,WEB,weblink Touchdown! Rosetta's Philae Probe Lands on Comet, European Space Agency, 12 November 2014, 11 December 2017, and NASA's Deep Impact, which blasted a crater on Comet Tempel 1 to study its interior.{{TOC limit|3}}

Etymology

File:Anglo-Saxon Chronicle - cometa (British Library Cotton MS Tiberius A VI, folio 10v).jpg|thumb|A comet was mentioned in the Anglo-Saxon ChronicleAnglo-Saxon ChronicleThe word comet derives from the Old English cometa from the Latin comēta or comētēs. That, in turn, is a latinisation of the Greek κομήτης ("wearing long hair"), and the Oxford English Dictionary notes that the term (ἀστὴρ) κομήτης already meant "long-haired star, comet" in Greek. Κομήτης was derived from κομᾶν ("to wear the hair long"), which was itself derived from κόμη ("the hair of the head") and was used to mean "the tail of a comet".{{OED|comet}}DICTIONARY,weblink Comet (n.), Online Etymology Dictionary, Harper, Douglas, 30 July 2013, The astronomical symbol for comets is (File:U+2604.svg|17px) (in Unicode ☄ U+2604), consisting of a small disc with three hairlike extensions.BOOK,weblink The Encyclopedia Americana: A Library of Universal Knowledge, 26, The Encyclopedia Americana Corp., 162–163, 1920,

Physical characteristics

Nucleus

File:Comet Hartley 2.jpg|thumb|Nucleus of 103P/Hartley103P/HartleyThe solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen carbon dioxide, carbon monoxide, methane, and ammonia.JOURNAL, 1998A&A...330..375G, Making a comet nucleus, Greenberg, J. Mayo, 330, 1998, 375, Astronomy and Astrophysics, As such, they are popularly described as "dirty snowballs" after Fred Whipple's model.WEB,weblink Dirty Snowballs in Space, Starryskies, 15 August 2013, dead,weblink" title="web.archive.org/web/20130129035627weblink">weblink 29 January 2013, However, many comets have a higher dust content, leading them to be called "icy dirtballs".NEWS,weblink Evidence from ESA's Rosetta Spacecraft Suggests that Comets are more "Icy Dirtball" than "Dirty Snowball", 21 October 2005, Times Higher Education, Research conducted in 2014 suggests that comets are like "deep fried ice cream", in that their surfaces are formed of dense crystalline ice mixed with organic compounds, while the interior ice is colder and less dense.The surface of the nucleus is generally dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick. In addition to the gases already mentioned, the nuclei contain a variety of organic compounds, which may include methanol, hydrogen cyanide, formaldehyde, ethanol, and ethane and perhaps more complex molecules such as long-chain hydrocarbons and amino acids.WEB, Meech, M., 1997 Apparition of Comet Hale–Bopp: What We Can Learn from Bright Comets,weblink Planetary Science Research Discoveries, 24 March 1997, 30 April 2013, WEB, Stardust Findings Suggest Comets More Complex Than Thought,weblink NASA, 14 December 2006, 31 July 2013, In 2009, it was confirmed that the amino acid glycine had been found in the comet dust recovered by NASA's Stardust mission.JOURNAL, 10.1111/j.1945-5100.2009.tb01224.x, Cometary glycine detected in samples returned by Stardust, 2009, Elsila, Jamie E., Glavin, Daniel P., Dworkin, Jason P., Meteoritics & Planetary Science, 44, 9, 1323, 2009M&PS...44.1323E, In August 2011, a report, based on NASA studies of meteorites found on Earth, was published suggesting DNA and RNA components (adenine, guanine, and related organic molecules) may have been formed on asteroids and comets.JOURNAL, 10.1073/pnas.1106493108, Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases, 2011, Callahan, M. P., Smith, K. E., Cleaves, H. J., Ruzicka, J., Stern, J. C., Glavin, D. P., House, C. H., Dworkin, J. P., Proceedings of the National Academy of Sciences, 108, 34, 13995–8, 2011PNAS..10813995C, 21836052, 3161613, WEB, Steigerwald, John, NASA Researchers: DNA Building Blocks Can Be Made in Space,weblink NASA, 8 August 2011, 31 July 2013, File:Comet borrelly.jpg|thumb|left|Comet Borrelly exhibits jets, but has no surface ice.]]The outer surfaces of cometary nuclei have a very low albedo, making them among the least reflective objects found in the Solar System. The Giotto space probe found that the nucleus of Halley's Comet reflects about four percent of the light that falls on it,JOURNAL, 10.1126/science.275.5308.1900, The Activity and Size of the Nucleus of Comet Hale-Bopp (C/1995 O1), 1997, Weaver, H. A., Science, 275, 5308, 1900–4, 9072959, Feldman, PD, a'Hearn, MF, Arpigny, C, Brandt, JC, Festou, MC, Haken, M, McPhate, JB, Stern, SA, Tozzi, GP, 1997Sci...275.1900W, and Deep Space 1 discovered that Comet Borrelly's surface reflects less than 3.0%; by comparison, asphalt reflects seven percent. The dark surface material of the nucleus may consist of complex organic compounds. Solar heating drives off lighter volatile compounds, leaving behind larger organic compounds that tend to be very dark, like tar or crude oil. The low reflectivity of cometary surfaces causes them to absorb the heat that drives their outgassing processes.BOOK,weblink 91, Habitability and Cosmic Catastrophes, 978-3-540-76945-3, Hanslmeier, Arnold, 2008, Comet nuclei with radii of up to {{convert|30|km}} have been observed,JOURNAL, 10.1023/A:1021545031431, The Nucleus of Comet Hale-Bopp (C/1995 O1): Size and Activity, 2000, Fernández, Yanga R., Earth, Moon, and Planets, 89, 3–25, 2002EM&P...89....3F, but ascertaining their exact size is difficult.WEB,weblink The Cometary Nucleus, Department of Earth and Space Sciences, UCLA, April 2003, 31 July 2013, The nucleus of 322P/SOHO is probably only {{convert|100|-|200|m}} in diameter.WEB, SOHO's new catch: its first officially periodic comet, European Space Agency,weblink 16 August 2013, A lack of smaller comets being detected despite the increased sensitivity of instruments has led some to suggest that there is a real lack of comets smaller than {{convert|100|m}} across.{{harvnb|Sagan|Druyan|1997|p=137}} Known comets have been estimated to have an average density of {{convert|0.6|g/cm3|oz/cuin|abbr=on}}.JOURNAL, 2006LPI....37.2214B, Small Body Density and Porosity: New Data, New Insights, Britt, D. T., Consolmagno, G. J., Merline, W. J., 37, 2006, 2214, 37th Annual Lunar and Planetary Science Conference,weblink Because of their low mass, comet nuclei do not become spherical under their own gravity and therefore have irregular shapes.WEB,weblink The Geology of Small Bodies, NASA, 15 August 2013, File:Comet wild 2.jpg|thumb|220px|Comet 81P/Wild81P/WildRoughly six percent of the near-Earth asteroids are thought to be extinct nuclei of comets that no longer experience outgassing,JOURNAL, 10.1016/j.icarus.2006.02.016, astro-ph/0603106v2, 2006, The size–frequency distribution of dormant Jupiter family comets, Whitman, K, Morbidelli, A, Jedicke, R, Icarus, 183, 1, 101–114, 2006Icar..183..101W, including 14827 Hypnos and 3552 Don Quixote.Results from the Rosetta and Philae spacecraft show that the nucleus of 67P/Churyumov–Gerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of planetesimals.NEWS,weblink Rosetta and Philae Find Comet Not Magnetised, European Space Agency, Markus, Bauer, 14 April 2015, 14 April 2015, JOURNAL, Rosetta's comet has no magnetic field, Nature (journal), Nature, Quirin, Schiermeier, 14 April 2015, 10.1038/nature.2015.17327, Further, the ALICE spectrograph on Rosetta determined that electrons (within {{convert|1|km|mi|abbr=on}} above the comet nucleus) produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma.WEB, Agle, DC, Brown, Dwayne, Fohn, Joe, Bauer, Markus, NASA Instrument on Rosetta Makes Comet Atmosphere Discovery,weblink 2 June 2015, NASA, 2 June 2015, JOURNAL, Feldman, Paul D., A'Hearn, Michael F., Bertaux, Jean-Loup, Feaga, Lori M., Parker, Joel Wm., Schindhelm, Eric, Steiffl, Andrew J., Stern, S. Alan, Weaver, Harold A., Sierks, Holger, Vincent, Jean-Baptiste, Measurements of the near-nucleus coma of comet 67P/Churyumov-Gerasimenko with the Alice far-ultraviolet spectrograph on Rosetta,weblink 2 June 2015, Astronomy and Astrophysics, 10.1051/0004-6361/201525925, 3 June 2015, 1506.01203, 2015A&A...583A...8F, 583, A8, Instruments on the Philae lander found at least sixteen organic compounds at the comet's surface, four of which (acetamide, acetone, methyl isocyanate and propionaldehyde) have been detected for the first time on a comet.NEWS,weblink Philae probe finds evidence that comets can be cosmic labs, The Washington Post, Associated Press, Frank, Jordans, 30 July 2015, 30 July 2015, WEB,weblink Science on the Surface of a Comet, European Space Agency, 30 July 2015, 30 July 2015, JOURNAL, Bibring, J.-P., Taylor, M.G.G.T., Alexander, C., Auster, U., Biele, J., Finzi, A. Ercoli, Goesmann, F., Klingehoefer, G., Kofman, W., Mottola, S., Seidenstiker, K.J., Spohn, T., Wright, I., Philae's First Days on the Comet – Introduction to Special Issue, 31 July 2015, Science (journal), Science, 349, 6247, 493, 10.1126/science.aac5116, 2015Sci...349..493B, 26228139, {| class="wikitable center" style="text-align: center; width: 530px; margin: 0.1em auto;"|+Properties of some comets! width=120 | Name! width=120 | Dimensions(km)! width=120 | Density(g/cm3)! width=120 | Mass(kg)Halley: Using the volume of an ellipsoid of 15×8×8 km * a rubble pile density of 0.6 g/cm3 yields a mass (m=d*v) of 3.02E+14 kg.Tempel 1: Using a spherical diameter of 6.25 km; volume of a sphere * a density of 0.62 g/cm3 yields a mass of 7.9E+13 kg.19P/Borrelly: Using the volume of an ellipsoid of 8x4x4km * a density of 0.3 g/cm3 yields a mass of 2.0E+13 kg.81P/Wild: Using the volume of an ellipsoid of 5.5x4.0x3.3 km * a density of 0.6 g/cm3 yields a mass of 2.28E+13 kg.!Refs
Halley's Comet| 15 × 8 × 8| 0.614}}DATE=1986 ACCESSDATE=4 OCTOBER 2013, 1988NATUR.331..240S >TITLE=IS THE NUCLEUS OF COMET HALLEY A LOW DENSITY BODY? FIRST1=R. Z. FIRST2=P. E. FIRST3=V. I. DATE=1988 JOURNAL=NATURE DOI=10.1038/331240A0, 6153,
Tempel 1| 7.6 × 4.9| 0.6213}}PUBLISHER=JPL, 16 August 2013,
19P/Borrelly| 8 × 4 × 4| 0.313}}|
81P/Wild| 5.5 × 4.0 × 3.3| 0.613}}URL=HTTP://WWW.PLANETARY.ORG/EXPLORE/TOPICS/ASTEROIDS_AND_COMETS/WILD2.HTML URL-STATUS=DEAD ARCHIVEDATE=6 JANUARY 2009,
67P/Churyumov–Gerasimenko| 4.1 × 3.3 × 1.8| 0.4713}}PUBLISHER=EUROPEAN SPACE AGENCY ACCESSDATE=24 JANUARY 2015, HTTP://BLOGS.ESA.INT/ROSETTA/2014/08/21/DETERMINING-THE-MASS-OF-COMET-67PC-G/ >TITLE=DETERMINING THE MASS OF COMET 67P/C-G FIRST=EMILY DATE=21 AUGUST 2014, 21 August 2014,

Coma

File:Hubble's Last Look at Comet ISON Before Perihelion.jpg|thumb|Hubble image of Comet ISON shortly before perihelionperihelionThe streams of dust and gas thus released form a huge and extremely thin atmosphere around the comet called the "coma". The force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous "tail" to form pointing away from the Sun.BOOK,weblink 66, A Complete Manual of Amateur Astronomy: Tools and Techniques for Astronomical Observations, 978-0-486-15216-5, Clay Sherrod, P. Clay, Koed, Thomas L., 2003, The coma is generally made of {{H2O}} and dust, with water making up to 90% of the volatiles that outflow from the nucleus when the comet is within {{convert|3|to|4|AU}} of the Sun.JOURNAL, 2004come.book..523C,weblink Gas dynamics and kinetics in the cometary coma: Theory and observations, Combi, Michael R., Harris, Walter M., Smyth, William H., 2004, 523, Comets II, The {{H2O}} parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization, with the solar wind playing a minor role in the destruction of water compared to photochemistry. Larger dust particles are left along the comet's orbital path whereas smaller particles are pushed away from the Sun into the comet's tail by light pressure.WEB,weblink Comet Definitions, Michael Gallagher, Morris, Charles S., 31 August 2013, Although the solid nucleus of comets is generally less than {{convert|60|km}} across, the coma may be thousands or millions of kilometres across, sometimes becoming larger than the Sun.JOURNAL, 10.1023/A:1021512317744, 2002, Rosine, Lallement, Bertaux, Jean-Loup, Szegö, Karöly, Nemeth, Szilvia, Earth, Moon, and Planets, 90, 67–76, The Shadow of Comet Hale–Bopp in Lyman-Alpha, For example, about a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun.WEB, David C. Jewitt, Jewitt, David,weblink The Splintering of Comet 17P/Holmes During a Mega-Outburst, University of Hawaii, 30 August 2013, The Great Comet of 1811 also had a coma roughly the diameter of the Sun.WEB, The Comet Primer, Gary W. Kronk's Cometography, Kronk, Gary W.,weblink 30 August 2013, dead,weblink" title="web.archive.org/web/20110517043903weblink">weblink 17 May 2011, Even though the coma can become quite large, its size can decrease about the time it crosses the orbit of Mars around {{convert|1.5|AU}} from the Sun. At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, and in doing so enlarging the tail. Ion tails have been observed to extend one astronomical unit (150 million km) or more.(File:PIA20119-CometChristensen-C2006W3-CO2-WISE-20100420.jpg|thumb|C/2006 W3 (Chistensen) emitting carbon gas (IR image))Both the coma and tail are illuminated by the Sun and may become visible when a comet passes through the inner Solar System, the dust reflects sunlight directly while the gases glow from ionisation.WEB,weblink Comets, University of Leicester, Brinkworth, Carolyn, Thomas, Claire, 31 July 2013, Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye.BOOK,weblink 75, A field guide to the stars and planets, 978-0-395-93432-6, Pasachoff, Jay M, 2000, Occasionally a comet may experience a huge and sudden outburst of gas and dust, during which the size of the coma greatly increases for a period of time. This happened in 2007 to Comet Holmes.WEB, Jewitt, David,weblink Comet Holmes Bigger Than The Sun, Institute for Astronomy at the University of Hawaii, 31 July 2013, In 1996, comets were found to emit X-rays.JOURNAL, 10.1126/science.274.5285.205, Discovery of X-ray and Extreme Ultraviolet Emission from Comet C/Hyakutake 1996 B2, 1996, Lisse, C. M., Dennerl, K., Englhauser, J., Harden, M., Marshall, F. E., Mumma, M. J., Petre, R., Pye, J. P., Ricketts, M. J., Science, 274, 5285, 205, Schmitt, J., Trumper, J., West, R. G., 1996Sci...274..205L,weblink This greatly surprised astronomers because X-ray emission is usually associated with very high-temperature bodies. The X-rays are generated by the interaction between comets and the solar wind: when highly charged solar wind ions fly through a cometary atmosphere, they collide with cometary atoms and molecules, "stealing" one or more electrons from the atom in a process called "charge exchange". This exchange or transfer of an electron to the solar wind ion is followed by its de-excitation into the ground state of the ion by the emission of X-rays and far ultraviolet photons.JOURNAL, 10.1126/science.292.5520.1343, Charge Exchange-Induced X-Ray Emission from Comet C/1999 S4 (LINEAR), 2001, Lisse, C. M., Science, 292, 5520, 1343–8, 11359004, Christian, DJ, Dennerl, K, Meech, KJ, Petre, R, Weaver, HA, Wolk, SJ, 2001Sci...292.1343L,

Bow shock

Bow shocks form as a result of the interaction between the solar wind and the cometary ionosphere, which is created by ionization of gases in the coma. As the comet approaches the Sun, increasing outgassing rates cause the coma to expand, and the sunlight ionizes gases in the coma. When the solar wind passes through this ion coma, the bow shock appears.The first observations were made in the 1980s and 90s as several spacecraft flew by comets 21P/Giacobini–Zinner,JOURNAL, The Bow wave of Comet Giacobini-Zinner – ICE magnetic field observations, Geophysical Research Letters, Jones, D. E., Smith, E. J., Slavin, J. A., Tsurutani, B. T., Siscoe, G. L., Mendis, D. A., 1, 13, 3, 243–246, March 1986, 1986GeoRL..13..243J, 10.1029/GL013i003p00243, 1P/Halley,JOURNAL, First in situ plasma and neutral gas measurements at comet Halley, Nature, Gringauz, K. I., Gombosi, T. I., Remizov, A. P., Szemerey, I., Verigin, M. I., L. I., Denchikova, A. V., Dyachkov, E., Keppler, I. N., Klimenko, A. K., Richter, A. J., Somogyi, K., Szego, S., Szendro, M., Tatrallyay, A., Varga, G. A., Vladimirova, 1, 321, 282–285, 15 May 1986, 1986Natur.321..282G, 10.1038/321282a0, and 26P/Grigg–Skjellerup.JOURNAL, First results from the Giotto magnetometer experiment during the P/Grigg-Skjellerup encounter, Astronomy and Astrophysics, F. M., Neubauer, H., Marschall, M., Pohl, K.-H., Glassmeier, G., Musmann, F., Mariani, M. H., Acuna, L. F., Burlaga, N. F., Ness, M. K., Wallis, H. U., Schmidt, E., Ungstrup, 1, 268, 2, L5–L8, February 1993, 1993A&A...268L...5N, It was then found that the bow shocks at comets are wider and more gradual than the sharp planetary bow shocks seen at, for example, Earth. These observations were all made near perihelion when the bow shocks already were fully developed.The Rosetta spacecraft observed the bow shock at comet 67P/Churyumov–Gerasimenko at an early stage of bow shock development when the outgassing increased during the comet's journey toward the Sun. This young bow shock was called the "infant bow shock". The infant bow shock is asymmetric and, relative to the distance to the nucleus, wider than fully developed bow shocks.JOURNAL, The infant bow shock: a new frontier at a weak activity comet, Astronomy and Astrophysics, Gunell, H., Goetz, C., Simon Wedlund, C., Lindkvist, J., Hamrin, M., Nilsson, H., LLera, K., Eriksson, A., Holmström, M., 1, 619, L2, November 2018, 10.1051/0004-6361/201834225,weblink

Tails

(File:Cometorbit01.svg|thumb|left|280px|Typical direction of tails during a comet's orbit near the Sun)In the outer Solar System, comets remain frozen and inactive and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from observations by the Hubble Space TelescopeJOURNAL, 1995ApJ...455..342C, The Discovery of Halley-sized Kuiper Belt Objects Using the Hubble Space Telescope, Cochran, Anita L., Levison, Harold F., Stern, S. Alan, Duncan, Martin J., 455, 1995, 342, Astrophysical Journal, 10.1086/176581, astro-ph/9509100, JOURNAL, 10.1086/311515, The Calibration of the Hubble Space Telescope Kuiper Belt Object Search:Setting the Record Straight, 1998, Cochran, Anita L., Levison, Harold F., Tamblyn, Peter, Stern, S. Alan, Duncan, Martin J., The Astrophysical Journal, 503, 1, L89, astro-ph/9806210, 1998ApJ...503L..89C, but these detections have been questioned.JOURNAL, 10.1086/311009, An Analysis of the Statistics of the ITAL Hubble Space Telescope/ITAL] Kuiper Belt Object Search, 1997, Brown, Michael E., Kulkarni, Shrinivas R., Liggett, Timothy J., The Astrophysical Journal, 490, 1, L119–L122, 1997ApJ...490L.119B, JOURNAL, 1996AJ....112.1225J, The Mauna Kea-Cerro-Tololo (MKCT) Kuiper Belt and Centaur Survey, Jewitt, David, Luu, Jane, Chen, Jun, 112, 1996, 1225, Astronomical Journal, 10.1086/118093, As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them.The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the type II or dust tail. At the same time, the ion or type I tail, made of gases, always points directly away from the Sun because this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory.BOOK,weblink 422, The Cambridge Guide to the Solar System, 978-1-139-49417-5, Lang, Kenneth R., 2011, On occasions—such as when the Earth passes through a comet's orbital plane, the antitail, pointing in the opposite direction to the ion and dust tails, may be seen.APOD, PanSTARRS: The Anti Tail Comet, 29 June 2013, 31 July 2013, File:Comet Parts.svg|thumb|upright|Diagram of a comet showing the dust trail, the dust tail, and the ion gas tail formed by solar windsolar windThe observation of antitails contributed significantly to the discovery of solar wind.JOURNAL, 10.1007/BF00225271, The plasma tails of comets and the interplanetary plasma, 1963, Biermann, L., Space Science Reviews, 1, 3, 553, 1963SSRv....1..553B, The ion tail is formed as a result of the ionisation by solar ultra-violet radiation of particles in the coma. Once the particles have been ionized, they attain a net positive electrical charge, which in turn gives rise to an "induced magnetosphere" around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles. Because the relative orbital speed of the comet and the solar wind is supersonic, a bow shock is formed upstream of the comet in the flow direction of the solar wind. In this bow shock, large concentrations of cometary ions (called "pick-up ions") congregate and act to "load" the solar magnetic field with plasma, such that the field lines "drape" around the comet forming the ion tail.BOOK, Carroll, B. W., Ostlie, D. A., An Introduction to Modern Astrophysics, 864–874, Addison-Wesley, 1996, 0-201-54730-9, If the ion tail loading is sufficient, the magnetic field lines are squeezed together to the point where, at some distance along the ion tail, magnetic reconnection occurs. This leads to a "tail disconnection event". This has been observed on a number of occasions, one notable event being recorded on 20 April 2007, when the ion tail of Encke's Comet was completely severed while the comet passed through a coronal mass ejection. This event was observed by the STEREO space probe.JOURNAL, 10.1007/s11207-008-9299-0, The Heliospheric Imagers Onboard the STEREO Mission, 2008, Eyles, C. J., Harrison, R. A., Davis, C. J., Waltham, N. R., Shaughnessy, B. M., Mapson-Menard, H. C. A., Bewsher, D., Crothers, S. R., Davies, J. A., Solar Physics, 254, 2, 387, Simnett, G. M., Howard, R. A., Moses, J. D., Newmark, J. S., Socker, D. G., Halain, J.-P., Defise, J.-M., Mazy, E., Rochus, P., 2009SoPh..254..387E, 2268/15675,weblink In 2013, ESA scientists reported that the ionosphere of the planet Venus streams outwards in a manner similar to the ion tail seen streaming from a comet under similar conditions."WEB, When A Planet Behaves Like A Comet,weblink 29 January 2013, ESA, 30 August 2013, Staff, WEB, Kramer, Miriam, Venus Can Have 'Comet-Like' Atmosphere,weblink 30 January 2013, Space.com, 30 August 2013,

Jets

File:Hartley2jets2 epoxi big.jpg|thumb|170px|Gas and snow jets of 103P/Hartley103P/HartleyUneven heating can cause newly generated gases to break out of a weak spot on the surface of comet's nucleus, like a geyser.WEB,weblink Comets and Jets, Hubblesite.org, 12 November 2013, These streams of gas and dust can cause the nucleus to spin, and even split apart. In 2010 it was revealed dry ice (frozen carbon dioxide) can power jets of material flowing out of a comet nucleus.NEWS,weblink Dry ice fuels comet jets, Astronomy Now, Emily, Baldwin, 11 November 2010,weblink" title="web.archive.org/web/20131217034053weblink">weblink 17 December 2013, Infrared imaging of Hartley 2 shows such jets exiting and carrying with it dust grains into the coma.NEWS,weblink Comet Hartley 2 Is Spewing Ice, NASA Photos Show, The New York Times, Chang, Kenneth, 18 November 2010,

Orbital characteristics

Most comets are small Solar System bodies with elongated elliptical orbits that take them close to the Sun for a part of their orbit and then out into the further reaches of the Solar System for the remainder.WEB,weblink The Orbit of a Comet, University of St Andrews, 1 September 2013, Comets are often classified according to the length of their orbital periods: The longer the period the more elongated the ellipse.

{{anchor|Short-period comet}} Short period

Periodic comets or short-period comets are generally defined as those having orbital periods of less than 200 years.JOURNAL, Duncan, Martin and Quinn, Thomas and Tremaine, Scott, 1988, The origin of short-period comets, The Astrophysical Journal, 328, L69{{ndash, L73 | url =weblink}} They usually orbit more-or-less in the ecliptic plane in the same direction as the planets.BOOK,weblink 117, Our Cosmic Origins: From the Big Bang to the Emergence of Life and Intelligence, 978-0-521-79480-0, Delsemme, Armand H., 2001, Their orbits typically take them out to the region of the outer planets (Jupiter and beyond) at aphelion; for example, the aphelion of Halley's Comet is a little beyond the orbit of Neptune. Comets whose aphelia are near a major planet's orbit are called its "family".JOURNAL, Wilson, H. C., The Comet Families of Saturn, Uranus and Neptune, Popular Astronomy, 17, 629–633, 1909, 1909PA.....17..629W, Such families are thought to arise from the planet capturing formerly long-period comets into shorter orbits.WEB,weblink Comets, Steven, Dutch, Natural and Applied Sciences, University of Wisconsin, 31 July 2013, dead,weblink" title="web.archive.org/web/20130729122906weblink">weblink 29 July 2013, At the shorter orbital period extreme, Encke's Comet has an orbit that does not reach the orbit of Jupiter, and is known as an Encke-type comet. Short-period comets with orbital periods less than 20 years and low inclinations (up to 30 degrees) to the ecliptic are called traditional Jupiter-family comets (JFCs).WEB,weblink The Jupiter Family Comets, Department of Terrestrial Magnetism Carnegie Institution of Washington, 11 August 2013, WEB,weblink Comets – where are they ?, 6 November 2012, British Astronomical Association, 11 August 2013, dead,weblink" title="web.archive.org/web/20130805211248weblink">weblink 5 August 2013, Those like Halley, with orbital periods of between 20 and 200 years and inclinations extending from zero to more than 90 degrees, are called Halley-type comets (HTCs).JOURNAL, 10.1007/s11214-008-9405-5, Dynamical Origin of Comets and Their Reservoirs, 2008, Duncan, Martin J., Space Science Reviews, 138, 1–4, 109–126, 2008SSRv..138..109D, JOURNAL, 10.1086/338692, From Kuiper Belt Object to Cometary Nucleus: The Missing Ultrared Matter, 2002, Jewitt, David C., The Astronomical Journal, 123, 2, 1039–1049, 2002AJ....123.1039J, {{As of|2019}}, 85 HTCs have been observed,WEB,weblink Constraints: orbital class (HTC), JPL Small-Body Database, NASA, 27 September 2018, compared with 664 identified JFCs.WEB,weblink Constraints: comets and orbital class (JFc), JPL Small-Body Database, NASA, 27 September 2018, Recently discovered main-belt comets form a distinct class, orbiting in more circular orbits within the asteroid belt.WEB, Reddy, Francis, New comet class in Earth's backyard,weblink Astronomy, 3 April 2006, 31 July 2013, Because their elliptical orbits frequently take them close to the giant planets, comets are subject to further gravitational perturbations.WEB,weblink Comets, The Pennsylvania State University, 8 August 2013, Short-period comets have a tendency for their aphelia to coincide with a giant planet's semi-major axis, with the JFCs being the largest group. It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined. These perturbations can deflect long-period comets into shorter orbital periods.{{harvnb|Sagan|Druyan|1997|pp=102–104}}BOOK,weblink 246, In Quest of the Solar System, 978-0-7637-9477-4, Koupelis, Theo, 2010, Based on their orbital characteristics, short-period comets are thought to originate from the centaurs and the Kuiper belt/scattered discWEB, Davidsson, Björn J. R., Comets – Relics from the birth of the Solar System,weblink Uppsala University, 2008, 30 July 2013, —a disk of objects in the trans-Neptunian region—whereas the source of long-period comets is thought to be the far more distant spherical Oort cloud (after the Dutch astronomer Jan Hendrik Oort who hypothesised its existence).JOURNAL, 1950BAN....11...91O, The structure of the cloud of comets surrounding the Solar System and a hypothesis concerning its origin, Oort, J. H., 11, 1950, 91, Bulletin of the Astronomical Institutes of the Netherlands, Vast swarms of comet-like bodies are thought to orbit the Sun in these distant regions in roughly circular orbits. Occasionally the gravitational influence of the outer planets (in the case of Kuiper belt objects) or nearby stars (in the case of Oort cloud objects) may throw one of these bodies into an elliptical orbit that takes it inwards toward the Sun to form a visible comet. Unlike the return of periodic comets, whose orbits have been established by previous observations, the appearance of new comets by this mechanism is unpredictable.BOOK,weblink 152, Habitability and Cosmic Catastrophes, 978-3-540-76945-3, Hanslmeier, Arnold, 2008,

Long period

{{See also|List of long-period comets|List of near-parabolic comets|List of hyperbolic comets}}File:Comet Kohoutek orbit p391.svg|thumb|Orbits of Comet Kohoutek (red) and the Earth (blue), illustrating the high eccentricity of its orbit and its rapid motion when close to the Sun.]]Long-period comets have highly eccentric orbits and periods ranging from 200 years to thousands of years. An eccentricity greater than 1 when near perihelion does not necessarily mean that a comet will leave the Solar System.WEB,weblink Influence of giant planets on the orbit of comet C/2010 X1, Leonid, Elenin, 7 March 2011, 11 August 2013, For example, Comet McNaught had a heliocentric osculating eccentricity of 1.000019 near its perihelion passage epoch in January 2007 but is bound to the Sun with roughly a 92,600-year orbit because the eccentricity drops below 1 as it moves farther from the Sun. The future orbit of a long-period comet is properly obtained when the osculating orbit is computed at an epoch after leaving the planetary region and is calculated with respect to the center of mass of the Solar System. By definition long-period comets remain gravitationally bound to the Sun; those comets that are ejected from the Solar System due to close passes by major planets are no longer properly considered as having "periods". The orbits of long-period comets take them far beyond the outer planets at aphelia, and the plane of their orbits need not lie near the ecliptic. Long-period comets such as Comet West and C/1999 F1 can have aphelion distances of nearly 70,000 AU with orbital periods estimated around 6 million years.Single-apparition or non-periodic comets are similar to long-period comets because they also have parabolic or slightly hyperbolic trajectoriesWEB, Small Bodies: Profile,weblink NASA/JPL, 29 October 2008, 11 August 2013, when near perihelion in the inner Solar System. However, gravitational perturbations from giant planets cause their orbits to change. Single-apparition comets have a hyperbolic or parabolic osculating orbit which allows them to permanently exit the Solar System after a single pass of the Sun.BOOK,weblink 21, Astronomy and Astrophysics, 978-0-7637-7786-9, Joardar, S, Bhattacharya, A. B, Bhattacharya, R, 2008, The Sun's Hill sphere has an unstable maximum boundary of 230,000 AU ({{convert|1.1|pc|ly|abbr=off}}).JOURNAL, 1964SvA.....7..618C, Gravitational Spheres of the Major Planets, Moon and Sun, Chebotarev, G. A., 7, 1964, 618, Soviet Astronomy, Only a few hundred comets have been seen to reach a hyperbolic orbit (e > 1) when near perihelionWEB, JPL Small-Body Database Search Engine: e > 1, JPL,weblink 13 August 2013, that using a heliocentric unperturbed two-body best-fit suggests they may escape the Solar System.As of 2019, only two objects have been discovered with an eccentricity significantly greater than one: 1I/Ê»Oumuamua and 2I/Borisov, indicating an origin outside the Solar System. While Ê»Oumuamua, with an eccentrcity of about 1.2, showed no optical signs of cometary activity during its passage through the inner Solar System in October 2017, changes to its trajectory—which suggests outgassing—indicate that it is probably a comet.NEWS,weblink Interstellar Visitor 'Oumuamua Is a Comet After All, Space.com, Chelsea, Gohd, 27 June 2018, 27 September 2018, On the other hand, 2I/Borisov, with an estimated eccentricity of about 3.36, has been observed to have the coma feature of comets, and is considered the first detected interstellar comet.WEB,weblink Astronomers have spotted a second interstellar object, Grossman, Lisa, 2019-09-12, Science News, en-US, 2019-09-16, WEB, Strickland, Ashley, 2nd interstellar visitor to our solar system confirmed and named,weblink Cable News Network, Turner Broadcasting System, Inc., English, Comet C/1980 E1 had an orbital period of roughly 7.1 million years before the 1982 perihelion passage, but a 1980 encounter with Jupiter accelerated the comet giving it the largest eccentricity (1.057) of any known hyperbolic comet.WEB,weblink C/1980 E1 (Bowell), JPL Small-Body Database, 1986-12-02 last obs, 13 August 2013, Comets not expected to return to the inner Solar System include C/1980 E1, C/2000 U5, C/2001 Q4 (NEAT), C/2009 R1, C/1956 R1, and C/2007 F1 (LONEOS).Some authorities use the term "periodic comet" to refer to any comet with a periodic orbit (that is, all short-period comets plus all long-period comets),WEB, Comet,weblink Encyclopædia Britannica Online, 13 August 2013, whereas others use it to mean exclusively short-period comets. Similarly, although the literal meaning of "non-periodic comet" is the same as "single-apparition comet", some use it to mean all comets that are not "periodic" in the second sense (that is, to also include all comets with a period greater than 200 years).Early observations have revealed a few genuinely hyperbolic (i.e. non-periodic) trajectories, but no more than could be accounted for by perturbations from Jupiter. If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of km per second). If such objects entered the Solar System, they would have positive specific orbital energy and would be observed to have genuinely hyperbolic trajectories. A rough calculation shows that there might be four hyperbolic comets per century within Jupiter's orbit, give or take one and perhaps two orders of magnitude.JOURNAL, 10.1086/185590, On the nondetection of extrasolar comets, 1989, McGlynn, Thomas A., Chapman, Robert D., The Astrophysical Journal, 346, L105, 1989ApJ...346L.105M, {| class="wikitable" style="width: 600px; text-align: center; font-size: 1em; margin: 0.4em auto;"
Hyperbolic comet discoveriesJPL SMALL-BODY DATABASE SEARCH ENGINE: E > 1 (SORTED BY NAME), JPL,weblink 27 September 2018,
! Year !! 2007 !! 2008 !! 2009 !! 2010 !! 2011 !! 2012 !! 2013 !! 2014 !! 2015 !! 2016 !! 2017 !! 2018
! Number
| 3

Oort cloud and Hills cloud

File:Kuiper oort-en.svg|thumb|The Oort cloud thought to surround the Solar SystemSolar SystemThe Oort cloud is thought to occupy a vast space starting from between {{convert|2000|and|5000|AU|ly|2|abbr=on}}BOOK, Harold F. Levison, Luke Donnes, Comet Populations and Cometary Dynamics, Lucy Ann Adams McFadden, Lucy-Ann Adams, Paul Robert Weissman, Torrence V. Johnson, 2007, Encyclopedia of the Solar System, 575–588, 2nd, Academic Press, Amsterdam; Boston, 978-0-12-088589-3, to as far as {{convert|50000|AU|ly|2|abbr=on}} from the Sun. Some estimates place the outer edge at between {{convert|100000|and|200000|AU|ly|2|abbr=on}}. The region can be subdivided into a spherical outer Oort cloud of {{convert|20000|-|50000|AU|ly|2|abbr=on}}, and a doughnut-shaped inner cloud, the Hills cloud, of {{convert|2000|-|20000|AU|ly|2|abbr=on}}.BOOK, Lisa, Randall, Dark matter and the dinosaurs: The astounding interconnectedness of the universe, 2015, Harper Collins Publishers, 978-0-06-232847-2, 115, The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets that fall to inside the orbit of Neptune. The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981. Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo;JOURNAL, Jack G. Hills, 1981, Comet showers and the steady-state infall of comets from the Oort Cloud, Astronomical Journal, 86, 1730–1740, 1981AJ.....86.1730H, 10.1086/113058, JOURNAL, Harold F. Levison, Luke Dones, Martin J. Duncan, 2001, The Origin of Halley-Type Comets: Probing the Inner Oort Cloud, Astronomical Journal, 121, 4, 2253–2267, 2001AJ....121.2253L, 10.1086/319943, BOOK, Thomas M. Donahue, Kathleen Kearney Trivers, and David M. Abramson, 1991, Planetary Sciences: American and Soviet Research, Proceedings from the U.S.–U.S.S.R. Workshop on Planetary Sciences,weblink National Academy Press, 251, 0-309-04333-6, 18 March 2008, it is seen as a possible source of new comets that resupply the relatively tenuous outer cloud as the latter's numbers are gradually depleted. The Hills cloud explains the continued existence of the Oort cloud after billions of years.JOURNAL, Julio A. Fernéndez, 1997, The Formation of the Oort Cloud and the Primitive Galactic Environment,weblink Icarus (journal), Icarus, 219, 1, 106–119, 18 March 2008, 1997Icar..129..106F, 10.1006/icar.1997.5754,

Exocomets

Exocomets beyond the Solar System have also been detected and may be common in the Milky Way.WEB, Exocomets may be as common as exoplanets,weblink 7 January 2013, UC Berkeley, 30 July 2013, Sanders, Robert, The first exocomet system detected was around Beta Pictoris, a very young A-type main-sequence star, in 1987.WEB, 'Exocomets' Common Across Milky Way Galaxy,weblink 7 January 2013, Space.com, 8 January 2013, dead,weblink" title="web.archive.org/web/20140916085824weblink">weblink 16 September 2014, JOURNAL, 1990A&A...236..202B, The Beta Pictoris circumstellar disk. X – Numerical simulations of infalling evaporating bodies, Beust, H., Lagrange-Henri, A.M., Vidal-Madjar, A., Ferlet, R., 236, 1990, 202–216, Astronomy and Astrophysics, 0004-6361, A total of 10 such exocomet systems have been identified {{as of|lc=y|2013}}, using the absorption spectrum caused by the large clouds of gas emitted by comets when passing close to their star.

Effects of comets

File:PSM V18 D201 Shower of perseids sept 6 and 7.jpg|thumb|Diagram of Perseids meteors ]]

Connection to meteor showers

As a result of outgassing, comets leave in their wake a trail of solid debris too large to be swept away by radiation pressure and the solar wind.{{harvnb|Sagan|Druyan|1997|p=235}} If the Earth's orbit sends it through that debris, there are likely to be meteor showers as Earth passes through. The Perseid meteor shower, for example, occurs every year between 9 and 13 August, when Earth passes through the orbit of Comet Swift–Tuttle. Halley's Comet is the source of the Orionid shower in October.WEB, Major Meteor Showers, Meteor Showers Online,weblink 31 July 2013,weblink" title="web.archive.org/web/20130724111133weblink">weblink 24 July 2013, dead,

Comets and impact on life

Many comets and asteroids collided with Earth in its early stages. Many scientists think that comets bombarding the young Earth about 4 billion years ago brought the vast quantities of water that now fill the Earth's oceans, or at least a significant portion of it. Others have cast doubt on this idea.WEB, Muir, Hazel, Earth's water brewed at home, not in space,weblink New Scientist, 25 September 2007, 30 August 2013, The detection of organic molecules, including polycyclic aromatic hydrocarbons,WEB, Clavin, Whitney, Why Comets Are Like Deep Fried Ice Cream,weblink 10 February 2015, NASA, 10 February 2015, in significant quantities in comets has led to speculation that comets or meteorites may have brought the precursors of life—or even life itself—to Earth.BOOK,weblink 315, Comets, 978-1-4020-3495-4, Fernández, Julio A., 2006, In 2013 it was suggested that impacts between rocky and icy surfaces, such as comets, had the potential to create the amino acids that make up proteins through shock synthesis.JOURNAL, 10.1038/ngeo1930, Shock synthesis of amino acids from impacting cometary and icy planet surface analogues, 2013, Martins, Zita, Price, Mark C., Goldman, Nir, Sephton, Mark A., Burchell, Mark J., Nature Geoscience, 2013NatGe...6.1045M, 6, 12, 1045–1049, In 2015, scientists found significant amounts of molecular oxygen in the outgassings of comet 67P, suggesting that the molecule may occur more often than had been thought, and thus less an indicator of life as has been supposed.Oregonian (29 Oct 2015), "Comet's oxygen shakes theories on solar system", p. A5It is suspected that comet impacts have, over long timescales, also delivered significant quantities of water to the Earth's Moon, some of which may have survived as lunar ice.WEB,weblink Water Discovered in Apollo Moon Rocks Likely Came from Comets, 7 September 2013, NASA, Comet and meteoroid impacts are also thought to be responsible for the existence of tektites and australites.WEB,weblink Australites, Museum Victoria, 7 September 2013,

Fear of comets

Fear of comets as acts of God and signs of impending doom was highest in Europe from AD 1200 to 1650.{{r|ley196710}} The year after the Great Comet of 1618, for example, Gotthard Arthusius published a pamphlet stating that it was a sign that the Day of Judgment was near.BOOK,weblink Cometa orientalis: Kurtze vnd eygentliche Beschreibung deß newen Cometen, so im November deß abgelauffenen 1618, Gallica.fr, Sigismund Latomus, Frankfurt on Main, Franckfurt-am-Mayn, Gothard, Arthusius, Gotthard Arthusius, 1619, He listed ten pages of comet-related disasters, including "earthquakes, floods, changes in river courses, hail storms, hot and dry weather, poor harvests, epidemics, war and treason and high prices". By 1700 most scholars concluded that such events occurred whether a comet was seen or not. Using Edmund Halley's records of comet sightings, however, William Whiston in 1711 wrote that the Great Comet of 1680 had a periodicity of 574 years and was responsible for the worldwide flood in the Book of Genesis, by pouring water on the Earth. His announcement revived for another century fear of comets, now as direct threats to the world instead of signs of disasters.MAGAZINE,weblink The Worst of All the Comets, For Your Information, Galaxy Science Fiction, Ley, Willy, 26, 1, 96–105, October 1967, Spectroscopic analysis in 1910 found the toxic gas cyanogen in the tail of Halley's Comet,NEWS,weblink Yerkes Observatory Finds Cyanogen in Spectrum of Halley's Comet, The New York Times, 8 February 1910, 8 January 2018, causing panicked buying of gas masks and quack "anti-comet pills" and "anti-comet umbrellas" by the public.NEWS,weblink Interesting Facts About Comets, Universe Today, Jerry, Coffey, 20 September 2009, 8 January 2018,

Fate of comets

Departure (ejection) from Solar System

If a comet is traveling fast enough, it may leave the Solar System. Such comets follow the open path of a hyperbola, and as such they are called hyperbolic comets. To date, comets are only known to be ejected by interacting with another object in the Solar System, such as Jupiter.JOURNAL, 1991JBAA..101..119H, On hyperbolic comets, Hughes, D. W., 101, 1991, 119, Journal of the British Astronomical Association, An example of this is thought to be Comet C/1980 E1, which was shifted from a predicted orbit of 7.1 million years around the Sun, to a hyperbolic trajectory, after a 1980 close pass by the planet Jupiter.WEB, JPL Horizons On-Line Ephemeris System, Horizons output,weblink Barycentric Osculating Orbital Elements for Comet C/1980 E1, 9 March 2011, (Solution using the Solar System Barycenter and barycentric coordinates. Select Ephemeris Type:Elements and Center:@0)

Volatiles exhausted

Jupiter-family comets and long-period comets appear to follow very different fading laws. The JFCs are active over a lifetime of about 10,000 years or ~1,000 orbits whereas long-period comets fade much faster. Only 10% of the long-period comets survive more than 50 passages to small perihelion and only 1% of them survive more than 2,000 passages. Eventually most of the volatile material contained in a comet nucleus evaporates, and the comet becomes a small, dark, inert lump of rock or rubble that can resemble an asteroid.WEB, Lyzenga, Greg, If comets melt, why do they seem to last for long periods of time,weblink Scientific American, 16 November 1998, 13 August 2013, Some asteroids in elliptical orbits are now identified as extinct comets.JOURNAL, 2002aste.book..669W,weblink Evolution of Comets into Asteroids, Bottke Jr, William F., Levison, Harold F., 2002, 669, Asteroids III, Davies, J. 1986. Are the IRAS-detected Apollo asteroids extinct comets? Mon. Not. R. astr. Soc: 221, 19–23.McFadden, L. 1993. The Comet – Asteroid Transition: Recent Telescopic Observations. In Milani, et al. (eds), Asteroids, Comets, meteors, 95–110McFadden, L., et al. 1993. The Enigmatic Object 2201 Oljato: Is it an Asteroid or an Evolved Comet? Journal of Geophysical Research: 98, 3031–3041. Roughly six percent of the near-Earth asteroids are thought to be extinct comet nuclei.

Breakup and collisions

The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart.NEWS,weblink Astronomers see comet break-up, 26 July 2002, BBC News, Whitehouse, David, A significant cometary disruption was that of Comet Shoemaker–Levy 9, which was discovered in 1993. A close encounter in July 1992 had broken it into pieces, and over a period of six days in July 1994, these pieces fell into Jupiter's atmosphere—the first time astronomers had observed a collision between two objects in the Solar System.WEB, Kronk, Gary W., D/1993 F2 Shoemaker–Levy 9,weblink Gary W. Kronk's Cometography, 27 April 2009, dead,weblink" title="web.archive.org/web/20080509145539weblink">weblink 9 May 2008, WEB,weblink Comet Shoemaker–Levy Background, JPL, 23 September 2013, Other splitting comets include 3D/Biela in 1846 and 73P/Schwassmann–Wachmann from 1995 to 2006.WEB, 10 May 2006, Spitzer Telescope Sees Trail of Comet Crumbs, Whitney, Clavin,weblink 16 August 2013, Greek historian Ephorus reported that a comet split apart as far back as the winter of 372–373 BC.WEB, Great Comets in History, Donald K., Yeomans, JPL,weblink April 2007, 16 August 2013, Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.JOURNAL, 2004come.book..301B,weblink Split comets, Boehnhardt, H., 2004, 301, Comets II, Comets 42P/Neujmin and 53P/Van Biesbroeck appear to be fragments of a parent comet. Numerical integrations have shown that both comets had a rather close approach to Jupiter in January 1850, and that, before 1850, the two orbits were nearly identical.JOURNAL, 2003DPS....35.4705P, Are Comets 42P/Neujmin 3 and 53P/Van Biesbroeck Parts of one Comet?, Pittichova, Jand, Meech, Karen J., Valsecchi, Giovanni B., Pittich, Eduard M., 35, 2003, 1011, Bulletin of the American Astronomical Society, Some comets have been observed to break up during their perihelion passage, including great comets West and Ikeya–Seki. Biela's Comet was one significant example, when it broke into two pieces during its passage through the perihelion in 1846. These two comets were seen separately in 1852, but never again afterward. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A minor meteor shower, the Andromedids, occurs annually in November, and it is caused when the Earth crosses the orbit of Biela's Comet.WEB,weblink The Andromedids, Meteor Showers Online, 27 April 2009,weblink" title="web.archive.org/web/20130122144223weblink">weblink 22 January 2013, dead, Some comets meet a more spectacular end – either falling into the SunWEB, SOHO analyses a kamikaze comet,weblink ESA, 23 February 2001, 30 August 2013, or smashing into a planet or other body. Collisions between comets and planets or moons were common in the early Solar System: some of the many craters on the Moon, for example, may have been caused by comets. A recent collision of a comet with a planet occurred in July 1994 when Comet Shoemaker–Levy 9 broke up into pieces and collided with Jupiter.WEB,weblink Comet Shoemaker–Levy 9 Collision with Jupiter, 30 August 2013, National Space Science Data Center, {{multiple image |direction=horizontal |align=center |total_width=800
|image1 =Jupiter showing SL9 impact sites.jpg
|image2 =Schwassman-Wachmann3-B-HST.gif
|image3 =C2015D1-SOHO.jpg
|image4 =14060-Asteroid-P2013R3-Disintegration-20140306.jpg
|caption1 =Brown spots mark impact sites of Comet Shoemaker–Levy 9 on Jupiter
|caption2 =The break up of 73P/Schwassmann–Wachmann within three days (1995)
|caption3 =Ghost tail of C/2015 D1 (SOHO) after passage at the Sun
|caption4 =Disintegration of P/2013 R3 (2014)WEB, Harrington, J.D., Villard, Ray, RELEASE 14-060 NASA's Hubble Telescope Witnesses Asteroid's Mysterious Disintegration,weblink 6 March 2014, NASA, 6 March 2014, }}
{{clear}}

Nomenclature

File:Halley's Comet, 1910.JPG|thumb|Halley's CometHalley's CometThe names given to comets have followed several different conventions over the past two centuries. Prior to the early 20th century, most comets were simply referred to by the year when they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "Great Comet of 1680", the "Great Comet of 1882", and the "Great January Comet of 1910".After Edmund Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759 by calculating its orbit, that comet became known as Halley's Comet.WEB, Ridpath, Ian, Ian Ridpath, Halley and his Comet,weblink A brief history of Halley's Comet, 3 July 2008, 14 August 2013, Similarly, the second and third known periodic comets, Encke's CometWEB, Kronk, Gary W., 2P/Encke,weblink Gary W. Kronk's Cometography, 14 August 2013, and Biela's Comet,WEB, Kronk, Gary W., 3D/Biela,weblink Gary W. Kronk's Cometography, 14 August 2013, were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their appearance.In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet can be named after its discoverers, or an instrument or program that helped to find it.WEB,weblink Comet Names and Designations; Comet Naming and Nomenclature; Names of Comets, Harvard University, 7 September 2013,

History of study

Early observations and thought

File:Bayeux Tapestry scene32 Halley comet.jpg|thumb|235px|Halley's Comet appeared in 1066, prior to the Battle of Hastings, and is depicted in the Bayeux TapestryBayeux TapestryFrom ancient sources, such as Chinese oracle bones, it is known that comets have been noticed by humans for millennia.WEB,weblink Chinese Oracle Bones, Cambridge University Library, 14 August 2013, Until the sixteenth century, comets were usually considered bad omens of deaths of kings or noble men, or coming catastrophes, or even interpreted as attacks by heavenly beings against terrestrial inhabitants.WEB, Ridpath, Ian, Ian Ridpath, Comet lore,weblink A brief history of Halley's Comet, 8 July 2008, 14 August 2013, {{harvnb|Sagan|Druyan|1997|p=14}}Aristotle believed that comets were atmospheric phenomena, due to the fact that they could appear outside of the Zodiac and vary in brightness over the course of a few days.{{harvnb|Sagan|Druyan|1997|p=24}} Pliny the Elder believed that comets were connected with political unrest and death.{{harvnb|Sagan|Druyan|1997|pp=27–28}}In India, by the 6th century astronomers believed that comets were celestial bodies that re-appeared periodically. This was the view expressed in the 6th century by the astronomers Varāhamihira and Bhadrabahu, and the 10th-century astronomer Bhaṭṭotpala listed the names and estimated periods of certain comets, but it is not known how these figures were calculated or how accurate they were.BOOK,weblink Exploring Ancient Skies: A Survey of Ancient and Cultural Astronomy, Springer Science+Business Media, Kelley, David H., Milone, Eugene F., 2nd, 293, 2011, 978-1-4419-7624-6, 10.1007/978-1-4419-7624-6, 710113366, In the 16th century Tycho Brahe demonstrated that comets must exist outside the Earth's atmosphere by measuring the parallax of the Great Comet of 1577 from observations collected by geographically separated observers. Within the precision of the measurements, this implied the comet must be at least four times more distant than from the Earth to the Moon.WEB, A Brief History of Comets I (until 1950),weblink European Southern Observatory, 14 August 2013, {{harvnb|Sagan|Druyan|1997|p=37}}

Orbital studies

{{further|Observational history of comets#Orbital studies}}Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of gravity must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.BOOK, Newton, Isaac, Isaac Newton, Lib. 3, Prop. 41, Philosophiæ Naturalis Principia Mathematica, Royal Society of London, 1687, 0-521-07647-1, Philosophiæ Naturalis Principia Mathematica, File:Newton Comet1680.jpg|thumb|290px|The orbit of the comet of 1680, fitted to a parabola, as shown in Newton's Principia]]In 1705, Edmond Halley (1656–1742) applied Newton's method to twenty-three cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation caused by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758–9.JOURNAL, 10.1098/rstl.1704.0064, Astronomiae Cometicae Synopsis, Autore Edmundo Halleio apud Oxonienses. Geometriae Professore Saviliano, & Reg. Soc. S, 1704, Halleio, E., Philosophical Transactions of the Royal Society of London, 24, 289–304, 1882,weblink Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy.On 1758 November 14, Alexis Clairaut announced to the Royal Academy of Sciences in Paris his prediction of the date at which Halley's comet would return:
  • Clairaut (January 1759) "Mémoire sur la cométe de 1682," Le Journal des Sçavans, pp. 38–45. On p. 44, Clairaut predicted that Halley's comet would return in mid April 1759. From p. 44: " … il me paroît que la Cométe attendue doit passer à son périhélie vers le milieu du mois d'Avril prochain." ( … it seems to me that the expected comet must pass its perihelion towards the middle of next April.) On p. 40, Clairaut stated that his prediction might be slightly incorrect due to the presence of unknown planets beyond Saturn: "Un corps qui passe dans des régions aussi éloignées, & qui échappe à nos yeux pendant des intervalles aussi longs, pourroit être soumis à des forces totalement inconnues; telles que l'action d'autres Cométes, ou même de quelque planéte toujours trop distante du Soleil pour être jamais apperçue." (A body [i.e., Halley's comet] which passes into regions so remote, and which escapes our eyes during such long intervals, might be subjected to totally unknown forces; such as the action of other comets, or even of some planet always too far from the sun to ever be perceived.)
On 1759 April 7, the French astronomer Joseph-Nicolas Delisle announced to the Royal Academy of Sciences in Paris that he and his assistant Charles Messier had observed the return of Halley's comet, as predicted: De l'Isle subsequently admitted that the comet's return had first been seen by a German amateur astronomer and farmer, Georg Palitzsch:
  • de l'Isle (August 1759) "Seconde lettre de M. de l'Isle," Le Journal des Sçavans, pp. 523–529. From p. 526: " … j'ai reçu une Lettre d'Heidelberg le premier Avril au soir, dans laquelle l'on m'écrit que l'on a publié à Leipsick le 24 Janvier de cette année un Mémoire Allemand dans lequel il est dit que cette Comète a été vue en Saxe par un Paysan, nommé Palisch, le 25 & 26 Décembre de l'année dernière; j'ai bien de la peine à concevoir comment ce Paysan aura pû la découvrir, cette Comète, … " ( … I received a letter from Heidelberg on the first of April in the evening, in which it is written to me that there had been published at Leipzig on the 24th of January of this year a German memoir in which it is said that this comet had been seen in Saxony by a peasant, named Palisch, on the 25th and 26th of December of last year; I can hardly conceive how this peasant could have discovered it, this comet, … )
The story behind the rediscovery of Halley's comet was given by Joseph Lalande in:
  • Delalande, Tables astronomiques de M. Halley, … Et l'Histoire de la Comete de 1759. [Astronomical tables of Mr. Halley, … and the history of the comet of 1759.] (Paris, France: Durand, 1759), pp. 91 ff. Lalande acknowledged the contributions of Madame Lepaute to predicting the return of Halley's comet on p. 110. From p. 110: " … mais il faut convenir que cette suite immense de détails m'eût semblé effrayante, si Madame LEPAUTE, appliquée depuis long-temps & avec succès aux calculs Astronomiques, n'en eût partagé le travail." ( … but it must be admitted that this immense series of details would have seemed frightening to me if Madame LEPAUTE, [who has] long applied [herself] successfully to astronomical calculations, had not shared in the work.)
See also:
  • Broughton, Peter (1985) "The first predicted return of comet Halley", Journal for the History of Astronomy, 16 : 123–132. Available at: Astrophysics Data System
See also:
  • Clairaut, du mouvement des comètes, … [Theory of the movement of comets, …] (Paris, France: Michel Lambert, 1760); see especially the preface.{{harvnb |Sagan |Druyan |1997 |p=93}} When the comet returned as predicted, it became known as Halley's Comet (with the latter-day designation of 1P/Halley). It will next appear in 2061.BOOK,weblink 35, The Greatest Comets in History: Broom Stars and Celestial Scimitars, 978-0-387-09513-4, Wong, Yau-Chuen, 2008,

Studies of physical characteristics

{{further|Observational history of comets#Studies of physical characteristics}}Isaac Newton described comets as compact and durable solid bodies moving in oblique orbit and their tails as thin streams of vapor emitted by their nuclei, ignited or heated by the Sun. Newton suspected that comets were the origin of the life-supporting component of air.{{harvnb|Sagan|Druyan|1997|pp=306–307}}
|style=padding: 8px 12px;
}}As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion.{{harvnb|Sagan|Druyan|1997|p=85}} In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor during the appearance of Halley's Comet in 1835, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit, and he argued that the non-gravitational movements of Encke's Comet resulted from this phenomenon.{{harvnb|Sagan|Druyan|1997|p=126}}In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.JOURNAL, 10.1086/145272, A comet model. I. The acceleration of Comet Encke, 1950, Whipple, F. L., The Astrophysical Journal, 111, 375, 1950ApJ...111..375W, This "dirty snowball" model soon became accepted and appeared to be supported by the observations of an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) that flew through the coma of Halley's Comet in 1986, photographed the nucleus, and observed jets of evaporating material.BOOK,weblink 156, Magic Universe:A Grand Tour of Modern Science, 978-0-19-162235-9, Calder, Nigel, 13 October 2005, On 22 January 2014, ESA scientists reported the detection, for the first definitive time, of water vapor on the dwarf planet Ceres, the largest object in the asteroid belt.JOURNAL, Küppers, Michael, O'Rourke, Laurence, Bockelée-Morvan, Dominique, Zakharov, Vladimir, Lee, Seungwon, von Allmen, Paul, Carry, Benoît, Teyssier, David, Marston, Anthony, Müller, Thomas, Crovisier, Jacques, Barucci, M. Antonietta, Moreno, Raphael, Localized sources of water vapour on the dwarf planet (1) Ceres, Nature, 505, 7484, 2014, 525–527, 0028-0836, 10.1038/nature12918, 2014Natur.505..525K, 24451541, The detection was made by using the far-infrared abilities of the Herschel Space Observatory.WEB, Harrington, J.D., Herschel Telescope Detects Water on Dwarf Planet – Release 14-021,weblink 22 January 2014, NASA, 22 January 2014, The finding is unexpected because comets, not asteroids, are typically considered to "sprout jets and plumes". According to one of the scientists, "The lines are becoming more and more blurred between comets and asteroids." On 11 August 2014, astronomers released studies, using the Atacama Large Millimeter/Submillimeter Array (ALMA) for the first time, that detailed the distribution of HCN, HNC, {{chem2|H2CO}}, and dust inside the comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON).WEB, Zubritsky, Elizabeth, Neal-Jones, Nancy, RELEASE 14-038 – NASA's 3-D Study of Comets Reveals Chemical Factory at Work,weblink 11 August 2014, NASA, 12 August 2014, JOURNAL, Cordiner, M.A., Mapping the Release of Volatiles in the Inner Comae of Comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON) Using the Atacama Large Millimeter/Submillimeter Array,weblink 11 August 2014, The Astrophysical Journal, 792, 1, 10.1088/2041-8205/792/1/L2, 12 August 2014, etal, 1408.2458, 2014ApJ...792L...2C, L2,

Spacecraft missions

{{See also|List of comets visited by spacecraft}}
  • The Halley Armada describes the collection of spacecraft missions that visited and/or made observations of Halley's Comet 1980s perihelion.
  • Deep Impact. Debate continues about how much ice is in a comet. In 2001, the Deep Space 1 spacecraft obtained high-resolution images of the surface of Comet Borrelly. It was found that the surface of comet Borrelly is hot and dry, with a temperature of between {{convert|26|to|71|C|F}}, and extremely dark, suggesting that the ice has been removed by solar heating and maturation, or is hidden by the soot-like material that covers Borrelly.WEB, NASA Spacecraft Finds Comet Has Hot, Dry Surface,weblink JPL, 5 April 2002, 22 August 2013, In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. The mission yielded results suggesting that the majority of a comet's water ice is below the surface and that these reservoirs feed the jets of vaporised water that form the coma of Tempel 1.WEB, NASA's 'Deep Impact' Team Reports First Evidence of Cometary Ice,weblink Brown University, 2 February 2006, 22 August 2013, Renamed EPOXI, it made a flyby of Comet Hartley 2 on 4 November 2010.
  • Stardust. Data from the Stardust mission show that materials retrieved from the tail of Wild 2 were crystalline and could only have been "born in fire", at extremely high temperatures of over {{convert|1000|C|F}}.NEWS, Rincon, Paul, Comets 'are born of fire and ice',weblink BBC News, 14 March 2006, 7 September 2013, WEB, Malik, T., NASA's Stardust Comet Samples Contain Minerals Born in Fire,weblink Space.com, 13 March 2006, 7 September 2013, Although comets formed in the outer Solar System, radial mixing of material during the early formation of the Solar System is thought to have redistributed material throughout the proto-planetary disk.
JOURNAL, 10.1038/nature03088, The building blocks of planets within the 'terrestrial' region of protoplanetary disks, 2004, Van Boekel, R., Min, M., Leinert, Ch., Waters, L.B.F.M., Richichi, A., Chesneau, O., Dominik, C., Jaffe, W., Dutrey, A., Graser, U., Henning, Th., De Jong, J., Köhler, R., De Koter, A., Lopez, B., Malbet, F., Morel, S., Paresce, F., Perrin, G., Preibisch, Th., Przygodda, F., Schöller, M., Wittkowski, M., Nature, 432, 7016, 479–82, 15565147, 2004Natur.432..479V, As a result, comets also contain crystalline grains that formed in the early, hot inner Solar System. This is seen in comet spectra as well as in sample return missions. More recent still, the materials retrieved demonstrate that the "comet dust resembles asteroid materials".WEB, Stardust comet dust resembles asteroid materials,weblink Lawrence Livermore National Laboratory, 24 January 2008, 7 September 2013, dead,weblink 28 May 2010, These new results have forced scientists to rethink the nature of comets and their distinction from asteroids.WEB, Dunham, Will, Dust samples prompt rethink about comets,weblink Reuters, 25 January 2008, 7 September 2013,
  • Rosetta. The Rosetta probe orbited Comet Churyumov–Gerasimenko. On 12 November 2014, its lander Philae successfully landed on the comet's surface, the first time a spacecraft has ever landed on such an object in history.WEB,weblink Rosetta Ready To Explore A Comet's Realm, ESA, 12 January 2004, 7 September 2013,

Great comets

File:Von einem Schrecklichen vnd Wunderbarlichen Cometen so sich den Dienstag nach Martini dieses lauffenden M. D. Lxxvij. Jahrs am Himmel erzeiget hat (grayscale).png|thumb|300px|Woodcut of the Great Comet of 1577Great Comet of 1577Approximately once a decade, a comet becomes bright enough to be noticed by a casual observer, leading such comets to be designated as great comets. Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions.BOOK,weblink 274, The World Almanac and Book of Facts 1996, 978-0-88687-780-4, Famighetti, Robert, 1995, Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it has a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular but failed to do so.WEB,weblink New 'Sun-Skirting' Comet Could Provide Dazzling Display in 2013, Universe Today, 7 September 2013, Atkinson, Nancy, Comet West, which appeared three years later, had much lower expectations but became an extremely impressive comet.WEB, Kronk, Gary W., C/1975 V1 (West),weblink Gary W. Kronk's Cometography, 7 September 2013, The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick succession—Comet Hyakutake in 1996, followed by Hale–Bopp, which reached maximum brightness in 1997 having been discovered two years earlier. The first great comet of the 21st century was C/2006 P1 (McNaught), which became visible to naked eye observers in January 2007. It was the brightest in over 40 years.WEB,weblink Great Moments in Comet History: Comet McNaught, Hubblesite, 15 August 2013,

Sungrazing comets

A sungrazing comet is a comet that passes extremely close to the Sun at perihelion, generally within a few million kilometres.BOOK,weblink 34, Hunting and Imaging Comets, 978-1-4419-6905-7, Mobberley, Martin, 2010, Although small sungrazers can be completely evaporated during such a close approach to the Sun, larger sungrazers can survive many perihelion passages. However, the strong tidal forces they experience often lead to their fragmentation.JOURNAL, 1966IrAJ....7..141O, Sun-Grazing Comets and Tidal Disruption, Opik, E. J., 7, 1966, 141, Irish Astronomical Journal, About 90% of the sungrazers observed with SOHO are members of the Kreutz group, which all originate from one giant comet that broke up into many smaller comets during its first passage through the inner Solar System.JOURNAL, Hahn, M. E., Chambers, J. E., Hahn, G., Origin of sungrazers: a frequent cometary end-state, Astronomy and Astrophysics, 257, 1, 315–322, 1992, 1992A&A...257..315B, The remainder contains some sporadic sungrazers, but four other related groups of comets have been identified among them: the Kracht, Kracht 2a, Marsden, and Meyer groups. The Marsden and Kracht groups both appear to be related to Comet 96P/Machholz, which is also the parent of two meteor streams, the Quadrantids and the Arietids.JOURNAL, Yoshikawa, K., Nakano, S., Yoshikawa, M., On the Association among Periodic Comet 96P/Machholz, Arietids, the Marsden Comet Group, and the Kracht Comet Group,weblink Publications of the Astronomical Society of Japan, 55, 1, 321–324, 2003, 10.1093/pasj/55.1.321, 2003PASJ...55..321O,

Unusual comets

File:Euler diagram of solar system bodies.svg|thumb|300px|right|Euler diagramEuler diagramOf the thousands of known comets, some exhibit unusual properties. Comet Encke (2P/Encke) orbits from outside the asteroid belt to just inside the orbit of the planet Mercury whereas the Comet 29P/Schwassmann–Wachmann currently travels in a nearly circular orbit entirely between the orbits of Jupiter and Saturn.WEB, Kronk, Gary W., 29P/Schwassmann–Wachmann 1,weblink Gary W. Kronk's Cometography, 22 September 2013, 2060 Chiron, whose unstable orbit is between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.WEB, Kronk, Gary W., 95P/Chiron,weblink Gary W. Kronk's Cometography, 27 April 2009, Similarly, Comet Shoemaker–Levy 2 was originally designated asteroid {{mp|1990 UL|3}}.WEB, Kronk, Gary W., 137P/Shoemaker–Levy 2,weblink Gary W. Kronk's Cometography, 27 April 2009, (See also Fate of comets, above)

Centaurs

Centaurs typically behave with characteristics of both asteroids and comets.JOURNAL, Horner, J., Evans, N.W., Bailey, M. E., Simulations of the Population of Centaurs I: The Bulk Statistics, 2004, astro-ph/0407400, 10.1111/j.1365-2966.2004.08240.x, Monthly Notices of the Royal Astronomical Society, 354, 3, 798–810, 2004MNRAS.354..798H, Centaurs can be classified as comets such as 60558 Echeclus, and 166P/NEAT. 166P/NEAT was discovered while it exhibited a coma, and so is classified as a comet despite its orbit, and 60558 Echeclus was discovered without a coma but later became active,Y-J. Choi, P.R. Weissman, and D. Polishook (60558) 2000 EC_98, IAU Circ., 8656 (Jan. 2006), 2. and was then classified as both a comet and an asteroid (174P/Echeclus). One plan for Cassini involved sending it to a centaur, but NASA decided to destroy it instead.WEB, Pappalardo, Bob, Spiker, Linda,weblink Cassini Proposed Extended-Extended Mission (XXM), Lunar and Planetary Institute, 15 March 2009, live, 18 July 2012,weblink" title="web.archive.org/web/20120718145449weblink">weblink

Observation

A comet may be discovered photographically using a wide-field telescope or visually with binoculars. However, even without access to optical equipment, it is still possible for the amateur astronomer to discover a sungrazing comet online by downloading images accumulated by some satellite observatories such as SOHO.WEB, Farmer, Steve E. Jr., Getting Started – SOHO Comet Hunting Techniques/Instructions,weblink Red Barn Observatory, 25 August 2013, dead,weblink" title="web.archive.org/web/20130404234938weblink">weblink 4 April 2013, SOHO's 2000th comet was discovered by Polish amateur astronomer Michał Kusiak on 26 December 2010WEB,weblink SOHO, 28 December 2010, NASA, 25 August 2013, and both discoverers of Hale–Bopp used amateur equipment (although Hale was not an amateur).

Lost

A number of periodic comets discovered in earlier decades or previous centuries are now lost comets. Their orbits were never known well enough to predict future appearances or the comets have disintegrated. However, occasionally a "new" comet is discovered, and calculation of its orbit shows it to be an old "lost" comet. An example is Comet 11P/Tempel–Swift–LINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001.WEB, Kronk, Gary W., 11P/Tempel–Swift–LINEAR,weblink Gary W. Kronk's Cometography, 27 April 2009, There are at least 18 comets that fit this category.WEB,weblink Lost periodic comets, The Catalogue of Comet Discoveries, Meyer, M., 2013, 18 July 2015,

In popular culture

{{See also|Comets in fiction|Category:Impact events in fiction}}The depiction of comets in popular culture is firmly rooted in the long Western tradition of seeing comets as harbingers of doom and as omens of world-altering change.BOOK,weblink 27–29, Science in Popular Culture: A Reference Guide, 978-0-313-31822-1, Bowdoin Van Riper, A, 2002, Halley's Comet alone has caused a slew of sensationalist publications of all sorts at each of its reappearances. It was especially noted that the birth and death of some notable persons coincided with separate appearances of the comet, such as with writers Mark Twain (who correctly speculated that he'd "go out with the comet" in 1910) and Eudora Welty, to whose life Mary Chapin Carpenter dedicated the song "Halley Came to Jackson".In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, the Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions,WEB, Ridpath, Ian, Ian Ridpath, Awaiting the Comet, A brief history of Halley's Comet,weblink 3 July 2008, 15 August 2013, whereas the appearance of Comet Hale–Bopp in 1997 triggered the mass suicide of the Heaven's Gate cult.NEWS, B. Drummond, Ayres Jr, Families Learning of 39 Cultists Who Died Willingly,weblink According to material the group posted on its Internet site, the timing of the suicides were probably related to the arrival of the Hale–Bopp comet, which members seemed to regard as a cosmic emissary beckoning them to another world, New York Times, 29 March 1997, 20 August 2013, In science fiction, the impact of comets has been depicted as a threat overcome by technology and heroism (as in the 1998 films Deep Impact and Armageddon), or as a trigger of global apocalypse (Lucifer's Hammer, 1979) or zombies (Night of the Comet, 1984). In Jules Verne's Off on a Comet a group of people are stranded on a comet orbiting the Sun, while a large manned space expedition visits Halley's Comet in Sir Arthur C. Clarke's novel (2061: Odyssey Three).NEWS,weblink The View From Halley's Comet : 2061: ODYSSEY THREE by Arthur C. Clarke, Los Angeles Times, Brin, David, 6 December 1987,

Gallery

File:Comet P1 McNaught02 - 23-01-07-edited.jpg|Comet C/2006 P1 (McNaught) taken from Victoria, Australia 2007File:Great Comet of 1882.jpg|The Great Comet of 1882 is a member of the Kreutz groupFile:Great Comet 1861.jpg|Great Comet 1861File:X-rays from Hyakutake.jpg|Comet Hyakutake (X-ray, ROSAT satellite)File:Asteroid P2013 P5 v2.jpg|"Active asteroid" 311P/PANSTARRS with several tails Active Asteroid P/2013 P5File:NASA-14090-Comet-C2013A1-SidingSpring-Hubble-20140311.jpg|Comet Siding Spring (Hubble; 11 March 2014)File:Comets WISE.jpg|Mosaic of 20 comets discovered by the WISE space telescopeFile:PIA22419-Neowise-1stFourYearsDataFromDec2013-20180420.gif|NEOWISE – first four years of data starting in December 2013File:Lovejoy-hi1a srem dec12 14.gif|C/2011 W3 (Lovejoy) heads towards the SunFile:ITS Impact.gif|View from the impactor in its last moments before hitting Comet Tempel 1 during the Deep Impact mission
Videos
File:NASA Developing Comet Harpoon for Sample Return.ogv|NASA is developing a comet harpoon for returning samples to EarthFile:Encke tail rip off.ogg|Comet Encke loses its tail{{clear}}

See also

References

{{Reflist|30em}}

Bibliography

  • BOOK,weblink Comet, Headline, London, Carl, Sagan, Carl Sagan, Ann, Druyan, Ann Druyan, 1997, 978-0-7472-7664-7, harv,

Further reading

  • BOOK, Comets, Popular Culture, and the Birth of Modern Cosmology, Princeton University Press, Sara J., Schechner, 1997, 978-0-691-01150-9,
  • BOOK, Introduction to Comets, Cambridge University Press, John C., Brandt, Robert D., Chapman, 2nd, 2004, 978-0-521-80863-7,

External links

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